Factor MessageTracker out of BdfIncomingMessageHook.

2026-02-13 11:19:04 +01:00 · 2016-11-11 16:55:06 +00:00
parent ab16ee7465
commit aa210fc555
46 changed files with 628 additions and 379 deletions
--- a/briar-core/src/org/briarproject/blogs/BlogManagerImpl.java
+++ b/briar-core/src/org/briarproject/blogs/BlogManagerImpl.java
@@ -27,6 +27,7 @@ import org.briarproject.api.identity.Author.Status;
 import org.briarproject.api.identity.AuthorId;
 import org.briarproject.api.identity.IdentityManager;
 import org.briarproject.api.identity.LocalAuthor;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupId;
 import org.briarproject.api.sync.Message;
@@ -69,6 +70,7 @@ import static org.briarproject.api.contact.ContactManager.AddContactHook;
 import static org.briarproject.api.contact.ContactManager.RemoveContactHook;
 import static org.briarproject.blogs.BlogPostValidator.authorToBdfDictionary;

+@NotNullByDefault
 class BlogManagerImpl extends BdfIncomingMessageHook implements BlogManager,
 		AddContactHook, RemoveContactHook, Client {

@@ -127,10 +129,8 @@ class BlogManagerImpl extends BdfIncomingMessageHook implements BlogManager,

 	@Override
 	public void removingContact(Transaction txn, Contact c) throws DbException {
-		if (c != null) {
-			Blog b = blogFactory.createBlog(c.getAuthor());
-			db.removeGroup(txn, b.getGroup());
-		}
+		Blog b = blogFactory.createBlog(c.getAuthor());
+		db.removeGroup(txn, b.getGroup());
 	}

 	@Override
--- a/briar-core/src/org/briarproject/blogs/BlogPostValidator.java
+++ b/briar-core/src/org/briarproject/blogs/BlogPostValidator.java
@@ -11,6 +11,7 @@ import org.briarproject.api.data.BdfEntry;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
 import org.briarproject.api.identity.Author;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupFactory;
 import org.briarproject.api.sync.InvalidMessageException;
@@ -42,6 +43,7 @@ import static org.briarproject.api.blogs.MessageType.COMMENT;
 import static org.briarproject.api.blogs.MessageType.POST;
 import static org.briarproject.api.identity.AuthorConstants.MAX_SIGNATURE_LENGTH;

+@NotNullByDefault
 class BlogPostValidator extends BdfMessageValidator {

 	private final GroupFactory groupFactory;
@@ -76,10 +78,10 @@ class BlogPostValidator extends BdfMessageValidator {
 				addMessageMetadata(c, m.getTimestamp());
 				break;
 			case WRAPPED_POST:
-				c = validateWrappedPost(m, g, body);
+				c = validateWrappedPost(body);
 				break;
 			case WRAPPED_COMMENT:
-				c = validateWrappedComment(m, g, body);
+				c = validateWrappedComment(body);
 				break;
 			default:
 				throw new InvalidMessageException("Unknown Message Type");
@@ -164,8 +166,8 @@ class BlogPostValidator extends BdfMessageValidator {
 		return new BdfMessageContext(meta, dependencies);
 	}

-	private BdfMessageContext validateWrappedPost(Message m, Group g,
-			BdfList body) throws InvalidMessageException, FormatException {
+	private BdfMessageContext validateWrappedPost(BdfList body)
+			throws InvalidMessageException, FormatException {

 		// p_group descriptor, p_timestamp, p_content, p_signature
 		checkSize(body, 4);
@@ -202,8 +204,8 @@ class BlogPostValidator extends BdfMessageValidator {
 		return new BdfMessageContext(meta);
 	}

-	private BdfMessageContext validateWrappedComment(Message m, Group g,
-			BdfList body) throws InvalidMessageException, FormatException {
+	private BdfMessageContext validateWrappedComment(BdfList body)
+			throws InvalidMessageException, FormatException {

 		// c_group descriptor, c_timestamp, c_comment, c_parent_original_id,
 		// c_parent_id, c_signature, parent_id
--- a/briar-core/src/org/briarproject/clients/BdfIncomingMessageHook.java
+++ b/briar-core/src/org/briarproject/clients/BdfIncomingMessageHook.java
@@ -3,31 +3,25 @@ package org.briarproject.clients;
 import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.MessageQueueManager.IncomingQueueMessageHook;
-import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.clients.QueueMessage;
 import org.briarproject.api.data.BdfDictionary;
-import org.briarproject.api.data.BdfEntry;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataParser;
 import org.briarproject.api.db.DatabaseComponent;
 import org.briarproject.api.db.DbException;
 import org.briarproject.api.db.Metadata;
 import org.briarproject.api.db.Transaction;
-import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.InvalidMessageException;
 import org.briarproject.api.sync.Message;
-import org.briarproject.api.sync.MessageId;
 import org.briarproject.api.sync.ValidationManager.IncomingMessageHook;

 import static org.briarproject.api.clients.QueueMessage.QUEUE_MESSAGE_HEADER_LENGTH;
 import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
-import static org.briarproject.clients.BdfConstants.GROUP_KEY_LATEST_MSG;
-import static org.briarproject.clients.BdfConstants.GROUP_KEY_MSG_COUNT;
-import static org.briarproject.clients.BdfConstants.GROUP_KEY_UNREAD_COUNT;
-import static org.briarproject.clients.BdfConstants.MSG_KEY_READ;

+@NotNullByDefault
 public abstract class BdfIncomingMessageHook implements IncomingMessageHook,
-		IncomingQueueMessageHook, MessageTracker {
+		IncomingQueueMessageHook {

 	protected final DatabaseComponent db;
 	protected final ClientHelper clientHelper;
@@ -44,16 +38,16 @@ public abstract class BdfIncomingMessageHook implements IncomingMessageHook,
 	 * Called once for each incoming message that passes validation.
 	 *
 	 * @throws DbException Should only be used for real database errors.
-	 * Do not rethrow
+	 * If this is thrown, delivery will be attempted again at next startup,
+	 * whereas if a FormatException is thrown, the message will be permanently
+	 * invalidated.
 	 * @throws FormatException Use this for any non-database error
 	 * that occurs while handling remotely created data.
 	 * This includes errors that occur while handling locally created data
 	 * in a context controlled by remotely created data
 	 * (for example, parsing the metadata of a dependency
 	 * of an incoming message).
-	 * Throwing this will delete the incoming message and its metadata
-	 * marking it as invalid in the database.
-	 * Never rethrow DbException as FormatException
+	 * Never rethrow DbException as FormatException!
 	 */
 	protected abstract boolean incomingMessage(Transaction txn, Message m,
 			BdfList body, BdfDictionary meta) throws DbException,
@@ -71,8 +65,12 @@ public abstract class BdfIncomingMessageHook implements IncomingMessageHook,

 	@Override
 	public void incomingMessage(Transaction txn, QueueMessage q, Metadata meta)
-			throws DbException, FormatException {
-		incomingMessage(txn, q, meta, QUEUE_MESSAGE_HEADER_LENGTH);
+			throws DbException, InvalidMessageException {
+		try {
+			incomingMessage(txn, q, meta, QUEUE_MESSAGE_HEADER_LENGTH);
+		} catch (FormatException e) {
+			throw new InvalidMessageException(e);
+		}
 	}

 	private boolean incomingMessage(Transaction txn, Message m, Metadata meta,
@@ -84,102 +82,4 @@ public abstract class BdfIncomingMessageHook implements IncomingMessageHook,
 		return incomingMessage(txn, m, body, metaDictionary);
 	}

-	protected void trackIncomingMessage(Transaction txn, Message m)
-			throws DbException {
-		trackMessage(txn, m.getGroupId(), m.getTimestamp(), false);
-	}
-
-	protected void trackOutgoingMessage(Transaction txn, Message m)
-			throws DbException {
-		trackMessage(txn, m.getGroupId(), m.getTimestamp(), true);
-	}
-
-	protected void trackMessage(Transaction txn, GroupId g, long time,
-			boolean read) throws DbException {
-		GroupCount c = getGroupCount(txn, g);
-		int msgCount = c.getMsgCount() + 1;
-		int unreadCount = c.getUnreadCount() + (read ? 0 : 1);
-		long latestTime =
-				time > c.getLatestMsgTime() ? time : c.getLatestMsgTime();
-		storeGroupCount(txn, g,
-				new GroupCount(msgCount, unreadCount, latestTime));
-	}
-
-	@Override
-	public GroupCount getGroupCount(GroupId g) throws DbException {
-		GroupCount count;
-		Transaction txn = db.startTransaction(true);
-		try {
-			count = getGroupCount(txn, g);
-			db.commitTransaction(txn);
-		}
-		finally {
-			db.endTransaction(txn);
-		}
-		return count;
-	}
-
-	protected GroupCount getGroupCount(Transaction txn, GroupId g)
-			throws DbException {
-		GroupCount count;
-		try {
-			BdfDictionary d = clientHelper.getGroupMetadataAsDictionary(txn, g);
-			count = new GroupCount(
-					d.getLong(GROUP_KEY_MSG_COUNT, 0L).intValue(),
-					d.getLong(GROUP_KEY_UNREAD_COUNT, 0L).intValue(),
-					d.getLong(GROUP_KEY_LATEST_MSG, 0L)
-			);
-		} catch (FormatException e) {
-			throw new DbException(e);
-		}
-		return count;
-	}
-
-	private void storeGroupCount(Transaction txn, GroupId g, GroupCount c)
-			throws DbException{
-		try {
-			BdfDictionary d = BdfDictionary.of(
-					new BdfEntry(GROUP_KEY_MSG_COUNT, c.getMsgCount()),
-					new BdfEntry(GROUP_KEY_UNREAD_COUNT, c.getUnreadCount()),
-					new BdfEntry(GROUP_KEY_LATEST_MSG, c.getLatestMsgTime())
-			);
-			clientHelper.mergeGroupMetadata(txn, g, d);
-		} catch (FormatException e) {
-			throw new DbException(e);
-		}
-	}
-
-	@Override
-	public void setReadFlag(GroupId g, MessageId m, boolean read)
-			throws DbException {
-		Transaction txn = db.startTransaction(false);
-		try {
-			// check current read status of message
-			BdfDictionary old =
-					clientHelper.getMessageMetadataAsDictionary(txn, m);
-			boolean wasRead = old.getBoolean(MSG_KEY_READ, false);
-
-			// if status changed
-			if (wasRead != read) {
-				// mark individual message as read
-				BdfDictionary meta = new BdfDictionary();
-				meta.put(MSG_KEY_READ, read);
-				clientHelper.mergeMessageMetadata(txn, m, meta);
-
-				// update unread counter in group metadata
-				GroupCount c = getGroupCount(txn, g);
-				BdfDictionary d = new BdfDictionary();
-				int count = c.getUnreadCount() + (read ? -1 : 1);
-				if (count < 0) throw new DbException();
-				d.put(GROUP_KEY_UNREAD_COUNT, count);
-				clientHelper.mergeGroupMetadata(txn, g, d);
-			}
-			db.commitTransaction(txn);
-		} catch (FormatException e) {
-			throw new DbException(e);
-		} finally {
-			db.endTransaction(txn);
-		}
-	}
-
 }
--- a/briar-core/src/org/briarproject/clients/BdfMessageValidator.java
+++ b/briar-core/src/org/briarproject/clients/BdfMessageValidator.java
@@ -9,6 +9,7 @@ import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
 import org.briarproject.api.db.Metadata;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.InvalidMessageException;
 import org.briarproject.api.sync.Message;
@@ -25,6 +26,7 @@ import static org.briarproject.api.clients.QueueMessage.QUEUE_MESSAGE_HEADER_LEN
 import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
 import static org.briarproject.api.transport.TransportConstants.MAX_CLOCK_DIFFERENCE;

+@NotNullByDefault
 public abstract class BdfMessageValidator implements MessageValidator,
 		QueueMessageValidator {

@@ -108,7 +110,7 @@ public abstract class BdfMessageValidator implements MessageValidator,
 		if (b != null && b.length != length) throw new FormatException();
 	}

-	protected void checkSize(BdfList list, int minSize, int maxSize)
+	protected void checkSize(@Nullable BdfList list, int minSize, int maxSize)
 			throws FormatException {
 		if (list != null) {
 			if (list.size() < minSize) throw new FormatException();
--- a/briar-core/src/org/briarproject/clients/ClientsModule.java
+++ b/briar-core/src/org/briarproject/clients/ClientsModule.java
@@ -3,6 +3,7 @@ package org.briarproject.clients;
 import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.ContactGroupFactory;
 import org.briarproject.api.clients.MessageQueueManager;
+import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.clients.QueueMessageFactory;
 import org.briarproject.api.crypto.CryptoComponent;
 import org.briarproject.api.data.BdfReaderFactory;
@@ -52,4 +53,8 @@ public class ClientsModule {
 		return new QueueMessageFactoryImpl(crypto);
 	}

+	@Provides
+	MessageTracker provideMessageTracker(MessageTrackerImpl messageTracker) {
+		return messageTracker;
+	}
 }
--- a/briar-core/src/org/briarproject/clients/ConversationClientImpl.java
+++ b/briar-core/src/org/briarproject/clients/ConversationClientImpl.java
@@ -1,6 +1,8 @@
 package org.briarproject.clients;

 import org.briarproject.api.clients.ClientHelper;
+import org.briarproject.api.clients.MessageTracker;
+import org.briarproject.api.clients.MessageTracker.GroupCount;
 import org.briarproject.api.contact.Contact;
 import org.briarproject.api.contact.ContactId;
 import org.briarproject.api.data.MetadataParser;
@@ -8,25 +10,34 @@ import org.briarproject.api.db.DatabaseComponent;
 import org.briarproject.api.db.DbException;
 import org.briarproject.api.db.Transaction;
 import org.briarproject.api.messaging.ConversationManager.ConversationClient;
-import org.briarproject.api.sync.Group;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.sync.MessageId;

+@NotNullByDefault
 public abstract class ConversationClientImpl extends BdfIncomingMessageHook
 		implements ConversationClient {

-	protected ConversationClientImpl(DatabaseComponent db,
-			ClientHelper clientHelper, MetadataParser metadataParser) {
-		super(db, clientHelper, metadataParser);
-	}
+	protected final MessageTracker messageTracker;

-	protected abstract Group getContactGroup(Contact contact);
+	protected ConversationClientImpl(DatabaseComponent db,
+			ClientHelper clientHelper, MetadataParser metadataParser,
+			MessageTracker messageTracker) {
+		super(db, clientHelper, metadataParser);
+		this.messageTracker = messageTracker;
+	}

 	@Override
 	public GroupCount getGroupCount(Transaction txn, ContactId contactId)
 			throws DbException {
 		Contact contact = db.getContact(txn, contactId);
 		GroupId groupId = getContactGroup(contact).getId();
-		return getGroupCount(txn, groupId);
+		return messageTracker.getGroupCount(txn, groupId);
 	}

+	@Override
+	public void setReadFlag(GroupId g, MessageId m, boolean read)
+			throws DbException {
+		messageTracker.setReadFlag(g, m, read);
+	}
 }
--- a/briar-core/src/org/briarproject/clients/MessageQueueManagerImpl.java
+++ b/briar-core/src/org/briarproject/clients/MessageQueueManagerImpl.java
@@ -11,6 +11,7 @@ import org.briarproject.api.db.DatabaseComponent;
 import org.briarproject.api.db.DbException;
 import org.briarproject.api.db.Metadata;
 import org.briarproject.api.db.Transaction;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.ClientId;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupId;
@@ -20,6 +21,7 @@ import org.briarproject.api.sync.MessageContext;
 import org.briarproject.api.sync.MessageId;
 import org.briarproject.api.sync.ValidationManager;
 import org.briarproject.api.sync.ValidationManager.IncomingMessageHook;
+import org.briarproject.api.sync.ValidationManager.MessageValidator;
 import org.briarproject.util.ByteUtils;

 import java.util.ArrayList;
@@ -29,12 +31,14 @@ import java.util.Map.Entry;
 import java.util.TreeMap;
 import java.util.logging.Logger;

+import javax.annotation.Nullable;
 import javax.inject.Inject;

 import static java.util.logging.Level.INFO;
 import static org.briarproject.api.clients.QueueMessage.QUEUE_MESSAGE_HEADER_LENGTH;
 import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;

+@NotNullByDefault
 class MessageQueueManagerImpl implements MessageQueueManager {

 	private static final String OUTGOING_POSITION_KEY = "nextOut";
@@ -139,6 +143,7 @@ class MessageQueueManagerImpl implements MessageQueueManager {
 			this.pending = pending;
 		}

+		@Nullable
 		MessageId popIncomingMessageId() {
 			Iterator<Entry<Long, MessageId>> it = pending.entrySet().iterator();
 			if (!it.hasNext()) {
@@ -161,8 +166,9 @@ class MessageQueueManagerImpl implements MessageQueueManager {
 		}
 	}

+	@NotNullByDefault
 	private static class DelegatingMessageValidator
-			implements ValidationManager.MessageValidator {
+			implements MessageValidator {

 		private final QueueMessageValidator delegate;

@@ -174,21 +180,24 @@ class MessageQueueManagerImpl implements MessageQueueManager {
 		public MessageContext validateMessage(Message m, Group g)
 				throws InvalidMessageException {
 			byte[] raw = m.getRaw();
-			if (raw.length < QUEUE_MESSAGE_HEADER_LENGTH) return null;
+			if (raw.length < QUEUE_MESSAGE_HEADER_LENGTH)
+				throw new InvalidMessageException();
 			long queuePosition = ByteUtils.readUint64(raw,
 					MESSAGE_HEADER_LENGTH);
-			if (queuePosition < 0) return null;
+			if (queuePosition < 0) throw new InvalidMessageException();
 			QueueMessage q = new QueueMessage(m.getId(), m.getGroupId(),
 					m.getTimestamp(), queuePosition, raw);
 			return delegate.validateMessage(q, g);
 		}
 	}

+	@NotNullByDefault
 	private class DelegatingIncomingMessageHook implements IncomingMessageHook {

 		private final IncomingQueueMessageHook delegate;

-		private DelegatingIncomingMessageHook(IncomingQueueMessageHook delegate) {
+		private DelegatingIncomingMessageHook(
+				IncomingQueueMessageHook delegate) {
 			this.delegate = delegate;
 		}

@@ -227,20 +236,16 @@ class MessageQueueManagerImpl implements MessageQueueManager {
 				// Save the queue state before passing control to the delegate
 				saveQueueState(txn, m.getGroupId(), queueState);
 				// Deliver the messages to the delegate
-				try {
-					delegate.incomingMessage(txn, q, meta);
-					for (MessageId id : consecutive) {
-						byte[] raw = db.getRawMessage(txn, id);
-						meta = db.getMessageMetadata(txn, id);
-						q = queueMessageFactory.createMessage(id, raw);
-						if (LOG.isLoggable(INFO)) {
-							LOG.info("Delivering pending message with position "
-									+ q.getQueuePosition());
-						}
-						delegate.incomingMessage(txn, q, meta);
+				delegate.incomingMessage(txn, q, meta);
+				for (MessageId id : consecutive) {
+					byte[] raw = db.getRawMessage(txn, id);
+					meta = db.getMessageMetadata(txn, id);
+					q = queueMessageFactory.createMessage(id, raw);
+					if (LOG.isLoggable(INFO)) {
+						LOG.info("Delivering pending message with position "
+								+ q.getQueuePosition());
 					}
-				} catch (FormatException e) {
-					throw new InvalidMessageException(e);
+					delegate.incomingMessage(txn, q, meta);
 				}
 			}
 			// message queues are only useful for groups with two members
--- a/briar-core/src/org/briarproject/clients/MessageTrackerImpl.java
+++ b/briar-core/src/org/briarproject/clients/MessageTrackerImpl.java
@@ -0,0 +1,132 @@
+package org.briarproject.clients;
+
+import org.briarproject.api.FormatException;
+import org.briarproject.api.clients.ClientHelper;
+import org.briarproject.api.clients.MessageTracker;
+import org.briarproject.api.data.BdfDictionary;
+import org.briarproject.api.data.BdfEntry;
+import org.briarproject.api.db.DatabaseComponent;
+import org.briarproject.api.db.DbException;
+import org.briarproject.api.db.Transaction;
+import org.briarproject.api.nullsafety.NotNullByDefault;
+import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.sync.Message;
+import org.briarproject.api.sync.MessageId;
+
+import javax.inject.Inject;
+
+import static org.briarproject.clients.BdfConstants.GROUP_KEY_LATEST_MSG;
+import static org.briarproject.clients.BdfConstants.GROUP_KEY_MSG_COUNT;
+import static org.briarproject.clients.BdfConstants.GROUP_KEY_UNREAD_COUNT;
+import static org.briarproject.clients.BdfConstants.MSG_KEY_READ;
+
+@NotNullByDefault
+class MessageTrackerImpl implements MessageTracker {
+
+	private final DatabaseComponent db;
+	private final ClientHelper clientHelper;
+
+	@Inject
+	MessageTrackerImpl(DatabaseComponent db, ClientHelper clientHelper) {
+		this.db = db;
+		this.clientHelper = clientHelper;
+	}
+
+	@Override
+	public void trackIncomingMessage(Transaction txn, Message m)
+			throws DbException {
+		trackMessage(txn, m.getGroupId(), m.getTimestamp(), false);
+	}
+
+	@Override
+	public void trackOutgoingMessage(Transaction txn, Message m)
+			throws DbException {
+		trackMessage(txn, m.getGroupId(), m.getTimestamp(), true);
+	}
+
+	@Override
+	public void trackMessage(Transaction txn, GroupId g, long time,
+			boolean read) throws DbException {
+		GroupCount c = getGroupCount(txn, g);
+		int msgCount = c.getMsgCount() + 1;
+		int unreadCount = c.getUnreadCount() + (read ? 0 : 1);
+		long latestMsgTime = Math.max(c.getLatestMsgTime(), time);
+		storeGroupCount(txn, g, new GroupCount(msgCount, unreadCount,
+				latestMsgTime));
+	}
+
+	@Override
+	public GroupCount getGroupCount(GroupId g) throws DbException {
+		GroupCount count;
+		Transaction txn = db.startTransaction(true);
+		try {
+			count = getGroupCount(txn, g);
+			db.commitTransaction(txn);
+		} finally {
+			db.endTransaction(txn);
+		}
+		return count;
+	}
+
+	@Override
+	public GroupCount getGroupCount(Transaction txn, GroupId g)
+			throws DbException {
+		try {
+			BdfDictionary d = clientHelper.getGroupMetadataAsDictionary(txn, g);
+			return new GroupCount(
+					d.getLong(GROUP_KEY_MSG_COUNT, 0L).intValue(),
+					d.getLong(GROUP_KEY_UNREAD_COUNT, 0L).intValue(),
+					d.getLong(GROUP_KEY_LATEST_MSG, 0L)
+			);
+		} catch (FormatException e) {
+			throw new DbException(e);
+		}
+	}
+
+	private void storeGroupCount(Transaction txn, GroupId g, GroupCount c)
+			throws DbException {
+		try {
+			BdfDictionary d = BdfDictionary.of(
+					new BdfEntry(GROUP_KEY_MSG_COUNT, c.getMsgCount()),
+					new BdfEntry(GROUP_KEY_UNREAD_COUNT, c.getUnreadCount()),
+					new BdfEntry(GROUP_KEY_LATEST_MSG, c.getLatestMsgTime())
+			);
+			clientHelper.mergeGroupMetadata(txn, g, d);
+		} catch (FormatException e) {
+			throw new DbException(e);
+		}
+	}
+
+	@Override
+	public void setReadFlag(GroupId g, MessageId m, boolean read)
+			throws DbException {
+		Transaction txn = db.startTransaction(false);
+		try {
+			// check current read status of message
+			BdfDictionary old =
+					clientHelper.getMessageMetadataAsDictionary(txn, m);
+			boolean wasRead = old.getBoolean(MSG_KEY_READ, false);
+
+			// if status changed
+			if (wasRead != read) {
+				// mark individual message as read
+				BdfDictionary meta = new BdfDictionary();
+				meta.put(MSG_KEY_READ, read);
+				clientHelper.mergeMessageMetadata(txn, m, meta);
+
+				// update unread counter in group metadata
+				GroupCount c = getGroupCount(txn, g);
+				int unreadCount = c.getUnreadCount() + (read ? -1 : 1);
+				if (unreadCount < 0) throw new DbException();
+				storeGroupCount(txn, g, new GroupCount(c.getMsgCount(),
+						unreadCount, c.getLatestMsgTime()));
+			}
+			db.commitTransaction(txn);
+		} catch (FormatException e) {
+			throw new DbException(e);
+		} finally {
+			db.endTransaction(txn);
+		}
+	}
+
+}
--- a/briar-core/src/org/briarproject/forum/ForumManagerImpl.java
+++ b/briar-core/src/org/briarproject/forum/ForumManagerImpl.java
@@ -2,6 +2,8 @@ package org.briarproject.forum;

 import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.ClientHelper;
+import org.briarproject.api.clients.MessageTracker;
+import org.briarproject.api.clients.MessageTracker.GroupCount;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataParser;
@@ -20,6 +22,7 @@ import org.briarproject.api.identity.Author.Status;
 import org.briarproject.api.identity.AuthorId;
 import org.briarproject.api.identity.IdentityManager;
 import org.briarproject.api.identity.LocalAuthor;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupId;
 import org.briarproject.api.sync.Message;
@@ -52,22 +55,25 @@ import static org.briarproject.api.identity.Author.Status.ANONYMOUS;
 import static org.briarproject.api.identity.Author.Status.OURSELVES;
 import static org.briarproject.clients.BdfConstants.MSG_KEY_READ;

+@NotNullByDefault
 class ForumManagerImpl extends BdfIncomingMessageHook implements ForumManager {

 	private final IdentityManager identityManager;
 	private final ForumFactory forumFactory;
 	private final ForumPostFactory forumPostFactory;
+	private final MessageTracker messageTracker;
 	private final List<RemoveForumHook> removeHooks;

 	@Inject
 	ForumManagerImpl(DatabaseComponent db, IdentityManager identityManager,
 			ClientHelper clientHelper, MetadataParser metadataParser,
-			ForumFactory forumFactory, ForumPostFactory forumPostFactory) {
+			ForumFactory forumFactory, ForumPostFactory forumPostFactory,
+			MessageTracker messageTracker) {
 		super(db, clientHelper, metadataParser);
-
 		this.identityManager = identityManager;
 		this.forumFactory = forumFactory;
 		this.forumPostFactory = forumPostFactory;
+		this.messageTracker = messageTracker;
 		removeHooks = new CopyOnWriteArrayList<RemoveForumHook>();
 	}

@@ -75,7 +81,7 @@ class ForumManagerImpl extends BdfIncomingMessageHook implements ForumManager {
 	protected boolean incomingMessage(Transaction txn, Message m, BdfList body,
 			BdfDictionary meta) throws DbException, FormatException {

-		trackIncomingMessage(txn, m);
+		messageTracker.trackIncomingMessage(txn, m);

 		ForumPostHeader post = getForumPostHeader(txn, m.getId(), meta);
 		ForumPostReceivedEvent event =
@@ -146,7 +152,7 @@ class ForumManagerImpl extends BdfIncomingMessageHook implements ForumManager {
 			meta.put(KEY_LOCAL, true);
 			meta.put(MSG_KEY_READ, true);
 			clientHelper.addLocalMessage(txn, p.getMessage(), meta, true);
-			trackOutgoingMessage(txn, p.getMessage());
+			messageTracker.trackOutgoingMessage(txn, p.getMessage());
 			db.commitTransaction(txn);
 		} catch (FormatException e) {
 			throw new RuntimeException(e);
@@ -204,6 +210,7 @@ class ForumManagerImpl extends BdfIncomingMessageHook implements ForumManager {
 		try {
 			// Parent ID, author, forum post body, signature
 			BdfList message = clientHelper.getMessageAsList(m);
+			if (message == null) throw new DbException();
 			return message.getString(2);
 		} catch (FormatException e) {
 			throw new DbException(e);
@@ -253,6 +260,17 @@ class ForumManagerImpl extends BdfIncomingMessageHook implements ForumManager {
 		removeHooks.add(hook);
 	}

+	@Override
+	public GroupCount getGroupCount(GroupId g) throws DbException {
+		return messageTracker.getGroupCount(g);
+	}
+
+	@Override
+	public void setReadFlag(GroupId g, MessageId m, boolean read)
+			throws DbException {
+		messageTracker.setReadFlag(g, m, read);
+	}
+
 	private Forum parseForum(Group g) throws FormatException {
 		byte[] descriptor = g.getDescriptor();
 		// Name, salt
--- a/briar-core/src/org/briarproject/forum/ForumPostValidator.java
+++ b/briar-core/src/org/briarproject/forum/ForumPostValidator.java
@@ -9,6 +9,7 @@ import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
 import org.briarproject.api.identity.Author;
 import org.briarproject.api.identity.AuthorFactory;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.InvalidMessageException;
 import org.briarproject.api.sync.Message;
@@ -25,6 +26,7 @@ import static org.briarproject.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENG
 import static org.briarproject.api.identity.AuthorConstants.MAX_PUBLIC_KEY_LENGTH;
 import static org.briarproject.api.identity.AuthorConstants.MAX_SIGNATURE_LENGTH;

+@NotNullByDefault
 class ForumPostValidator extends BdfMessageValidator {

 	private final AuthorFactory authorFactory;
--- a/briar-core/src/org/briarproject/introduction/IntroductionManagerImpl.java
+++ b/briar-core/src/org/briarproject/introduction/IntroductionManagerImpl.java
@@ -3,6 +3,7 @@ package org.briarproject.introduction;
 import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.Client;
 import org.briarproject.api.clients.ClientHelper;
+import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.contact.Contact;
 import org.briarproject.api.contact.ContactId;
@@ -23,6 +24,7 @@ import org.briarproject.api.introduction.IntroductionManager;
 import org.briarproject.api.introduction.IntroductionMessage;
 import org.briarproject.api.introduction.IntroductionRequest;
 import org.briarproject.api.introduction.IntroductionResponse;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupId;
 import org.briarproject.api.sync.Message;
@@ -73,6 +75,7 @@ import static org.briarproject.api.introduction.IntroductionConstants.TYPE_REQUE
 import static org.briarproject.api.introduction.IntroductionConstants.TYPE_RESPONSE;
 import static org.briarproject.clients.BdfConstants.MSG_KEY_READ;

+@NotNullByDefault
 class IntroductionManagerImpl extends ConversationClientImpl
 		implements IntroductionManager, Client, AddContactHook,
 		RemoveContactHook {
@@ -86,11 +89,12 @@ class IntroductionManagerImpl extends ConversationClientImpl

 	@Inject
 	IntroductionManagerImpl(DatabaseComponent db, ClientHelper clientHelper,
-			MetadataParser metadataParser, IntroducerManager introducerManager,
+			MetadataParser metadataParser, MessageTracker messageTracker,
+			IntroducerManager introducerManager,
 			IntroduceeManager introduceeManager,
 			IntroductionGroupFactory introductionGroupFactory) {

-		super(db, clientHelper, metadataParser);
+		super(db, clientHelper, metadataParser, messageTracker);
 		this.introducerManager = introducerManager;
 		this.introduceeManager = introduceeManager;
 		this.introductionGroupFactory = introductionGroupFactory;
@@ -213,7 +217,7 @@ class IntroductionManagerImpl extends ConversationClientImpl
 					introduceeManager.initialize(txn, groupId, message);
 			try {
 				introduceeManager.incomingMessage(txn, state, message);
-				trackIncomingMessage(txn, m);
+				messageTracker.trackIncomingMessage(txn, m);
 			} catch (DbException e) {
 				if (LOG.isLoggable(WARNING)) LOG.log(WARNING, e.toString(), e);
 				introduceeManager.abort(txn, state);
@@ -239,7 +243,8 @@ class IntroductionManagerImpl extends ConversationClientImpl
 						LOG.warning("Unknown role '" + role + "'");
 					throw new DbException();
 				}
-				if (type == TYPE_RESPONSE) trackIncomingMessage(txn, m);
+				if (type == TYPE_RESPONSE)
+					messageTracker.trackIncomingMessage(txn, m);
 			} catch (DbException e) {
 				if (LOG.isLoggable(WARNING)) LOG.log(WARNING, e.toString(), e);
 				if (role == ROLE_INTRODUCER) introducerManager.abort(txn, state);
@@ -260,7 +265,7 @@ class IntroductionManagerImpl extends ConversationClientImpl
 	}

 	@Override
-	protected Group getContactGroup(Contact contact) {
+	public Group getContactGroup(Contact contact) {
 		return introductionGroupFactory.createIntroductionGroup(contact);
 	}

@@ -274,8 +279,8 @@ class IntroductionManagerImpl extends ConversationClientImpl
 			introducerManager.makeIntroduction(txn, c1, c2, msg, timestamp);
 			Group g1 = getContactGroup(c1);
 			Group g2 = getContactGroup(c2);
-			trackMessage(txn, g1.getId(), timestamp, true);
-			trackMessage(txn, g2.getId(), timestamp, true);
+			messageTracker.trackMessage(txn, g1.getId(), timestamp, true);
+			messageTracker.trackMessage(txn, g2.getId(), timestamp, true);
 			db.commitTransaction(txn);
 		} finally {
 			db.endTransaction(txn);
@@ -295,7 +300,7 @@ class IntroductionManagerImpl extends ConversationClientImpl
 					getSessionState(txn, g.getId(), sessionId.getBytes());

 			introduceeManager.acceptIntroduction(txn, state, timestamp);
-			trackMessage(txn, g.getId(), timestamp, true);
+			messageTracker.trackMessage(txn, g.getId(), timestamp, true);
 			db.commitTransaction(txn);
 		} finally {
 			db.endTransaction(txn);
@@ -315,7 +320,7 @@ class IntroductionManagerImpl extends ConversationClientImpl
 					getSessionState(txn, g.getId(), sessionId.getBytes());

 			introduceeManager.declineIntroduction(txn, state, timestamp);
-			trackMessage(txn, g.getId(), timestamp, true);
+			messageTracker.trackMessage(txn, g.getId(), timestamp, true);
 			db.commitTransaction(txn);
 		} finally {
 			db.endTransaction(txn);
--- a/briar-core/src/org/briarproject/introduction/IntroductionValidator.java
+++ b/briar-core/src/org/briarproject/introduction/IntroductionValidator.java
@@ -7,6 +7,7 @@ import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.system.Clock;
@@ -39,6 +40,7 @@ import static org.briarproject.api.introduction.IntroductionConstants.TYPE_RESPO
 import static org.briarproject.api.properties.TransportPropertyConstants.MAX_PROPERTIES_PER_TRANSPORT;
 import static org.briarproject.api.properties.TransportPropertyConstants.MAX_PROPERTY_LENGTH;

+@NotNullByDefault
 class IntroductionValidator extends BdfMessageValidator {

 	IntroductionValidator(ClientHelper clientHelper,
--- a/briar-core/src/org/briarproject/messaging/ConversationManagerImpl.java
+++ b/briar-core/src/org/briarproject/messaging/ConversationManagerImpl.java
@@ -6,12 +6,14 @@ import org.briarproject.api.db.DatabaseComponent;
 import org.briarproject.api.db.DbException;
 import org.briarproject.api.db.Transaction;
 import org.briarproject.api.messaging.ConversationManager;
+import org.briarproject.api.nullsafety.NotNullByDefault;

 import java.util.Set;
 import java.util.concurrent.CopyOnWriteArraySet;

 import javax.inject.Inject;

+@NotNullByDefault
 class ConversationManagerImpl implements ConversationManager {

 	private final DatabaseComponent db;
@@ -32,9 +34,7 @@ class ConversationManagerImpl implements ConversationManager {
 	}

 	@Override
-	public GroupCount getGroupCount(ContactId contactId)
-			throws DbException {
-
+	public GroupCount getGroupCount(ContactId contactId) throws DbException {
 		int msgCount = 0, unreadCount = 0;
 		long latestTime = 0;
 		Transaction txn = db.startTransaction(true);
--- a/briar-core/src/org/briarproject/messaging/MessagingManagerImpl.java
+++ b/briar-core/src/org/briarproject/messaging/MessagingManagerImpl.java
@@ -4,6 +4,7 @@ import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.Client;
 import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.ContactGroupFactory;
+import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.contact.Contact;
 import org.briarproject.api.contact.ContactId;
 import org.briarproject.api.contact.ContactManager.AddContactHook;
@@ -18,6 +19,7 @@ import org.briarproject.api.event.PrivateMessageReceivedEvent;
 import org.briarproject.api.messaging.MessagingManager;
 import org.briarproject.api.messaging.PrivateMessage;
 import org.briarproject.api.messaging.PrivateMessageHeader;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupId;
 import org.briarproject.api.sync.Message;
@@ -33,6 +35,7 @@ import javax.inject.Inject;

 import static org.briarproject.clients.BdfConstants.MSG_KEY_READ;

+@NotNullByDefault
 class MessagingManagerImpl extends ConversationClientImpl
 		implements MessagingManager, Client, AddContactHook, RemoveContactHook {

@@ -40,9 +43,9 @@ class MessagingManagerImpl extends ConversationClientImpl

 	@Inject
 	MessagingManagerImpl(DatabaseComponent db, ClientHelper clientHelper,
-			MetadataParser metadataParser,
+			MetadataParser metadataParser, MessageTracker messageTracker,
 			ContactGroupFactory contactGroupFactory) {
-		super(db, clientHelper, metadataParser);
+		super(db, clientHelper, metadataParser, messageTracker);
 		this.contactGroupFactory = contactGroupFactory;
 	}

@@ -72,7 +75,7 @@ class MessagingManagerImpl extends ConversationClientImpl
 	}

 	@Override
-	protected Group getContactGroup(Contact c) {
+	public Group getContactGroup(Contact c) {
 		return contactGroupFactory.createContactGroup(CLIENT_ID, c);
 	}

@@ -95,7 +98,7 @@ class MessagingManagerImpl extends ConversationClientImpl
 		PrivateMessageReceivedEvent event = new PrivateMessageReceivedEvent(
 				header, contactId, groupId);
 		txn.attach(event);
-		trackIncomingMessage(txn, m);
+		messageTracker.trackIncomingMessage(txn, m);

 		// don't share message
 		return false;
@@ -110,7 +113,7 @@ class MessagingManagerImpl extends ConversationClientImpl
 			meta.put("local", true);
 			meta.put("read", true);
 			clientHelper.addLocalMessage(txn, m.getMessage(), meta, true);
-			trackOutgoingMessage(txn, m.getMessage());
+			messageTracker.trackOutgoingMessage(txn, m.getMessage());
 			db.commitTransaction(txn);
 		} catch (FormatException e) {
 			throw new RuntimeException(e);
@@ -195,20 +198,11 @@ class MessagingManagerImpl extends ConversationClientImpl
 		try {
 			// 0: private message body
 			BdfList message = clientHelper.getMessageAsList(m);
+			if (message == null) throw new DbException();
 			return message.getString(0);
 		} catch (FormatException e) {
 			throw new DbException(e);
 		}
 	}

-	@Override
-	public GroupCount getGroupCount(Transaction txn, ContactId contactId)
-			throws DbException {
-
-		Contact contact = db.getContact(txn, contactId);
-		GroupId groupId = getContactGroup(contact).getId();
-
-		return getGroupCount(txn, groupId);
-	}
-
 }
--- a/briar-core/src/org/briarproject/messaging/PrivateMessageValidator.java
+++ b/briar-core/src/org/briarproject/messaging/PrivateMessageValidator.java
@@ -6,6 +6,7 @@ import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.system.Clock;
@@ -14,6 +15,7 @@ import org.briarproject.clients.BdfMessageValidator;
 import static org.briarproject.api.messaging.MessagingConstants.MAX_PRIVATE_MESSAGE_BODY_LENGTH;
 import static org.briarproject.clients.BdfConstants.MSG_KEY_READ;

+@NotNullByDefault
 class PrivateMessageValidator extends BdfMessageValidator {

 	PrivateMessageValidator(ClientHelper clientHelper,
--- a/briar-core/src/org/briarproject/privategroup/GroupMessageValidator.java
+++ b/briar-core/src/org/briarproject/privategroup/GroupMessageValidator.java
@@ -8,6 +8,7 @@ import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
 import org.briarproject.api.identity.Author;
 import org.briarproject.api.identity.AuthorFactory;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.privategroup.MessageType;
 import org.briarproject.api.privategroup.PrivateGroup;
 import org.briarproject.api.privategroup.PrivateGroupFactory;
@@ -38,6 +39,7 @@ import static org.briarproject.privategroup.GroupConstants.KEY_READ;
 import static org.briarproject.privategroup.GroupConstants.KEY_TIMESTAMP;
 import static org.briarproject.privategroup.GroupConstants.KEY_TYPE;

+@NotNullByDefault
 class GroupMessageValidator extends BdfMessageValidator {

 	private final PrivateGroupFactory privateGroupFactory;
--- a/briar-core/src/org/briarproject/privategroup/PrivateGroupManagerImpl.java
+++ b/briar-core/src/org/briarproject/privategroup/PrivateGroupManagerImpl.java
@@ -2,6 +2,8 @@ package org.briarproject.privategroup;

 import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.ClientHelper;
+import org.briarproject.api.clients.MessageTracker;
+import org.briarproject.api.clients.MessageTracker.GroupCount;
 import org.briarproject.api.clients.ProtocolStateException;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfEntry;
@@ -75,17 +77,18 @@ public class PrivateGroupManagerImpl extends BdfIncomingMessageHook implements

 	private final PrivateGroupFactory privateGroupFactory;
 	private final IdentityManager identityManager;
+	private final MessageTracker messageTracker;
 	private final List<PrivateGroupHook> hooks;

 	@Inject
 	PrivateGroupManagerImpl(ClientHelper clientHelper,
 			MetadataParser metadataParser, DatabaseComponent db,
 			PrivateGroupFactory privateGroupFactory,
-			IdentityManager identityManager) {
+			IdentityManager identityManager, MessageTracker messageTracker) {
 		super(db, clientHelper, metadataParser);
-
 		this.privateGroupFactory = privateGroupFactory;
 		this.identityManager = identityManager;
+		this.messageTracker = messageTracker;
 		hooks = new CopyOnWriteArrayList<PrivateGroupHook>();
 	}

@@ -126,7 +129,7 @@ public class PrivateGroupManagerImpl extends BdfIncomingMessageHook implements
 		meta.put(KEY_TYPE, JOIN.getInt());
 		addMessageMetadata(meta, m, true);
 		clientHelper.addLocalMessage(txn, m.getMessage(), meta, true);
-		trackOutgoingMessage(txn, m.getMessage());
+		messageTracker.trackOutgoingMessage(txn, m.getMessage());
 		addMember(txn, m.getMessage().getGroupId(), m.getMember(), VISIBLE);
 		setPreviousMsgId(txn, m.getMessage().getGroupId(),
 				m.getMessage().getId());
@@ -218,7 +221,7 @@ public class PrivateGroupManagerImpl extends BdfIncomingMessageHook implements

 			// track message
 			setPreviousMsgId(txn, g, m.getMessage().getId());
-			trackOutgoingMessage(txn, m.getMessage());
+			messageTracker.trackOutgoingMessage(txn, m.getMessage());

 			// broadcast event
 			attachGroupMessageAddedEvent(txn, m.getMessage(), meta, true);
@@ -437,6 +440,17 @@ public class PrivateGroupManagerImpl extends BdfIncomingMessageHook implements
 		return false;
 	}

+	@Override
+	public GroupCount getGroupCount(GroupId g) throws DbException {
+		return messageTracker.getGroupCount(g);
+	}
+
+	@Override
+	public void setReadFlag(GroupId g, MessageId m, boolean read)
+			throws DbException {
+		messageTracker.setReadFlag(g, m, read);
+	}
+
 	@Override
 	public void relationshipRevealed(Transaction txn, GroupId g, AuthorId a,
 			boolean byContact) throws FormatException, DbException {
@@ -500,7 +514,7 @@ public class PrivateGroupManagerImpl extends BdfIncomingMessageHook implements
 		}
 		addMember(txn, m.getGroupId(), member, v);
 		// track message and broadcast event
-		trackIncomingMessage(txn, m);
+		messageTracker.trackIncomingMessage(txn, m);
 		attachJoinMessageAddedEvent(txn, m, meta, false, v);
 	}

@@ -537,7 +551,7 @@ public class PrivateGroupManagerImpl extends BdfIncomingMessageHook implements
 		if (previousType != JOIN && previousType != POST)
 			throw new FormatException();
 		// track message and broadcast event
-		trackIncomingMessage(txn, m);
+		messageTracker.trackIncomingMessage(txn, m);
 		attachGroupMessageAddedEvent(txn, m, meta, false);
 	}

--- a/briar-core/src/org/briarproject/privategroup/invitation/GroupInvitationManagerImpl.java
+++ b/briar-core/src/org/briarproject/privategroup/invitation/GroupInvitationManagerImpl.java
@@ -4,6 +4,7 @@ import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.Client;
 import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.ContactGroupFactory;
+import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.contact.Contact;
 import org.briarproject.api.contact.ContactId;
@@ -74,13 +75,14 @@ class GroupInvitationManagerImpl extends ConversationClientImpl
 	@Inject
 	protected GroupInvitationManagerImpl(DatabaseComponent db,
 			ClientHelper clientHelper, MetadataParser metadataParser,
+			MessageTracker messageTracker,
 			ContactGroupFactory contactGroupFactory,
 			PrivateGroupFactory privateGroupFactory,
 			PrivateGroupManager privateGroupManager,
 			MessageParser messageParser, SessionParser sessionParser,
 			SessionEncoder sessionEncoder,
 			ProtocolEngineFactory engineFactory) {
-		super(db, clientHelper, metadataParser);
+		super(db, clientHelper, metadataParser, messageTracker);
 		this.contactGroupFactory = contactGroupFactory;
 		this.privateGroupFactory = privateGroupFactory;
 		this.privateGroupManager = privateGroupManager;
@@ -131,7 +133,7 @@ class GroupInvitationManagerImpl extends ConversationClientImpl
 	}

 	@Override
-	protected Group getContactGroup(Contact c) {
+	public Group getContactGroup(Contact c) {
 		return contactGroupFactory.createContactGroup(CLIENT_ID, c);
 	}

--- a/briar-core/src/org/briarproject/properties/TransportPropertyValidator.java
+++ b/briar-core/src/org/briarproject/properties/TransportPropertyValidator.java
@@ -1,11 +1,12 @@
 package org.briarproject.properties;

 import org.briarproject.api.FormatException;
-import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.BdfMessageContext;
+import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.system.Clock;
@@ -15,6 +16,7 @@ import static org.briarproject.api.TransportId.MAX_TRANSPORT_ID_LENGTH;
 import static org.briarproject.api.properties.TransportPropertyConstants.MAX_PROPERTIES_PER_TRANSPORT;
 import static org.briarproject.api.properties.TransportPropertyConstants.MAX_PROPERTY_LENGTH;

+@NotNullByDefault
 public class TransportPropertyValidator extends BdfMessageValidator {

 	TransportPropertyValidator(ClientHelper clientHelper,
--- a/briar-core/src/org/briarproject/sharing/BlogSharingManagerImpl.java
+++ b/briar-core/src/org/briarproject/sharing/BlogSharingManagerImpl.java
@@ -12,6 +12,7 @@ import org.briarproject.api.blogs.BlogSharingMessage.BlogInvitation;
 import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.ContactGroupFactory;
 import org.briarproject.api.clients.MessageQueueManager;
+import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.contact.Contact;
 import org.briarproject.api.contact.ContactId;
@@ -28,6 +29,7 @@ import org.briarproject.api.identity.Author;
 import org.briarproject.api.identity.AuthorFactory;
 import org.briarproject.api.identity.IdentityManager;
 import org.briarproject.api.identity.LocalAuthor;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sharing.InvitationMessage;
 import org.briarproject.api.sync.ClientId;
 import org.briarproject.api.sync.GroupId;
@@ -43,6 +45,7 @@ import static org.briarproject.api.blogs.BlogConstants.BLOG_PUBLIC_KEY;
 import static org.briarproject.api.sharing.SharingConstants.INVITATION_ID;
 import static org.briarproject.api.sharing.SharingConstants.RESPONSE_ID;

+@NotNullByDefault
 class BlogSharingManagerImpl extends
 		SharingManagerImpl<Blog, BlogInvitation, BlogInviteeSessionState, BlogSharerSessionState, BlogInvitationReceivedEvent, BlogInvitationResponseReceivedEvent>
 		implements BlogSharingManager, RemoveBlogHook {
@@ -63,10 +66,11 @@ class BlogSharingManagerImpl extends
 			DatabaseComponent db, MessageQueueManager messageQueueManager,
 			MetadataEncoder metadataEncoder, MetadataParser metadataParser,
 			ContactGroupFactory contactGroupFactory, SecureRandom random,
-			IdentityManager identityManager) {
+			IdentityManager identityManager, MessageTracker messageTracker) {

 		super(db, messageQueueManager, clientHelper, metadataParser,
-				metadataEncoder, random, contactGroupFactory, clock);
+				metadataEncoder, random, contactGroupFactory, messageTracker,
+				clock);

 		this.blogManager = blogManager;
 		this.identityManager = identityManager;
--- a/briar-core/src/org/briarproject/sharing/BlogSharingValidator.java
+++ b/briar-core/src/org/briarproject/sharing/BlogSharingValidator.java
@@ -7,6 +7,7 @@ import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.system.Clock;
@@ -30,6 +31,7 @@ import static org.briarproject.api.sharing.SharingConstants.SHARE_MSG_TYPE_LEAVE
 import static org.briarproject.api.sharing.SharingConstants.TIME;
 import static org.briarproject.api.sharing.SharingConstants.TYPE;

+@NotNullByDefault
 class BlogSharingValidator extends BdfMessageValidator {

 	@Inject
--- a/briar-core/src/org/briarproject/sharing/ForumSharingManagerImpl.java
+++ b/briar-core/src/org/briarproject/sharing/ForumSharingManagerImpl.java
@@ -4,6 +4,7 @@ import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.ContactGroupFactory;
 import org.briarproject.api.clients.MessageQueueManager;
+import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.contact.ContactId;
 import org.briarproject.api.data.BdfDictionary;
@@ -20,8 +21,10 @@ import org.briarproject.api.forum.ForumFactory;
 import org.briarproject.api.forum.ForumInvitationRequest;
 import org.briarproject.api.forum.ForumInvitationResponse;
 import org.briarproject.api.forum.ForumManager;
+import org.briarproject.api.forum.ForumManager.RemoveForumHook;
 import org.briarproject.api.forum.ForumSharingManager;
 import org.briarproject.api.forum.ForumSharingMessage.ForumInvitation;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sharing.InvitationMessage;
 import org.briarproject.api.sync.ClientId;
 import org.briarproject.api.sync.GroupId;
@@ -37,9 +40,10 @@ import static org.briarproject.api.forum.ForumConstants.FORUM_SALT;
 import static org.briarproject.api.sharing.SharingConstants.INVITATION_ID;
 import static org.briarproject.api.sharing.SharingConstants.RESPONSE_ID;

+@NotNullByDefault
 class ForumSharingManagerImpl extends
 		SharingManagerImpl<Forum, ForumInvitation, ForumInviteeSessionState, ForumSharerSessionState, ForumInvitationReceivedEvent, ForumInvitationResponseReceivedEvent>
-		implements ForumSharingManager, ForumManager.RemoveForumHook {
+		implements ForumSharingManager, RemoveForumHook {

 	private final SFactory sFactory;
 	private final IFactory iFactory;
@@ -57,9 +61,10 @@ class ForumSharingManagerImpl extends
 			MetadataEncoder metadataEncoder,
 			MetadataParser metadataParser,
 			ContactGroupFactory contactGroupFactory,
-			SecureRandom random) {
+			SecureRandom random, MessageTracker messageTracker) {
 		super(db, messageQueueManager, clientHelper, metadataParser,
-				metadataEncoder, random, contactGroupFactory, clock);
+				metadataEncoder, random, contactGroupFactory, messageTracker,
+				clock);

 		sFactory = new SFactory(forumFactory, forumManager);
 		iFactory = new IFactory();
--- a/briar-core/src/org/briarproject/sharing/ForumSharingValidator.java
+++ b/briar-core/src/org/briarproject/sharing/ForumSharingValidator.java
@@ -7,6 +7,7 @@ import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.MetadataEncoder;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.system.Clock;
@@ -30,6 +31,7 @@ import static org.briarproject.api.sharing.SharingConstants.SHARE_MSG_TYPE_LEAVE
 import static org.briarproject.api.sharing.SharingConstants.TIME;
 import static org.briarproject.api.sharing.SharingConstants.TYPE;

+@NotNullByDefault
 class ForumSharingValidator extends BdfMessageValidator {

 	@Inject
--- a/briar-core/src/org/briarproject/sharing/SharingManagerImpl.java
+++ b/briar-core/src/org/briarproject/sharing/SharingManagerImpl.java
@@ -6,6 +6,7 @@ import org.briarproject.api.clients.Client;
 import org.briarproject.api.clients.ClientHelper;
 import org.briarproject.api.clients.ContactGroupFactory;
 import org.briarproject.api.clients.MessageQueueManager;
+import org.briarproject.api.clients.MessageTracker;
 import org.briarproject.api.clients.SessionId;
 import org.briarproject.api.contact.Contact;
 import org.briarproject.api.contact.ContactId;
@@ -25,6 +26,7 @@ import org.briarproject.api.event.Event;
 import org.briarproject.api.event.InvitationRequestReceivedEvent;
 import org.briarproject.api.event.InvitationResponseReceivedEvent;
 import org.briarproject.api.identity.LocalAuthor;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sharing.InvitationMessage;
 import org.briarproject.api.sharing.Shareable;
 import org.briarproject.api.sharing.SharingInvitationItem;
@@ -52,6 +54,8 @@ import java.util.Map;
 import java.util.Set;
 import java.util.logging.Logger;

+import javax.annotation.Nullable;
+
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static org.briarproject.api.clients.ProtocolEngine.StateUpdate;
@@ -86,6 +90,7 @@ import static org.briarproject.api.sharing.SharingMessage.Invitation;
 import static org.briarproject.clients.BdfConstants.MSG_KEY_READ;
 import static org.briarproject.sharing.InviteeSessionState.State.AWAIT_LOCAL_RESPONSE;

+@NotNullByDefault
 abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS extends InviteeSessionState, SS extends SharerSessionState, IR extends InvitationRequestReceivedEvent, IRR extends InvitationResponseReceivedEvent>
 		extends ConversationClientImpl
 		implements SharingManager<S>, Client, AddContactHook,
@@ -105,9 +110,8 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 			MessageQueueManager messageQueueManager, ClientHelper clientHelper,
 			MetadataParser metadataParser, MetadataEncoder metadataEncoder,
 			SecureRandom random, ContactGroupFactory contactGroupFactory,
-			Clock clock) {
-		super(db, clientHelper, metadataParser);
-
+			MessageTracker messageTracker, Clock clock) {
+		super(db, clientHelper, metadataParser, messageTracker);
 		this.messageQueueManager = messageQueueManager;
 		this.metadataEncoder = metadataEncoder;
 		this.random = random;
@@ -226,7 +230,7 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 					new InviteeEngine<IS, IR>(getIRFactory(), clock);
 			processInviteeStateUpdate(txn, m.getId(),
 					engine.onMessageReceived(state, msg));
-			trackIncomingMessage(txn, m);
+			messageTracker.trackIncomingMessage(txn, m);
 		} else if (msg.getType() == SHARE_MSG_TYPE_ACCEPT ||
 				msg.getType() == SHARE_MSG_TYPE_DECLINE) {
 			// we are a sharer who just received a response
@@ -237,7 +241,7 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 							getIRRFactory(), clock);
 			processSharerStateUpdate(txn, m.getId(),
 					engine.onMessageReceived(state, msg));
-			trackIncomingMessage(txn, m);
+			messageTracker.trackIncomingMessage(txn, m);
 		} else if (msg.getType() == SHARE_MSG_TYPE_LEAVE ||
 				msg.getType() == SHARE_MSG_TYPE_ABORT) {
 			// we don't know who we are, so figure it out
@@ -296,7 +300,8 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 			// track message
 			// TODO handle this properly without engine hacks (#376)
 			long time = update.toSend.get(0).getTime();
-			trackMessage(txn, localState.getGroupId(), time, true);
+			messageTracker.trackMessage(txn, localState.getGroupId(), time,
+					true);

 			db.commitTransaction(txn);
 		} catch (FormatException e) {
@@ -359,7 +364,7 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 		// track message
 		// TODO handle this properly without engine hacks (#376)
 		long time = update.toSend.get(0).getTime();
-		trackMessage(txn, localState.getGroupId(), time, true);
+		messageTracker.trackMessage(txn, localState.getGroupId(), time, true);
 	}

 	@Override
@@ -581,16 +586,6 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 		}
 	}

-	@Override
-	public GroupCount getGroupCount(Transaction txn, ContactId contactId)
-			throws DbException {
-
-		Contact contact = db.getContact(txn, contactId);
-		GroupId groupId = getContactGroup(contact).getId();
-
-		return getGroupCount(txn, groupId);
-	}
-
 	void removingShareable(Transaction txn, S f) throws DbException {
 		try {
 			for (Contact c : db.getContacts(txn)) {
@@ -827,7 +822,8 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 		throw new FormatException();
 	}

-	private void processStateUpdate(Transaction txn, MessageId messageId,
+	private void processStateUpdate(Transaction txn,
+			@Nullable MessageId messageId,
 			StateUpdate<SharingSessionState, BaseMessage> result, S f)
 			throws DbException, FormatException {

@@ -859,8 +855,8 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 		}
 	}

-	private void processSharerStateUpdate(Transaction txn, MessageId messageId,
-			StateUpdate<SS, BaseMessage> result)
+	private void processSharerStateUpdate(Transaction txn,
+			@Nullable MessageId messageId, StateUpdate<SS, BaseMessage> result)
 			throws DbException, FormatException {

 		StateUpdate<SharingSessionState, BaseMessage> r =
@@ -874,8 +870,8 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 		processStateUpdate(txn, messageId, r, f);
 	}

-	private void processInviteeStateUpdate(Transaction txn, MessageId messageId,
-			StateUpdate<IS, BaseMessage> result)
+	private void processInviteeStateUpdate(Transaction txn,
+			@Nullable MessageId messageId, StateUpdate<IS, BaseMessage> result)
 			throws DbException, FormatException {

 		StateUpdate<SharingSessionState, BaseMessage> r =
@@ -947,7 +943,7 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 	}

 	@Override
-	protected Group getContactGroup(Contact c) {
+	public Group getContactGroup(Contact c) {
 		return contactGroupFactory.createContactGroup(getClientId(), c);
 	}

--- a/briar-core/src/org/briarproject/sync/MessageFactoryImpl.java
+++ b/briar-core/src/org/briarproject/sync/MessageFactoryImpl.java
@@ -1,19 +1,22 @@
 package org.briarproject.sync;

-
 import org.briarproject.api.UniqueId;
 import org.briarproject.api.crypto.CryptoComponent;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.GroupId;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.sync.MessageFactory;
 import org.briarproject.api.sync.MessageId;
 import org.briarproject.util.ByteUtils;

+import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;

 import static org.briarproject.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
 import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;

+@Immutable
+@NotNullByDefault
 class MessageFactoryImpl implements MessageFactory {

 	private final CryptoComponent crypto;
--- a/briar-core/src/org/briarproject/sync/ValidationManagerImpl.java
+++ b/briar-core/src/org/briarproject/sync/ValidationManagerImpl.java
@@ -1,6 +1,5 @@
 package org.briarproject.sync;

-import org.briarproject.api.UniqueId;
 import org.briarproject.api.crypto.CryptoExecutor;
 import org.briarproject.api.db.DatabaseComponent;
 import org.briarproject.api.db.DatabaseExecutor;
@@ -13,15 +12,15 @@ import org.briarproject.api.event.Event;
 import org.briarproject.api.event.EventListener;
 import org.briarproject.api.event.MessageAddedEvent;
 import org.briarproject.api.lifecycle.Service;
+import org.briarproject.api.nullsafety.NotNullByDefault;
 import org.briarproject.api.sync.ClientId;
 import org.briarproject.api.sync.Group;
-import org.briarproject.api.sync.GroupId;
 import org.briarproject.api.sync.InvalidMessageException;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.sync.MessageContext;
+import org.briarproject.api.sync.MessageFactory;
 import org.briarproject.api.sync.MessageId;
 import org.briarproject.api.sync.ValidationManager;
-import org.briarproject.util.ByteUtils;

 import java.util.Collection;
 import java.util.LinkedList;
@@ -33,15 +32,17 @@ import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.logging.Logger;

+import javax.annotation.concurrent.ThreadSafe;
 import javax.inject.Inject;

 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
-import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
 import static org.briarproject.api.sync.ValidationManager.State.DELIVERED;
 import static org.briarproject.api.sync.ValidationManager.State.INVALID;
 import static org.briarproject.api.sync.ValidationManager.State.PENDING;

+@ThreadSafe
+@NotNullByDefault
 class ValidationManagerImpl implements ValidationManager, Service,
 		EventListener {

@@ -51,6 +52,7 @@ class ValidationManagerImpl implements ValidationManager, Service,
 	private final DatabaseComponent db;
 	private final Executor dbExecutor;
 	private final Executor cryptoExecutor;
+	private final MessageFactory messageFactory;
 	private final Map<ClientId, MessageValidator> validators;
 	private final Map<ClientId, IncomingMessageHook> hooks;
 	private final AtomicBoolean used = new AtomicBoolean(false);
@@ -58,10 +60,12 @@ class ValidationManagerImpl implements ValidationManager, Service,
 	@Inject
 	ValidationManagerImpl(DatabaseComponent db,
 			@DatabaseExecutor Executor dbExecutor,
-			@CryptoExecutor Executor cryptoExecutor) {
+			@CryptoExecutor Executor cryptoExecutor,
+			MessageFactory messageFactory) {
 		this.db = db;
 		this.dbExecutor = dbExecutor;
 		this.cryptoExecutor = cryptoExecutor;
+		this.messageFactory = messageFactory;
 		validators = new ConcurrentHashMap<ClientId, MessageValidator>();
 		hooks = new ConcurrentHashMap<ClientId, IncomingMessageHook>();
 	}
@@ -134,7 +138,8 @@ class ValidationManagerImpl implements ValidationManager, Service,
 			try {
 				MessageId id = unvalidated.poll();
 				byte[] raw = db.getRawMessage(txn, id);
-				m = parseMessage(id, raw);
+				if (raw == null) throw new DbException();
+				m = messageFactory.createMessage(id, raw);
 				g = db.getGroup(txn, m.getGroupId());
 				db.commitTransaction(txn);
 			} finally {
@@ -210,7 +215,9 @@ class ValidationManagerImpl implements ValidationManager, Service,
 						invalidateMessage(txn, id);
 						invalidate = getDependentsToInvalidate(txn, id);
 					} else if (allDelivered) {
-						Message m = parseMessage(id, db.getRawMessage(txn, id));
+						byte[] raw = db.getRawMessage(txn, id);
+						if (raw == null) throw new DbException();
+						Message m = messageFactory.createMessage(id, raw);
 						Group g = db.getGroup(txn, m.getGroupId());
 						ClientId c = g.getClientId();
 						Metadata meta = db.getMessageMetadataForValidator(txn,
@@ -247,15 +254,6 @@ class ValidationManagerImpl implements ValidationManager, Service,
 		}
 	}

-	private Message parseMessage(MessageId id, byte[] raw) {
-		if (raw.length <= MESSAGE_HEADER_LENGTH)
-			throw new IllegalArgumentException();
-		byte[] groupId = new byte[UniqueId.LENGTH];
-		System.arraycopy(raw, 0, groupId, 0, UniqueId.LENGTH);
-		long timestamp = ByteUtils.readUint64(raw, UniqueId.LENGTH);
-		return new Message(id, new GroupId(groupId), timestamp, raw);
-	}
-
 	private void validateMessageAsync(final Message m, final Group g) {
 		cryptoExecutor.execute(new Runnable() {
 			@Override
@@ -412,7 +410,7 @@ class ValidationManagerImpl implements ValidationManager, Service,

 	/**
 	 * Shares the next message from the toShare queue asynchronously.
-	 *
+	 * <p>
 	 * This method should only be called for messages that have all their
 	 * dependencies delivered and have been delivered themselves.
 	 */