Improved encapsulation of thread synchronisation as follows

- replaced use of Object instance mutex with a private final Lock object
- replaced Object signaling with specific condition signalling

briar-core/src/org/briarproject/lifecycle/ShutdownManagerImpl.java

@@ -2,6 +2,8 @@ package org.briarproject.lifecycle;
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.briarproject.api.lifecycle.ShutdownManager;
 
@@ -10,26 +12,42 @@ class ShutdownManagerImpl implements ShutdownManager {
 	protected final Map<Integer, Thread> hooks; // Locking: this
 
 	private int nextHandle = 0; // Locking: this
+	
+	private final Lock synchLock = new ReentrantLock();
 
 	ShutdownManagerImpl() {
 		hooks = new HashMap<Integer, Thread>();
 	}
 
-	public synchronized int addShutdownHook(Runnable r) {
-		int handle = nextHandle++;
-		Thread hook = createThread(r);
-		hooks.put(handle, hook);
-		Runtime.getRuntime().addShutdownHook(hook);
-		return handle;
+	public int addShutdownHook(Runnable r) {
+		synchLock.lock();
+		try{
+			int handle = nextHandle++;
+			Thread hook = createThread(r);
+			hooks.put(handle, hook);
+			Runtime.getRuntime().addShutdownHook(hook);
+			return handle;
+		}
+		finally{
+			synchLock.unlock();
+		}
+
 	}
 
 	protected Thread createThread(Runnable r) {
 		return new Thread(r, "ShutdownManager");
 	}
 
-	public synchronized boolean removeShutdownHook(int handle) {
-		Thread hook = hooks.remove(handle);
-		if(hook == null) return false;
-		else return Runtime.getRuntime().removeShutdownHook(hook);
+	public boolean removeShutdownHook(int handle) {
+		synchLock.lock();
+		try{
+			Thread hook = hooks.remove(handle);
+			if(hook == null) return false;
+			else return Runtime.getRuntime().removeShutdownHook(hook);
+		}
+		finally{
+			synchLock.unlock();
+		}
+
 	}
 }