Added null safety annotations.

briar-api/src/org/briarproject/api/nullsafety/NotNullByDefault.java

@@ -0,0 +1,32 @@
+package org.briarproject.api.nullsafety;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+
+import javax.annotation.Nonnull;
+import javax.annotation.meta.TypeQualifierDefault;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/**
+ * This annotation can be applied to a package or class to indicate that
+ * the fields, methods and parameters in that element are non-null by default
+ * unless:
+ * <ul>
+ * <li> There is an explicit nullness annotation
+ * <li> The method overrides a method in a superclass (in which case the
+ * annotation of the corresponding method or parameter in the superclass
+ * applies)
+ * <li> There is a default nullness annotation applied to a more tightly
+ * nested element.
+ * </ul>
+ */
+@Documented
+@Nonnull
+@TypeQualifierDefault({FIELD, METHOD, PARAMETER})
+@Retention(RUNTIME)
+public @interface NotNullByDefault {
+}