Confidence objects now pin themselves when a listener is attached, eliminating a certain class of GC-triggered heisenbug.

core/src/main/java/org/bitcoinj/core/TransactionConfidence.java

@@ -174,6 +174,15 @@ public class TransactionConfidence implements Serializable {
         public void onConfidenceChanged(TransactionConfidence confidence, ChangeReason reason);
     }
 
+    // This is used to ensure that confidence objects which aren't referenced from anywhere but which have an event
+    // listener set on them don't become eligible for garbage collection. Otherwise the TxConfidenceTable, which only
+    // has weak references to these objects, would not be enough to keep the event listeners working as transactions
+    // propagate around the network - it cannot know directly if the API user is interested in the object, so it uses
+    // heap reachability as a proxy for interest.
+    //
+    // We add ourselves to this set when a listener is added and remove ourselves when the listener list is empty.
+    private static final Set<TransactionConfidence> pinnedConfidenceObjects = Collections.synchronizedSet(new HashSet<TransactionConfidence>());
+
     /**
      * <p>Adds an event listener that will be run when this confidence object is updated. The listener will be locked and
      * is likely to be invoked on a peer thread.</p>
@@ -186,6 +195,7 @@ public class TransactionConfidence implements Serializable {
     public void addEventListener(Listener listener, Executor executor) {
         checkNotNull(listener);
         listeners.addIfAbsent(new ListenerRegistration<Listener>(listener, executor));
+        pinnedConfidenceObjects.add(this);
     }
 
     /**
@@ -204,7 +214,10 @@ public class TransactionConfidence implements Serializable {
 
     public boolean removeEventListener(Listener listener) {
         checkNotNull(listener);
-        return ListenerRegistration.removeFromList(listener, listeners);
+        boolean removed = ListenerRegistration.removeFromList(listener, listeners);
+        if (listeners.isEmpty())
+            pinnedConfidenceObjects.remove(this);
+        return removed;
     }
 
     /**

core/src/main/java/org/bitcoinj/utils/ListenerRegistration.java

@@ -33,6 +33,7 @@ public class ListenerRegistration<T> {
         this.executor = checkNotNull(executor);
     }
 
+    /** Returns true if the listener was removed, else false. */
     public static <T> boolean removeFromList(T listener, List<? extends ListenerRegistration<T>> list) {
         checkNotNull(listener);
 

core/src/test/java/org/bitcoinj/core/TxConfidenceTableTest.java

@@ -35,7 +35,8 @@ public class TxConfidenceTableTest {
     @Before
     public void setup() throws Exception {
         BriefLogFormatter.init();
-        table = Context.getOrCreate().getConfidenceTable();
+        Context context = new Context();
+        table = context.getConfidenceTable();
 
         Address to = new ECKey().toAddress(params);
         Address change = new ECKey().toAddress(params);
@@ -48,7 +49,26 @@ public class TxConfidenceTableTest {
         address2 = new PeerAddress(InetAddress.getByAddress(new byte[] { 127, 0, 0, 2 }));
         address3 = new PeerAddress(InetAddress.getByAddress(new byte[] { 127, 0, 0, 3 }));
     }
-    
+
+    @Test
+    public void pinHandlers() throws Exception {
+        Transaction tx = new Transaction(params, tx1.bitcoinSerialize());
+        Sha256Hash hash = tx.getHash();
+        table.seen(hash, address1);
+        assertEquals(1, tx.getConfidence().numBroadcastPeers());
+        final int[] seen = new int[1];
+        tx.getConfidence().addEventListener(new TransactionConfidence.Listener() {
+            @Override
+            public void onConfidenceChanged(TransactionConfidence confidence, ChangeReason reason) {
+                seen[0] = confidence.numBroadcastPeers();
+            }
+        }, Threading.SAME_THREAD);
+        tx = null;
+        System.gc();
+        table.seen(hash, address2);
+        assertEquals(2, seen[0]);
+    }
+
     @Test
     public void invAndDownload() throws Exception {
         // Base case: we see a transaction announced twice and then download it. The count is in the confidence object.