1 files changed, 135 insertions, 0 deletions
diff --git a/qpid/java/client/src/main/java/org/apache/qpid/client/util/FlowControllingBlockingQueue.java b/qpid/java/client/src/main/java/org/apache/qpid/client/util/FlowControllingBlockingQueue.java
new file mode 100644
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+++ b/qpid/java/client/src/main/java/org/apache/qpid/client/util/FlowControllingBlockingQueue.java
@@ -0,0 +1,135 @@
+/*
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ *
+ */
+package org.apache.qpid.client.util;
+
+import java.util.Iterator;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * A blocking queue that emits events above a user specified threshold allowing the caller to take action (e.g. flow
+ * control) to try to prevent the queue growing (much) further. The underlying queue itself is not bounded therefore the
+ * caller is not obliged to react to the events. <p/> This implementation is <b>only</b> safe where we have a single
+ * thread adding items and a single (different) thread removing items.
+ *
+ * @todo Make this implement java.util.Queue and hide the implementation. Then different queue types can be substituted.
+ */
+public class FlowControllingBlockingQueue
+{
+	private static final Logger _logger = LoggerFactory.getLogger(FlowControllingBlockingQueue.class);
+	
+    /** This queue is bounded and is used to store messages before being dispatched to the consumer */
+    private final Queue _queue = new ConcurrentLinkedQueue();
+
+    private final int _flowControlHighThreshold;
+    private final int _flowControlLowThreshold;
+
+    private final ThresholdListener _listener;
+
+    /** We require a separate count so we can track whether we have reached the threshold */
+    private int _count;
+    
+    private boolean disableFlowControl; 
+
+    public boolean isEmpty()
+    {
+        return _queue.isEmpty();
+    }
+
+    public interface ThresholdListener
+    {
+        void aboveThreshold(int currentValue);
+
+        void underThreshold(int currentValue);
+    }
+
+    public FlowControllingBlockingQueue(int threshold, ThresholdListener listener)
+    {
+        this(threshold, threshold, listener);
+    }
+
+    public FlowControllingBlockingQueue(int highThreshold, int lowThreshold, ThresholdListener listener)
+    {
+        _flowControlHighThreshold = highThreshold;
+        _flowControlLowThreshold = lowThreshold;
+        _listener = listener;
+        if (highThreshold == 0)
+        {
+        	disableFlowControl = true;
+        }
+    }
+
+    public Object take() throws InterruptedException
+    {
+        Object o = _queue.poll();
+        if(o == null)
+        {
+            synchronized(this)
+            {
+                while((o = _queue.poll())==null)
+                {
+                    wait();
+                }
+            }
+        }
+        if (!disableFlowControl && _listener != null)
+        {
+            synchronized (_listener)
+            {
+                if (_count-- == _flowControlLowThreshold)
+                {
+                    _listener.underThreshold(_count);
+                }
+            }
+            
+        }
+
+        return o;
+    }
+
+    public void add(Object o)
+    {
+        synchronized(this)
+        {
+            _queue.add(o);
+
+            notifyAll();
+        }
+        if (!disableFlowControl && _listener != null)
+        {
+            synchronized (_listener)
+            {
+                if (++_count == _flowControlHighThreshold)
+                {
+                    _listener.aboveThreshold(_count);
+                }
+            }
+        }
+    }
+
+    public Iterator iterator()
+    {
+        return _queue.iterator();
+    }
+}