Adding a Promise class that can allow us to use callbacks to get future computation results.

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1482954 13f79535-47bb-0310-9956-ffa450edef68

activemq-broker/src/main/java/org/apache/activemq/util/Promise.java

@@ -0,0 +1,123 @@
+/**
+ * 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.activemq.util;
+
+
+import java.util.ArrayList;
+import java.util.concurrent.*;
+
+/**
+ * <p>
+ *   Used to implement callback based result passing of a promised computation.
+ *   Can be converted to a future using the future() method.
+ * </p>
+ *
+ * @author <a href="http://hiramchirino.com">Hiram Chirino</a>
+ */
+public class Promise<T> extends PromiseCallback<T> {
+
+    ArrayList<PromiseCallback<T>> callbacks = new ArrayList<PromiseCallback<T>>(1);
+    T value;
+    Throwable error;
+    Future<T> future=null;
+
+    private class PromiseFuture extends PromiseCallback<T> implements Future<T> {
+        CountDownLatch latch = new CountDownLatch(1);
+
+        public boolean cancel(boolean mayInterruptIfRunning) {
+            return false;
+        }
+
+        public boolean isCancelled() {
+            return false;
+        }
+
+        public boolean isDone() {
+            return latch.getCount() == 0;
+        }
+
+        public T get() throws InterruptedException, ExecutionException {
+            latch.await();
+            return value();
+        }
+
+        public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
+            if (latch.await(timeout, unit)) {
+                return value();
+            } else {
+                throw new TimeoutException();
+            }
+        }
+
+        public void onComplete(T value, Throwable error) {
+            latch.countDown();
+        }
+
+        private T value() throws ExecutionException {
+            if( error!=null ) {
+                throw new ExecutionException(error);
+            }
+            return value;
+        }
+    }
+
+    public Future<T> future() {
+        if( future == null ) {
+            PromiseFuture future = new PromiseFuture();
+            watch(future);
+            this.future = future;
+        }
+        return future;
+    }
+
+    public void watch(PromiseCallback<T> callback) {
+        if (callback == null)
+            throw new IllegalArgumentException("callback cannot be null");
+        boolean queued = false;
+        synchronized (this) {
+            if (callbacks != null) {
+                callbacks.add(callback);
+                queued = true;
+            }
+        }
+        if (!queued) {
+            callback.onComplete(value, error);
+        }
+    }
+
+    @Override
+    public void onComplete(T value, Throwable error) {
+        if( value!=null && error !=null ) {
+            throw new IllegalArgumentException("You can not have both a vaule and error");
+        }
+        ArrayList<PromiseCallback<T>> callbacks;
+        synchronized (this) {
+            callbacks = this.callbacks;
+            if (callbacks != null) {
+                this.value = value;
+                this.error = error;
+                this.callbacks = null;
+            }
+        }
+        if (callbacks != null) {
+            for (PromiseCallback callback : callbacks) {
+                callback.onComplete(this.value, this.error);
+            }
+        }
+    }
+
+}

activemq-broker/src/main/java/org/apache/activemq/util/PromiseCallback.java

@@ -0,0 +1,21 @@
+/**
+ * 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.activemq.util;
+
+public abstract class PromiseCallback<T> {
+    abstract public void onComplete(T value, Throwable error);
+}