[LANG-501] Added CallableBackgroundInitializer class with unit tests.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@883100 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/commons/lang/concurrent/CallableBackgroundInitializer.java

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+/*
+ * 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.commons.lang.concurrent;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+
+/**
+ * <p>
+ * A specialized {@link BackgroundInitializer} implementation that wraps a
+ * {@code Callable} object.
+ * </p>
+ * <p>
+ * An instance of this class is initialized with a {@code Callable} object when
+ * it is constructed. The implementation of the {@link #initialize()} method
+ * defined in the super class delegates to this {@code Callable} so that the
+ * {@code Callable} is executed in the background thread.
+ * </p>
+ * <p>
+ * The {@code java.util.concurrent.Callable} interface is a standard mechanism
+ * of the JDK to define tasks to be executed by another thread. The {@code
+ * CallableBackgroundInitializer} class allows combining this standard interface
+ * with the background initializer API.
+ * </p>
+ * <p>
+ * Usage of this class is very similar to the default usage pattern of the
+ * {@link BackgroundInitializer} class: Just create an instance and provide the
+ * {@code Callable} object to be executed, then call the initializer's
+ * {@link #start()} method. This causes the {@code Callable} to be executed in
+ * another thread. When the results of the {@code Callable} are needed the
+ * initializer's {@link #get()} method can be called (which may block until
+ * background execution is complete). The following code fragment shows a
+ * typical usage example:
+ *
+ * <pre>
+ * // a Callable that performs a complex computation
+ * Callable<Integer> computationCallable = new MyComputationCallable();
+ *
+ * // setup the background initializer
+ * CallableBackgroundInitializer<Integer> initializer =
+ *     new CallableBackgroundInitializer(computationCallable);
+ * initializer.start();
+ *
+ * // Now do some other things. Initialization runs in a parallel thread
+ * ...
+ * // Wait for the end of initialization and access the result
+ * Integer result = initializer.get();
+ * </pre>
+ *
+ * </p>
+ *
+ * @version $Id: $
+ * @param <T> the type of the object managed by this initializer class
+ */
+public class CallableBackgroundInitializer<T> extends BackgroundInitializer<T> {
+    /** The Callable to be executed. */
+    private final Callable<T> callable;
+
+    /**
+     * Creates a new instance of {@code CallableBackgroundInitializer} and sets
+     * the {@code Callable} to be executed in a background thread.
+     *
+     * @param call the {@code Callable} (must not be <b>null</b>)
+     * @throws IllegalArgumentException if the {@code Callable} is <b>null</b>
+     */
+    public CallableBackgroundInitializer(Callable<T> call) {
+        checkCallable(call);
+        callable = call;
+    }
+
+    /**
+     * Creates a new instance of {@code CallableBackgroundInitializer} and
+     * initializes it with the {@code Callable} to be executed in a background
+     * thread and the {@code ExecutorService} for managing the background
+     * execution.
+     *
+     * @param call the {@code Callable} (must not be <b>null</b>)
+     * @param exec an external {@code ExecutorService} to be used for task
+     * execution
+     * @throws IllegalArgumentException if the {@code Callable} is <b>null</b>
+     */
+    public CallableBackgroundInitializer(Callable<T> call, ExecutorService exec) {
+        super(exec);
+        checkCallable(call);
+        callable = call;
+    }
+
+    /**
+     * Performs initialization in a background thread. This implementation
+     * delegates to the {@code Callable} passed at construction time of this
+     * object.
+     *
+     * @return the result of the initialization
+     * @throws Exception if an error occurs
+     */
+    @Override
+    protected T initialize() throws Exception {
+        return callable.call();
+    }
+
+    /**
+     * Tests the passed in {@code Callable} and throws an exception if it is
+     * undefined.
+     *
+     * @param call the object to check
+     * @throws IllegalArgumentException if the {@code Callable} is <b>null</b>
+     */
+    private void checkCallable(Callable<T> call) {
+        if (call == null) {
+            throw new IllegalArgumentException("Callable must not be null!");
+        }
+    }
+}

src/test/org/apache/commons/lang/concurrent/CallableBackgroundInitializerTest.java

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+/*
+ * 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.commons.lang.concurrent;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import junit.framework.TestCase;
+
+/**
+ * Test class for {@code CallableBackgroundInitializer}
+ *
+ * @version $Id$
+ */
+public class CallableBackgroundInitializerTest extends TestCase {
+    /** Constant for the result of the call() invocation. */
+    private static final Integer RESULT = 42;
+
+    /**
+     * Tries to create an instance without a Callable. This should cause an
+     * exception.
+     */
+    public void testInitNullCallable() {
+        try {
+            new CallableBackgroundInitializer<Object>(null);
+            fail("Could create instance without a Callable!");
+        } catch (IllegalArgumentException iex) {
+            // ok
+        }
+    }
+
+    /**
+     * Tests whether the executor service is correctly passed to the super
+     * class.
+     */
+    public void testInitExecutor() {
+        ExecutorService exec = Executors.newSingleThreadExecutor();
+        CallableBackgroundInitializer<Integer> init = new CallableBackgroundInitializer<Integer>(
+                new TestCallable(), exec);
+        assertEquals("Executor not set", exec, init.getExternalExecutor());
+    }
+
+    /**
+     * Tries to pass a null Callable to the constructor that takes an executor.
+     * This should cause an exception.
+     */
+    public void testInitExecutorNullCallable() {
+        ExecutorService exec = Executors.newSingleThreadExecutor();
+        try {
+            new CallableBackgroundInitializer<Integer>(null, exec);
+            fail("Could create instance without a Callable!");
+        } catch (IllegalArgumentException iex) {
+            // ok
+        }
+    }
+
+    /**
+     * Tests the implementation of initialize().
+     */
+    public void testInitialize() throws Exception {
+        TestCallable call = new TestCallable();
+        CallableBackgroundInitializer<Integer> init = new CallableBackgroundInitializer<Integer>(
+                call);
+        assertEquals("Wrong result", RESULT, init.initialize());
+        assertEquals("Wrong number of invocations", 1, call.callCount);
+    }
+
+    /**
+     * A test Callable implementation for checking the initializer's
+     * implementation of the initialize() method.
+     */
+    private static class TestCallable implements Callable<Integer> {
+        /** A counter for the number of call() invocations. */
+        int callCount;
+
+        /**
+         * Records this invocation and returns the test result.
+         */
+        public Integer call() throws Exception {
+            callCount++;
+            return RESULT;
+        }
+    }
+}