[LANG-609] Added AtomicSafeInitializer class. Thanks to alexander dot apanasovich at gmail dot com for the proposal.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1006172 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/lang3/concurrent/AtomicSafeInitializer.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.lang3.concurrent;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+/**
+ * <p>
+ * A specialized {@code ConcurrentInitializer} implementation which is similar
+ * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
+ * method is called only once.
+ * </p>
+ * <p>
+ * As {@link AtomicInitializer} this class is based on atomic variables, so it
+ * can create an object under concurrent access without synchronization.
+ * However, it implements an additional check to guarantee that the
+ * {@link #initialize()} method which actually creates the object cannot be
+ * called multiple times.
+ * </p>
+ * <p>
+ * Because of this additional check this implementation is slightly less
+ * efficient than {@link AtomicInitializer}, but if the object creation in the
+ * {@code initialize()} method is expensive or if multiple invocations of
+ * {@code initialize()} are problematic, it is the better alternative.
+ * </p>
+ * <p>
+ * From its semantics this class has the same properties as
+ * {@link LazyInitializer}. It is a &quot;save&quot; implementation of the lazy
+ * initializer pattern. Comparing both classes in terms of efficiency is
+ * difficult because which one is faster depends on multiple factors. Because
+ * {@code AtomicSafeInitializer} does not use synchronization at all it probably
+ * outruns {@link LazyInitializer}, at least under low or moderate concurrent
+ * access. Developers should run their own benchmarks on the expected target
+ * platform to decide which implementation is suitable for their specific use
+ * case.
+ * </p>
+ *
+ * @author Apache Software Foundation
+ * @version $Id$
+ * @param <T> the type of the object managed by this initializer class
+ */
+public abstract class AtomicSafeInitializer<T> implements
+        ConcurrentInitializer<T> {
+    /** A guard which ensures that initialize() is called only once. */
+    private final AtomicReference<AtomicSafeInitializer<T>> factory =
+            new AtomicReference<AtomicSafeInitializer<T>>();
+
+    /** Holds the reference to the managed object. */
+    private final AtomicReference<T> reference = new AtomicReference<T>();
+
+    /**
+     * Get (and initialize, if not initialized yet) the required object
+     *
+     * @return lazily initialized object
+     */
+    public final T get() throws ConcurrentException {
+        T result;
+
+        while ((result = reference.get()) == null) {
+            if (factory.compareAndSet(null, this)) {
+                reference.set(initialize());
+            }
+        }
+
+        return result;
+    }
+
+    /**
+     * Creates and initializes the object managed by this
+     * {@code AtomicInitializer}. This method is called by {@link #get()} when
+     * the managed object is not available yet. An implementation can focus on
+     * the creation of the object. No synchronization is needed, as this is
+     * already handled by {@code get()}. This method is guaranteed to be called
+     * only once.
+     *
+     * @return the managed data object
+     * @throws ConcurrentException if an error occurs during object creation
+     */
+    protected abstract T initialize() throws ConcurrentException;
+}

src/test/java/org/apache/commons/lang3/concurrent/AtomicSafeInitializerTest.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.lang3.concurrent;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.junit.Before;
+import org.junit.Test;
+
+/**
+ * Test class for {@code AtomicSafeInitializer}.
+ *
+ * @author Apache Software Foundation
+ * @version $Id$
+ */
+public class AtomicSafeInitializerTest extends
+        AbstractConcurrentInitializerTest {
+    /** The instance to be tested. */
+    private AtomicSafeInitializerTestImpl initializer;
+
+    @Before
+    public void setUp() throws Exception {
+        initializer = new AtomicSafeInitializerTestImpl();
+    }
+
+    /**
+     * Returns the initializer to be tested.
+     *
+     * @return the {@code AtomicSafeInitializer} under test
+     */
+    @Override
+    protected ConcurrentInitializer<Object> createInitializer() {
+        return initializer;
+    }
+
+    /**
+     * Tests that initialize() is called only once.
+     */
+    @Test
+    public void testNumberOfInitializeInvocations() throws ConcurrentException,
+            InterruptedException {
+        testGetConcurrent();
+        assertEquals("Wrong number of invocations", 1,
+                initializer.initCounter.get());
+    }
+
+    /**
+     * A concrete test implementation of {@code AtomicSafeInitializer}. This
+     * implementation also counts the number of invocations of the initialize()
+     * method.
+     */
+    private static class AtomicSafeInitializerTestImpl extends
+            AtomicSafeInitializer<Object> {
+        /** A counter for initialize() invocations. */
+        final AtomicInteger initCounter = new AtomicInteger();
+
+        @Override
+        protected Object initialize() throws ConcurrentException {
+            initCounter.incrementAndGet();
+            return new Object();
+        }
+    }
+}