Add putIfAbsent on Cache abstraction

This commit adds a putIfAbsent method to the Cache interface. This
method offers an atomic put if the key is not already associated in
the cache.

Issue: SPR-11400

spring-context/src/test/java/org/springframework/cache/concurrent/ConcurrentCacheTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2014 the original author or authors.
+ * Copyright 2002-2014 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,6 +29,7 @@ import static org.junit.Assert.*;
 /**
  * @author Costin Leau
  * @author Juergen Hoeller
+ * @author Stephane Nicoll
  */
 public class ConcurrentCacheTests  {
 
@@ -79,6 +80,18 @@ public class ConcurrentCacheTests  {
 		assertNull(cache.get(key, Object.class));
 	}
 
+	@Test
+	public void testCachePutIfAbsent() throws Exception {
+		Object key = new Object();
+		Object value = "initialValue";
+
+		assertNull(cache.get(key));
+		assertNull(cache.putIfAbsent(key, value));
+		assertEquals(value, cache.get(key).get());
+		assertEquals("initialValue", cache.putIfAbsent(key, "anotherValue").get());
+		assertEquals(value, cache.get(key).get()); // not changed
+	}
+
 	@Test
 	public void testCacheRemove() throws Exception {
 		Object key = "enescu";