Use ConcurrentHashMaps in DefaultContextCache

The changes made in 0cb22fc8f3 would
result in contexts not being properly closed if evicted from the
ConcurrentReferenceHashMap by the Garbage Collector.

This commit reverts those changes and returns to using standard
ConcurrentHashMaps for the time being.

Issue: SPR-7687

spring-test/src/main/java/org/springframework/test/context/support/DefaultContextCache.java

@@ -21,10 +21,12 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.style.ToStringCreator;
@@ -32,19 +34,16 @@ import org.springframework.test.annotation.DirtiesContext.HierarchyMode;
 import org.springframework.test.context.ContextCache;
 import org.springframework.test.context.MergedContextConfiguration;
 import org.springframework.util.Assert;
-import org.springframework.util.ConcurrentReferenceHashMap;
 
 /**
  * Default implementation of the {@link ContextCache} API.
  *
- * <p>Uses Spring's {@link ConcurrentReferenceHashMap} to store
- * {@linkplain java.lang.ref.SoftReference soft references} to cached
- * contexts and {@code MergedContextConfiguration} instances.
+ * <p>Uses {@link ConcurrentHashMap ConcurrentHashMaps} to cache
+ * {@link ApplicationContext} and {@link MergedContextConfiguration} instances.
  *
  * @author Sam Brannen
  * @author Juergen Hoeller
  * @since 2.5
- * @see ConcurrentReferenceHashMap
  */
 public class DefaultContextCache implements ContextCache {
 
@@ -54,7 +53,7 @@ public class DefaultContextCache implements ContextCache {
 	 * Map of context keys to Spring {@code ApplicationContext} instances.
 	 */
 	private final Map<MergedContextConfiguration, ApplicationContext> contextMap =
-			new ConcurrentReferenceHashMap<MergedContextConfiguration, ApplicationContext>(64);
+			new ConcurrentHashMap<MergedContextConfiguration, ApplicationContext>(64);
 
 	/**
 	 * Map of parent keys to sets of children keys, representing a top-down <em>tree</em>
@@ -63,7 +62,7 @@ public class DefaultContextCache implements ContextCache {
 	 * of other contexts.
 	 */
 	private final Map<MergedContextConfiguration, Set<MergedContextConfiguration>> hierarchyMap =
-			new ConcurrentReferenceHashMap<MergedContextConfiguration, Set<MergedContextConfiguration>>(64);
+			new ConcurrentHashMap<MergedContextConfiguration, Set<MergedContextConfiguration>>(64);
 
 	private final AtomicInteger hitCount = new AtomicInteger();