Related to #116. As per Iwao's suggestion, now removeObject is used to

notify the rollback to a blocking cache. EhCache adapter has been fixed
also.

Author: emacarron

src/main/java/org/apache/ibatis/cache/Cache.java

@@ -59,10 +59,18 @@ public interface Cache {
   Object getObject(Object key);
 
   /**
-   * Optional. It is not called by the core.
+   * As of 3.3.0 this method is only called during a rollback 
+   * for any previous value that was missing in the cache.
+   * This lets any blocking cache to release the lock that 
+   * may have previously put on the key.
+   * A blocking cache puts a lock when a value is null 
+   * and releases it when the value is back again.
+   * This way other threads will wait for the value to be 
+   * available instead of hitting the database.
+   *
    * 
    * @param key The key
-   * @return The object that was removed
+   * @return Not used
    */
   Object removeObject(Object key);
 

src/main/java/org/apache/ibatis/cache/decorators/BlockingCache.java

@@ -27,7 +27,7 @@
 /**
  * Simple blocking decorator 
  * 
- * Sipmle and inefficient version of EhCache's BlockingCache decorator.
+ * Simple and inefficient version of EhCache's BlockingCache decorator.
  * It sets a lock over a cache key when the element is not found in cache.
  * This way, other threads will wait until this element is filled instead of hitting the database.
  * 
@@ -76,7 +76,9 @@ public Object getObject(Object key) {
 
   @Override
   public Object removeObject(Object key) {
-    return delegate.removeObject(key);
+    // despite of its name, this method is called only to release locks
+    releaseLock(key);
+    return null;
   }
 
   @Override

src/main/java/org/apache/ibatis/cache/decorators/TransactionalCache.java

@@ -22,6 +22,8 @@
 import java.util.concurrent.locks.ReadWriteLock;
 
 import org.apache.ibatis.cache.Cache;
+import org.apache.ibatis.logging.Log;
+import org.apache.ibatis.logging.LogFactory;
 
 /**
  * The 2nd level cache transactional buffer.
@@ -36,6 +38,8 @@
  */
 public class TransactionalCache implements Cache {
 
+  private static final Log log = LogFactory.getLog(TransactionalCache.class);
+
   private Cache delegate;
   private boolean clearOnCommit;
   private Map<Object, Object> entriesToAddOnCommit;
@@ -126,7 +130,12 @@ private void flushPendingEntries() {
 
   private void unlockMissedEntries() {
     for (Object entry : entriesMissedInCache) {
-      delegate.putObject(entry, null);
+      try {
+        delegate.removeObject(entry);
+      } catch (Exception e) {
+        log.warn("Unexpected exception while notifiying a rollback to the cache adapter."
+            + "Consider upgrading your cache adapter to the latest version.  Cause: " + e);
+      }
     }
   }