X-Git-Url: http://wamblee.org/gitweb/?a=blobdiff_plain;f=support%2Fgeneral%2Fsrc%2Fmain%2Fjava%2Forg%2Fwamblee%2Fcache%2FCachedObject.java;h=87feaf0e003f957d0f3310071fae9aab47c4cac4;hb=3a6e2ead110ac7944f60f449b4a68d27b67fc19e;hp=3d200b5ad38d1ba9818c9d4c3d53ebb11f85eaf0;hpb=233b206571da3d744844bac769e04733112b9dd1;p=utils

diff --git a/support/general/src/main/java/org/wamblee/cache/CachedObject.java b/support/general/src/main/java/org/wamblee/cache/CachedObject.java
index 3d200b5a..87feaf0e 100644
--- a/support/general/src/main/java/org/wamblee/cache/CachedObject.java
+++ b/support/general/src/main/java/org/wamblee/cache/CachedObject.java
@@ -12,23 +12,24 @@
  * 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.wamblee.cache;
 
-import org.apache.log4j.Logger;
-
 import java.io.Serializable;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 
 /**
- * Represents a cached object. The object is either retrieved from the cache if
- * the cache has it, or a call back is invoked to get the object (and put it in
- * the cache).
+ * Represents a cached object identified by the key it has in a certain
+ * {@link Cache}. The object is either retrieved from the cache if the cache has
+ * it, or a call back is invoked to get the object (and put it in the cache).
  * 
  * @author Erik Brakkee
  * 
  */
 public class CachedObject<KeyType extends Serializable, ValueType extends Serializable> {
-    private static final Logger LOGGER = Logger.getLogger(CachedObject.class);
+    private static final Logger LOGGER = Logger.getLogger(CachedObject.class
+        .getName());
 
     /**
      * Cache to use.
@@ -40,6 +41,16 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
      */
     private KeyType objectKey;
 
+    /**
+     * Last known value.
+     */
+    private ValueType value;
+
+    /**
+     * Are we now computing the value or not?
+     */
+    private boolean computing;
+
     /**
      * Computation used to obtain the object if it is not found in the cache.
      */
@@ -67,33 +78,52 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
      * Gets the object. Since the object is cached, different calls to this
      * method may return different objects.
      * 
+     * If the object is expired from the cache it is recomputed using the
+     * callback. In case the callback throws an exception the
+     * last known value is used. In case an exception is thrown, the problem is
+     * also logged. In case a recomputation is already being done by another
+     * thread, the last known value is immediately returned.
+     * 
      * @return Object.
      */
     public ValueType get() {
-        ValueType object = (ValueType) cache.get(objectKey); // the used
-        // cache is
-        // thread safe.
-
-        if (object == null) {
-            // synchronize the computation to make sure that the object is only
-            // computed
-            // once when multiple concurrent threads detect that the entry must
-            // be
-            // recomputed.
+        synchronized (this) {
+            if (computing) {
+                // always return old value while computing.
+                return value;
+            }
+
+            ValueType cachedValue = cache.get(objectKey);
+            if (cachedValue == null) {
+                // expired
+                computing = true;
+            } else {
+                // no change.
+                return value;
+            }
+        }
+        try {
+
+            // we only get here if we are computing
+            // do the computation without holding the lock.
+            LOGGER.fine("Refreshing cache for '" + objectKey + "'");
+            ValueType object = computation.getObject(objectKey);
+            cache.put(objectKey, object);
+
+            synchronized (this) {
+                value = object;
+            }
+        } catch (Exception e) {
+            LOGGER.log(Level.INFO,
+                "Recomputation of cached item failed for key '" + objectKey +
+                    "'", e);
+        } finally {
             synchronized (this) {
-                object = (ValueType) cache.get(objectKey);
-
-                if (object == null) {
-                    // No other thread did a recomputation so we must do this
-                    // now.
-                    LOGGER.debug("Refreshing cache for '" + objectKey + "'");
-                    object = computation.getObject(objectKey);
-                    cache.put(objectKey, object);
-                }
+                computing = false;
             }
         }
 
-        return object;
+        return value;
     }
 
     /**
@@ -122,12 +152,14 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
     public static interface Computation<Key extends Serializable, Value extends Serializable> {
         /**
          * Gets the object. Called when the object is not in the cache.
+         * In case computation fails, an exception should be thrown to ensure that the last 
+         * known value will be used. 
          * 
          * @param aObjectKey
          *            Id of the object in the cache.
          * 
          * @return Object, must be non-null.
          */
-        Value getObject(Key aObjectKey);
+        Value getObject(Key aObjectKey) throws Exception;
     }
 }