X-Git-Url: http://wamblee.org/gitweb/?a=blobdiff_plain;f=support%2Fgeneral%2Fsrc%2Fmain%2Fjava%2Forg%2Fwamblee%2Fcache%2FCachedObject.java;h=7ec87534f08e521ec5c95bcccc6ec4b0479277a0;hb=02f5f47eb96320baf55a6cb2e5141360e304b504;hp=58daf8c918ee3c53ff4ade7d31db0ee9d90fe279;hpb=32a62ca2c752e33a7873ac868a7a1f289caedcd4;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 58daf8c9..7ec87534 100644
--- a/support/general/src/main/java/org/wamblee/cache/CachedObject.java
+++ b/support/general/src/main/java/org/wamblee/cache/CachedObject.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2005 the original author or authors.
+ * Copyright 2005-2010 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.
@@ -13,55 +13,48 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.wamblee.cache;
 
 import java.io.Serializable;
-
-import org.apache.log4j.Logger;
+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
+        .getName());
 
-    private static final Logger LOGGER = Logger.getLogger(CachedObject.class);
+    /**
+     * Cache to use.
+     */
+    private Cache<KeyType, ValueType> cache;
 
     /**
-     * Callback invoked to compute an object if it was not found in the cache.
-     * 
-     * @param <T>
-     *            Type of the object
+     * Key of the object in the cache.
      */
-    public static interface Computation<Key extends Serializable, Value extends Serializable> {
-        /**
-         * Gets the object. Called when the object is not in the cache.
-         * 
-         * @param aObjectKey
-         *            Id of the object in the cache.
-         * @return Object, must be non-null.
-         */
-        Value getObject(Key aObjectKey);
-    }
+    private KeyType objectKey;
 
     /**
-     * Cache to use.
+     * Last known value.
      */
-    private Cache<KeyType, ValueType> _cache;
+    private ValueType value;
 
     /**
-     * Key of the object in the cache.
+     * Are we now computing the value or not?
      */
-    private KeyType _objectKey;
+    private boolean computing;
 
     /**
      * Computation used to obtain the object if it is not found in the cache.
      */
-    private Computation<KeyType, ValueType> _computation;
+    private Computation<KeyType, ValueType> computation;
 
     /**
      * Constructs the cached object.
@@ -75,49 +68,71 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
      *            cache.
      */
     public CachedObject(Cache<KeyType, ValueType> aCache, KeyType aObjectKey,
-            Computation<KeyType, ValueType> aComputation) {
-        _cache = aCache;
-        _objectKey = aObjectKey;
-        _computation = aComputation;
+        Computation<KeyType, ValueType> aComputation) {
+        cache = aCache;
+        objectKey = aObjectKey;
+        computation = aComputation;
     }
 
     /**
      * 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;
+        synchronized(this) { 
+            return value; 
+        }
     }
 
     /**
      * Invalidates the cache for the object so that it is recomputed the next
      * time it is requested.
-     * 
      */
     public void invalidate() {
-        _cache.remove(_objectKey);
+        cache.remove(objectKey);
     }
 
     /**
@@ -126,6 +141,26 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
      * @return Cache.
      */
     public Cache getCache() {
-        return _cache;
+        return cache;
+    }
+
+    /**
+     * Callback invoked to compute an object if it was not found in the cache.
+     * 
+     * @param <T>
+     *            Type of the object
+     */
+    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) throws Exception;
     }
 }