X-Git-Url: http://wamblee.org/gitweb/?a=blobdiff_plain;f=support%2Fgeneral%2Fsrc%2Fmain%2Fjava%2Forg%2Fwamblee%2Fcache%2FCachedObject.java;h=70baf67ca9c298c6de19ea0a2967c6a6b184e876;hb=HEAD;hp=f8eb8b0fad49c6c900f351ba6c6325c7168f503c;hpb=8de36ff0206c996baf3ee4adc3e2293b12ff5f39;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 f8eb8b0f..70baf67c 100644
--- a/support/general/src/main/java/org/wamblee/cache/CachedObject.java
+++ b/support/general/src/main/java/org/wamblee/cache/CachedObject.java
@@ -1,12 +1,12 @@
 /*
- * 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.
  * You may obtain a copy of the License at
- *
+ * 
  *      http://www.apache.org/licenses/LICENSE-2.0
- *
+ * 
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
@@ -15,20 +15,19 @@
  */
 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 +39,16 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
      */
     private KeyType objectKey;
 
+    /**
+     * Last known value. We only use this to return the last known value in case recomputation of the value fails.
+     */
+    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.
      */
@@ -47,17 +56,14 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
 
     /**
      * Constructs the cached object.
-     * 
-     * @param aCache
-     *            Cache to use.
-     * @param aObjectKey
-     *            Key of the object in the cache.
-     * @param aComputation
-     *            Computation to get the object in case the object is not in the
-     *            cache.
+     *
+     * @param aCache       Cache to use.
+     * @param aObjectKey   Key of the object in the cache.
+     * @param aComputation Computation to get the object in case the object is not in the
+     *                     cache.
      */
     public CachedObject(Cache<KeyType, ValueType> aCache, KeyType aObjectKey,
-        Computation<KeyType, ValueType> aComputation) {
+            Computation<KeyType, ValueType> aComputation) {
         cache = aCache;
         objectKey = aObjectKey;
         computation = aComputation;
@@ -66,34 +72,56 @@ 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.
-     * 
+     * <p/>
+     * 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) {
-                object = (ValueType) cache.get(objectKey);
+        synchronized (this) {
+            if (computing) {
+                // always return old value while computing.
+                return value;
+            }
 
-                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);
-                }
+            ValueType cachedValue = cache.get(objectKey);
+            if (cachedValue == null) {
+                // expired
+                computing = true;
+            } else {
+                // Two different instances of cached object might share the same cache and so it can occur
+                // that the value in one of the instances it out of date.
+                value = cachedValue;
+                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);
 
-        return object;
+            synchronized (this) {
+                value = object;
+            }
+        }
+        catch (Exception e) {
+            LOGGER.log(Level.INFO, "Recomputation of cached item failed for key '" + objectKey +
+                    "'", e);
+        } finally {
+            synchronized (this) {
+                computing = false;
+            }
+        }
+        synchronized (this) {
+            return value;
+        }
     }
 
     /**
@@ -106,7 +134,7 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
 
     /**
      * Gets the cache.
-     * 
+     *
      * @return Cache.
      */
     public Cache getCache() {
@@ -115,19 +143,18 @@ public class CachedObject<KeyType extends Serializable, ValueType extends Serial
 
     /**
      * Callback invoked to compute an object if it was not found in the cache.
-     * 
-     * @param <T>
-     *            Type of the object
+     *
+     * @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.
-         * 
-         * @param aObjectKey
-         *            Id of the object in the cache.
-         * 
+         * 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;
     }
 }