(no commit message)

[utils] / support / general / src / main / java / org / wamblee / cache / CachedObject.java

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 f8eb8b0fad49c6c900f351ba6c6325c7168f503c..da86298d6598115f9f5b4a683a27053c8f08da73 100644 (file)
--- 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 +15,21 @@
  */
 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 +78,53 @@ 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) {
-                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 {
+                // no change.
+                return value;
             }
         }
+        try {
 
-        return object;
+            // 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) {
+                computing = false;
+            }
+        }
+        synchronized (this) {
+            return value;
+        }
     }
 
     /**
@@ -121,13 +152,15 @@ 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.
+         * 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;
     }
 }