(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
new file mode 100644 (file)
index 0000000..58daf8c
--- /dev/null
+++ b/support/general/src/main/java/org/wamblee/cache/CachedObject.java
@@ -0,0 +1,131 @@
+/*
+ * Copyright 2005 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.
+ * 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;
+
+/**
+ * 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).
+ *
+ * @author Erik Brakkee
+ */
+public class CachedObject<KeyType extends Serializable, ValueType extends Serializable> {
+
+    private static final Logger LOGGER = Logger.getLogger(CachedObject.class);
+
+    /**
+     * 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.
+         * 
+         * @param aObjectKey
+         *            Id of the object in the cache.
+         * @return Object, must be non-null.
+         */
+        Value getObject(Key aObjectKey);
+    }
+
+    /**
+     * Cache to use.
+     */
+    private Cache<KeyType, ValueType> _cache;
+
+    /**
+     * Key of the object in the cache.
+     */
+    private KeyType _objectKey;
+
+    /**
+     * Computation used to obtain the object if it is not found in the cache.
+     */
+    private Computation<KeyType, ValueType> _computation;
+
+    /**
+     * 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.
+     */
+    public CachedObject(Cache<KeyType, ValueType> aCache, KeyType aObjectKey,
+            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.
+     * 
+     * @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);
+                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);
+                }
+            }
+        }
+        return object;
+    }
+
+    /**
+     * Invalidates the cache for the object so that it is recomputed the next
+     * time it is requested.
+     * 
+     */
+    public void invalidate() {
+        _cache.remove(_objectKey);
+    }
+
+    /**
+     * Gets the cache.
+     * 
+     * @return Cache.
+     */
+    public Cache getCache() {
+        return _cache;
+    }
+}