Java tutorial
/* * Copyright (C) 2011 The Android Open Source Project * * 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 android.util; import android.annotation.UnsupportedAppUsage; import java.util.LinkedHashMap; import java.util.Map; /** * A cache that holds strong references to a limited number of values. Each time * a value is accessed, it is moved to the head of a queue. When a value is * added to a full cache, the value at the end of that queue is evicted and may * become eligible for garbage collection. * * <p>If your cached values hold resources that need to be explicitly released, * override {@link #entryRemoved}. * * <p>If a cache miss should be computed on demand for the corresponding keys, * override {@link #create}. This simplifies the calling code, allowing it to * assume a value will always be returned, even when there's a cache miss. * * <p>By default, the cache size is measured in the number of entries. Override * {@link #sizeOf} to size the cache in different units. For example, this cache * is limited to 4MiB of bitmaps: * <pre> {@code * int cacheSize = 4 * 1024 * 1024; // 4MiB * LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) { * protected int sizeOf(String key, Bitmap value) { * return value.getByteCount(); * } * }}</pre> * * <p>This class is thread-safe. Perform multiple cache operations atomically by * synchronizing on the cache: <pre> {@code * synchronized (cache) { * if (cache.get(key) == null) { * cache.put(key, value); * } * }}</pre> * * <p>This class does not allow null to be used as a key or value. A return * value of null from {@link #get}, {@link #put} or {@link #remove} is * unambiguous: the key was not in the cache. * * <p>This class appeared in Android 3.1 (Honeycomb MR1); it's available as part * of <a href="http://developer.android.com/sdk/compatibility-library.html">Android's * Support Package</a> for earlier releases. */ public class LruCache<K, V> { @UnsupportedAppUsage private final LinkedHashMap<K, V> map; /** Size of this cache in units. Not necessarily the number of elements. */ private int size; private int maxSize; private int putCount; private int createCount; private int evictionCount; private int hitCount; private int missCount; /** * @param maxSize for caches that do not override {@link #sizeOf}, this is * the maximum number of entries in the cache. For all other caches, * this is the maximum sum of the sizes of the entries in this cache. */ public LruCache(int maxSize) { if (maxSize <= 0) { throw new IllegalArgumentException("maxSize <= 0"); } this.maxSize = maxSize; this.map = new LinkedHashMap<K, V>(0, 0.75f, true); } /** * Sets the size of the cache. * * @param maxSize The new maximum size. */ public void resize(int maxSize) { if (maxSize <= 0) { throw new IllegalArgumentException("maxSize <= 0"); } synchronized (this) { this.maxSize = maxSize; } trimToSize(maxSize); } /** * Returns the value for {@code key} if it exists in the cache or can be * created by {@code #create}. If a value was returned, it is moved to the * head of the queue. This returns null if a value is not cached and cannot * be created. */ public final V get(K key) { if (key == null) { throw new NullPointerException("key == null"); } V mapValue; synchronized (this) { mapValue = map.get(key); if (mapValue != null) { hitCount++; return mapValue; } missCount++; } /* * Attempt to create a value. This may take a long time, and the map * may be different when create() returns. If a conflicting value was * added to the map while create() was working, we leave that value in * the map and release the created value. */ V createdValue = create(key); if (createdValue == null) { return null; } synchronized (this) { createCount++; mapValue = map.put(key, createdValue); if (mapValue != null) { // There was a conflict so undo that last put map.put(key, mapValue); } else { size += safeSizeOf(key, createdValue); } } if (mapValue != null) { entryRemoved(false, key, createdValue, mapValue); return mapValue; } else { trimToSize(maxSize); return createdValue; } } /** * Caches {@code value} for {@code key}. The value is moved to the head of * the queue. * * @return the previous value mapped by {@code key}. */ public final V put(K key, V value) { if (key == null || value == null) { throw new NullPointerException("key == null || value == null"); } V previous; synchronized (this) { putCount++; size += safeSizeOf(key, value); previous = map.put(key, value); if (previous != null) { size -= safeSizeOf(key, previous); } } if (previous != null) { entryRemoved(false, key, previous, value); } trimToSize(maxSize); return previous; } /** * Remove the eldest entries until the total of remaining entries is at or * below the requested size. * * @param maxSize the maximum size of the cache before returning. May be -1 * to evict even 0-sized elements. */ public void trimToSize(int maxSize) { while (true) { K key; V value; synchronized (this) { if (size < 0 || (map.isEmpty() && size != 0)) { throw new IllegalStateException( getClass().getName() + ".sizeOf() is reporting inconsistent results!"); } if (size <= maxSize) { break; } Map.Entry<K, V> toEvict = map.eldest(); if (toEvict == null) { break; } key = toEvict.getKey(); value = toEvict.getValue(); map.remove(key); size -= safeSizeOf(key, value); evictionCount++; } entryRemoved(true, key, value, null); } } /** * Removes the entry for {@code key} if it exists. * * @return the previous value mapped by {@code key}. */ public final V remove(K key) { if (key == null) { throw new NullPointerException("key == null"); } V previous; synchronized (this) { previous = map.remove(key); if (previous != null) { size -= safeSizeOf(key, previous); } } if (previous != null) { entryRemoved(false, key, previous, null); } return previous; } /** * Called for entries that have been evicted or removed. This method is * invoked when a value is evicted to make space, removed by a call to * {@link #remove}, or replaced by a call to {@link #put}. The default * implementation does nothing. * * <p>The method is called without synchronization: other threads may * access the cache while this method is executing. * * @param evicted true if the entry is being removed to make space, false * if the removal was caused by a {@link #put} or {@link #remove}. * @param newValue the new value for {@code key}, if it exists. If non-null, * this removal was caused by a {@link #put}. Otherwise it was caused by * an eviction or a {@link #remove}. */ protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) { } /** * Called after a cache miss to compute a value for the corresponding key. * Returns the computed value or null if no value can be computed. The * default implementation returns null. * * <p>The method is called without synchronization: other threads may * access the cache while this method is executing. * * <p>If a value for {@code key} exists in the cache when this method * returns, the created value will be released with {@link #entryRemoved} * and discarded. This can occur when multiple threads request the same key * at the same time (causing multiple values to be created), or when one * thread calls {@link #put} while another is creating a value for the same * key. */ protected V create(K key) { return null; } private int safeSizeOf(K key, V value) { int result = sizeOf(key, value); if (result < 0) { throw new IllegalStateException("Negative size: " + key + "=" + value); } return result; } /** * Returns the size of the entry for {@code key} and {@code value} in * user-defined units. The default implementation returns 1 so that size * is the number of entries and max size is the maximum number of entries. * * <p>An entry's size must not change while it is in the cache. */ protected int sizeOf(K key, V value) { return 1; } /** * Clear the cache, calling {@link #entryRemoved} on each removed entry. */ public final void evictAll() { trimToSize(-1); // -1 will evict 0-sized elements } /** * For caches that do not override {@link #sizeOf}, this returns the number * of entries in the cache. For all other caches, this returns the sum of * the sizes of the entries in this cache. */ public synchronized final int size() { return size; } /** * For caches that do not override {@link #sizeOf}, this returns the maximum * number of entries in the cache. For all other caches, this returns the * maximum sum of the sizes of the entries in this cache. */ public synchronized final int maxSize() { return maxSize; } /** * Returns the number of times {@link #get} returned a value that was * already present in the cache. */ public synchronized final int hitCount() { return hitCount; } /** * Returns the number of times {@link #get} returned null or required a new * value to be created. */ public synchronized final int missCount() { return missCount; } /** * Returns the number of times {@link #create(Object)} returned a value. */ public synchronized final int createCount() { return createCount; } /** * Returns the number of times {@link #put} was called. */ public synchronized final int putCount() { return putCount; } /** * Returns the number of values that have been evicted. */ public synchronized final int evictionCount() { return evictionCount; } /** * Returns a copy of the current contents of the cache, ordered from least * recently accessed to most recently accessed. */ public synchronized final Map<K, V> snapshot() { return new LinkedHashMap<K, V>(map); } @Override public synchronized final String toString() { int accesses = hitCount + missCount; int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0; return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]", maxSize, hitCount, missCount, hitPercent); } }