LruCache

package cn.seaboot.common.core;

import java.util.LinkedHashMap;
import java.util.Map;

/**
 * 后进先出缓存，选自Android
 * @author ChenSS 2017年11月22日
 */
public class LruCache<K, V> {
    private final LinkedHashMap<K, V> map;

    /** Size of this cache in units. Not necessarily the number of elements. */
    private int size;
    /**
     * cache最大大小
     */
    private int maxSize; 

    /**
     * put的次数
     */
    private int putCount; 
    /**
     * create的次数
     */
    private int createCount; 
    /**
     * 回收的次数
     */
    private int evictionCount;
    /**
     * 命中的次数
     */
    private int hitCount;
    /**
     * 未命中次数
     */
    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;
        // 将LinkedHashMap的accessOrder设置为true来实现LRU
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
    }

    /**
     * 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. 通过key获取相应的item，或者创建返回相应的item。相应的item会移动到队列的尾部，
     * 如果item的value没有被cache或者不能被创建，则返回null。
     */
    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) {
                // mapValue不为空表示命中，hitCount+1并返回mapValue对象
                hitCount++;
                return mapValue;
            }
            missCount++; // 未命中
        }

        /*
         * 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.
         * 如果未命中，则试图创建一个对象，这里create方法返回null,并没有实现创建对象的方法
         * 如果需要事项创建对象的方法可以重写create方法。因为图片缓存时内存缓存没有命中会去 文件缓存中去取或者从网络下载，所以并不需要创建。
         */
        V createdValue = create(key);
        if (createdValue == null) {
            return null;
        }
        // 假如创建了新的对象，则继续往下执行
        synchronized (this) {
            createCount++;
            // 将createdValue加入到map中，并且将原来键为key的对象保存到mapValue
            mapValue = map.put(key, createdValue);
            if (mapValue != null) {
                // There was a conflict so undo that last put
                // 如果mapValue不为空，则撤销上一步的put操作。
                map.put(key, mapValue);
            } else {
                // 加入新创建的对象之后需要重新计算size大小
                size += safeSizeOf(key, createdValue);
            }
        }

        if (mapValue != null) {
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue;
        } else {
            // 每次新加入对象都需要调用trimToSize方法看是否需要回收
            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); // size加上预put对象的大小
            previous = map.put(key, value);
            if (previous != null) {
                // 如果之前存在键为key的对象，则size应该减去原来对象的大小
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, value);
        }
        // 每次新加入对象都需要调用trimToSize方法看是否需要回收
        trimToSize(maxSize);
        return previous;
    }

    /**
     * @param maxSize
     *            the maximum size of the cache before returning. May be -1 to
     *            evict even 0-sized elements.
     *            此方法根据maxSize来调整内存cache的大小，如果maxSize传入-1，则清空缓存中的所有对象
     */
    private 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!");
                }
                // 如果当前size小于maxSize或者map没有任何对象,则结束循环
                if (size <= maxSize || map.isEmpty()) {
                    break;
                }
                // 移除链表头部的元素，并进入下一次循环
                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++; // 回收次数+1
            }

            entryRemoved(true, key, value, null);
        }
    }

    /**
     * Removes the entry for {@code key} if it exists.
     *
     * @return the previous value mapped by {@code key}.
     *         从内存缓存中根据key值移除某个对象并返回该对象
     */
    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.
     * 用来计算单个对象的大小，这里默认返回1，一般需要重写该方法来计算对象的大小
     * xUtils中创建LruMemoryCache时就重写了sizeOf方法来计算bitmap的大小 mMemoryCache = new
     * LruMemoryCache<MemoryCacheKey, Bitmap>(globalConfig.getMemoryCacheSize())
     * {
     * 
     * @Override protected int sizeOf(MemoryCacheKey key, Bitmap bitmap) { if
     *           (bitmap == null) return 0; return bitmap.getRowBytes() *
     *           bitmap.getHeight(); } };
     *
     */
    protected int sizeOf(K key, V value) {
        return 1;
    }

    /**
     * Clear the cache, calling {@link #entryRemoved} on each removed entry.
     * 清空内存缓存
     */
    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.
     */
    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);
    }
}