LruCache介绍

LruCache介绍

Lru–>(Least recent used)最少最近使用算法

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/**
    * @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);
   }

构造传入缓存容量，LinkedHashMap维护键值。

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/**
    * 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);
   }

重设缓存容量，加锁控制并发访问。调用了trimToSize方法

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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);
        }
    }

实际容量为负数，或者map为空但是实际容量却不为空。抛出异常。

map.eldest();取出末端数据，然后移除，evictionCount移除数加一。实际容量缩减。调用了safeSizeOf

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private int safeSizeOf(K key, V value) {
        int result = sizeOf(key, value);
        if (result < 0) {
            throw new IllegalStateException("Negative size: " + key + "=" + value);
        }
        return result;
    }

再调用了sizeOf方法，默认返回1,一般重写这个方法，如果是图片缓存，bitmap是值，这里重写返回bitmap的大小

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/**
     * 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;
    }

trimToSize方法最后调用了entryRemoved方法，默认空实现。

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/**
   * 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) {}

此方法没有控制并发访问，第一个个传true代表已经移除了，用来压缩空间。如果是false，代表是put，或者remove方法调用了此方法。

看下put方法

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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;
    }

LruCache不允许key，value为null值。同步操作，把key放入map，size增加，如果map已经有值，即previous不是null，则size不能增加，要反扣掉。再调用entryRemoved和trimTosize方法。

再看get方法

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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;
        }
    }

在map取到就直接返回，如果取不到，调用了create方法

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/**
     * 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;
    }

create默认返回null，没有控制并发。

如果有重写，则会添加到map，试图把create返回的createdValue添加到map，但是返回mapValue不是null，说明，有别的线程也访问了create，并且已经添加了一个一样的key到map，所以当前线程不能把这个值覆盖，所以又调用了

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// There was a conflict so undo that last put
               map.put(key, mapValue);

用来撤回刚才的put操作。当mapValue不为null时候，调用entryRemoved通知外部，如果为null，则调用trimToSize检查是否超过容量，是否要移除尾部。