学步园

今天闲着没事看看 android v4包，结果发现一个类Pools.java，看了一下，从字面上的意思来看应该也是池，我自然而然的就会联想到线程池，看了一下，Pools类其实是对象池

里面维护了一个数组，废话不多说，看代码：

public final class Pools {

    /**
     * Interface for managing a pool of objects.
     *
     * @param <T> The pooled type.
     */
    public static interface Pool<T> {

        /**
         * @return An instance from the pool if such, null otherwise.
         */
        public T acquire();

        /**
         * Release an instance to the pool.
         *
         * @param instance The instance to release.
         * @return Whether the instance was put in the pool.
         *
         * @throws IllegalStateException If the instance is already in the pool.
         */
        public boolean release(T instance);
    }

    private Pools() {
        /* do nothing - hiding constructor */
    }

    /**
     * Simple (non-synchronized) pool of objects.
     *
     * @param <T> The pooled type.
     */
    public static class SimplePool<T> implements Pool<T> {
        private final Object[] mPool;

        private int mPoolSize;

        /**
         * Creates a new instance.
         *
         * @param maxPoolSize The max pool size.
         *
         * @throws IllegalArgumentException If the max pool size is less than zero.
         */
        public SimplePool(int maxPoolSize) {
            if (maxPoolSize <= 0) {
                throw new IllegalArgumentException("The max pool size must be > 0");
            }
            mPool = new Object[maxPoolSize];
        }

        @Override
        @SuppressWarnings("unchecked")
        public T acquire() {
            if (mPoolSize > 0) {
                final int lastPooledIndex = mPoolSize - 1;
                T instance = (T) mPool[lastPooledIndex];
                mPool[lastPooledIndex] = null;
                mPoolSize--;
                return instance;
            }
            return null;
        }

        @Override
        public boolean release(T instance) {
            if (isInPool(instance)) {
                throw new IllegalStateException("Already in the pool!");
            }
            if (mPoolSize < mPool.length) {
                mPool[mPoolSize] = instance;
                mPoolSize++;
                return true;
            }
            return false;
        }

        private boolean isInPool(T instance) {
            for (int i = 0; i < mPoolSize; i++) {
                if (mPool[i] == instance) {
                    return true;
                }
            }
            return false;
        }
    }

    /**
     * Synchronized) pool of objects.
     *
     * @param <T> The pooled type.
     */
    public static class SynchronizedPool<T> extends SimplePool<T> {
        private final Object mLock = new Object();

        /**
         * Creates a new instance.
         *
         * @param maxPoolSize The max pool size.
         *
         * @throws IllegalArgumentException If the max pool size is less than zero.
         */
        public SynchronizedPool(int maxPoolSize) {
            super(maxPoolSize);
        }

        @Override
        public T acquire() {
            synchronized (mLock) {
                return super.acquire();
            }
        }

        @Override
        public boolean release(T element) {
            synchronized (mLock) {
                return super.release(element);
            }
        }
    }
}

这个类首先定义了一个Pool接口，其中acquare 方法返回T类型对象，release方法，返回true/false

然后还定义了两个内部类SimplePool,SynchronizedPool

先来说SynchronizedPool：

这个类继承了SimplePool,只是对存取过程加锁，可以这样使用

 public class MyPooledClass {
 
     private static final SynchronizedPool<MyPooledClass> sPool =
              new SynchronizedPool<MyPooledClass>(10);
 
      public static MyPooledClass obtain() {
          MyPooledClass instance = sPool.acquire();
          return (instance != null) ? instance : new MyPooledClass();
      }
 
      public void recycle() {
           // Clear state if needed.
           sPool.release(this);
      }

SimplePool 实现了Pool接口

首先获取数组大小，然后取出一个对象mPoolSize-1

        public T acquire() {
            if (mPoolSize > 0) {
                final int lastPooledIndex = mPoolSize - 1;
                T instance = (T) mPool[lastPooledIndex];
                mPool[lastPooledIndex] = null;
                mPoolSize--;
                return instance;
            }
            return null;
        }

释放的时候再把数组的那个元素赋值，mPoolSize+1

        @Override
        public boolean release(T instance) {
            if (isInPool(instance)) {
                throw new IllegalStateException("Already in the pool!");
            }
            if (mPoolSize < mPool.length) {
                mPool[mPoolSize] = instance;
                mPoolSize++;
                return true;
            }
            return false;
        }