android android.support.v4.util.Pools 源码解析

android android.support.v4.util.Pools 源码解析

路径是在 android.support.v4.util，该类属于工具类。代码比较简单，忽然翻到，就简单分析一下。

#####首先 Pools只是一个空壳子。然后里面的代码才是真正的代码。

Pool是一个接口，带泛型的接口。里面就两个方法，一个是取数据，一个是存数据。

SimplePool是具体实现，类如其名，实现的逻辑非常简单。

• 首先通过构造方法创建一个指定大小的数组：Object[size];
• T acquire()方法就是去获取Object[size-1]对应的数据，每次获取之后，size-- , 如果一直获取，超过了size个数了，就会获取null. (实际上，Object[index] 也是null ,看代码里面只是初始化了数组，并没有初始化数组中的元素。。。。)
• boolean release(T obj)的操作是acquire()的反操作，也就是将数据放回之前的数组Object[size]中，然后放进去之后，size++。放之前先判断当前元素是不是已经存在于数组中了。
• boolean isInPool(T t) 逻辑也很简单的，就是遍历当前数组，把数组的元素拿出来一个个与目标元素比较，如果有相等的，就说明已存在，否则不存在。

SynchronizedPool是对SimplePool的同步实现

其实SimplePool代码更简单，完全是利用了SimplePool，只是在 acquire() / release() 前加了同步关键字保护。

---

就目前而言，我非常怀疑这个类的存在意义。得到实际场景才能看到其存在的意义。

下面是该类的源码。如你所见，非常的简单。

/*
 * Copyright (C) 2013 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.support.v4.util;


/**
 * Helper class for creating pools of objects. An example use looks like this:
 * <pre>
 * 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);
 *     }
 *
 *     . . .
 * }
 * </pre>
 *
 */
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);
            }
        }
    }
}