Android的Message Pool是个什么鬼——源码角度分析

Android中，我们在线程之间通信传递通常采用Android的消息机制，而这机制传递的正是Message。

通常，我们使用Message.obtain()和Handler.obtainMessage()从Message Pool中获取Message，避免直接构造Message。

• 那么Android会否因为Message Pool缓存的Message对象而造成OOM呢？对于这个问题，我可以明确的说APP不会因Message Pool而OOM。至于为什么，可以一步步往下看，心急的可以直接看最后一节——Message Pool如何存放Message。

Message Obtain分析

Message.obtain()源码

    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

从代码片中，可以看到Message是直接由sPool赋值的。

Handler.obtainMessage()源码

    /**
     * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
     * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
     *  If you don‘t want that facility, just call Message.obtain() instead.
     */
    public final Message obtainMessage()
    {
        return Message.obtain(this);
    }

Handler.obtain()最终还是调用Message.obtain()来获取的。

Message Pool相关源码分析

Message Pool数据结构

    // sometimes we store linked lists of these things
    /*package*/ Message next;

    private static final Object sPoolSync = new Object();
    private static Message sPool;
    private static int sPoolSize = 0;

    private static final int MAX_POOL_SIZE = 50;

    private static boolean gCheckRecycle = true;

从代码中可以很明确的看到，Message Pool的数据结构实际就是一个链表。sPool就是一个全局的消息池，sPoolSize记录链表长度，MAX_POOL_SIZE表示链表的最大长度为50。

Message Pool如何存放Message

    /** @hide */
    public static void updateCheckRecycle(int targetSdkVersion) {
        if (targetSdkVersion < Build.VERSION_CODES.LOLLIPOP) {
            gCheckRecycle = false;
        }
    }

    /**
     * Return a Message instance to the global pool.
     * <p>
     * You MUST NOT touch the Message after calling this function because it has
     * effectively been freed.  It is an error to recycle a message that is currently
     * enqueued or that is in the process of being delivered to a Handler.
     * </p>
     */
    public void recycle() {
        if (isInUse()) {
            if (gCheckRecycle) {
                throw new IllegalStateException("This message cannot be recycled because it "
                        + "is still in use.");
            }
            return;
        }
        recycleUnchecked();
    }

    /**
     * Recycles a Message that may be in-use.
     * Used internally by the MessageQueue and Looper when disposing of queued Messages.
     */
    void recycleUnchecked() {
        // Mark the message as in use while it remains in the recycled object pool.
        // Clear out all other details.
        flags = FLAG_IN_USE;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        sendingUid = -1;
        when = 0;
        target = null;
        callback = null;
        data = null;

        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }

从代码分析上看，消息池存放的核心方法就是上面的recycleUnchecked()方法：

1、将待回收的Message对象字段置空（避免因Message过大，使静态的消息池内存泄漏）。因此无论原先的Message对象有多大，最终被缓存进Message Pool前都被置空，那么这些缓存的Message对象所占内存大小对于一个app内存来说基本可以忽略。所以说，Message Pool并不会造成App的OOM。

2、以内置锁对方式（线程安全），判断当前线程池的大小是否小于50。若小于50，直接将Mesaage插入到消息池链表尾部；若大于等于50，则直接丢弃掉，那么这些被丢弃的Message将交由GC处理。

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