Android的Message Pool是個什麼鬼——源碼角度分析，androidpool

Android的Message Pool是個什麼鬼——源碼角度分析，androidpool

Android中，我們線上程之間通訊傳遞通常採用Android的訊息機制，而這機制傳遞的正是Message。

通常，我們使用Message.obtain()和Handler.obtainMessage()從Message Pool中擷取Message，避免直接構造Message。

• 那麼Android會否因為Message Pool緩衝的Message對象而造成OOM呢？對於這個問題，我可以明確的說APP不會因Message Pool而OOM。至於為什麼，可以一步步往下看，心急的可以直接看最後一節——Message Pool如何存放Message。

Message Obtain分析Message.obtain()源碼

    /**     * 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();    }

從代碼片中，可以看到Message是直接由sPool賦值的。

Handler.obtainMessage()源碼

    /**     * 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()最終還是調用Message.obtain()來擷取的。

Message Pool相關源碼分析Message Pool資料結構

    // 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;

從代碼中可以很明確的看到，Message Pool的資料結構實際就是一個鏈表。sPool就是一個全域的訊息池，sPoolSize記錄鏈表長度，MAX_POOL_SIZE表示鏈表的最大長度為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++;            }        }    }

從程式碼分析上看，訊息池存放的核心方法就是上面的recycleUnchecked()方法：

1、將待回收的Message對象欄位置空（避免因Message過大，使靜態訊息池記憶體流失）。因此無論原先的Message對象有多大，最終被緩衝進Message Pool前都被置空，那麼這些緩衝的Message對象所佔記憶體大小對於一個app記憶體來說基本可以忽略。所以說，Message Pool並不會造成App的OOM。

2、以內建鎖的方式（安全執行緒），判斷當前線程池的大小是否小於50。若小於50，直接將Mesaage插入到訊息池鏈表尾部；若大於等於50，則直接丟棄掉，那麼這些被丟棄的Message將交由GC處理。

著作權聲明：本文為博主原創文章，未經博主允許不得轉載。