Android中,我们在线程之间通信传递通常採用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会否由于Message Pool缓存的Message对象而造成OOM呢?
对于这个问题,我能够明白的说APP不会因Message Pool而OOM。至于为什么,能够一步步往下看,心急的能够直接看最后一节——Message Pool怎样存放Message。
Handler.obtainMessage()源代码:
/** * 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); }
显然。Handler.obtain()是调用Message.obtain()来获取的。那么我门再来看下Message.obtain()源代码:
/** * 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(); }
上述代码给我们透露几个个关键信息:
剧透下这里的sPool事实上就是Message Pool
public final class Message implements Parcelable { // 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;}
看到关键信息了没?Message的成员有next、sPool和sPoolSize。这对于略微学过一点数据结构的,非常快就能判断出这是一个典型的链表结构的实现。sPool就是一个全局的消息池即链表。next记录链表中的下一个元素,sPoolSize记录链表长度。MAXPOOLSIZE表示链表的最大长度为50。
public final class Message implements Parcelable { private static boolean gCheckRecycle = true; /** @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:
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)。因此不管原先的Message对象有多大。终于被缓存进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内存来说基本能够忽略。所以说。Message Pool并不会造成App的OOM。
以内置锁的方式(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。