当在学习跨线程机制 Handler 时，一定会接触到 Message.obtain() 方法
当在学习Message.obtain 时，可能有几个疑问：
- Message的集合的具体存储结构是怎样的？
- 两种Message构建方法：Message.obtain() 和 new Message() 之间的区别？
- Message.obtain() 调用后获取的链表节点缓存对象会不会形成脏数据？
- new Handler().obtainMessage() 和 Message.obtain() 区别？
弄清这几个问题的方式还是从源码入手比较干脆：
问题1：Message的集合的具体存储结构是怎样的？
先抛结论：单链表(MAX_POOL_SIZE = 50)
- 第1段关键代码位于 Handler.java
在Client调用new Handler().postDelayed()等方法后，沿着调用链，会调用到Handler类关键方法enqueueMessage()，把Message塞入MessageQueue
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
- 第2段关键代码位于 MessageQueue.java
MessageQueue虽命名为Queue（队列），但从源码和Message角度看出，调用MessageQueue postSyncBarrier、enqueueMessage等方法时，逻辑结构是线性表，存储结构是链表而不是队列；但从注册的回调监听 - mIdleHandlers看，倒是符合队列性质；我们姑且叫它为队列吧
在enqueueMessage中调整消息顺序：
新插入消息(节点=msg)的后继指针(指针=msg.next)指向当前消息(节点=mMessages) - msg.next = p;
当前消息指针(指针=mMessages)指向新插入的消息(节点=msg) - prev.next = msg;
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
问题2：两种Message构建方法：Message.obtain() 和 new Message() 之间的区别？
先抛结论：Message.obtain() 比 new Message() 更高效
关键代码：位于 Message.java
调用obtain方法后，优先查找Message单链表表头是否已有Message对象，有则利用，无则创建；所以Message.obtain() 和 new Message() 之间的区别：Message.obtain()更高效，因为节省了为Message分配、创建、调整内存的操作
/**
* 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();
}
问题3：Message.obtain() 调用后获取的链表节点缓存对象会不会形成脏数据？
先抛结论：不会
关键代码位于：Looper.java
由问题2知，Message.obtain是取缓存操作，Message会持有what、when等变量，如果我们重用表头的Message，岂不是取到了脏数据？
Android SDK当然会解决这个问题：Looper机制，Android的Looper机制原理和设计思想 可类比 iOS中的Runloop，开启一条线程，同时开启一个消息循环，制造一个常驻线程
只是Runloop的业务场景更底层些（内核所启动的消息循环，控制内核态和用户态切换）
loop()方法实现的最后一行msg.recycleUnchecked()正解决了问题3，每一次loop结尾，Android SDK都会帮我们把执行完毕的消息所持有的变量重置到初值，以使下次使用时保证数据整洁
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycleUnchecked();
}
}
问题4. new Handler().obtainMessage() 和 Message.obtain() 区别
先抛结论：没有区别
关键代码位于 Handler.java
public final Message obtainMessage()
{
return Message.obtain(this);
}
