Handler 核心知识点
1.为什么在不能再子线程直接初始化handler,主线程却可以?
查看handler的源码会发现
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
,在创建handler的时候,会检查looper对象,如果为空,会抛出一个运行时异常,也就是说,在子线程创建handler的时候,looper是没有创建的,这也就解释了,在子线程创建handler的时候需要 执行如下代码:
Looper.prepare();
...
Loop.loop();
为什么主线程不需要呢?
需要看那一下ActivityThread 这个类,ActivityThread就是我们常说的主线程或UI线程，ActivityThread的main方法是整个APP的入口!可以看看在这个类中,main函数中做了什么操作
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("");
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
不懂没关细,只需要看下咱们关心的内容就可以了, == Looper.prepareMainLooper();== 以及 ==Looper.loop();==,这也就跟之前的对用上了,为什么在子线程的创建handler的时候需要 Looper.parpare(),以及Looper.loop(),而主线程不需要;其实在ActivityThread类中在main函数调用的prepareMainLooper()和在子线程中调用的parpare()还是略微有点区别,看源码
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
代码不懂没关系,看注释,大致意思就是:
==初始化当前线程作为一个looper,让它作为整个应用的main looper,(个人理解,就是主线程吧),这个 main looper 是有Android 环境创建的,因此你没有必要调用你自己的方法 ,比如 prepare();==
很清晰了,也就解释了为什么主线程不需要调用 prepare();
2.handler中延时消息是怎么实现的?
handler 中延时消息的发送时通过 ==postDelayed(Runnable r, Object token, long delayMillis)==实现的,但最终调用的==sendMessageAtTime(Message msg, long uptimeMillis)==函数,然后加入到消息队列中,到目前看没有发现什么异常,可以看看,这个延时消息是怎么处理的!
handler 的中消息的分发处理是通过 Looper 中的loop()函数以及MessageQueue中的next()函数进行处理的,可以先看loop()函数:
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();
}
}
代码不少,没有必要全部看明白,如果感觉不像是在这里处理的,那就看看,MessageQueuede next()函数,
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
没有贴出全部代码,可以看到，在这个方法内，如果头部的这个Message是有延迟而且延迟时间没到的（now < msg.when），会计算一下时间（保存为变量nextPollTimeoutMillis），然后在循环开始的时候判断如果这个Message有延迟，就调用nativePollOnce(ptr, nextPollTimeoutMillis)进行阻塞。nativePollOnce()的作用类似与object.wait()，只不过是使用了Native的方法对这个线程精确时间的唤醒。
3.handler MessageQueue是如何被阻塞的以及被唤醒的?
在MessageQueue的next函数中可以看到，里面不过是有一个 for (;;) 死循环结果，循环体内部调用了一个 ==nativePollOnce(long, int)****== 方法。这是一个 Native 方法，实际作用是通过 Native 层的 MessageQueue 阻塞当前调用栈线程 nextPollTimeoutMillis 毫秒的时间。
==可以看到nextPollTimeoutMillis== 取值不同情况下的阻塞表现
-
小于 0
一直被阻塞,知道被唤醒
-
等于 0
不会被阻塞
-
大于 0
最长阻塞 nextPollTimeoutMillis 毫秒，期间如被唤醒会立即返回。
==nextPollTimeoutMillis==的默认值为0，所以不会阻塞，会直接去取 Message 对象,如果没有取到 Message 对象数据，则直接会把 ==nextPollTimeoutMillis== 置为 -1,此时满足小于 0 的条件，会被一直阻塞，直到其他地方调用另外一个 Native 方法 nativeWake(long) 进行唤醒 ,调用这个函数的地方是在 ==MessageQueue==中的 ==enqueueMessage== 中被调用的!
4.Lopper中的loop函数为什么不会阻塞主线程?
有上面ActivityThread的main函数中源码可以知道,在程序启动的时候回初始化一个Looper,这个looper 就是一个main looper ,为什么说是main looper呢?因为这个lopper在应用的整个生命周期中不是唯一,可以看看looper 的parpare函数:
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
可以看到,只要调用这个函数,就会新创建一个Looper 保存起来!至于为什么loop不会阻塞主线程,可以参考上面的内容,在主线程的这个loop,如果有消息事件就处理,如果没有就休眠,释放cpu资源,等待下一次被唤醒!直至这个程序退出!