其实,写这篇文章的初衷还是上一篇关于ANR问题剖析的时分,想到其实ANR中心实质便是让UI线程(主线程)等了太久,导致体系判定在主线程做了耗时操作导致ANR。当咱们履行任何一个使命的时分,在Framework底层是经过音讯机制来保护使命的分发,从下面这个日志能够看到,
"main" prio=5 tid=1 Blocked
| group="main" sCount=1 dsCount=0 flags=1 obj=0x7583df30 self=0xe36f4000
| sysTid=6084 nice=-10 cgrp=default sched=0/0 handle=0xe83b5494
| state=S schedstat=( 4210489664 1169737873 12952 ) utm=123 stm=298 core=2 HZ=100
| stack=0xff753000-0xff755000 stackSize=8MB
| held mutexes=
at com.lay.datastore.DataStoreActivity.onCreate$lambda-1(DataStoreActivity.kt:29)
- waiting to lock <0x0493299a> (a java.lang.Object) held by thread 15
at com.lay.datastore.DataStoreActivity.$r8$lambda$IFZrCDzOUja7d5eTPj5Nq-CEC-8(DataStoreActivity.kt:-1)
at com.lay.datastore.DataStoreActivity$$ExternalSyntheticLambda0.onClick(D8$$SyntheticClass:-1)
at android.view.View.performClick(View.java:6597)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1219)
at android.view.View.performClickInternal(View.java:6574)
at android.view.View.access$3100(View.java:778)
at android.view.View$PerformClick.run(View.java:25885)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
每个使命履行,都是经过Handler来分发音讯,一旦使命堵塞无法履行下去,那么就会导致main thread被挂起,便是Blocked状况,所以把握Handler的事情分发机制,关于咱们剖析ANR日志会有很大的帮助。
1 Handler的事情分发机制
咱们知道,UI的改写有必要要在主线程,而关于耗时操作,例如网络恳求,往往都是发生在子线程,所以关于数据的改写有必要要涉及到线程切换,像Rxjava、EventBus、协程,都具备线程的上下文切换的才能,其实归结到底层都是Handler。
1.1 sendMessage办法剖析
咱们在运用Handler的时分,通常都是创立一个Handler目标,在handleMessage中接收其他线程发送来的音讯。
class HandlerActivity : AppCompatActivity() {
private val handler by lazy {
Handler(Looper.getMainLooper()) { message ->
when (message.what) {
1 -> {
}
}
return@Handler true
}
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_handler)
Thread {
handler.sendEmptyMessage(1)
}.start()
}
}
那么在子线程中,发送音讯的办法都是sendxxx办法,看下图:
如此多的send办法,咱们肯定都熟悉他们的用法,经过源码咱们能够看到,每个办法终究都调用了sendMessageAtTime办法。
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
在这个办法中,首要拿到了一个MessageQueue目标,这个是一个音讯行列,具体数据结构稍后剖析,然后调用了enqueueMessage办法。
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
enqueueMessage从字面意思上,便是将音讯入列,即将音讯刺进音讯行列;首要会给Message目标添加一个target特点,这个需求注意一下,由于咱们可能会创立多个Handler,那么这个target特点就标记了音讯终究交给哪个Handler处理,这里就有可能发生内存走漏。
最后调用了MessageQueue的enqueueMessage办法;
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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;
}
咱们看到这里是一个同步办法,由于存在多个线程的音讯入队,所以这里是线程安全的;在enqueueMessage办法中传入了一个when参数,这个参数的效果便是音讯在入队的时分,会依据履行时刻的先后进行排队,依据时刻先后顺次履行,这样子线程的音讯发送就完结了。
总结一下:子线程调用send办法时,其实便是将数据封装为Message结构体,往音讯行列中刺进这条message音讯就完结使命了,剩余的就交给子线程来处理。
1.2 Android体系心跳机制
当子线程将音讯入队之后,主线程怎样去取的呢?当app发动之后,体系会经过zygote进程fork出一个app进程,剩余的发动使命就交给ActivityThread来完结,经过反射调用了ActivityThread的main办法。
public static void main(String[] args) {
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");
}
在main办法中,其实便是创立了Looper目标,调用loop办法敞开了死循环。
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
if (me.mInLoop) {
Slog.w(TAG, "Loop again would have the queued messages be executed"
+ " before this one completed.");
}
me.mInLoop = true;
// 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);
me.mSlowDeliveryDetected = false;
for (;;) {
if (!loopOnce(me, ident, thresholdOverride)) {
return;
}
}
}
这里敞开的死循环,能够理解为Android体系的一个心跳机制,经过死循环不断地取出音讯处理音讯,确保咱们这个进程是活着的。当然回到文章最初说的,当处理音讯发生堵塞性问题时,就会导致ANR。
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
Message msg = me.mQueue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
// 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);
}
// ......
try {
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
// ..
return true;
}
咱们能够看到,在死循环中,会重复调用loopOnce办法,在这个办法中,会从MessageQueue中不断取出Message,在子线程音讯入队的时分,设置了target特点,咱们看到终究其实在音讯处理的时分,便是调用了Handler的dispatchMessage办法。
1.3 dispatchMessage办法剖析
咱们简单看下dispatchMessage的代码,很简单,大致分为3种类型。
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
1.3.1 假如Message的callback目标不为空;
什么情况下,会对Message的callback参数赋值?咱们看下handleCallback的源码,终究履行了callback的run办法,其实便是履行了Runnable的run办法。
private static void handleCallback(Message message) {
message.callback.run();
}
咱们看下Handler的post办法,在这个办法中其实便是传入了一个Runnable目标,
public final boolean post(@NonNull Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
然后getPostMessage办法中,也是创立了一个Message目标,并将Runnable目标作为callback参数赋值。
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
也便是说,当子线程调用post办法的时分,就会走到这部分逻辑中,经过这种办法能够完结线程的切换。
1.3.2 假如mCallback不为空;
从Handler的结构函数中中,发现能够传入一个Callback目标,
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
boolean handleMessage(@NonNull Message msg);
}
Callback是一个接口,内部办法为handleMessage,也便是说假如在Handler中传入了Callback目标,那么就会运用Callback的handleMessage进行音讯处理;假如没有传入Callback,那么就直接调用handleMessage,类似于文章最初那种运用办法,当然大部分场景下,咱们都会这样运用。
所以经过子线程音讯发送,主线程处理音讯,咱们大概就能理解Handler跨线程的实质,两者之间便是经过MessageQueue作为纽带来关联起来的,类似于一个传送带,而MessageQueue是什么时分创立的呢?
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
在创立Looper的时分就现已创立了,既然是在主线程创立,那么MessageQueue便是在主线程,子线程就能够往这个行列中刺进Message,主线程调用MessageQueue的next办法去获取自然也是在主线程了。
2 Handler处理多线程并发问题
前面,咱们提到的都是常规用法,在主线程中创立Handler,在主线程中进行音讯的处理,主要用于子线程发送数据,主线程更新UI,那么咱们只能在主线程创立Handler吗?假如在子线程中创立Handler,需求做什么处理?
2.1 子线程创立Handler
像下面这样,咱们能直接在子线程中创立Handler目标吗?
Thread {
val handler = Handler{
return@Handler true
}
}.start()
咱们看下Handler的结构函数源码,在判别逻辑处,
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> 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());
}
}
// 判别 --
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;
}
会判别mLooper是否为空,在获取Looper的时分,是从sThreadLocal目标中取,它其实是一个与线程相关的Map集合,在调用get办法的时分,会以当时线程为key,获取对应的Looper目标。
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
像主线程中的Looper目标,其实便是在ActivityThread中创立并加入到主线程中的ThreadLocal中,其实便是调用了下面的prepare办法。
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));
}
也便是说在每个线程中,只能有一个Looper目标,对应一个MessageQueue,所以假如在子线程中想要创立Handler目标,就有必要要调用prepare办法。
ok,那么咱们能够这样运用,就先调用一下Looper的prepare办法不就行了吗?
Thread {
Looper.prepare()
val handler = Handler{
return@Handler true
}
Looper.loop()
}.start()
这样写法正确吗?是不对的!咱们是在匿名内部类中调用了prepare办法,可是线程目标是无法获取到的,就无法在ThreadLocal中存储对应的Looper目标。
所以这个时分,就需求自行创立一个Thread子类,
class MyHandlerThread : Thread() {
private var mLooper: Looper? = null
override fun run() {
super.run()
Looper.prepare()
mLooper = Looper.myLooper()
Looper.loop()
}
fun getLooper(): Looper? {
return mLooper
}
}
这样当MyHandlerThread运转之后,子线程的Looper也就顺畅创立了。
val handlerThread = MyHandlerThread()
handlerThread.start()
//第三行
val handler = Handler(handlerThread.getLooper()!!){
return@Handler true
}
由于这部分都是在主线程中创立的,可是Looper的创立以及存储都是在线程中进行,那么第三行代码履行时,怎么确保getLooper时拿到了现已创立的Looper,这就涉及到了线程同步的问题。
加延迟?太low了。
class MyHandlerThread : Thread() {
private var mLooper: Looper? = null
private var lock = java.lang.Object()
override fun run() {
super.run()
Looper.prepare()
synchronized(lock){
Log.e("TAG","开始创立Looper----")
mLooper = Looper.myLooper()
//创立完结
Log.e("TAG","创立Looper完结,唤醒----")
lock.notifyAll()
}
Looper.loop()
}
fun getLooper(): Looper? {
synchronized(lock){
while (mLooper == null){
Log.e("TAG","获取Looper失利,mLooper == null 等候----")
lock.wait()
}
}
Log.e("TAG","获取Looper成功")
return mLooper
}
}
处理线程的并发问题,自然要想到锁机制,其实这里咱们只需求在获取和创立的时分加锁,当在获取Looper目标的时分,就会判别Looper是否创立成功,假如没有就调用wait释放锁,等候创立成功之后,就回来。
2023-04-22 14:00:04.244 10067-10067/com.lay.layzproject E/TAG: 获取Looper失利,mLooper == null 等候----
2023-04-22 14:00:04.247 10067-10124/com.lay.layzproject E/TAG: 开始创立Looper----
2023-04-22 14:00:04.250 10067-10124/com.lay.layzproject E/TAG: 创立Looper完结,唤醒----
2023-04-22 14:00:04.253 10067-10067/com.lay.layzproject E/TAG: 获取Looper成功
这才是子线程创立Handler的正确姿势,当然体系也帮咱们提供了对应的HandlerThread类,不需求咱们自己去自定义。
2.2 音讯行列中无音讯时,主线程和子线程怎么高雅处理
咱们先看子线程,由于咱们Looper是在子线程中创立的,所以MessageQueue也是在子线程处理音讯,
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// block
nativePollOnce(ptr, nextPollTimeoutMillis);
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;
}
// 中心代码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);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
当从MessageQueue中取出音讯时,会调用next办法,假如音讯行列中为空,那么会调用nativePollOnce办法,此刻线程就处于Block的状况,此刻退出页面时,线程不会被收回会导致内存走漏。
所以,想要解决这个问题,就需求在页面退出之后,调用Looper的quitSafely办法,
public void quitSafely() {
mQueue.quit(true);
}
咱们先看源码中是怎么处理的。
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
其实终究是调用了MessageQueue的quit办法,在这个办法中,会把mQuitting = true,并调用了nativeWake办法,其实跟nativePollOnce是相对应的;
在nativePollOnce处于block时,调用nativeWake会唤醒,并继续往下履行,履行到中心代码1时,由于mQuitting = true,所以直接return,并调用dispose。
在loopOnce办法中,假如调用next办法拿到了msg为空,直接return退出for循环,那么此刻线程也就履行完毕了,终究被收回掉。
主线程能够这么处理吗?显然不行!假如主线程都被退出了,那么整个app将无法运转,所以在quit办法中,
Main thread not allowed to quit
榜首行代码就在判别,假如是主线程就不能退出。
2.3 Handler怎么确保多线程安全
由于咱们在创立Handler之后,在经过Handler发送音讯的时分,可能会在不同的线程,那么Handler是怎么确保线程安全的呢?
其实咱们前面提到过,假设是在主线程创立Handler,那么Looper便是在ActivityThread的main函数中创立的,此刻在sThreadLocal中,保护的便是<main,mainLooper>这样的映射联系,也便是说一个线程只能有一个looper目标,假如接连创立就会报错,能够看下Looper的prepare源码。
由于MessageQueue也是在Looper的结构办法中创立,也便是说 线程 – Looper – MessageQueue 是意义对应的联系,不会存在多个,所以中心就在于音讯的入队;经过enqueueMessage办法,咱们发现在入队的时分,其实是加锁的,拿到的是MessageQueue的目标锁,由于MessageQueue只有一个,所有的线程都会竞争这把锁,所以都是互斥的。
即便是创立多个Handler,也是同理。
3 Handler与ANR的恩怨情仇
3.1 Handler的音讯堵塞机制
当主线程堵塞超越5s之后,就会触发ANR;前面咱们知道,在Looper敞开死循环取音讯的时分,假如音讯行列中没有音讯的时分,就可能会被block,调用了nativePollOnce,那么为什么没有堵塞主线程呢?
其实咱们应该把这分为两件事来看,looper.loop是用来处理音讯,当没有音讯的时分,主线程就休息了,不需求干任何事;像input事情,其实便是一个Message,当它加入到音讯行列的时分,会调用nativeWake唤醒主线程,主线程来处理这个音讯,只有处理这个音讯超时,才会发生ANR,而不是死循环会导致ANR。
3.2 ANR日志中看Handler音讯机制
"main" prio=5 tid=1 Native
| group="main" sCount=1 dsCount=0 flags=1 obj=0x7185b6a8 self=0xb400007375b4bbe0
| sysTid=3433 nice=0 cgrp=default sched=0/0 handle=0x749c9844f8
| state=S schedstat=( 800801640 66783841 881 ) utm=60 stm=19 core=0 HZ=100
| stack=0x7fc20cb000-0x7fc20cd000 stackSize=8192KB
| held mutexes=
native: #00 pc 000000000009ca68 /apex/com.android.runtime/lib64/bionic/libc.so (__epoll_pwait+8)
native: #01 pc 0000000000019d88 /system/lib64/libutils.so (android::Looper::pollInner(int)+184)
native: #02 pc 0000000000019c68 /system/lib64/libutils.so (android::Looper::pollOnce(int, int*, int*, void**)+112)
native: #03 pc 0000000000112194 /system/lib64/libandroid_runtime.so (android::android_os_MessageQueue_nativePollOnce(_JNIEnv*, _jobject*, long, int)+44)
at android.os.MessageQueue.nativePollOnce(Native method)
at android.os.MessageQueue.next(MessageQueue.java:335)
at android.os.Looper.loop(Looper.java:183)
at android.app.ActivityThread.main(ActivityThread.java:7723)
at java.lang.reflect.Method.invoke(Native method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:612)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:997)
在咱们剖析ANR日志时,常常会看到这样表现,结合上面咱们关于Handler的了解,这个时分其实便是没有音讯了,咱们看现已调用了nativePollOnce办法,此刻主线程就休眠了,等候下一个音讯到来。
"main" prio=5 tid=1 Blocked
| group="main" sCount=1 dsCount=0 flags=1 obj=0x7185b6a8 self=0xb400007375b4bbe0
| sysTid=3906 nice=-10 cgrp=default sched=0/0 handle=0x749c9844f8
| state=S schedstat=( 2591708189 61276010 2414 ) utm=220 stm=38 core=5 HZ=100
| stack=0x7fc20cb000-0x7fc20cd000 stackSize=8192KB
| held mutexes=
// ......
- waiting to lock <0x0167ghe6d> (a java.lang.Object) held by thread 5
// ...... 办法调用,保密
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7723)
at java.lang.reflect.Method.invoke(Native method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:612)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:997)
在这段日志中,咱们看到主线程现已是出问题了,处于Blocked的状况,那么在Handler调用dispatchMessage办法的时分,是调用了handleCallback,说明此刻是调用了post办法,在post办法中,主线程一向想要获取其他线程持有的一把锁,导致了超时产生了ANR。
从日志看,就能知道,其实ANR跟Looper.loop彻底便是两回事。
其实Handler作为面试中的高频问点,关于其中的原理咱们需求把握,尤其是多线程并发的原理,可能是许多同伴们忽视的重点。
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