的声明：
ActivityThread 管理应用进程中主线程的执行，根据AMS调度执行广播和其他操作
Handler 发送消息并处理消息
MessageQueue 用于存放消息的消息队列
Looper 循环消息队列中的消息循环器
Message 可在整个app中传递的消息
ActivityThread
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// Install selective syscall interception
AndroidOs.install();
// 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();
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
// Call per-process mainline module initialization.
initializeMainlineModules();
Process.setArgV0("");
/**
* 初始化Looper消息循环器，且初始化MessageQueue消息队列
* prepare(false) false说明不可以移除消息队列中的所有消息
* mQueue = new MessageQueue(quitAllowed);
* mThread = Thread.currentThread()
*/
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()));
}
}
}
/**
* 初始化ActivityThread 绑定一些类 比如：Instrumentation等等
* ....
* 通过AMS发送绑定Application的请求完成Application的创建
* 注意：在创建Application之前会先去判断应用内是否存在ContentProvider的注册并启动它
* 故：ContentProvider中的onCreate会比Application中的onCreate要先执行的原因
*/
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
/**
* 启动消息循环启 如果启动失败直接退出
*/
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
Looper
public static void prepareMainLooper() {
// 准备一个Looper
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}
/**
* 创建一个MessageQueue消息队列
* @param quitAllowed
*/
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
// 通过消息队列中next指针获取消息 如果没有消息会调native方法去阻塞
Message msg = me.mQueue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
/**
* 可以考虑做卡顿问题的分析
* 可以Looper.getMainLooper().setMessageLogging(printer)打印出这个日志
*
* logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
* 存在字符串拼接，频繁调用会创建大量对象造成内存抖动
* 后台线程频繁获取主线程堆栈对性能有一定影响，获取主线程堆栈会暂停主线程的运行
*/
// 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);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
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;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
// 获取消息中的Handler进行消息的分发
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);
}
}
if (logSlowDelivery) {
if (me.mSlowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
me.mSlowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
me.mSlowDeliveryDetected = 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);
}
/**
* 该消息使用完之后 清空该消息里面的数据
*/
msg.recycleUnchecked();
return true;
}
MessageQueue
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
/**
* 初始化Native方法，返回值NativeMessageQueue指针地址
*/
mPtr = nativeInit();
}
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();
}
/***
*
* 阻塞操作，nextPollTimeoutMillisddd等待时长
* nativePollOnce用于“等待”, 直到有下一条消息可用为止
* nextPollTimeoutMillis 为-1则阻塞
*/
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;
// 判断是不是异步消息
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) {
// 从消息队列中拿到的Message的执行时间，比当前时间还后面，则计算其差值，用于后面休眠。
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 {
// 如果从消息队列中获取的Message小于当前时间，则返回给Looper进行派发和处理
// 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.
// 消息队列中没有更多的消息了。则进行长时间休眠 -1代表长时间等待
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);
}
/**
* 如果没有消息处理
* 判断是否存在闲时处理器调用queueIdle方法
*/
// 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 {
// 如果返回false则只处理一次 并且移除这个闲时处理器
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;
}
}
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) {
// 如果p为空，表明消息队列中没有消息，那么msg将是第一个消息，needwake需要根据mBlocked复值
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// 如果p不为空，表明消息队列中还有剩余的消息，需要将新的msg添加到对应的位置。
// 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;
// 从消息队列中取出消息，判断该消息的触发时间，与要新添加的Message的时间对比。
// 如果新添加的Message时间小于 当前消息队列中获取的Message的时间，则直接break。
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
// 将新添加的Message添加到该队列的指定位置
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// 判断是否需要唤醒
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
// 调用nativeWake，触发nativePollOnce函数结束等待。
nativeWake(mPtr);
}
}
return true;
}
Handler
public Handler() {this(null, false);}
/**
* 外部传递一个looper
* @param looper
*/
public Handler(@NonNull Looper looper) {
this(looper, null, false);
}
public Handler(@Nullable Callback callback) {
this(callback, false);
}
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper; // Handler中会持有消息循环器
mQueue = looper.mQueue;// Handler中会持有消息队列
mCallback = callback;
mAsynchronous = async;
}
public final boolean post(@NonNull Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postAtTime(
@NonNull Runnable r, @Nullable Object token, long uptimeMillis) {
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean postDelayed(Runnable r, int what, long delayMillis) {
return sendMessageDelayed(getPostMessage(r).setWhat(what), delayMillis);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
MessageQueue queue = mQueue; // 如果消息队列为null直接退出
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);
}
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;// 把当前Handler对象复值
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
Handler
Handler(Looper looper):有传递消息循环器,那就直接使用该looper
Handler(Callback callback, boolean async):没有如looper就去Looper.myLooper()获取一个
Looper mLooper=Looper.myLooper()
MessageQueue mQueue= mLooper.mQueue
post(Runnable r):会自动生成一个msg然后发送出去
postAtTime(Runnable r, long uptimeMillis):会自动生成一个msg然后延迟x发送出去
sendMessageDelayed(getPostMessage(r), 0):得到msg发送消息
sendMessage(Message msg):发送一个指定msg
sendMessageDelayed(Message msg, long delayMillis)延迟发送一个指定msg
sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis):方法中会得到mQueue 即消息队列MessageQueue
enqueueMessage(queue, msg, uptimeMillis):该方法msg.target=this;意味着msg中持有一个handler;通过queue.enqueueMessage(msg, uptimeMillis)把msg消息以链表的结构存储(先进先出原则)
Looper
Looper(boolean quitAllowed)唯一的构造方法,quitAllowed是否支持停止消息循环
mQueue = new MessageQueue(quitAllowed);构造方法中构造消息队列对象
mThread = Thread.currentThread();
loop():开启消息循环唯一入口
Looper me = myLooper()判断当前Looper是否存在不存在抛出异常
for()开启循环
Message msg = me.mQueue.next():获取消息队列中的消息
msg.target.dispatchMessage(msg)把消息分发下去
msg.callback != null->handleCallback(msg):callback实现Runnable其run()做处理工作；
mCallback != null->mCallback.handleMessage(msg): mCallback实现Callback由其handleMessage进行处理；
handleMessage(msg):通过重写Handler中handleMessage(msg)处理消息.
MessageQueue
Message next():开启消息读取
nativePollOnce(ptr, nextPollTimeoutMillis):表示下一次轮询超时时间,nextPollTimeoutMillis=-1没有消息需阻塞,0开启新一轮或已经执行完idle闲时事件处理
keep = idler.queueIdle():如没消息执行该方法,返回false移除这个接口,否则不移除
boolean enqueueMessage(Message msg, long when)
msg.target == null 如果Handler为null直接抛出异常
needWake为true说明有消息加入需要处理nativeWake(mPtr)通知其next()中需要处理数据
注意：可以使用闲暇handler做一些不是很急需的事情
Looper.myQueue().addIdleHandler(IdleHandler接口 handler)
IdleHandler接口实现queueIdle()返回值:false该queueIdle只执行一次，true该queueIdle可在空闲时直执行
针对app主线程崩溃问题
我们可以对dispatchMessage方法进行异常捕获，但是该方法是由Looper.loop中的消息触发，所以无法捕获该异常。
但是我们可以对Looper的loop方法进行一个异常捕获，保证所有主线程的异常全部被捕获到。
为什么可以这么说？
因为Looper.loop是开启所有主线程消息循环，在ActivityThread.main中被开启执行。
故我们可以发送一个消息去执行自己开启的loop方法
比如：
Log.e(TAG, "通过handler发送数据执行run 如果不通过发送消息方式可能会阻塞后面运行的程序");
new Handler(Looper.getMainLooper()).post(() -> {
Log.e(TAG, "死循环为保证每次捕获崩溃之后能再次开启消息循环");
while (true) {
try {
Log.e(TAG, "保证崩溃之后被try住异常 同时记录主线程崩溃次数和崩溃信息");
Looper.loop();
} catch (Throwable e) {
mainCount++;
Log.e(TAG, "当前主线程崩溃次数==" + mainCount + "==崩溃信息：" + e.getMessage());
}
}
});
