android blockcanary 原理,blockCanary原理

blockCanary

对于android里面的性能优化，最主要的问题就是UI线程的阻塞导致的，对于如何准确的计算UI的绘制所耗费的时间，是非常有必要的，blockCanary是基于这个需求出现的，同样的，也是基于LeakCanary，和LeakCanary有着显示页面和堆栈信息。

使用

首先在gradle引入

implementation 'com.github.markzhai:blockcanary-android:1.5.0'

然后Application里面进行初始化和start

BlockCanary.install(this, new BlockCanaryContext()).start();

原理：

其中BlockCanaryContext表示的就是我们监测的某些参数,包括卡顿的阈值、输出文件的路径等等

//默认卡顿阈值为1000ms

public int provideBlockThreshold() {

return 1000;

}

//输出的log

public String providePath() {

return "/blockcanary/";

}

//支持文件上传

public void upload(File zippedFile) {

throw new UnsupportedOperationException();

}

//可以在卡顿提供自定义操作

@Override

public void onBlock(Context context, BlockInfo blockInfo) {

}

其中，init只是创建出BlockCanary实例。主要是start方法的操作。

/**

* Start monitoring.

*/

public void start() {

if (!mMonitorStarted) {

mMonitorStarted = true;

Looper.getMainLooper().setMessageLogging(mBlockCanaryCore.monitor);

}

}

其实就是给主线程的Looper设置一个monitor。

我们可以先看看主线程的looper实现。

public static void loop() {

...

for (;;) {

...

// This must be in a local variable, in case a UI event sets the logger

Printer logging = me.mLogging;

if (logging != null) {

logging.println(">>>>> Dispatching to " + msg.target + " " +

msg.callback + ": " + msg.what);

}

msg.target.dispatchMessage(msg);

if (logging != null) {

logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);

}

...

}

}

在上面的loop循环的代码中，msg.target.dispatchMessage就是我们UI线程收到每一个消息需要执行的操作，都在其内部执行。

系统也在其执行的前后都会执行logging类的print的方法，这个方法是我们可以自定义的。所以只要我们在运行的前后都添加一个时间戳，用运行后的时间减去运行前的时间，一旦这个时间超过了我们设定的阈值，那么就可以说这个操作卡顿，阻塞了UI线程，最后通过dump出此时的各种信息，来分析各种性能瓶颈。

那么接下来可以看看这个monitor的println方法。

@Override

public void println(String x) {

//如果当前是在调试中，那么直接返回，不做处理

if (mStopWhenDebugging && Debug.isDebuggerConnected()) {

return;

}

if (!mPrintingStarted) {

//执行操作前

mStartTimestamp = System.currentTimeMillis();

mStartThreadTimestamp = SystemClock.currentThreadTimeMillis();

mPrintingStarted = true;

startDump();

} else {

//执行操作后

final long endTime = System.currentTimeMillis();

mPrintingStarted = false;

//是否卡顿

if (isBlock(endTime)) {

notifyBlockEvent(endTime);

}

stopDump();

}

}

在ui操作执行前，将会记录当前的时间戳，同时会startDump。

在ui操作执行后，将会计算当前是否卡顿了，如果卡顿了，将会回调到onBlock的onBlock方法。同时将会停止dump。

为什么操作之前就开启了startDump，而操作执行之后就stopDump呢?

private void startDump() {

if (null != BlockCanaryInternals.getInstance().stackSampler) {

BlockCanaryInternals.getInstance().stackSampler.start();

}

if (null != BlockCanaryInternals.getInstance().cpuSampler) {

BlockCanaryInternals.getInstance().cpuSampler.start();

}

}

public void start() {

if (mShouldSample.get()) {

return;

}

mShouldSample.set(true);

HandlerThreadFactory.getTimerThreadHandler().removeCallbacks(mRunnable);

HandlerThreadFactory.getTimerThreadHandler().postDelayed(mRunnable,

BlockCanaryInternals.getInstance().getSampleDelay());

}

其实startDump的时候并没有马上start，而是会postDelay一个runnable，这个runnable就是执行dump的真正的操作，delay的时间就是我们设置的阈值的0.8

也就是，一旦我们的stop在设置的延迟时间之前执行，就不会真正的执行dump操作。

public void stop() {

if (!mShouldSample.get()) {

return;

}

mShouldSample.set(false);

HandlerThreadFactory.getTimerThreadHandler().removeCallbacks(mRunnable);

}

只有当stop操作在设置的延迟时间之后执行，才会执行dump操作。

private Runnable mRunnable = new Runnable() {

@Override

public void run() {

doSample();

if (mShouldSample.get()) {

HandlerThreadFactory.getTimerThreadHandler()

.postDelayed(mRunnable, mSampleInterval);

}

}

};

这个doSameple分别会dump出stack信息和cpu信息。

cpu:

try {

cpuReader = new BufferedReader(new InputStreamReader(

new FileInputStream("/proc/stat")), BUFFER_SIZE);

String cpuRate = cpuReader.readLine();

if (cpuRate == null) {

cpuRate = "";

}

if (mPid == 0) {

mPid = android.os.Process.myPid();

}

pidReader = new BufferedReader(new InputStreamReader(

new FileInputStream("/proc/" + mPid + "/stat")), BUFFER_SIZE);

String pidCpuRate = pidReader.readLine();

if (pidCpuRate == null) {

pidCpuRate = "";

}

parse(cpuRate, pidCpuRate);

}

stack:

protected void doSample() {

StringBuilder stringBuilder = new StringBuilder();

for (StackTraceElement stackTraceElement : mCurrentThread.getStackTrace()) {

stringBuilder

.append(stackTraceElement.toString())

.append(BlockInfo.SEPARATOR);

}

synchronized (sStackMap) {

if (sStackMap.size() == mMaxEntryCount && mMaxEntryCount > 0) {

sStackMap.remove(sStackMap.keySet().iterator().next());

}

sStackMap.put(System.currentTimeMillis(), stringBuilder.toString());

}

}

这样，整个blockCanary的执行过程就完毕了。

ANR

当卡顿时间大于一定值之后，将会造成ANR，那么Android系统的ANR是如何检测出来的呢？其实就是通过Watchdog来实现的，这个Watchdog是一个线程。

public class Watchdog extends Thread {

}

我们主要看一下其中的run方法的实现。

@Override

public void run() {

boolean waitedHalf = false;

while (true) {

final ArrayList blockedCheckers;

final String subject;

final boolean allowRestart;

int debuggerWasConnected = 0;

synchronized (this) {

long timeout = CHECK_INTERVAL;

// Make sure we (re)spin the checkers that have become idle within

// this wait-and-check interval

for (int i=0; i

HandlerChecker hc = mHandlerCheckers.get(i);

hc.scheduleCheckLocked();

}

if (debuggerWasConnected > 0) {

debuggerWasConnected--;

}

// NOTE: We use uptimeMillis() here because we do not want to increment the time we

// wait while asleep. If the device is asleep then the thing that we are waiting

// to timeout on is asleep as well and won't have a chance to run, causing a false

// positive on when to kill things.

long start = SystemClock.uptimeMillis();

while (timeout > 0) {

if (Debug.isDebuggerConnected()) {

debuggerWasConnected = 2;

}

try {

wait(timeout);

} catch (InterruptedException e) {

Log.wtf(TAG, e);

}

if (Debug.isDebuggerConnected()) {

debuggerWasConnected = 2;

}

timeout = CHECK_INTERVAL - (SystemClock.uptimeMillis() - start);

}

在这个run方法中，会开启一个死循环,主要用于持续检测ANR

while (true) {

}

通过wait，设置每一次休眠时间，

long start = SystemClock.uptimeMillis();

while (timeout > 0) {

if (Debug.isDebuggerConnected()) {

debuggerWasConnected = 2;

}

try {

wait(timeout);

} catch (InterruptedException e) {

Log.wtf(TAG, e);

}

if (Debug.isDebuggerConnected()) {

debuggerWasConnected = 2;

}

timeout = CHECK_INTERVAL - (SystemClock.uptimeMillis() - start);

}

当timeout计算完毕之后，会尝试获取当前各个的线程的状态

final int waitState = evaluateCheckerCompletionLocked();

private int evaluateCheckerCompletionLocked() {

int state = COMPLETED;

for (int i=0; i

HandlerChecker hc = mHandlerCheckers.get(i);

state = Math.max(state, hc.getCompletionStateLocked());

}

return state;

}

public int getCompletionStateLocked() {

if (mCompleted) {

return COMPLETED;

} else {

long latency = SystemClock.uptimeMillis() - mStartTime;

if (latency < mWaitMax/2) {

return WAITING;

} else if (latency < mWaitMax) {

return WAITED_HALF;

}

}

return OVERDUE;

}

一旦有线程等待时间超过了最大等待时间，则表示当前已经有ANR。需要dump此时的堆栈信息。

if (waitState == COMPLETED) {

// The monitors have returned; reset

waitedHalf = false;

continue;

} else if (waitState == WAITING) {

// still waiting but within their configured intervals; back off and recheck

continue;

} else if (waitState == WAITED_HALF) {

if (!waitedHalf) {

// We've waited half the deadlock-detection interval. Pull a stack

// trace and wait another half.

ArrayList pids = new ArrayList();

pids.add(Process.myPid());

ActivityManagerService.dumpStackTraces(true, pids, null, null,

getInterestingNativePids());

waitedHalf = true;

}

continue;

}

此外，还有一个第三方库，ANRWatchDog，也是用来检测Anr的，其实原理更加简单，

public void run() {

setName("|ANR-WatchDog|");

int lastTick;

int lastIgnored = -1;

while (!isInterrupted()) {

lastTick = _tick;

_uiHandler.post(_ticker);

try {

Thread.sleep(_timeoutInterval);

}

catch (InterruptedException e) {

_interruptionListener.onInterrupted(e);

return ;

}

// If the main thread has not handled _ticker, it is blocked. ANR.

if (_tick == lastTick) {

if (!_ignoreDebugger && Debug.isDebuggerConnected()) {

if (_tick != lastIgnored)

Log.w("ANRWatchdog", "An ANR was detected but ignored because the debugger is connected (you can prevent this with setIgnoreDebugger(true))");

lastIgnored = _tick;

continue ;

}

ANRError error;

if (_namePrefix != null)

error = ANRError.New(_namePrefix, _logThreadsWithoutStackTrace);

else

error = ANRError.NewMainOnly();

_anrListener.onAppNotResponding(error);

return;

}

}

}

它会在线程中，利用uiHanlder抛出一个计数器，然后wait指定时间，一旦等待时间到达，那么它会检查计数的值是否发生改变，如果没有发生改变，表示uiHandler的计算方法并没有执行到。也就是出现了Anr，此时需要dump堆栈信息。