Android 的主线程为什么不会因为looper的死循环堵塞

前言：

每一种创伤，都是一种成熟。

 在我们学习handler的时候，looper是一个死循环的

上图中是源码

咋们这边再做一个测试

开启一个子线程

new Thread(new Testhah(),"我是一个子线程的测试线程").start();

 

 

 

/**
 * @author MR.su
 * @create 2019/4/28
 * @Describe
 */
public class Testhah implements Runnable {
   @Override
   public void run() {
      String name = Thread.currentThread().getName();
      Log.e("++", name);
      Looper.prepare();
      Looper.loop();
      Handler handler = new Handler(Looper.myLooper());
      Log.e("++", "走了");
   }
}

在上面的子线程中，我开启了一个looper，调用了looper的方法去死循环取消息

发现没有走Log.e("++", "走了");  

说明子线程被堵塞了，

问题

1.为什么android默认给主线程开启的looper没有造成堵塞呢？Activity的生命周期怎么执行呀

2.如果android主线没有造成堵塞，线程是如何进行保活的呀（线程池同问）

根据这些，我首先猜测是开启了一个新的线程去执行系统服务（AMS）发送过来的消息，就是咋们所说的手势，跳转之类的，然后通过handler传递给主线程

我们走进android应用开启调用的代码处

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);                             -------------调用了这个方法

    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();                                               ----------------开启轮询
 
    throw new RuntimeException("Main thread loop unexpectedly exited");
}

我们发现在轮询之前调用thread.attach(false, startSeq);

这个方法是干嘛的呢？

private void attach(boolean system, long startSeq) {
    sCurrentActivityThread = this;
    mSystemThread = system;
    if (!system) {
        ViewRootImpl.addFirstDrawHandler(new Runnable() {
            @Override
            public void run() {
                ensureJitEnabled();
            }
        });
        android.ddm.DdmHandleAppName.setAppName("",
                                                UserHandle.myUserId());
        RuntimeInit.setApplicationObject(mAppThread.asBinder());
        final IActivityManager mgr = ActivityManager.getService();
        try {
            mgr.attachApplication(mAppThread, startSeq);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }
        // Watch for getting close to heap limit.
        BinderInternal.addGcWatcher(new Runnable() {
            @Override public void run() {
                if (!mSomeActivitiesChanged) {
                    return;
                }
                Runtime runtime = Runtime.getRuntime();
                long dalvikMax = runtime.maxMemory();
                long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
                if (dalvikUsed > ((3*dalvikMax)/4)) {
                    if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                            + " total=" + (runtime.totalMemory()/1024)
                            + " used=" + (dalvikUsed/1024));
                    mSomeActivitiesChanged = false;
                    try {
                        mgr.releaseSomeActivities(mAppThread);
                    } catch (RemoteException e) {
                        throw e.rethrowFromSystemServer();
                    }
                }
            }
        });
    } else {
        // Don't set application object here -- if the system crashes,
        // we can't display an alert, we just want to die die die.
        android.ddm.DdmHandleAppName.setAppName("system_process",
                UserHandle.myUserId());
        try {
            mInstrumentation = new Instrumentation();
            mInstrumentation.basicInit(this);
            ContextImpl context = ContextImpl.createAppContext(
                    this, getSystemContext().mPackageInfo);
            mInitialApplication = context.mPackageInfo.makeApplication(true, null);
            mInitialApplication.onCreate();
        } catch (Exception e) {
            throw new RuntimeException(
                    "Unable to instantiate Application():" + e.toString(), e);
        }
    }

方法实在太长了，看到下面一句就明白了

// Watch for getting close to heap limit.
BinderInternal.addGcWatcher(new Runnable() {                 --------------------开启一个子线程
    @Override public void run() {
        if (!mSomeActivitiesChanged) {
            return;
        }
        Runtime runtime = Runtime.getRuntime();
        long dalvikMax = runtime.maxMemory();
        long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
        if (dalvikUsed > ((3*dalvikMax)/4)) {
            if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                    + " total=" + (runtime.totalMemory()/1024)
                    + " used=" + (dalvikUsed/1024));
            mSomeActivitiesChanged = false;
            try {
                mgr.releaseSomeActivities(mAppThread);
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        }
    }
});

相关的博客推荐：

http://blog.csdn.net/lmj623565791/article/details/38377229              ---鸿阳大神的handler讲解