Create 主线程为什么没有 ANR.md

blogs/Android/Framework/Interview/线程通信相关/主线程为什么没有 ANR.md

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+---
+主线程为什么没有 ANR？
+---
+
+1. 了解 ANR 的触发的原理
+2. 了解应用大致启动流程
+3. 了解线程的消息循环机制
+4. 了解应用和系统服务通信过程
+
+#### ANR 是什么？
+
+```java
+final void appNotResponding(ProcessRecord app, ...){
+    Message msg = Message.obtain();
+    msg.what = SHOW_NOT_RESPONDING_MSG;
+    mUiHandler.sendMessage(msg);
+}
+// SystemService 进程
+Dialog d = new AppNotRespondingDialog(...);
+d.show();
+```
+
+ANR 场景有哪些？
+
+1. Service Timeout
+2. BroadcastQueue Timeout
+3. ContentProvider Timeout
+4. InputDispatching Timeout
+
+以 Service 为例，看一下 ANR 是怎么触发的？
+
+```java
+void realStartServiceLocked(ServiceRecord r, ProcessRecord app, boolean execInfg){
+	bumpServiceExecutingLocked(r, execInFg, "create");
+    app.thread.scheduleCreateService(r, r.serviceInfo, ...);
+}
+void bumpServiceExecutingLocked(ServiceRecord r, boolean fg, String why){
+    scheduleServiceTimeoutLocked(r.app);
+}
+void scheduleServiceTimeoutLocked(ProcessRecord proc){
+    long now = SystemClock.uptimeMillis();
+    Message msg = mAm.mHandler.obtainMessage(SERVICE_TIMEOUT_MSG);
+    mAm.mHandler.sendMessageAtTime(msg, now+SERVICE_TIMEOUT);
+}
+void serviceTimeout(ProcessRecord proc){
+    mAm.appNotResponding(proc, ...);
+}
+// 应用端处理 Service 请求
+private void handleCreateService(CreateServiceData data) {
+    service = (Service)cl.loadClass(data.info.name).newInstance();
+    ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
+    Application app = packageInfo.makeApplication(false, mInstrumentation);
+    service.attach(context, this, data.info.name, data.token, ...);
+    service.onCreate();
+    ActivityManagerNative.getDefault().serviceDoneExecuting(...);
+}
+private void serviceDoneExecutingLocked(ServiceRecord r, ...){
+    mAm.mHandler.removeMessage(SERVICE_TIMEOUT_MSG, r.app);
+}
+```
+
+往主线程发消息：
+
+```java
+@Override
+void scheduleLaunchActivity(Intent intent, IBinder token, ...){
+    sendMessage(H.LAUNCH_ACTIVITY, r);
+}
+void scheduleCreateService(IBinder token, ...){
+    sendMessage(H.CREATE_SERVICE, s);
+}
+void scheduleReceiver(Intent intent, ActivityInfo info, ...){
+    sendMessage(H.RECEIVER, r);
+}
+```
+
+#### 总结
+
+1. ANR 是应用没有在规定的时间内完成 AMS 指定的任务导致的
+2. AMS 请求调用应用端 binder 线程，再丢消息去唤醒主线程来处理
+3. ANR 不是因为主线程 loop 循环，而是因为主线程有耗时任务