架构设计三之app应用的启动过程分析

前言

首先说一下看源码的方式，首先明确看源码的目的是为了更好的指导上层开发。 看android的系统源码，重要的是理解整体的执行流程， 不要纠结于代码细节。太纠结于代码细节， 对指导上层开发并没有指导意义。

从入口main()到Applicaiton的onCreate()方法被调用

ActivityThread提供两个核心内部类 ApplicationThread和 H

public final class ActivityThread { final ApplicationThread mAppThread = new ApplicationThread(); final H mH = new H(); private class ApplicationThread extends ApplicationThreadNative {} private class H extends Handler {} }

提供了main()方法作为app启动的入口

public static void main(String[] args) { //创建Looper对象, 创建MessageQueue对象 Looper.prepareMainLooper(); //创建自己的ActivityThread对象 ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } AsyncTask.init(); if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } //进入消息循环 Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); } } private void attach(boolean system) { ... //获得AMS(ActivityManagerService)实例, AMS的log tag: "ActivityManager" IActivityManager mgr = ActivityManagerNative.getDefault(); //把ApplicationThread对象传给AMS mgr.attachApplication(mAppThread); ... }

下面进入ActivityManagerService的attachApplication(IApplicationThread thread), ActivityThread把binder对象传给ActivityManagerService.

public final class ActivityManagerService extends ActivityManagerNative { ... public final void attachApplication(IApplicationThread thread) { ... attachApplicationLocked(thread, callingPid); ... } ... private final boolean attachApplicationLocked(IApplicationThread thread, int pid) { .... //通过binder，跨进程调用ApplicationThread的bindApplication()方法, 下面代码逻辑重回ActivityThread.java thread.bindApplication(processName, appInfo, providers, app.instrumentationClass, profileFile, profileFd, profileAutoStop, app.instrumentationArguments, app.instrumentationWatcher, app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace, isRestrictedBackupMode || !normalMode, app.persistent, new Configuration(mConfiguration), app.compat, getCommonServicesLocked(), mCoreSettingsObserver.getCoreSettingsLocked()); .... } } //ActivityThread.java private class ApplicationThread extends Binder implements IApplicationThread{ ... public final void bindApplication(String processName, ApplicationInfo appInfo, List<ProviderInfo> providers, ComponentName instrumentationName, String profileFile, ParcelFileDescriptor profileFd, boolean autoStopProfiler, Bundle instrumentationArgs, IInstrumentationWatcher instrumentationWatcher, IUiAutomationConnection instrumentationUiConnection, int debugMode, boolean enableOpenGlTrace, boolean isRestrictedBackupMode, boolean persistent, Configuration config, CompatibilityInfo compatInfo, Map<String, IBinder> services, Bundle coreSettings) { AppBindData data = new AppBindData(); data.processName = processName; data.appInfo = appInfo; data.providers = providers; data.instrumentationName = instrumentationName; data.instrumentationArgs = instrumentationArgs; data.instrumentationWatcher = instrumentationWatcher; data.instrumentationUiAutomationConnection = instrumentationUiConnection; data.debugMode = debugMode; data.enableOpenGlTrace = enableOpenGlTrace; data.restrictedBackupMode = isRestrictedBackupMode; data.persistent = persistent; data.config = config; data.compatInfo = compatInfo; data.initProfileFile = profileFile; data.initProfileFd = profileFd; data.initAutoStopProfiler = false; //发消息 sendMessage(H.BIND_APPLICATION, data); } ... private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) { Message msg = Message.obtain(); msg.what = what; msg.obj = obj; msg.arg1 = arg1; msg.arg2 = arg2; if (async) { msg.setAsynchronous(true); } //通过mH把BIND_APPLICATION消息发给H处理 mH.sendMessage(msg); } ... }

使用H的目的是，把代码执行的逻辑从binder线程池里的线程切换到main线程里去执行.

//ActivityThread.java private class H extends Handler { ... public static final int BIND_APPLICATION = 110; ... public void handleMessage(Message msg) { switch (msg.what) { ... case BIND_APPLICATION: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication"); AppBindData data = (AppBindData)msg.obj; handleBindApplication(data);//调用ActivityThread的handleBindApplication()方法处理 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; ... } } } public final class ActivityThread { ... private void handleBindApplication(AppBindData data) { ... //创建Instrumentation 对象 java.lang.ClassLoader cl = instrContext.getClassLoader(); mInstrumentation = (Instrumentation) cl.loadClass(data.instrumentationName.getClassName()).newInstance(); data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo); //创建app运行时的上下文对象，并对其进行初始化. final ContextImpl appContext = new ContextImpl(); appContext.init(data.info, null, this); //这里的data.info是LoadedApk类的对象 //在这里创建了上层开发者的代码中所涉及的Applicaiton类的对象 Application app = data.info.makeApplication(data.restrictedBackupMode, null); ... //如果有ContentProvider的话, 先加载ContentProvider，后调用Application的onCreate()方法 List<ProviderInfo> providers = data.providers; if (providers != null) { installContentProviders(app, providers); } //调Application的生命周期函数 onCreate() mInstrumentation.callApplicationOnCreate(app); } ... } // LoadedApk.java public final class LoadedApk { ... public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) { Application app = null; app = mActivityThread.mInstrumentation.newApplication( cl, appClass, appContext); return app; } ... } // Instrumentation.java public class Instrumentation { ... public void callApplicationOnCreate(Application app) { app.onCreate(); } ... }

实验结果: 在Application的onCreate()方法打个断点.

最终总结, 应用启动的流程:

入口是ActivityThread.main(), 创建UI线程的消息循环， 并最终进入消息循环. ActivityThread的内部类ApplicationThread是一个Binder，获得系统的ActivityManagerService， 调用它的attachApplication(mAppThread)并把ApplicationThread的对象mAppThread(是一个binder对象)传给ActivityManagerService, AcitivityManagerService拿到这个Binder对象后做了3件事，

attachApplication()方法内

1.

准备数据, 创建一个ProcessRecord app的对象记录一个进程的所有信息, 对各个字段进行赋值.

2.

调用mAppThread这个Binder对象的bindApplication(ProcessRecord app)方法, 把记录着进程信息的对象ProcessRecord传给ApplicationThread处理, 在bindApplication()内通过Handler mH发送一个”BIND_APPLICATION”消息让mH处理.mH处理”BIND_APPLICATION”时调用handleBindApplication(), 在里面创建Applicaiton app对象， 调用Application的onCreate(), 到这里ActivityThread的handleBindApplication()方法执行完毕.

3.

在ActivityManagerService的attachApplication()处理完第二步之后, 调用mStackSupervisor.attachApplicationLocked(app), 在里面再调用realStartActivityLocked(), 里面再调用app.thread.scheduleLaunchActivity()， 也就是mAppThread的scheduleLaunchActivity(), 在ApplicationThread的scheduleLaunchActivity()内，发送一个”LAUNCH_ACTIVITY”消息, mH处理”LAUNCH_ACTIVITY”时调用handleLaunchActivity(), handleLaunchActivity()分两步, 第一步调performLaunchActivity(), 创建Activity的对象， 依次调用它的onCreate(), onStart(). 第二步调handleResumeActivity(), 调用Activity对象的onResume().

至此， 应用启动的完整流程就分析完整了.