12.源码阅读（app启动流程-android api 26）

activity的启动流程之前已经通过源码了解了，那么app的启动流程是怎样的，从我们按下app的图标，到应用启动起来显示出画面，中间都经历了什么？

安卓是基于java的，所以和java有一定的相似性，一个java程序的起点是main，那么android是不是也是如此？事实上，正如猜想我们找到ActivityTread这个类中的main方法，这里就是app启动的源头

public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingProfilerIntegration.start(); CloseGuard.setEnabled(false); //初始化内存卡环境 Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); //处理签名相关的东西，验证之类的饿 AndroidKeyStoreProvider.install(); // Make sure TrustedCertificateStore looks in the right place for CA certificates //用户目录相关的东西 final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); //应用程序一启动就初始化全局的looper Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); //这个是主要的方法，false表示不是系统应用，第三方app默认都是false thread.attach(false); //初始化主线程handler，在ActivityTread加载的时候就已经生成了 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(); throw new RuntimeException("Main thread loop unexpectedly exited"); } 

正式开始加载app

private void attach(boolean system) { sCurrentActivityThread = this; mSystemThread = system; if (!system) { //第三方应用 //ViewRootImpl相关 ...... //这个已经很熟悉了，我们要去找IActivityManager的实现类ActivityManagerService final IActivityManager mgr = ActivityManagerNative.getDefault(); try { //进入ActivityManagerService mgr.attachApplication(mAppThread); } catch (RemoteException ex) { // Ignore } //可以看到这里是内存使用量的一个监听，当app已经占用的空间大于系统分配给 //这个app最大空间的四分之三的时候，java虚拟机会触发gc清理空间 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) { } } } }); } else { //系统应用 // 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(); 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); } } // add dropbox logging to libcore DropBox.setReporter(new DropBoxReporter()); //设置Configuration改变的监听 ViewRootImpl.addConfigCallback(new ComponentCallbacks2() { @Override public void onConfigurationChanged(Configuration newConfig) { synchronized (mResourcesManager) { // We need to apply this change to the resources // immediately, because upon returning the view // hierarchy will be informed about it. if (mResourcesManager.applyConfigurationToResourcesLocked(newConfig, null)) { // This actually changed the resources! Tell // everyone about it. if (mPendingConfiguration == null || mPendingConfiguration.isOtherSeqNewer(newConfig)) { mPendingConfiguration = newConfig; sendMessage(H.CONFIGURATION_CHANGED, newConfig); } } } } @Override public void onLowMemory() { } @Override public void onTrimMemory(int level) { } }); } 

进入ActivityManagerService

@Override public final void attachApplication(IApplicationThread thread) { synchronized (this) { int callingPid = Binder.getCallingPid(); final long origId = Binder.clearCallingIdentity(); attachApplicationLocked(thread, callingPid); Binder.restoreCallingIdentity(origId); } } 

private final boolean attachApplicationLocked(IApplicationThread thread, int pid) { ...... //thread就是传过来的final ApplicationThread mAppThread = new ApplicationThread();它在ActivityThread加载的时候就已经创建 thread.bindApplication(processName, appInfo, providers, app.instrumentationClass, profilerInfo, app.instrumentationArguments, app.instrumentationWatcher, app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace, isRestrictedBackupMode || !normalMode, app.persistent, new Configuration(mConfiguration), app.compat, getCommonServicesLocked(app.isolated), mCoreSettingsObserver.getCoreSettingsLocked()); ...... // See if the top visible activity is waiting to run in this process... if (normalMode) { try { if (mStackSupervisor.attachApplicationLocked(app)) { didSomething = true; } } catch (Exception e) { Slog.wtf(TAG, "Exception thrown launching activities in " + app, e); badApp = true; } } // Find any services that should be running in this process... if (!badApp) { try { didSomething |= mServices.attachApplicationLocked(app, processName); } catch (Exception e) { Slog.wtf(TAG, "Exception thrown starting services in " + app, e); badApp = true; } } ...... 

回到ActivityThread中

 public final void bindApplication(String processName, ApplicationInfo appInfo, List<ProviderInfo> providers, ComponentName instrumentationName, ProfilerInfo profilerInfo, Bundle instrumentationArgs, IInstrumentationWatcher instrumentationWatcher, IUiAutomationConnection instrumentationUiConnection, int debugMode, boolean enableOpenGlTrace, boolean isRestrictedBackupMode, boolean persistent, Configuration config, CompatibilityInfo compatInfo, Map<String, IBinder> services, Bundle coreSettings) { if (services != null) { // Setup the service cache in the ServiceManager ServiceManager.initServiceCache(services); } setCoreSettings(coreSettings); /* * Two possible indications that this package could be * sharing its runtime with other packages: * * 1.) the sharedUserId attribute is set in the manifest, * indicating a request to share a VM with other * packages with the same sharedUserId. * * 2.) the application element of the manifest has an * attribute specifying a non-default process name, * indicating the desire to run in another packages VM. * * If sharing is enabled we do not have a unique application * in a process and therefore cannot rely on the package * name inside the runtime. */ IPackageManager pm = getPackageManager(); android.content.pm.PackageInfo pi = null; try { pi = pm.getPackageInfo(appInfo.packageName, 0, UserHandle.myUserId()); } catch (RemoteException e) { } if (pi != null) { boolean sharedUserIdSet = (pi.sharedUserId != null); boolean processNameNotDefault = (pi.applicationInfo != null && !appInfo.packageName.equals(pi.applicationInfo.processName)); boolean sharable = (sharedUserIdSet || processNameNotDefault); // Tell the VMRuntime about the application, unless it is shared // inside a process. if (!sharable) { VMRuntime.registerAppInfo(appInfo.packageName, appInfo.dataDir, appInfo.processName); } } 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.initProfilerInfo = profilerInfo; sendMessage(H.BIND_APPLICATION, data); } 

handler接收消息进行处理

case BIND_APPLICATION: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication"); AppBindData data = (AppBindData)msg.obj; handleBindApplication(data); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; 

private void handleBindApplication(AppBindData data) { ...... //反射加载到Instrumentation对象或者当条件不满足时，直接new出来 try { java.lang.ClassLoader cl = instrContext.getClassLoader(); mInstrumentation = (Instrumentation) cl.loadClass(data.instrumentationName.getClassName()).newInstance(); } catch (Exception e) { throw new RuntimeException( "Unable to instantiate instrumentation " + data.instrumentationName + ": " + e.toString(), e); } .... else { mInstrumentation = new Instrumentation(); } ...... try { //调用Application的onCreate 方法 mInstrumentation.callApplicationOnCreate(app); } catch (Exception e) { if (!mInstrumentation.onException(app, e)) { throw new RuntimeException( "Unable to create application " + app.getClass().getName() + ": " + e.toString(), e); } } } 

Application是启动了，那么到了这里似乎已经无法继续往下走了，activity是如何启动的，我们回到ActivityManagerService中，在上边的那代码下如下,现在已经知道了bindApplication最终执行到了Application的onCreate，那么下边的方法attachApplicationLocked将会是寻找Activity启动的线索

private final boolean attachApplicationLocked(IApplicationThread thread, int pid) { ...... //thread就是传过来的final ApplicationThread mAppThread = new ApplicationThread();它在ActivityThread加载的时候就已经创建 thread.bindApplication(processName, appInfo, providers, app.instrumentationClass, profilerInfo, app.instrumentationArguments, app.instrumentationWatcher, app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace, isRestrictedBackupMode || !normalMode, app.persistent, new Configuration(mConfiguration), app.compat, getCommonServicesLocked(app.isolated), mCoreSettingsObserver.getCoreSettingsLocked()); ...... // See if the top visible activity is waiting to run in this process... if (normalMode) { try { if (mStackSupervisor.attachApplicationLocked(app)) { didSomething = true; } } catch (Exception e) { Slog.wtf(TAG, "Exception thrown launching activities in " + app, e); badApp = true; } } // Find any services that should be running in this process... if (!badApp) { try { didSomething |= mServices.attachApplicationLocked(app, processName); } catch (Exception e) { Slog.wtf(TAG, "Exception thrown starting services in " + app, e); badApp = true; } } ...... 

attachApplicationLocked

boolean attachApplicationLocked(ProcessRecord app) throws RemoteException { final String processName = app.processName; boolean didSomething = false; for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) { ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks; for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) { final ActivityStack stack = stacks.get(stackNdx); if (!isFrontStack(stack)) { continue; } ActivityRecord hr = stack.topRunningActivityLocked(null); if (hr != null) { if (hr.app == null && app.uid == hr.info.applicationInfo.uid && processName.equals(hr.processName)) { try { //真的开始启动activity了 if (realStartActivityLocked(hr, app, true, true)) { didSomething = true; } } catch (RemoteException e) { Slog.w(TAG, "Exception in new application when starting activity " + hr.intent.getComponent().flattenToShortString(), e); throw e; } } } } } if (!didSomething) { ensureActivitiesVisibleLocked(null, 0); } return didSomething; } 

到了realStartActivityLocked这个方法这里，后边的东西就和之前看过的activity的启动流程相同了，所以就不再往下看了，感兴趣可以看看我的另一篇03.源码阅读（Activity启动流程--android api 23）