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）