Android App启动流程和源码详解

前言

之前看了些App启动流程的文章，但是看得很浅显，隔了没多久就忘了，自己抓耳挠腮的终于看完了，看得头疼哦。因为很多是个人理解，大哥们主打一个7分信，2分思考，1分怀疑哈。

主要看的源码是Android 9 的源码，因为很多framework的代码在AS里面看需要配置，我没配好，于是找了几个网站，最后综合下来用的这个

Android社区 - https://www.androidos.net.cn

一、启动方式

App启动分为冷启动和热启动

1. 冷启动：点击桌面图标，手机系统不存在该应用进程，这时系统会重新fork一个子进程来加载Application并启动Activity，这个启动方式就是冷启动。
2. 热启动：应用的热启动比冷启动简单得多，开销也更低。在热启动中，因为系统里已有该应用的进程，所以系统的所有工作就是将您的 Activity 带到前台。 

二、App冷启动的简要流程

这是找的网上的图片：

 如果只是了解，纯应付简单的面试，上面这张图就够了。

简单地讲，从 用户手指触摸点击桌面图标到 Activity启动 可以用下面 4 步概括：

1.启动进程

点击图标发生在Launcher应用的进程，startActivity()函数最终是由Instrumentation通过Android的Binder跨进程通信机制 发送消息给 system_server 进程； 在 system_server 中，启动进程的操作在ActivityManagerService。AMS发现ProcessRecord不存在时，就会执行Process.start()，最终是通过 socket 通信告知 Zygote 进程 fork 子进程（app进程，这儿用的Socket）

第一步：Activity的startActivity方法

源码位置：

源码

@Override
public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }
    @Override
    public void startActivityForResult(
            String who, Intent intent, int requestCode, @Nullable Bundle options) {
        Uri referrer = onProvideReferrer();
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        options = transferSpringboardActivityOptions(options);
        Instrumentation.ActivityResult ar =
            mInstrumentation.execStartActivity(
                this, mMainThread.getApplicationThread(), mToken, who,
                intent, requestCode, options);//关键代码
        if (ar != null) {
            mMainThread.sendActivityResult(
                mToken, who, requestCode,
                ar.getResultCode(), ar.getResultData());
        }
        cancelInputsAndStartExitTransition(options);
    }

第二步：Instrumnetation调用ActivityManager.getService().startActivity()方法

源码位置：

    public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        ···
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            int result = ActivityManager.getService()//关键代码
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

ActivityManager.java

    public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

第三步：ActivityManagerService里的调用

    @Override
    public final int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions,
                UserHandle.getCallingUserId());
    }

    @Override
    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        enforceNotIsolatedCaller("startActivity");
        userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
                userId, false, ALLOW_FULL_ONLY, "startActivity", null);
        // TODO: Switch to user app stacks here.
        return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
                resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
                profilerInfo, null, null, bOptions, false, userId, null, null,
                "startActivityAsUser");
    }

第四步：mActivityStarter.startActivityMayWait()方法

    final int startActivityMayWait(IApplicationThread caller, int callingUid,
            String callingPackage, Intent intent, String resolvedType,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            IBinder resultTo, String resultWho, int requestCode, int startFlags,
            ProfilerInfo profilerInfo, WaitResult outResult,
            Configuration globalConfig, Bundle bOptions, boolean ignoreTargetSecurity, int userId,
            IActivityContainer iContainer, TaskRecord inTask, String reason) {
        ···
            int res = startActivityLocked(caller, in