這里介紹一些Android的一些冷知識(shí)。

App進(jìn)程是從哪里開(kāi)始啟動(dòng)的

ActivityManagerService類(lèi)的源碼在Sdk里有，是Android中最核心的服務(wù)之一，負(fù)責(zé)系統(tǒng)中四大組件的啟動(dòng)、切換、調(diào)度及應(yīng)用進(jìn)程的管理和調(diào)度等等工作。這個(gè)類(lèi)是運(yùn)行在系統(tǒng)進(jìn)程里，由系統(tǒng)管理，通過(guò)跨進(jìn)程通信的方式與我們的App進(jìn)行通信。這個(gè)類(lèi)有個(gè)方法

private final void startProcessLocked(ProcessRecord app, String hostingType, String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
    ......
    Process.ProcessStartResult startResult = Process.start(entryPoint,
            app.processName, uid, uid, gids, debugFlags, mountExternal,
            app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
            app.info.dataDir, entryPointArgs);
    ......
}

我們的App進(jìn)程最初就是由這個(gè)方法創(chuàng)建的。

App的入口方法在哪里？

任何語(yǔ)言寫(xiě)的任何程序都會(huì)有執(zhí)行入口。Android App的入口是ActivityThread的main方法。具體定義是在ActivityManagerService的startProcessLocked方法里。

private final void startProcessLocked(ProcessRecord app, String hostingType, String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
    ......
    // Start the process.  It will either succeed and return a result containing
    // the PID of the new process, or else throw a RuntimeException.
    boolean isActivityProcess = (entryPoint == null);
    if (entryPoint == null) entryPoint = "android.app.ActivityThread";
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
            app.processName);
    checkTime(startTime, "startProcess: asking zygote to start proc");
    Process.ProcessStartResult startResult = Process.start(entryPoint,
            app.processName, uid, uid, gids, debugFlags, mountExternal,
            app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
            app.info.dataDir, entryPointArgs);
    checkTime(startTime, "startProcess: returned from zygote!");
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    ......
}

if (entryPoint == null) entryPoint = "android.app.ActivityThread";這一行定義了App的入口類(lèi)。

App創(chuàng)建Activity的地方在哪里

創(chuàng)建Activity的消息是從ActivityManagerService通過(guò)跨進(jìn)程通信的方式發(fā)送到

-> ApplicationThread 
     -> scheduleLaunchActivity
     mH->handleMessage -> LAUNCH_ACTIVITY
     -> handleLaunchActivity
     -> performLaunchActivity
        -> mInstrumentation.newActivity
            -> (Activity)cl.loadClass(className).newInstance();

最終通過(guò)反射的方式創(chuàng)建出了Activity實(shí)例。

Activity實(shí)例引用被誰(shuí)持有？

Activity創(chuàng)建了實(shí)例對(duì)象后，肯定會(huì)被某個(gè)對(duì)象持有，否則引用計(jì)數(shù)為0后，Activity實(shí)例就會(huì)被GC回收掉。

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ......
      Activity activity = null;
      ......
      activity = mInstrumentation.newActivity(cl,component.getClassName(), r.intent);
      ......
      r.activity = activity;
      mActivities.put(r.token, r);
      ......
}

Activity在創(chuàng)建之后就被mActivities持有。mActivities被ActivityThread持有。ActivityThread對(duì)象是在App的入口main方法實(shí)例化的。main方法除非進(jìn)程被殺死否則main是不會(huì)結(jié)束的。

public static void main(String[] args) {
    ......
    ActivityThread thread = new ActivityThread();
    ......
}

Activity實(shí)例什么時(shí)候被銷(xiāo)毀？

前面說(shuō)到Activity被ActivityThread的mActivities持有。那什么時(shí)候Activity實(shí)例被銷(xiāo)毀呢？說(shuō)到銷(xiāo)毀大家可能會(huì)想到Activity生命周期的onDestory方法。

-> ApplicationThread 
     -> scheduleDestroyActivity
     mH->handleMessage -> DESTROY_ACTIVITY
     -> handleDestroyActivity
     -> performDestroyActivity
        // 從這里繼續(xù)會(huì)執(zhí)行到Activity的onDestory方法
        -> mInstrumentation.callActivityOnDestroy(r.activity);
     // Activity實(shí)例從mActivities被移除
     -> mActivities.remove(token);

當(dāng)Activity實(shí)例從mActivities里移除后，如果當(dāng)前Activity實(shí)例沒(méi)有被其他對(duì)象強(qiáng)引用或者說(shuō)Activity沒(méi)有內(nèi)存泄漏，那么下次GC的時(shí)候Activity實(shí)例就會(huì)被銷(xiāo)毀掉。