谈谈Activity如何启动的

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调用栈如下

ActivityManagerService.java

public void systemReady(final Runnable goingCallback) 传入goingCallback = null，

ActivityStack.java

final boolean resumeTopActivityLocked(ActivityRecord prev) 传入 null

    ActivityRecord next = topRunningActivityLocked(null); //判断有没有Activity在运行。

                    
        if (next == null) {
                             // There are no more activities!  Let's just start up the
                     // Launcher...
            if (mMainStack) {
                return mService.startHomeActivityLocked();
            }
        }

 mMainStack.startActivityLocked(null, intent, null, null, 0, aInfo,
                        null, null, 0, 0, 0, false, false, null);

然后是

ActivityStack.java 中的

    final int startActivityLocked(IApplicationThread caller,
            Intent intent, String resolvedType,
            Uri[] grantedUriPermissions,
            int grantedMode, ActivityInfo aInfo, IBinder resultTo,
            String resultWho, int requestCode,
            int callingPid, int callingUid, boolean onlyIfNeeded,
            boolean componentSpecified, ActivityRecord[] outActivity)

 if (mMainStack) {
            if (mResumedActivity == null
                    || mResumedActivity.info.applicationInfo.uid != callingUid) {
                if (!mService.checkAppSwitchAllowedLocked(callingPid, callingUid, "Activity start")) {
                    PendingActivityLaunch pal = new PendingActivityLaunch();
                    pal.r = r;
                    pal.sourceRecord = sourceRecord;
                    pal.grantedUriPermissions = grantedUriPermissions;
                    pal.grantedMode = grantedMode;
                    pal.onlyIfNeeded = onlyIfNeeded;
                    mService.mPendingActivityLaunches.add(pal);
                    mDismissKeyguardOnNextActivity = false;
                    return START_SWITCHES_CANCELED;
                }
            }
        
            if (mService.mDidAppSwitch) {
                // This is the second allowed switch since we stopped switches,
                // so now just generally allow switches.  Use case: user presses
                // home (switches disabled, switch to home, mDidAppSwitch now true);
                // user taps a home icon (coming from home so allowed, we hit here
                // and now allow anyone to switch again).
                mService.mAppSwitchesAllowedTime = 0;
            } else {
                mService.mDidAppSwitch = true;
            }
         
            mService.doPendingActivityLaunchesLocked(false);
        }
        
        err = startActivityUncheckedLocked(r, sourceRecord,
                grantedUriPermissions, grantedMode, onlyIfNeeded, true);
        if (mDismissKeyguardOnNextActivity && mPausingActivity == null) {
            // Someone asked to have the keyguard dismissed on the next
            // activity start, but we are not actually doing an activity
            // switch...  just dismiss the keyguard now, because we
            // probably want to see whatever is behind it.
            mDismissKeyguardOnNextActivity = false;
            mService.mWindowManager.dismissKeyguard();
        }

进程启动的方法

private final void startProcessLocked(ProcessRecord app,
            String hostingType, String hostingNameStr) {
        if (app.pid > 0 && app.pid != MY_PID) {
            synchronized (mPidsSelfLocked) {
                mPidsSelfLocked.remove(app.pid);
                mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
            }
            app.pid = 0;
        }

        if (DEBUG_PROCESSES && mProcessesOnHold.contains(app)) Slog.v(TAG,
                "startProcessLocked removing on hold: " + app);
        mProcessesOnHold.remove(app);

        updateCpuStats();
        
        System.arraycopy(mProcDeaths, 0, mProcDeaths, 1, mProcDeaths.length-1);
        mProcDeaths[0] = 0;
        
        try {
            int uid = app.info.uid;
            int[] gids = null;
            try {
                gids = mContext.getPackageManager().getPackageGids(
                        app.info.packageName);
            } catch (PackageManager.NameNotFoundException e) {
                Slog.w(TAG, "Unable to retrieve gids", e);
            }
            if (mFactoryTest != SystemServer.FACTORY_TEST_OFF) {
                if (mFactoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL
                        && mTopComponent != null
                        && app.processName.equals(mTopComponent.getPackageName())) {
                    uid = 0;
                }
                if (mFactoryTest == SystemServer.FACTORY_TEST_HIGH_LEVEL
                        && (app.info.flags&ApplicationInfo.FLAG_FACTORY_TEST) != 0) {
                    uid = 0;
                }
            }
            int debugFlags = 0;
            if ((app.info.flags & ApplicationInfo.FLAG_DEBUGGABLE) != 0) {
                debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;
                // Also turn on CheckJNI for debuggable apps. It's quite
                // awkward to turn on otherwise.
                debugFlags |= Zygote.DEBUG_ENABLE_CHECKJNI;
            }
            // Run the app in safe mode if its manifest requests so or the
            // system is booted in safe mode.
            if ((app.info.flags & ApplicationInfo.FLAG_VM_SAFE_MODE) != 0 ||
                Zygote.systemInSafeMode == true) {
                debugFlags |= Zygote.DEBUG_ENABLE_SAFEMODE;
            }
            if ("1".equals(SystemProperties.get("debug.checkjni"))) {
                debugFlags |= Zygote.DEBUG_ENABLE_CHECKJNI;
            }
            if ("1".equals(SystemProperties.get("debug.jni.logging"))) {
                debugFlags |= Zygote.DEBUG_ENABLE_JNI_LOGGING;
            }
            if ("1".equals(SystemProperties.get("debug.assert"))) {
                debugFlags |= Zygote.DEBUG_ENABLE_ASSERT;
            }

            // Start the process.  It will either succeed and return a result containing
            // the PID of the new process, or else throw a RuntimeException.
            Process.ProcessStartResult startResult = Process.start("android.app.ActivityThread",
                    app.processName, uid, uid, gids, debugFlags,
                    app.info.targetSdkVersion, null);

            BatteryStatsImpl bs = app.batteryStats.getBatteryStats();
            synchronized (bs) {
                if (bs.isOnBattery()) {
                    app.batteryStats.incStartsLocked();
                }
            }
            
            EventLog.writeEvent(EventLogTags.AM_PROC_START, startResult.pid, uid,
                    app.processName, hostingType,
                    hostingNameStr != null ? hostingNameStr : "");
            
            if (app.persistent) {
                Watchdog.getInstance().processStarted(app.processName, startResult.pid);
            }
            
            StringBuilder buf = mStringBuilder;
            buf.setLength(0);
            buf.append("Start proc ");
            buf.append(app.processName);
            buf.append(" for ");
            buf.append(hostingType);
            if (hostingNameStr != null) {
                buf.append(" ");
                buf.append(hostingNameStr);
            }
            buf.append(": pid=");
            buf.append(startResult.pid);
            buf.append(" uid=");
            buf.append(uid);
            buf.append(" gids={");
            if (gids != null) {
                for (int gi=0; gi<gids.length; gi++) {
                    if (gi != 0) buf.append(", ");
                    buf.append(gids[gi]);

                }
            }
            buf.append("}");
            Slog.i(TAG, buf.toString());
            app.pid = startResult.pid;
            app.usingWrapper = startResult.usingWrapper;
            app.removed = false;
            synchronized (mPidsSelfLocked) {
                this.mPidsSelfLocked.put(startResult.pid, app);
                Message msg = mHandler.obtainMessage(PROC_START_TIMEOUT_MSG);
                msg.obj = app;
                mHandler.sendMessageDelayed(msg, startResult.usingWrapper
                        ? PROC_START_TIMEOUT_WITH_WRAPPER : PROC_START_TIMEOUT);
            }
        } catch (RuntimeException e) {
            // XXX do better error recovery.
            app.pid = 0;
            Slog.e(TAG, "Failure starting process " + app.processName, e);
        }
    }

Process.java

    public static final ProcessStartResult start(final String processClass,
                                  final String niceName,
                                  int uid, int gid, int[] gids,
                                  int debugFlags, int targetSdkVersion,
                                  String[] zygoteArgs) {
        try {
            return startViaZygote(processClass, niceName, uid, gid, gids,
                    debugFlags, targetSdkVersion, zygoteArgs);
        } catch (ZygoteStartFailedEx ex) {
            Log.e(LOG_TAG,
                    "Starting VM process through Zygote failed");
            throw new RuntimeException(
                    "Starting VM process through Zygote failed", ex);
        }
    }

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