一：在談到Service保活的時候，需要理解出如下幾個概念
1)Activity的finish方法

    /**
     * Call this when your activity is done and should be closed.  
     * The ActivityResult is propagated back to whoever launched you via
     * onActivityResult().
     */
    public void finish() {
        finish(DONT_FINISH_TASK_WITH_ACTIVITY);
    }
 

    /** @hide Task isn't finished when activity is finished */
    public static final int DONT_FINISH_TASK_WITH_ACTIVITY = 0;

    /**
     * Finishes the current activity and specifies whether to remove the task associated with this
     * activity.
     */
    private void finish(int finishTask) {
        if
 
 (mParent == null) {
            int resultCode;
            Intent resultData;
            synchronized (this) {
                resultCode = mResultCode;
                resultData = mResultData;
            }
            if (false) Log.v(TAG, "Finishing self: token=" + mToken);
            try {
                if
 
 (resultData != null) {
                    resultData.prepareToLeaveProcess(this);
                }
                if (ActivityManagerNative.getDefault()
                        .finishActivity(mToken, resultCode, resultData, finishTask)) {
                    mFinished = true;
                }
            } catch (RemoteException e) {
                // Empty
            }
        } else {
            mParent.finishFromChild(this);
        }
    }

finish方法主要作用是關閉當前activity，並將當前activity從相關的task中移除，如果是通過startActivityForResult的方式start activity，則會回撥onActivityResult(int requestCode, int resultCode,Intent data)；

finish(int finishTask)是如何完成上述動作的？我們接著分ActivityManagerNative的finishActivity方法

ActivityManagerNative.getDefault()的方法將返回IActivityManager介面，其具體實現為ActivityManagerService.java，ActivityManagerService.java中finishActivity的方法體如下

    /**
     * This is the internal entry point for handling Activity.finish().
     *
     * @param token The Binder token referencing the Activity we want to finish.
     * @param resultCode Result code, if any, from this Activity.
     * @param resultData Result data (Intent), if any, from this Activity.
     * @param finishTask Whether to finish the task associated with this Activity.
     *
     * @return Returns true if the activity successfully finished, or false if it is still running.
     */
    @Override
    public final boolean finishActivity(IBinder token, int resultCode, Intent resultData,
            int finishTask) {
        // Refuse possible leaked file descriptors
        if (resultData != null && resultData.hasFileDescriptors() == true) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }

        synchronized(this) {
            ActivityRecord r = ActivityRecord.isInStackLocked(token);
            if (r == null) {
                return true;
            }
            // Keep track of the root activity of the task before we finish it
            TaskRecord tr = r.task;
            ActivityRecord rootR = tr.getRootActivity();
            if (rootR == null) {
                Slog.w(TAG, "Finishing task with all activities already finished");
            }
            // Do not allow task to finish if last task in lockTask mode. Launchable priv-apps can
            // finish.
            if (tr.mLockTaskAuth != LOCK_TASK_AUTH_LAUNCHABLE_PRIV && rootR == r &&
                    mStackSupervisor.isLastLockedTask(tr)) {
                Slog.i(TAG, "Not finishing task in lock task mode");
                mStackSupervisor.showLockTaskToast();
                return false;
            }
            if (mController != null) {
                // Find the first activity that is not finishing.
                ActivityRecord next = r.task.stack.topRunningActivityLocked(token, 0);
                if (next != null) {
                    // ask watcher if this is allowed
                    boolean resumeOK = true;
                    try {
                        resumeOK = mController.activityResuming(next.packageName);
                    } catch (RemoteException e) {
                        mController = null;
                        Watchdog.getInstance().setActivityController(null);
                    }

                    if (!resumeOK) {
                        Slog.i(TAG, "Not finishing activity because controller resumed");
                        return false;
                    }
                }
            }
            final long origId = Binder.clearCallingIdentity();
            try {
                boolean res;
                final boolean finishWithRootActivity =
                        finishTask == Activity.FINISH_TASK_WITH_ROOT_ACTIVITY;
                if (finishTask == Activity.FINISH_TASK_WITH_ACTIVITY
                        || (finishWithRootActivity && r == rootR)) {
                    // If requested, remove the task that is associated to this activity only if it
                    // was the root activity in the task. The result code and data is ignored
                    // because we don't support returning them across task boundaries. Also, to
                    // keep backwards compatibility we remove the task from recents when finishing
                    // task with root activity.
                    res = removeTaskByIdLocked(tr.taskId, false, finishWithRootActivity);
                    if (!res) {
                        Slog.i(TAG, "Removing task failed to finish activity");
                    }
                } else {
                    res = tr.stack.requestFinishActivityLocked(token, resultCode,
                            resultData, "app-request", true);
                    if (!res) {
                        Slog.i(TAG, "Failed to finish by app-request");
                    }
                }
                return res;
            } finally {
                Binder.restoreCallingIdentity(origId);
            }
        }
    }

該方法是一個同步的方法，如果activity成功被finish，則返回true，否則返回false；
該方法實際呼叫的是ActivityStack.java中的requestFinishActivityLocked完成動作

    /**
     * @return Returns true if the activity is being finished, false if for
     * some reason it is being left as-is.
     */
    final boolean requestFinishActivityLocked(IBinder token, int resultCode,
            Intent resultData, String reason, boolean oomAdj) {
        ActivityRecord r = isInStackLocked(token);
        if (DEBUG_RESULTS || DEBUG_STATES) Slog.v(TAG_STATES,
                "Finishing activity token=" + token + " r="
                + ", result=" + resultCode + ", data=" + resultData
                + ", reason=" + reason);
        if (r == null) {
            return false;
        }

        finishActivityLocked(r, resultCode, resultData, reason, oomAdj);
        return true;
    }

該方法中主要呼叫finishActivityLocked方法，finishActivityLocked中將呼叫startPausingLocked方法，startPausingLocked將呼叫如下程式碼

                prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing,
                        userLeaving, prev.configChangeFlags, dontWait);

這裡實際上呼叫的是IApplicationThread的schedulePauseActivity方法，IApplicationThread也是一個Binder物件，其實現者為：ActivityThread，所以最終會呼叫的是ActivityThread的schedulePauseActivity方法

        public final void schedulePauseActivity(IBinder token, boolean finished,
                boolean userLeaving, int configChanges, boolean dontReport) {
            int seq = getLifecycleSeq();
            if (DEBUG_ORDER) Slog.d(TAG, "pauseActivity " + ActivityThread.this
                    + " operation received seq: " + seq);
            sendMessage(
                    finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
                    token,
                    (userLeaving ? USER_LEAVING : 0) | (dontReport ? DONT_REPORT : 0),
                    configChanges,
                    seq);
        }

通過傳送非同步訊息的方式 在handler中處理如下case

                case PAUSE_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handlePauseActivity((IBinder) args.arg1, false,
                            (args.argi1 & USER_LEAVING) != 0, args.argi2,
                            (args.argi1 & DONT_REPORT) != 0, args.argi3);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;

handlePauseActivity程式碼實現如下

    private void handlePauseActivity(IBinder token, boolean finished,
            boolean userLeaving, int configChanges, boolean dontReport, int seq) {
        ActivityClientRecord r = mActivities.get(token);
        if (DEBUG_ORDER) Slog.d(TAG, "handlePauseActivity " + r + ", seq: " + seq);
        if (!checkAndUpdateLifecycleSeq(seq, r, "pauseActivity")) {
            return;
        }
        if (r != null) {
            //Slog.v(TAG, "userLeaving=" + userLeaving + " handling pause of " + r);
            if (userLeaving) {
                performUserLeavingActivity(r);
            }

            r.activity.mConfigChangeFlags |= configChanges;
            performPauseActivity(token, finished, r.isPreHoneycomb(), "handlePauseActivity");

            // Make sure any pending writes are now committed.
            if (r.isPreHoneycomb()) {
                QueuedWork.waitToFinish();
            }

            // Tell the activity manager we have paused.
            if (!dontReport) {
                try {
                    ActivityManagerNative.getDefault().activityPaused(token);
                } catch (RemoteException ex) {
                    throw ex.rethrowFromSystemServer();
                }
            }
            mSomeActivitiesChanged = true;
        }
    }

performPauseActivity的實現如下

    final Bundle performPauseActivity(IBinder token, boolean finished,
            boolean saveState, String reason) {
        ActivityClientRecord r = mActivities.get(token);
        return r != null ? performPauseActivity(r, finished, saveState, reason) : null;
    }

呼叫performPauseActivity方法體如下

    final Bundle performPauseActivity(ActivityClientRecord r, boolean finished,
            boolean saveState, String reason) {
        if (r.paused) {
            if (r.activity.mFinished) {
                // If we are finishing, we won't call onResume() in certain cases.
                // So here we likewise don't want to call onPause() if the activity
                // isn't resumed.
                return null;
            }
            RuntimeException e = new RuntimeException(
                    "Performing pause of activity that is not resumed: "
                    + r.intent.getComponent().toShortString());
            Slog.e(TAG, e.getMessage(), e);
        }
        if (finished) {
            r.activity.mFinished = true;
        }

        // Next have the activity save its current state and managed dialogs...
        if (!r.activity.mFinished && saveState) {
            callCallActivityOnSaveInstanceState(r);
        }

        performPauseActivityIfNeeded(r, reason);

        // Notify any outstanding on paused listeners
        ArrayList<OnActivityPausedListener> listeners;
        synchronized (mOnPauseListeners) {
            listeners = mOnPauseListeners.remove(r.activity);
        }
        int size = (listeners != null ? listeners.size() : 0);
        for (int i = 0; i < size; i++) {
            listeners.get(i).onPaused(r.activity);
        }

        return !r.activity.mFinished && saveState ? r.state : null;
    }

該方法將觸發Activity的生命週期變化：onPaused
在handlePauseActivity方法中，最後將呼叫

ActivityManagerNative.getDefault().activityPaused(token);

完成activity的回收，其實際觸發的是ActivityManagerService的activityPaused方法

    @Override
    public final void activityPaused(IBinder token) {
        final long origId = Binder.clearCallingIdentity();
        synchronized(this) {
            ActivityStack stack = ActivityRecord.getStackLocked(token);
            if (stack != null) {
                stack.activityPausedLocked(token, false);
            }
        }
        Binder.restoreCallingIdentity(origId);
    }

ActivityStack .java中的activityPausedLocked方法將呼叫completePauseLocked方法，在completePauseLocked方法中，將會呼叫resumeTopActivitiesLocked方法，這樣在棧頂的activity將觸發activity生命週期的onresume方法

綜述：activity的生命週期裡面涉及到ActivityStack，ActivityThread，ActivityManagerService，其中ActivityThread中的

private class H extends Handler{}

將統管activity的生命週期相關內容

2)System.exit

    /**
     * Terminates the currently running Java Virtual Machine. The
     * argument serves as a status code; by convention, a nonzero status
     * code indicates abnormal termination.
     * <p>
     * This method calls the <code>exit</code> method in class
     * <code>Runtime</code>. This method never returns normally.
     * <p>
     * The call <code>System.exit(n)</code> is effectively equivalent to
     * the call:
     * <blockquote><pre>
     * Runtime.getRuntime().exit(n)
     * </pre></blockquote>
     *
     * @param      status   exit status.
     * @throws  SecurityException
     *        if a security manager exists and its <code>checkExit</code>
     *        method doesn't allow exit with the specified status.
     * @see        java.lang.Runtime#exit(int)
     */
    public static void exit(int status) {
        Runtime.getRuntime().exit(status);
    }

終止當前執行的java虛擬機器，引數status如果非0，則表示異常退出

3)Process.killProcess

    /**
     * Kill the process with the given PID.
     * Note that, though this API allows us to request to
     * kill any process based on its PID, the kernel will
     * still impose standard restrictions on which PIDs you
     * are actually able to kill.  Typically this means only
     * the process running the caller's packages/application
     * and any additional processes created by that app; packages
     * sharing a common UID will also be able to kill each
     * other's processes.
     */
    public static final void killProcess(int pid) {
        sendSignal(pid, SIGNAL_KILL);
    }

用給定的PID殺死過程。雖然這個API允許我們請求基於它的PID來殺死任何程序，但是kernal將嚴格限制哪一個PIDS可以被殺死；典型的有呼叫者的包/application的程序，由這個app建立的其他程序，分享的一個共享UID都可以被殺死