Android之Service自啟動流程
阿新 • • 發佈：2019-01-09
當Service被異常銷燬後，預設都會自動重啟，但Service的自動重啟是如何實現的尼？下面就來梳理一下Service的自動重啟流程。
無論Service是啟動還是繫結成功，都會呼叫ActiveServices的serviceDoneExecutingLocked方法，來看一下這個方法的實現。
void serviceDoneExecutingLocked(ServiceRecord r, int type, int startId, int res) {
boolean inDestroying = mDestroyingServices.contains 當Service啟動或繫結成功後就會給當前Service給stopIfKilled這個標誌位賦值，該標誌位決定當Service被異常kill掉後是否需要重啟。
Service被意外kill後會呼叫ActiveServices的killServicesLocked方法，來看看這個方法的實現。
final void killServicesLocked(ProcessRecord app, boolean allowRestart) {
...
ServiceMap smap = getServiceMapLocked(app.userId);
// Now do remaining service cleanup.
for (int i=app.services.size()-1; i>=0; i--) {
//拿到Service的資訊
ServiceRecord sr = app.services.valueAt(i);
...
// Any services running in the application may need to be placed
// back in the pending list.
//如果連續兩次重啟失敗就不再重啟Service
if (allowRestart && sr.crashCount >= 2 && (sr.serviceInfo.applicationInfo.flags
&ApplicationInfo.FLAG_PERSISTENT) == 0) {
...
//不允許重啟
bringDownServiceLocked(sr);
} else if (!allowRestart//不允許重啟
|| !mAm.mUserController.isUserRunningLocked(sr.userId, 0)) {
//不允許重啟
bringDownServiceLocked(sr);
} else {
//嘗試重啟
boolean canceled = scheduleServiceRestartLocked(sr, true);
//在多次嘗試傳送失敗的命令的極端情況下，也會在這裡停止它。
if (sr.startRequested && (sr.stopIfKilled || canceled)) {
if (sr.pendingStarts.size() == 0) {
sr.startRequested = false;
if (sr.tracker != null) {
sr.tracker.setStarted(false, mAm.mProcessStats.getMemFactorLocked(),
SystemClock.uptimeMillis());
}
if (!sr.hasAutoCreateConnections()) {
// Whoops, no reason to restart!
//哎呀，沒理由重啟！
bringDownServiceLocked(sr);
}
}
}
}
}
//做一些清理操作
...
}
從上面程式碼可以看出，嘗試重啟的具體實現是在scheduleServiceRestartLocked這個方法裡，來看看它的實現。
private final boolean scheduleServiceRestartLocked(ServiceRecord r, boolean allowCancel) {
boolean canceled = false;
...
final long now = SystemClock.uptimeMillis();
if ((r.serviceInfo.applicationInfo.flags
&ApplicationInfo.FLAG_PERSISTENT) == 0) {
long minDuration = mAm.mConstants.SERVICE_RESTART_DURATION;
long resetTime = mAm.mConstants.SERVICE_RESET_RUN_DURATION;
// Any delivered but not yet finished starts should be put back
// on the pending list.
final int N = r.deliveredStarts.size();
if (N > 0) {
for (int i=N-1; i>=0; i--) {
ServiceRecord.StartItem si = r.deliveredStarts.get(i);
si.removeUriPermissionsLocked();
if (si.intent == null) {
// We'll generate this again if needed.
} else if (!allowCancel || (si.deliveryCount < ServiceRecord.MAX_DELIVERY_COUNT
&& si.doneExecutingCount < ServiceRecord.MAX_DONE_EXECUTING_COUNT)) {
//重啟的時候，deliveredStarts被pendingStarts替換掉了，這樣就能拿到Intent的值
// 也就說，這個時候START_REDELIVER_INTENT由死轉生
r.pendingStarts.add(0, si);
long dur = SystemClock.uptimeMillis() - si.deliveredTime;
//將重啟間隔時間拉大，被kill掉次數越多，重啟所需要時間越長
dur *= 2;
if (minDuration < dur) minDuration = dur;
if (resetTime < dur) resetTime = dur;
} else {
Slog.w(TAG, "Canceling start item " + si.intent + " in service "
+ r.name);
canceled = true;
}
}
r.deliveredStarts.clear();
}
//重啟次數加1
r.totalRestartCount++;
if (r.restartDelay == 0) {
r.restartCount++;
r.restartDelay = minDuration;
} else {
// If it has been a "reasonably long time" since the service
// was started, then reset our restart duration back to
// the beginning, so we don't infinitely increase the duration
// on a service that just occasionally gets killed (which is
// a normal case, due to process being killed to reclaim memory).
if (now > (r.restartTime+resetTime)) {
r.restartCount = 1;
r.restartDelay = minDuration;
} else {
r.restartDelay *= mAm.mConstants.SERVICE_RESTART_DURATION_FACTOR;
if (r.restartDelay < minDuration) {
r.restartDelay = minDuration;
}
}
}
r.nextRestartTime = now + r.restartDelay;
//兩個Service啟動至少間隔10秒，這裡的意義其實不是很大，主要是為了Service啟動失敗的情況，如果啟動成功，其他要啟動的Service會被一併直接重新喚起
boolean repeat;
do {
repeat = false;
final long restartTimeBetween = mAm.mConstants.SERVICE_MIN_RESTART_TIME_BETWEEN;
for (int i=mRestartingServices.size()-1; i>=0; i--) {
ServiceRecord r2 = mRestartingServices.get(i);
if (r2 != r && r.nextRestartTime >= (r2.nextRestartTime-restartTimeBetween)
&& r.nextRestartTime < (r2.nextRestartTime+restartTimeBetween)) {
r.nextRestartTime = r2.nextRestartTime + restartTimeBetween;
r.restartDelay = r.nextRestartTime - now;
repeat = true;
break;
}
}
} while (repeat);
} else {
// Persistent processes are immediately restarted, so there is no
// reason to hold of on restarting their services.
r.totalRestartCount++;
r.restartCount = 0;
r.restartDelay = 0;
r.nextRestartTime = now;
}
if (!mRestartingServices.contains(r)) {
r.createdFromFg = false;
mRestartingServices.add(r);
r.makeRestarting(mAm.mProcessStats.getMemFactorLocked(), now);
}
cancelForegroundNotificationLocked(r);
mAm.mHandler.removeCallbacks(r.restarter);
//執行Service重啟操作，這裡的restarter就是一個ServiceRestarter物件，在ServiceRecord建立時新增進去的
mAm.mHandler.postAtTime(r.restarter, r.nextRestartTime);
r.nextRestartTime = SystemClock.uptimeMillis() + r.restartDelay;
...
return canceled;
}
private class ServiceRestarter implements Runnable {
private ServiceRecord mService;
void setService(ServiceRecord service) {
mService = service;
}
public void run() {
synchronized(mAm) {
//開始重啟
performServiceRestartLocked(mService);
}
}
}
final void performServiceRestartLocked(ServiceRecord r) {
...
try {
bringUpServiceLocked(r, r.intent.getIntent().getFlags(), r.createdFromFg, true, false);
} catch (TransactionTooLargeException e) {
...
}
}
最後就在ServiceRestarter執行重新建立Service操作，在run方法裡呼叫了performServiceRestartLocked來進行重啟，而performServiceRestartLocked裡直接呼叫了bringUpServiceLocked這個方法，這個就是重新建立Service的方法，關於這個方法的詳細實現可以去閱讀Android原始碼之Service啟動流程這篇文章。
Android的服務（Service）（二）Service的自動重啟問題
Android Service重啟恢復（Service程序重啟）原理解析