和你一起终身学习，这里是程序员Android
Android O 推出出了Background Execution Limits，减少后台应用内存使用及耗电，一个很明显的应用就是不准后台应用通过startService启动服务，这里有两个问题需要弄清楚，第一：什么状态下startService的属于后台启动service；第二：如果想要在后台startService，如何兼容，因此分如下几个问题分析下
- 后台startService的场景
- 后台startService的Crash原理分析
- 如何修改达到兼容
对于普通APP而言，我们不考虑系统的各种白名单，一般后台startService服务分下面两种：
- 通过其他应用startService
- 通过自己应用startService
而每种又可以分不同的小场景，通过其他应用startService已经不被推荐，所以先看看自己应用startService。
通过自己应用在后台startService限制
可以通过一个简单的实验观察什么情况属于后台startService，注意：如果是自己APP启动Service，那么自身应用必定已经起来了。通过延迟执行就复现该场景。比如：通过click事件，延迟执行一个startService操作，延迟时间是65s（要超过一分钟，后面会看到这是个阈值），然后点击Home键，回到桌面，之后等待一分钟就可复现Crash：
@OnClick(R.id.first)
void first() {
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Intent intent = new Intent(LabApplication.getContext(), BackGroundService.class);
startService(intent);
LogUtils.v("延迟执行");
}
},1000*65);
}
大概一分多钟之后，延迟消息被执行，然后会有如下Crash日志被打印：
--------- beginning of crash
2019-06-17 19:47:43.148 25916-25916/com.snail.labaffinity E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.snail.labaffinity, PID: 25916
java.lang.IllegalStateException: Not allowed to start service Intent { cmp=com.snail.labaffinity/.service.BackGroundService }: app is in background uid UidRecord{9048c2c u0a73 LAST bg:+1m4s376ms idle change:idle procs:1 seq(0,0,0)}
at android.app.ContextImpl.startServiceCommon(ContextImpl.java:1577)
at android.app.ContextImpl.startService(ContextImpl.java:1532)
at android.content.ContextWrapper.startService(ContextWrapper.java:664)
at com.snail.labaffinity.activity.MainActivity$2.run(MainActivity.java:41)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
就会看到很经典的startService限制信息：
Not allowed to start service Intent XXX : app is in background uid UidRecord
也就是说，当前退到后台的APP已经属于后台应用，不能通过startService启动服务。Why？跟踪源码看下 startService会调用ContextImpl 的startServiceCommon，进而通过Binder调用AMS启动Service，根据返回值选择性抛出IllegalStateException异常：
ContextImpl.java
private ComponentName startServiceCommon(Intent service, boolean requireForeground,
UserHandle user) {
try {
validateServiceIntent(service);
service.prepareToLeaveProcess(this);
ComponentName cn = ActivityManager.getService().startService(
mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
getContentResolver()), requireForeground,
getOpPackageName(), user.getIdentifier());
if (cn != null) {
<!--返回值是？的情况下就是后台启动service的异常-->
if (cn.getPackageName().equals("?")) {
throw new IllegalStateException(
"Not allowed to start service " + service + ": " + cn.getClassName());
}
}
什么时候ActivityManager.getService().startService的返回值包名 ？，核心代码在AMS端，AMS进一步调用ActiveServices.java的startServiceLocked：
ActiveServices.java
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
int callingPid, int callingUid, boolean fgRequired, String callingPackage, final int userId)
throws TransactionTooLargeException {
final boolean callerFg;
if (caller != null) {
final ProcessRecord callerApp = mAm.getRecordForAppLocked(caller);
...
callerFg = callerApp.setSchedGroup != ProcessList.SCHED_GROUP_BACKGROUND;
} else {
callerFg = true;
}
ServiceLookupResult res =
retrieveServiceLocked(service, resolvedType, callingPackage,
callingPid, callingUid, userId, true, callerFg, false, false);
...
ServiceRecord r = res.record;
// If we're starting indirectly (e.g. from PendingIntent), figure out whether
// we're launching into an app in a background state. This keys off of the same
// idleness state tracking as e.g. O+ background service start policy.
<!--通过PendingIntent启动的也要检查-->
// 是否当前Uid Active 不过不是activity就是后台启动
final boolean bgLaunch = !mAm.isUidActiveLocked(r.appInfo.uid);
// If the app has strict background restrictions, we treat any bg service
// start analogously to the legacy-app forced-restrictions case, regardless
// of its target SDK version.
boolean forcedStandby = false;
<!--appRestrictedAnyInBackground 一般人不会主动设置，所以这个经常是返回false-->
if (bgLaunch && appRestrictedAnyInBackground(r.appInfo.uid, r.packageName)) {
...
forcedStandby = true;
}
<!--forcedStandby可以先无视 这里注意两点，第一点 ：r.startRequested标志是通过startService调用启动过，第一次进来的时候是false，第二：对于普通是starServicefgRequired是false-->
if (forcedStandby || (!r.startRequested && !fgRequired)) {
<!--检测当前app是否允许后台启动-->
final int allowed = mAm.getAppStartModeLocked(r.appInfo.uid, r.packageName,
r.appInfo.targetSdkVersion, callingPid, false, false, forcedStandby);
<!--如果不允许 Background start not allowed-->
if (allowed != ActivityManager.APP_START_MODE_NORMAL) {
...
<!--返回 ? 告诉客户端现在处于后台启动状态，禁止你-->
return new ComponentName("?", "app is in background uid " + uidRec);
}
}
假设我们是第一次startService，那么(!r.startRequested && !fgRequired)就等于true，进而走进mAm.getAppStartModeLocked，看看当前进程是否处于后台非激活状态，如果是的话 ，就不会允许startService：
ActivityManagerService.java
int getAppStartModeLocked(int uid, String packageName, int packageTargetSdk,
int callingPid, boolean alwaysRestrict, boolean disabledOnly, boolean forcedStandby) {
UidRecord uidRec = mActiveUids.get(uid);
<!--UidRecord是关键 alwaysRestrict || forcedStandby 传入的都是false，忽略 -->
if (uidRec == null || alwaysRestrict || forcedStandby || uidRec.idle) {
boolean ephemeral;
...
final int startMode = (alwaysRestrict)
? appRestrictedInBackgroundLocked(uid, packageName, packageTargetSdk)
: appServicesRestrictedInBackgroundLocked(uid, packageName,
packageTargetSdk);
...
return startMode;
}
return ActivityManager.APP_START_MODE_NORMAL;
}
这里UidRecord是关键，UidRecord为null，则说明整个APP没有被启动，那么就一定属于后台启动Service，如果UidRecord非null，则要判断应用是否属于后台应用，而这个关键就是uidRec.idle，如果idle是true，就说明应用处于后台状态，继续调用 appServicesRestrictedInBackgroundLocked看看是否是O以后的，走Crash逻辑：
int appServicesRestrictedInBackgroundLocked(int uid, String packageName, int packageTargetSdk) {
<!--永久进程 -->
// Persistent app?
if (mPackageManagerInt.isPackagePersistent(packageName)) {
return ActivityManager.APP_START_MODE_NORMAL;
}
<!--白名单-->
// Non-persistent but background whitelisted?
if (uidOnBackgroundWhitelist(uid)) {
return ActivityManager.APP_START_MODE_NORMAL;
}
<!--白名单-->
// Is this app on the battery whitelist?
if (isOnDeviceIdleWhitelistLocked(uid, /*allowExceptIdleToo=*/ false)) {
return ActivityManager.APP_START_MODE_NORMAL;
}
// 普通进程
return appRestrictedInBackgroundLocked(uid, packageName, packageTargetSdk);
}
对于普通进程看看O限制
int appRestrictedInBackgroundLocked(int uid, String packageName, int packageTargetSdk) {
<!--对于targetSDKVersion>O 的直接 返回ActivityManager.APP_START_MODE_DELAYED_RIGID-->
if (packageTargetSdk >= Build.VERSION_CODES.O) {
return ActivityManager.APP_START_MODE_DELAYED_RIGID;
}
// 否则仅仅对老版本做兼容性限制
int appop = mAppOpsService.noteOperation(AppOpsManager.OP_RUN_IN_BACKGROUND,
uid, packageName);
if (DEBUG_BACKGROUND_CHECK) {
Slog.i(TAG, "Legacy app " + uid + "/" + packageName + " bg appop " + appop);
}
switch (appop) {
case AppOpsManager.MODE_ALLOWED:
// If force-background-check is enabled, restrict all apps that aren't whitelisted.
if (mForceBackgroundCheck &&
!UserHandle.isCore(uid) &&
!isOnDeviceIdleWhitelistLocked(uid, /*allowExceptIdleToo=*/ true)) {
return ActivityManager.APP_START_MODE_DELAYED;
}
...
}
appServicesRestrictedInBackgroundLocked仅仅是根据是否是O以后，返回ActivityManager.APP_START_MODE_DELAYED_RIGID，只是兼容，核心还是UidRecord的idle，下面就重点看看UidRecord跟其idle的值，这个值是应用是否位于后台的核心指标，应用未启动的不考虑，未启动肯定也属于”后台“的一种极端。
不是特别老的Android版本都不允许没有LAUNCHER Activity的应用，不然压根没法编译运行，也就说普通场景通过桌面启动应用的时候，都是通过startActivity直接启动APP的，在启动App的时候，UidRecord会被新建（AMS端），UidRecord构造函数中默认 idle = true。
public UidRecord(int _uid) {
uid = _uid;
idle = true;
reset();
}
其启动流程调用堆栈如下：
image
也就是启动APP时候，刚好一开UidRecord中idle的值是true，被看做后台应用，那么一定有某个地方设置为false，设置为前台应用。
前后台应用切换时机与原理
一个应用可以有一个或者多个进程，当任何一个进程变为被转换成前台可见进程的时候，APP都会被认作前台应用（对于startService应用而言），resumetopActivity是一个非常明确的切换时机，
会调用
final void scheduleIdleLocked() {
mHandler.sendEmptyMessage(IDLE_NOW_MSG);
}
会通过updateOomAdjLocked修改当前即将可见Activity应用的idle状态，updateOomAdjLocked在期间可能会被调用多次，
@GuardedBy("this")
final void updateOomAdjLocked() {
...
for (int i=mActiveUids.size()-1; i>=0; i--) {
final UidRecord uidRec = mActiveUids.valueAt(i);
int uidChange = UidRecord.CHANGE_PROCSTATE;
if (uidRec.curProcState != ActivityManager.PROCESS_STATE_NONEXISTENT
&& (uidRec.setProcState != uidRec.curProcState
|| uidRec.setWhitelist != uidRec.curWhitelist)) {
...
if (ActivityManager.isProcStateBackground(uidRec.curProcState)
&& !uidRec.curWhitelist) {
...
} else {
<!--设置为false 标记为前台进程-->
if (uidRec.idle) {
uidChange = UidRecord.CHANGE_ACTIVE;
EventLogTags.writeAmUidActive(uidRec.uid);
uidRec.idle = false;
}
<!--清零后台进程锚点时间-->
uidRec.lastBackgroundTime = 0;
}
对于即将可见的APP而言 ActivityManager.isProcStateBackground为false，所以走else逻辑设置uidRec.idle = false,uidChange = UidRecord.CHANGE_ACTIVE，之后通过enqueueUidChangeLocked最后设置相对应的idle = false。相对应，上一个被切换走的应用可能会触发设置idle = true的操作，不过设置为true的操作不是即可执行的，而是延迟执行的，延迟时间60s：
final void updateOomAdjLocked() {
...
for (int i=mActiveUids.size()-1; i>=0; i--) {
final UidRecord uidRec = mActiveUids.valueAt(i);
int uidChange = UidRecord.CHANGE_PROCSTATE;
if (uidRec.curProcState != ActivityManager.PROCESS_STATE_NONEXISTENT
&& (uidRec.setProcState != uidRec.curProcState
|| uidRec.setWhitelist != uidRec.curWhitelist)) {
if (ActivityManager.isProcStateBackground(uidRec.curProcState)
&& !uidRec.curWhitelist) {
// UID is now in the background (and not on the temp whitelist). Was it
// previously in the foreground (or on the temp whitelist)?
if (!ActivityManager.isProcStateBackground(uidRec.setProcState)
|| uidRec.setWhitelist) {
<!--切换后台时候更新lastBackgroundTime-->
uidRec.lastBackgroundTime = nowElapsed;
if (!mHandler.hasMessages(IDLE_UIDS_MSG)) {
<!--60s后更新-->
mHandler.sendEmptyMessageDelayed(IDLE_UIDS_MSG,
mConstants.BACKGROUND_SETTLE_TIME);
}
}
延迟60s是为了防止60s之内多次切换APP导致的重复更新，系统只要保证60s内有一次就可以了。
private static final long DEFAULT_BACKGROUND_SETTLE_TIME = 60*1000;
60s之后调用idleUids更新idle字段
final void idleUids() {
synchronized (this) {
final int N = mActiveUids.size();
if (N <= 0) {
return;
}
final long nowElapsed = SystemClock.elapsedRealtime();
final long maxBgTime = nowElapsed - mConstants.BACKGROUND_SETTLE_TIME;
long nextTime = 0;
for (int i=N-1; i>=0; i--) {
final UidRecord uidRec = mActiveUids.valueAt(i);
<!--刚才切后台的时候已经更新过uidRec.lastBackgroundTime-->
final long bgTime = uidRec.lastBackgroundTime;
if (bgTime > 0 && !uidRec.idle) {
<!--标准：后台存在时间超过mConstants.BACKGROUND_SETTLE_TIME-->
if (bgTime <= maxBgTime) {
uidRec.idle = true;
uidRec.setIdle = true;
doStopUidLocked(uidRec.uid, uidRec);
} else {
如果被提前执行了，则在下一个60s到达的时候执行
if (nextTime == 0 || nextTime > bgTime) {
nextTime = bgTime;
}
}
}
}
if (nextTime > 0) {
mHandler.removeMessages(IDLE_UIDS_MSG);
mHandler.sendEmptyMessageDelayed(IDLE_UIDS_MSG,
nextTime + mConstants.BACKGROUND_SETTLE_TIME - nowElapsed);
}
}
}
之前是前台，现在变后台，那么uidRec.lastBackgroundTime = nowElapsed赋值，再次切前台，uidRec.lastBackgroundTime清零，简而言之， 应用变为前台，UID状态马上变更为active状态，应用变为后台，即procState大于等于PROCESS_STATE_TRANSIENT_BACKGROUND时，如果持续在后台60s后，UID状态会变更为idle=true状态,不能startService；
通过其他应用startService的情况
跨应用startService已经不被推荐了，不过也容易模拟，在A应用中通过setAction+setPackage就可以startService：
var intent = Intent();
intent.setAction("com.snail.BackGroundService");
intent.setPackage("com.snail.labaffinity");
startService(intent)
当然在B应用中AndroidManifest要暴露出来：
<service
android:name=".service.BackGroundService"
<!--是否独立进程，无关紧要-->
android:process=":service"
android:exported="true">
<intent-filter>
<action android:name="com.snail.BackGroundService" />
</intent-filter>
</service>
这样A中startService同样要遵守不准后台启动的条件。比如如果B没启动过，直接在A中startService，则会Crash，如果B启动了，还没变成后台应用（退到后台没超过60S），则不会Crash。个人觉得通过adb命令startService也属于这种范畴，通过如下命令可以达到相同的效果。
am startservice -n com.snail.labaffinity/com.snail.labaffinity.service.BackGroundService
如果APP没有启动就会看到如下日志：
app is in background uid null
如果启动了，但是属于后台应用，就会看到如下日志，跟自己APP后台启动Service类似：
Not allowed to start service Intent { cmp=com.snail.labaffinity/.service.BackGroundService }: app is in background uid UidRecord{72bb30d u0a238 SVC idle change:idle|uncached procs:1 seq(0,0,0)}
其实，startService不是看调用的APP处于何种状态，而是看Servic所在APP处于何种状态，因为看的是Servic所处的UidRecord的状态，UidRecord仅仅跟APP安装有关系，跟进程pid没关系。
特殊场景：进程通过Service恢复的场景
先看下如下代码，APP在启动的时候，在Application的onCreate中通过startService启动了一个服务，并且没有stop，这种场景下第一次通过Launcher冷启动没问题，如果我们在后台杀死APP，由于存在一个未stop的服务，系统会重新拉起该服务，也就是会重启一个进程，然后启动服务。
public class LabApplication extends Application {
@Override
public void onCreate() {
super.onCreate();
Intent intent = new Intent( this, BackGroundService.class);
startService(intent);
}
}
public class BackGroundService extends Service {
@Nullable
@Override
public IBinder onBind(Intent intent) {
return null;
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
LogUtils.v("onStartCommand");
}
}
在这个过程中，应用重启会复现如下Crash（禁止后台启动Service的Crash Log）：
java.lang.RuntimeException: Unable to create application com.snail.labaffinity.app.LabApplication: java.lang.IllegalStateException: Not allowed to start service Intent { cmp=com.snail.labaffinity/.service.BackGroundService }: app is in background uid UidRecord{72bb30d u0a238 SVC idle change:idle|uncached procs:1 seq(0,0,0)}
at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5925)
at android.app.ActivityThread.access$1100(ActivityThread.java:200)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1656)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6718)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Why？为什么冷启动没问题，后台杀死自启动恢复就有问题，看日志是因为当app is in background，Not allowed to start service，也就是后台进程不能通过startService启动服务，在LabApplication的onCreate中我们确实主动startService(intent)，这个就是crash的原因，那为什么第一次没问题？在前文我们知道，通过Laucher启动应用是通过startActivity启动的，也就是存在一个resumeTopActivity的时机，在这个时机，APP的idle会被设置为false，也就是非后台应用，但是对于后台杀死又恢复的场景，他不是通过startActivity启动的，所以APP就算重启了，APP的idle还是true，是非激活的状态，也就是属于后台应用，不准通过startService启动服务（假设单进程）。
因为第一次冷启动时候，走正常启动Activity流程，新建进程，然后去AMS attachApplication，
@GuardedBy("this")
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid, int callingUid, long startSeq) {
...
<!-- 通知APP端创建Application-->
thread.bindApplication(processName, appInfo, providers,
app.instr.mClass,
profilerInfo, app.instr.mArguments,...);
...
boolean badApp = false;
boolean didSomething = false;
// See if the top visible activity is waiting to run in this process...
if (normalMode) {
try {
<!-- 需要启动的Activity 关键点 -->
if (mStackSupervisor.attachApplicationLocked(app)) {
didSomething = true;
}
if (!badApp) {
try {
<!-- 需要恢复的Service-->
didSomething |= mServices.attachApplicationLocked(app, processName);
checkTime(startTime, "attachApplicationLocked: after mServices.attachApplicationLocked");
}
第一次启动APP的时候，thread.bindApplication首先通知APP端启动Application，并执行onCreate，不过onCreate中的startService要等待AMS端上一个消息执行完毕（Handler保证），这个过程中mStackSupervisor.attachApplicationLocked(app)中会调用realStartActivityLocked启动Activity，先将UidRecord的idle给更新为false，attachApplicationLocked执行之后，才有可能轮到下一个消息startService执行，这个时候APP已经不是后台应用了，所以不会Crash。
boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
final String processName = app.processName;
boolean didSomething = false;
...
final int size = mTmpActivityList.size();
<!--存在要启动的Activity-->
for (int i = 0; i < size; i++) {
final ActivityRecord activity = mTmpActivityList.get(i);
if (activity.app == null && app.uid == activity.info.applicationInfo.uid
&& processName.equals(activity.processName)) {
try {
<!--走realStartActivityLocked-->
if (realStartActivityLocked(activity, app,
top == activity /* andResume */, true /* checkConfig */))
realStartActivityLocked会更新oom，并设置idle为false，因为有Activity要启动，就不在是后台进程，调用流程如下：
image
但是对于而对于杀死并通过Service恢复的进程，没有明确的startActivity，所以size = mTmpActivityList.size()这里size是0，不会走realStartActivityLocked，也就在进程恢复阶段，不会将APP归为前台应用，这个时候再AMS执行下一个消息启动Service的时候，就会告诉APP端，不能在后台启动应用。
如何解决这个问题
既然不能再后台偷偷启动，那只能显示启动，Google提供的方案是：startForegroundService()。并且在系统创建Service后，需要在一定时间内调用startForeground()让Service为用户可见通知，否则则系统将停止此Service，抛出ANR，如果不像让用户可见可以参考JobScheduler。不过本篇只看startForegroundService：
@Override
public ComponentName startService(Intent service) {
warnIfCallingFromSystemProcess();
return startServiceCommon(service, false, mUser);
}
@Override
public ComponentName startForegroundService(Intent service) {
warnIfCallingFromSystemProcess();
return startServiceCommon(service, true, mUser);
}
同普通startService的区别那就是startServiceCommon的第二参数boolean requireForeground 是true：
ComponentName startServiceLocked(IApplicationThread caller, Intent service ...}
<!--fgRequired为true，不会检测启动后台限制-->
if (forcedStandby || (!r.startRequested && !fgRequired)) {
final int allowed = mAm.getAppStartModeLocked(r.appInfo.uid, r.packageName,
r.appInfo.targetSdkVersion, callingPid, false, false, forcedStandby);
if (allowed != ActivityManager.APP_START_MODE_NORMAL) {
return new ComponentName("?", "app is in background uid " + uidRec);
}
}
...
<!--ServiceRecord赋值r.fgRequired 后面会用到-->
r.fgRequired = fgRequired;
<!--添加后面回调StartItem-->
r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(),
service, neededGrants, callingUid));
在AMS端startForegroundService跟普通startService区别， ServiceRecord的fgRequired被设置为true，然后走后续流程bringUpServiceLocked->realStartServiceLocked-> sendServiceArgsLocked，在sendServiceArgsLocked的时候，Service其实已经创建并启动（可以看Service启动流程），
private final void sendServiceArgsLocked(ServiceRecord r, boolean execInFg,
boolean oomAdjusted) throws TransactionTooLargeException {
...
ArrayList<ServiceStartArgs> args = new ArrayList<>();
while (r.pendingStarts.size() > 0) {
ServiceRecord.StartItem si = r.pendingStarts.remove(0);
...
if (r.fgRequired && !r.fgWaiting) {
if (!r.isForeground) {
<!--监听是否5S内startForeground-->
scheduleServiceForegroundTransitionTimeoutLocked(r);
} ...
try {
r.app.thread.scheduleServiceArgs(r, slice);
}
可以看到对于要求前台启动的Service fgRequired = true，并且第一次r.fgWaiting=false，所以会走scheduleServiceForegroundTransitionTimeoutLocked，
void scheduleServiceForegroundTransitionTimeoutLocked(ServiceRecord r) {
if (r.app.executingServices.size() == 0 || r.app.thread == null) {
return;
}
Message msg = mAm.mHandler.obtainMessage(
ActivityManagerService.SERVICE_FOREGROUND_TIMEOUT_MSG);
msg.obj = r;
r.fgWaiting = true;
mAm.mHandler.sendMessageDelayed(msg, SERVICE_START_FOREGROUND_TIMEOUT);
}
r.fgWaiting会被设置为true，scheduleServiceForegroundTransitionTimeoutLocked过一次后，就不会再次走。
static final int SERVICE_START_FOREGROUND_TIMEOUT = 10*1000;
看9.0代码，是10s完成调用startForeground，否则在10s后Handler处理这一消息的时候，会停止该服务，并抛出Service的ANR异常。
void serviceForegroundTimeout(ServiceRecord r) {
ProcessRecord app;
synchronized (mAm) {
if (!r.fgRequired || r.destroying) {
return;
}
app = r.app;
r.fgWaiting = false;
stopServiceLocked(r);
}
if (app != null) {
mAm.mAppErrors.appNotResponding(app, null, null, false,
"Context.startForegroundService() did not then call Service.startForeground(): "
+ r);
}
}
抛出异常栈如下
--------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.snail.labaffinity, PID: 21513
android.app.RemoteServiceException: Context.startForegroundService() did not then call Service.startForeground()
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1768)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
解决方案就是及时调用startForeground，对于O以后的还要注意Notification需要一个ChannelID
public class BackGroundService extends Service {
@Override
public void onCreate() {
super.onCreate();
startForeground();
}
private void startForeground() {
String CHANNEL_ONE_ID = "com.snail.labaffinity";
String CHANNEL_ONE_NAME = "Channel One";
NotificationChannel notificationChannel = null;
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
notificationChannel = new NotificationChannel(CHANNEL_ONE_ID,
CHANNEL_ONE_NAME, NotificationManager.IMPORTANCE_DEFAULT);
NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
assert manager != null;
manager.createNotificationChannel(notificationChannel);
startForeground(1, new NotificationCompat.Builder(this, CHANNEL_ONE_ID).build());
}
}
}
startForeground主要就是讲Service至于前台可见，同时取消掉刚才的那个延时Message，这样就不会检测并抛出异常了。
private void setServiceForegroundInnerLocked(final ServiceRecord r, int id,
Notification notification, int flags) {
<!--id不能为0-->
if (id != 0) {
...
if (r.fgRequired) {
<!--设置fgRequired = false-->
r.fgRequired = false;
<!--设置 fgWaiting = false-->
r.fgWaiting = false;
alreadyStartedOp = true;
<!--移除ActivityManagerService.SERVICE_FOREGROUND_TIMEOUT_MSG消息-->
mAm.mHandler.removeMessages(
ActivityManagerService.SERVICE_FOREGROUND_TIMEOUT_MSG, r);
}
不过不过这样的话，状态栏会有一个xxx正在运行的通知，体验不太好，如果是要完成某项任务完成后，最好主动stop掉。还有一个要注意的问题：在调用startForGround前不准调stop，否则也会抛出异常：
private final void bringDownServiceLocked(ServiceRecord r) {
...
if (r.fgRequired) {
r.fgRequired = false;
r.fgWaiting = false;
mAm.mAppOpsService.finishOperation(AppOpsManager.getToken(mAm.mAppOpsService),
AppOpsManager.OP_START_FOREGROUND, r.appInfo.uid, r.packageName);
mAm.mHandler.removeMessages(
ActivityManagerService.SERVICE_FOREGROUND_TIMEOUT_MSG, r);
if (r.app != null) {
Message msg = mAm.mHandler.obtainMessage(
ActivityManagerService.SERVICE_FOREGROUND_CRASH_MSG);
msg.obj = r.app;
msg.getData().putCharSequence(
ActivityManagerService.SERVICE_RECORD_KEY, r.toString());
mAm.mHandler.sendMessage(msg);
}
}
如果调用了startForegroundService，但是没有调用startForGround，此时调用stopService时，r.fgRequired = true，那么bringDownServiceLocked就会直接移除ActivityManagerService.SERVICE_FOREGROUND_TIMEOUT_MSG消息，并抛出ActivityManagerService.SERVICE_FOREGROUND_CRASH_MSG异常，其实只要在onCreate中startForeground就行了。
总结
- startService抛异常不是看调用的APP处于何种状态，而是看Servic所在APP处于何种状态，因为看的是UID的状态，所以这里重要的是APP而不仅仅是进程状态
- 不要通过Handler延迟太久再startService，否则可能会有问题
- 应用进入后台，60s之后就会变成idle状态，无法start其中的Service，但是可以通过startForegroundService来启动
- Application里面不要startService，否则恢复的时候可能有问题
- startForGround 要及时配合startForegroundService，否则会有各种异常
原文链接：https://www.jianshu.com/p/f2db0f58d47f
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