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) {
                if (cn.getPackageName().equals("!")) {
                    throw new SecurityException(
                            "Not allowed to start service " + service
                            + " without permission " + cn.getClassName());
                } else if (cn.getPackageName().equals("!!")) {
                    throw new SecurityException(
                            "Unable to start service " + service
                            + ": " + cn.getClassName());
                 //此处就是曝出异常的地方，非法状态，不允许启动服务
                } else if (cn.getPackageName().equals("?")) {
                    throw new IllegalStateException(
                            "Not allowed to start service " + service + ": " + cn.getClassName());
                }
            }
            return cn;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

@Override
public ComponentName startService(IApplicationThread caller, Intent service,
            String resolvedType, boolean requireForeground, String callingPackage, int userId)
           throws TransactionTooLargeException {
        enforceNotIsolatedCaller("startService");
        // Refuse possible leaked file descriptors
       if (service != null && service.hasFileDescriptors() == true) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
       }

        if (callingPackage == null) {
            throw new IllegalArgumentException("callingPackage cannot be null");
       }

        if (DEBUG_SERVICE) Slog.v(TAG_SERVICE,
                "*** startService: " + service + " type=" + resolvedType + " fg=" + requireForeground);
       synchronized(this) {
            final int callingPid = Binder.getCallingPid();
            final int callingUid = Binder.getCallingUid();
            final long origId = Binder.clearCallingIdentity();
            ComponentName res;
            try {
            //调用ActiveServices的startServiceLocked
                res = mServices.startServiceLocked(caller, service,
                        resolvedType, callingPid, callingUid,
                        requireForeground, callingPackage, userId);
            } finally {
                Binder.restoreCallingIdentity(origId);
            }
            return res;
        }
    }

    ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
            int callingPid, int callingUid, boolean fgRequired, String callingPackage, final int userId)
            throws TransactionTooLargeException {
        //...
        // 启动服务之前有2个判断一个是startRequested，一个是fgRequired。
        // startRequested代表的是：是否已经启动过服务，一般出现问题都是启动一个没有运行的服务，
        // 那么这个就是false。
        // fgRequired这个就是启动服务传递的requireForeground，
        if (!r.startRequested && !fgRequired) {
            // 这里面有个关键函数getAppStartModeLocked，判断是否运行启动服务
            // 注意此处传递的最后2个参数：alwaysRestrict和disabledOnly都是false
            final int allowed = mAm.getAppStartModeLocked(r.appInfo.uid, r.packageName,
                    r.appInfo.targetSdkVersion, callingPid, false, false);
            // 如果不允许启动服务则会运行到里面
            if (allowed != ActivityManager.APP_START_MODE_NORMAL) {
                //...
                UidRecord uidRec = mAm.mActiveUids.get(r.appInfo.uid);
                // 此处就是不允许运行服务返回的原因"app is in background"
                // 找到了原因就在这里
                return new ComponentName("?", "app is in background uid " + uidRec);
            }
        }
        //...
    }

int getAppStartModeLocked(int uid, String packageName, int packageTargetSdk,
            int callingPid, boolean alwaysRestrict, boolean disabledOnly) {
        UidRecord uidRec = mActiveUids.get(uid);
        if (DEBUG_BACKGROUND_CHECK) Slog.d(TAG, "checkAllowBackground: uid=" + uid + " pkg="
                + packageName + " rec=" + uidRec + " always=" + alwaysRestrict + " idle="
                + (uidRec != null ? uidRec.idle : false));
        if (uidRec == null || alwaysRestrict || uidRec.idle) {
            boolean ephemeral;
            if (uidRec == null) {
                ephemeral = getPackageManagerInternalLocked().isPackageEphemeral(
                        UserHandle.getUserId(uid), packageName);
            } else {
                ephemeral = uidRec.ephemeral;
            }

            if (ephemeral) {
                // We are hard-core about ephemeral apps not running in the background.
                return ActivityManager.APP_START_MODE_DISABLED;
            } else {
                if (disabledOnly) {
                    // The caller is only interested in whether app starts are completely
                    // disabled for the given package (that is, it is an instant app).  So
                    // we don‘t need to go further, which is all just seeing if we should
                    // apply a "delayed" mode for a regular app.
                    return ActivityManager.APP_START_MODE_NORMAL;
                }
　　　　　　　　　 // 此处alwaysRestrict==false，于是调用的是appServicesRestrictedInBackgroundLocked
                final int startMode = (alwaysRestrict)
                        ? appRestrictedInBackgroundLocked(uid, packageName, packageTargetSdk)
                        : appServicesRestrictedInBackgroundLocked(uid, packageName,
                                packageTargetSdk);
                if (DEBUG_BACKGROUND_CHECK) Slog.d(TAG, "checkAllowBackground: uid=" + uid
                        + " pkg=" + packageName + " startMode=" + startMode
                        + " onwhitelist=" + isOnDeviceIdleWhitelistLocked(uid));
                if (startMode == ActivityManager.APP_START_MODE_DELAYED) {
                    // This is an old app that has been forced into a "compatible as possible"
                    // mode of background check.  To increase compatibility, we will allow other
                    // foreground apps to cause its services to start.
                    if (callingPid >= 0) {
                        ProcessRecord proc;
                        synchronized (mPidsSelfLocked) {
                            proc = mPidsSelfLocked.get(callingPid);
                        }
                        if (proc != null &&
                                !ActivityManager.isProcStateBackground(proc.curProcState)) {
                            // Whoever is instigating this is in the foreground, so we will allow it
                            // to go through.
                            return ActivityManager.APP_START_MODE_NORMAL;
                        }
                    }
                }
                return startMode;
            }
        }
        return ActivityManager.APP_START_MODE_NORMAL;
    }