【 Android 10 四大组件 】系列 -- Service 的 " 启动流程 "

前言

由于源码分析的代码量比较大，大部分博客网站的内容显示页面都比较窄，显示出来的效果都异常丑陋，所以您也可以直接查看 《 Thinking in Android 》 来阅读这边文章，希望这篇文章能帮你梳理清楚 “Service 的启动流程”。

核心源码

关键类	路径
ContextImpl.java	frameworks/base/core/java/android/app/ContextImpl.java
ContextWrapper.java	frameworks/base/core/java/android/content/ContextWrapper.java
ContextThemeWrapper.java	frameworks/base/core/java/android/view/ContextThemeWrapper.java
Activity.java	frameworks/base/core/java/android/app/Activity.java
ActivityManager.java	frameworks/base/core/java/android/app/ActivityManager.java
ActivityManagerService.java	frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
ActiveServices.java	frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
ActivityThread.java	frameworks/base/core/java/android/app/ActivityThread.java

Service 的启动过程相对 Activity 的启动过程来说简单了很多，我们都知道怎么去创建和启动一个 Service，那么你有没有从源码角度研究过 Service 启动后在系统层是如何运作的？

Activity.startService()

首先我们知道：要启动一个 Service 的时候，一般都是在 Activity 中通过 startService() 来启动：

Intent intent = new Intent(MainActivity.this, xxxService.class);
startService(intent);

但是我们在 Activity 源码中并没有实现 startService() 方法，那它在哪里被调用的？找不到我们就去 Activity 的父类中找。

// frameworks/base/core/java/android/app/Activity.java

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback, WindowControllerCallback,
        AutofillManager.AutofillClient, ContentCaptureManager.ContentCaptureClient {
        
}

// frameworks/base/core/java/android/view/ContextThemeWrapper.java

public class ContextThemeWrapper extends ContextWrapper {

}

// frameworks/base/core/java/android/content/ContextWrapper.java
public class ContextWrapper extends Context {

}

Activity 继承了 ContextThemeWrapper 类，ContextThemeWrapper 又继承了 ContextWrapper 类，ContextWrapper 又继承了 Context 类。

ContextWrapper.startService()

在 ContextWrapper 中实现了 startService() 方法：

// frameworks/base/core/java/android/content/ContextWrapper.java

public class ContextWrapper extends Context {

    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }

    @Override
    public ComponentName startService(Intent service) {
        return mBase.startService(service);
    }

}

所以 startService() 方法其实是由 ContextWrapper 实现的，紧接着又调用了 mBase.startService() 方法，mBase 对象是 Context 的子类 ContextImpl，所以调用最终进入 ContextImpl 类的 startService() 方法。

ContextImpl.startService()

// frameworks/base/core/java/android/app/ContextImpl.java

class ContextImpl extends Context {

    @Override
    public ComponentName startService(Intent service) {
        warnIfCallingFromSystemProcess();
        return startServiceCommon(service, false, mUser);
    }

    private ComponentName startServiceCommon(Intent service, boolean requireForeground, UserHandle user) {
        try {
            validateServiceIntent(service);
            service.prepareToLeaveProcess(this);
            // 通过 Binder 进程间通信进入 ActivityManagerService 的 startService() 方法
            ComponentName cn = ActivityManager.getService().startService(
                mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
                    getContentResolver()), requireForeground, getOpPackageName(), user.getIdentifier());
            ... ...

            return cn;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

}

ActivityManager.getService()

// frameworks/base/core/java/android/app/ActivityManager.java

public class ActivityManager {

    @UnsupportedAppUsage
    public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

    @UnsupportedAppUsage
    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

}

ActivityManagerService.startService()

接下来就执行到 ActivityManagerService 的 startService() 方法：

// frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

    final ActiveServices mServices;

    public ComponentName startService(IApplicationThread caller, Intent service,
            String resolvedType, boolean requireForeground, String callingPackage, int userId)
            throws TransactionTooLargeException {
        ... ...

        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() 方法
                res = mServices.startServiceLocked(caller, service, resolvedType, callingPid,
                            callingUid, requireForeground, callingPackage, userId);
            } finally {
                Binder.restoreCallingIdentity(origId);
            }
            return res;
        }
    }

}

ActiveServices.startServiceLocked()

// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

public final class ActiveServices {

    ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
            int callingPid, int callingUid, boolean fgRequired, String callingPackage, final int userId)
            throws TransactionTooLargeException {
        return startServiceLocked(caller, service, resolvedType, callingPid, callingUid, fgRequired,
                callingPackage, userId, false);
    }

    ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
            int callingPid, int callingUid, boolean fgRequired, String callingPackage,
            final int userId, boolean allowBackgroundActivityStarts)
            throws TransactionTooLargeException {
        ... ...
        
        // 解析 AndroidManifest.xml 文件中配置 Service 的 intent-filter 相关内容信息
        ServiceLookupResult res = retrieveServiceLocked(service, null, resolvedType, callingPackage,
                    callingPid, callingUid, userId, true, callerFg, false, false);
        if (res == null) {
            return null;
        }
        ... ...

        ServiceRecord r = res.record;
        ... ...

        // 调用 startServiceInnerLocked() 方法
        ComponentName cmp = startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
        return cmp;
    }

}

通过 retrieveServiceLocked() 方法来解析 AndroidManifest.xml 文件中配置 Service 的 intent-filter 相关内容信息。

当解析完 Service 的 intent-filter 相关内容信息后，解析的结果会保存在 res.record 变量中。而 res 变量是一个 ServiceLookupResult 类型的对象，它的 record 变量则是一个 ServiceRecord 类型对象，用来表示一个 Service。

ActiveServices.startServiceInnerLocked()

ActiveServices 的 startServiceLocked() 方法最后调用了 startServiceInnerLocked() 方法：

// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

public final class ActiveServices {

    ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
            boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
        ... ...

        // 调用 bringUpServiceLocked() 方法
        String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
        ... ...

    }
}

ActiveServices.bringUpServiceLocked()

调用 bringUpServiceLocked() 方法进一步处理：

// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

public final class ActiveServices {

    private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
            boolean whileRestarting, boolean permissionsReviewRequired)
            throws TransactionTooLargeException {
        ... ...

        ProcessRecord app;

        if (!isolated) {
            app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);    // 获取 app 对象
            if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid + " app=" + app);
            if (app != null && app.thread != null) {
                try {
                    app.addPackage(r.appInfo.packageName, r.appInfo.longVersionCode, mAm.mProcessStats);
                    // 运行 service 的进程已经启动，调用 realStartServiceLocked() 方法
                    realStartServiceLocked(r, app, execInFg);
                    return null;
                } catch (TransactionTooLargeException e) {
                    throw e;
                } catch (RemoteException e) {
                    Slog.w(TAG, "Exception when starting service " + r.shortInstanceName, e);
                }

                // If a dead object exception was thrown -- fall through to
                // restart the application.
            }
        } else {
            ... ...

        }

        // Not running -- get it started, and enqueue this service record
        // to be executed when the app comes up.
        if (app == null && !permissionsReviewRequired) {
            if ((app = mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
                    hostingRecord, false, isolated, false)) == null) {
                String msg = "Unable to launch app "
                        + r.appInfo.packageName + "/"
                        + r.appInfo.uid + " for service "
                        + r.intent.getIntent() + ": process is bad";
                Slog.w(TAG, msg);
                bringDownServiceLocked(r);
                return msg;
            }
            if (isolated) {
                r.isolatedProc = app;
            }
        }
        ... ...

        return null;
    }

}

bringUpServiceLocked() 方法首先通过 getProcessRecordLocked() 方法去获取 app 对象，它是一个 ProcessRecord 类型的对象，如果它不为空，说明 Service 要运行的进程已经存在。

Service 运行的进程有两种：

（1）一种是默认的，即运行在 Activity 启动 Service 的那个进程里，也就是说在哪个进程里调用了 startService() 方法，启动的 service 就运行在哪个进程里。

（2）一种是给 Service 一个单独的进程运行，比如在 AndroidManifest 文件里配置了如下内容：

//

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="com.android.documentsui">

    <application
        android:name=".DocumentsApplication"
        android:label="@string/app_label"
        android:icon="@drawable/app_icon"
        android:supportsRtl="true"
        android:allowBackup="true"
        android:backupAgent=".prefs.BackupAgent"
        android:fullBackupOnly="false">

        <!-- Run FileOperationService in a separate process so that we can use FileLock class to
            wait until jumbo clip is done writing to disk before reading it. See ClipStorage for
            details. -->
        <service
            android:name=".services.FileOperationService"
            android:exported="false"
            android:process=":com.android.documentsui.services">
        </service>

    </application>
</manifest>

在这段配置里有 android:process="xxx" 声明，这个声明用来实现 service 单独运行在 "xxx" 进程里。这样做的好处是，即使应用程序无法工作，由于 service 单独运行在一个进程里，所以会继续工作。

回到前面的方法，如果变量 app 为空，就代表 service 要运行的进程还没有启动，于是调用 startProcessLocked() 方法去启动一个新的进程。

如果 app 不为空，采用默认的方式启动 Service，最终调用到 realStartServiceLocked() 方法：

ActiveServices.realStartServiceLocked()

// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

public final class ActiveServices {

    private final void realStartServiceLocked(ServiceRecord r,
            ProcessRecord app, boolean execInFg) throws RemoteException {
        ... ...

        try {
            ... ...

            app.thread.scheduleCreateService(r, r.serviceInfo,
                    mAm.compatibilityInfoForPackage(r.serviceInfo.applicationInfo), app.getReportedProcState());
            ... ...

        }
        ... ...

    }

}

在这个方法中，app 对象的 thread 变量是一个 ApplicationThread Binder 对象，调用它的 scheduleCreateService() 方法之后，会进入客户端的 ActivityThread 中。

ActivityThread.scheduleCreateService()

// frameworks/base/core/java/android/app/ActivityThread.java

public final class ActivityThread extends ClientTransactionHandler {

    // ApplicationThread 是一个 Binder
    private class ApplicationThread extends IApplicationThread.Stub {

        public final void scheduleCreateService(IBinder token,
                ServiceInfo info, CompatibilityInfo compatInfo, int processState) {
            updateProcessState(processState, false);
            CreateServiceData s = new CreateServiceData();
            s.token = token;
            s.info = info;
            s.compatInfo = compatInfo;

            sendMessage(H.CREATE_SERVICE, s);
        }

    }
    
}

ApplicationThread 的 scheduleCreateService() 方法通过调用 sendMessage() 方法来发送一个 msg 消息，当 Handler 接收到了 msg 消息之后，它会调用 handleCreateService() 方法来做进一步处理。

H.handleMessage()

// frameworks/base/core/java/android/app/ActivityThread.java

public final class ActivityThread extends ClientTransactionHandler {

    class H extends Handler {

        public void handleMessage(Message msg) {
            switch (msg.what) {

                case CREATE_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
                    handleCreateService((CreateServiceData)msg.obj);    // 调用 handleCreateService() 方法
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;

            }
        }

    }
}

ActivityThread.handleCreateService()

// frameworks/base/core/java/android/app/ActivityThread.java

public final class ActivityThread extends ClientTransactionHandler {

    private void handleCreateService(CreateServiceData data) {
        ... ...

        LoadedApk packageInfo = getPackageInfoNoCheck(data.info.applicationInfo, data.compatInfo);
        Service service = null;
        try {
            java.lang.ClassLoader cl = packageInfo.getClassLoader();
            service = packageInfo.getAppFactory().instantiateService(cl, data.info.name, data.intent);
        } catch (Exception e) {
            if (!mInstrumentation.onException(service, e)) {
                throw new RuntimeException(
                    "Unable to instantiate service " + data.info.name
                    + ": " + e.toString(), e);
            }
        }

        try {
            if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);

            ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
            context.setOuterContext(service);

            Application app = packageInfo.makeApplication(false, mInstrumentation);
            service.attach(context, this, data.info.name, data.token, app, ActivityManager.getService());
            service.onCreate();    // 进入 Service.onCreate() 方法
            mServices.put(data.token, service);
            try {
                ActivityManager.getService().serviceDoneExecuting(
                        data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(service, e)) {
                throw new RuntimeException(
                    "Unable to create service " + data.info.name
                    + ": " + e.toString(), e);
            }
        }
    }

}

在 ActivityThread 类的 handleCreateService() 方法中，首先通过 ClassLoader 类把 Service 加载进来，而参数 data.info.name 表示这个 Service 的名字，instantiateService() 方法是创建一个 Service 实例。接着，创建一个 Context 对象，作为上下文环境之用。

handleCreateService() 方法最后调用了 service 的 onCreate() 方法，当这个方法被调用之后，就会进入应用程序里 Service 的 onCreate() 方法。

至此，Service 的启动就分析完毕，这个过程与启动 Activity 相比简单了很多。