如何实现一个 System Services？

吴小龙同學

发布于 2021-12-20 10:06:29

9920

发布于 2021-12-20 10:06:29

《Android 系统开发做什么？》写到 Android System Services 是专注于特定功能的模块化组件，应用框架 API 所提供的功能可与系统服务通信，以访问底层硬件。Android System Services 是如何写的？来以DisplayManagerService 为例，具体来看看。

System Service 是如何写的？

应用调用

123	DisplayManager dm = getSystemService(DisplayManager.class);dm.setTemporaryBrightness(0.0f);Settings.System.putInt(getContentResolver(), Settings.System.SCREEN_BRIGHTNESS, 0);

看下 getSystemService 方法，在 Context 类里。

Context#getSystemService

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public final @Nullable <T> T getSystemService(@NonNull Class<T> serviceClass) { // Because subclasses may override getSystemService(String) we cannot // perform a lookup by class alone. We must first map the class to its // service name then invoke the string-based method. String serviceName = getSystemServiceName(serviceClass); return serviceName != null ? (T)getSystemService(serviceName) : null;}public abstract @Nullable String getSystemServiceName(@NonNull Class<?> serviceClass);

ContextImpl#getSystemService

1234	@Overridepublic String getSystemServiceName(Class<?> serviceClass) { return SystemServiceRegistry.getSystemServiceName(serviceClass);}

继续跟 SystemServiceRegistry.getSystemServiceName。

SystemServiceRegistry#getSystemServiceName

1234567891011	public static String getSystemServiceName(Class<?> serviceClass) { if (serviceClass == null) { return null; } final String serviceName = SYSTEM_SERVICE_NAMES.get(serviceClass); if (sEnableServiceNotFoundWtf && serviceName == null) { // This should be a caller bug. Slog.wtf(TAG, "Unknown manager requested: " + serviceClass.getCanonicalName()); } return serviceName;}

什么时候 registerService 的？

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public final class SystemServiceRegistry { static { registerService(Context.DISPLAY_SERVICE, DisplayManager.class, new CachedServiceFetcher<DisplayManager>() { @Override public DisplayManager createService(ContextImpl ctx) { return new DisplayManager(ctx.getOuterContext()); } }); }}private static <T> void registerService(@NonNull String serviceName, @NonNull Class<T> serviceClass, @NonNull ServiceFetcher<T> serviceFetcher) { SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName); SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher); SYSTEM_SERVICE_CLASS_NAMES.put(serviceName, serviceClass.getSimpleName());}

结合上面的分析代码可以知道 getSystemService(DisplayManager.class)得到的是一个 DisplayManager 的实例。

接下来看 dm.setTemporaryBrightness 方法。

DisplayManager#setTemporaryBrightness

123	public void setTemporaryBrightness(float brightness) { mGlobal.setTemporaryBrightness(brightness);}

mGlobal 是 DisplayManagerGlobal 对象。

DisplayManagerGlobal#setTemporaryBrightness

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private final IDisplayManager mDm;private DisplayManagerGlobal(IDisplayManager dm) { mDm = dm;}public static DisplayManagerGlobal getInstance() { synchronized (DisplayManagerGlobal.class) { if (sInstance == null) { IBinder b = ServiceManager.getService(Context.DISPLAY_SERVICE); if (b != null) { sInstance = new DisplayManagerGlobal(IDisplayManager.Stub.asInterface(b)); } } return sInstance; }}public void setTemporaryBrightness(float brightness) { try { mDm.setTemporaryBrightness(brightness); } catch (RemoteException ex) { throw ex.rethrowFromSystemServer(); }}

mDm 是 IDisplayManager 对象，初始化在IDisplayManager.Stub.asInterface(ServiceManager.getService(Context.DISPLAY_SERVICE))，看到 IDisplayManager 是一个 aidl 文件：frameworks/base/core/java/android/hardware/display/IDisplayManager.aidl，AIDL (Android Interface Definition Language) 是 Android 中的接口定义文件，为系统提供了一种简单跨进程通信方法，先不管 AIDL。

IDisplayManager

IDisplayManager 定义了包括 setTemporaryBrightness 的几个接口。

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interface IDisplayManager { //…… void registerCallback(in IDisplayManagerCallback callback); // Requires CONFIGURE_WIFI_DISPLAY permission. // The process must have previously registered a callback. void startWifiDisplayScan(); // Requires CONFIGURE_WIFI_DISPLAY permission. void stopWifiDisplayScan(); // Requires CONFIGURE_WIFI_DISPLAY permission. void connectWifiDisplay(String address); // No permissions required. void disconnectWifiDisplay(); // Temporarily sets the display brightness. void setTemporaryBrightness(float brightness); //……}

IDisplayManager 只是接口，需要找下哪里实现了它，搜索是在 BinderService，BinderService 是 DisplayManagerService 内部类。

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final class BinderService extends IDisplayManager.Stub { @Override // Binder call public void setTemporaryBrightness(float brightness) { mContext.enforceCallingOrSelfPermission( Manifest.permission.CONTROL_DISPLAY_BRIGHTNESS, "Permission required to set the display's brightness"); final long token = Binder.clearCallingIdentity(); try { synchronized (mSyncRoot) { mDisplayPowerController.setTemporaryBrightness(brightness); } } finally { Binder.restoreCallingIdentity(token); } }}

mDisplayPowerController.setTemporaryBrightness(brightness)后面经过一系列调用会到 LightsService#setLight_native，通过 JNI 调用到 native 层，调用底层进行背光调节，关于背光调节后面文章再细讲。

SystemServer

DisplayManagerService 是继承了 SystemService，DisplayManagerService 是怎么注册为系统服务的呢？在 SystemServer 里面：

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private void startBootstrapServices(@NonNull TimingsTraceAndSlog t) { t.traceBegin("StartDisplayManager"); //开启DisplayManagerService mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class); t.traceEnd();}private void startOtherServices(@NonNull TimingsTraceAndSlog t) { //通知服务系统启动完成 t.traceBegin("MakeDisplayManagerServiceReady"); try { // TODO: use boot phase and communicate these flags some other way mDisplayManagerService.systemReady(safeMode, mOnlyCore); } catch (Throwable e) { reportWtf("making Display Manager Service ready", e); } t.traceEnd();}

看完 DisplayManagerService 是怎么写的，不妨模仿写个。所谓看着代码，感觉还是挺简单的，实际操作起来，各种编译报错……

如何写个 System Service

先上图：

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@startumltitle \n如何实现一个 System Services？\nskinparam backgroundColor #EEEBDCskinparam handwritten truehide empty descriptionstate 1.编写AIDL文件1.编写AIDL文件 --> 2.Context定义变量1.编写AIDL文件:IWuXiaolongManager.aidl2.Context定义变量 --> 3.编写系统服务类2.Context定义变量:String WUXIAOLONG_SERVICE = "wuxiaolong"note left of 2.Context定义变量 : 执行make update-api，\n更新接口3.编写系统服务类 --> 4.注册系统服务类3.编写系统服务类:WuXiaolongManagerService.java4.注册系统服务类--> 5.编写Manager类note right of 4.注册系统服务类涉及SELinux权限end note5.编写Manager类 --> 6.注册Managernote right of 5.编写Manager类 1.写成单例 2.@Nullable注解end note5.编写Manager类:WuXiaolongManager.java6.注册Manager --> 7.应用调用@enduml

1.编写 AIDL 文件

新建 frameworks/base/core/java/android/hardware/wuxiaolong/IWuXiaolongManager.aidl，内容如下:

123456	package android.hardware.wuxiaolong;/** @hide */interface IWuXiaolongManager { String getName();}

2.Context 定义变量

在 Context 里定义一个代表 wuxiaolong 服务的字符串 frameworks/base/core/java/android/content/Context.java

1	public static final String WUXIAOLONG_SERVICE = "wuxiaolong";

3.编写系统服务

frameworks/base/services/core/java/com/android/server/wuxiaolong/WuXiaolongManagerService.java

12345678910111213141516171819	package com.android.server.wuxiaolong;import android.content.Context;import android.hardware.wuxiaolong.IWuXiaolongManager;public class WuXiaolongManagerService extends IWuXiaolongManager.Stub { private final Context mContext; public WuXiaolongManagerService(Context context) { super(); mContext = context; } @Override public String getName() { String name = "WuXiaolong.."; return name; }}

4.注册系统服务

frameworks/base/services/java/com/android/server/SystemServer.java

1234567891011	import com.android.server.wuxiaolong.WuXiaolongManagerService;private void startOtherServices() { // 部分代码省略... try { android.util.Log.d("wxl","SystemServer WuXiaolongManagerService"); ServiceManager.addService(Context.WUXIAOLONG_SERVICE, new WuXiaolongManagerService(context)); } catch (Throwable e) { reportWtf("starting WuXiaolongManagerService", e); } // 部分代码省略...}

5.编写 Manager 类

frameworks/base/core/java/android/hardware/wuxiaolong/WuXiaolongManager.java

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package android.hardware.wuxiaolong;import android.os.IBinder;import android.os.ServiceManager;import android.hardware.wuxiaolong.IWuXiaolongManager;import android.content.Context;import android.os.RemoteException;import android.compat.annotation.UnsupportedAppUsage;import android.annotation.Nullable;import android.os.ServiceManager.ServiceNotFoundException;import android.annotation.SystemService;@SystemService(Context.WUXIAOLONG_SERVICE)public class WuXiaolongManager { private static WuXiaolongManager sInstance; private final IWuXiaolongManager mService; private Context mContext; /** * @hide */ public WuXiaolongManager(IWuXiaolongManager iWuXiaolongManager) { mService = iWuXiaolongManager; } /** * Gets an instance of the WuXiaolong manager. * * @return The WuXiaolong manager instance. * @hide */ @UnsupportedAppUsage public static WuXiaolongManager getInstance() { android.util.Log.d("wxl", "WuXiaolongManager getInstance"); synchronized (WuXiaolongManager.class) { if (sInstance == null) { try { IBinder b = ServiceManager.getServiceOrThrow(Context.WUXIAOLONG_SERVICE); sInstance = new WuXiaolongManager(IWuXiaolongManager.Stub .asInterface(ServiceManager.getServiceOrThrow(Context.WUXIAOLONG_SERVICE))); } catch (ServiceNotFoundException e) { throw new IllegalStateException(e); } } return sInstance; } } @Nullable public String getName() { android.util.Log.d("wxl", "WuXiaolongManager getName"); try { return mService.getName(); } catch (RemoteException e) { throw e.rethrowFromSystemServer(); } }}

6.注册 Manager

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

1234567891011	import android.hardware.wuxiaolong.WuXiaolongManager;static { registerService(Context.WUXIAOLONG_SERVICE, WuXiaolongManager.class, new CachedServiceFetcher<WuXiaolongManager>() { @Override public WuXiaolongManager createService(ContextImpl ctx) throws ServiceNotFoundException { android.util.Log.d("wxl","SystemServiceRegistry registerService"); return WuXiaolongManager.getInstance(); }});}

7.应用调用

12	WuXiaolongManager mWuXiaolongManager = (WuXiaolongManager)mContext.getSystemService(Context.WUXIAOLONG_SERVICE);android.util.Log.d("wxl","Name="+ mWuXiaolongManager.getName());

8.解决报错

编译报错

报错 1：

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******************************You have tried to change the API from what has been previously approved.To make these errors go away, you have two choices: 1. You can add '@hide' javadoc comments (and remove @SystemApi/@TestApi/etc) to the new methods, etc. shown in the above diff. 2. You can update current.txt and/or removed.txt by executing the following command: make api-stubs-docs-non-updatable-update-current-api To submit the revised current.txt to the main Android repository, you will need approval.******************************

需要执行 make update-api，更新接口，会多出来：

frameworks/base/api/current.txt

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diff --git a/api/current.txt b/api/current.txtindex 6b1a96c..0779378 100755--- a/api/current.txt+++ b/api/current.txt@@ -10256,6 +10256,7 @@ package android.content { field public static final String WIFI_RTT_RANGING_SERVICE = "wifirtt"; field public static final String WIFI_SERVICE = "wifi"; field public static final String WINDOW_SERVICE = "window";+ field public static final String WUXIAOLONG_SERVICE = "wuxiaolong"; } public class ContextWrapper extends android.content.Context {@@ -18318,6 +18319,14 @@ package android.hardware.usb { }+package android.hardware.wuxiaolong {++ public class WuXiaolongManager {+ method @Nullable public String getName();+ }++}+ package android.icu.lang {

frameworks/base/non-updatable-api/current.txt

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diff --git a/non-updatable-api/current.txt b/non-updatable-api/current.txtindex adf1bb5..e738c02 100755--- a/non-updatable-api/current.txt+++ b/non-updatable-api/current.txt@@ -10256,6 +10256,7 @@ package android.content { field public static final String WIFI_RTT_RANGING_SERVICE = "wifirtt"; field public static final String WIFI_SERVICE = "wifi"; field public static final String WINDOW_SERVICE = "window";+ field public static final String WUXIAOLONG_SERVICE = "wuxiaolong"; } public class ContextWrapper extends android.content.Context {@@ -18318,6 +18319,14 @@ package android.hardware.usb { }+package android.hardware.wuxiaolong {++ public class WuXiaolongManager {+ method @Nullable public String getName();+ }++}+ package android.icu.lang {

报错 2：

1	[0mManagers must always be obtained from Context; no direct constructors [ManagerConstructor]

编写 Manager 类需写成单例。

报错 3：

1	Missing nullability on method `getName` return [MissingNullability]

getName 方法加上@Nullable注解。

运行报错

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04-08 15:41:38.798 297 297 E SELinux : avc: denied { find } for pid=12717 uid=1000 name=wuxiaolong scontext=u:r:system_server:s0 tcontext=u:object_r:default_android_service:s0 tclass=service_manager permissive=104-08 15:41:38.802 12717 12758 E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: PowerManagerService04-08 15:41:38.802 12717 12758 E AndroidRuntime: java.lang.IllegalStateException: android.os.ServiceManager$ServiceNotFoundException: No service published for: wuxiaolong04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.hardware.wuxiaolong.WuXiaolongManager.getInstance(WuXiaolongManager.java:47)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.app.SystemServiceRegistry$27.createService(SystemServiceRegistry.java:497)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.app.SystemServiceRegistry$27.createService(SystemServiceRegistry.java:493)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.app.SystemServiceRegistry$CachedServiceFetcher.getService(SystemServiceRegistry.java:1760)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.app.SystemServiceRegistry.getSystemService(SystemServiceRegistry.java:1440)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.app.ContextImpl.getSystemService(ContextImpl.java:1921)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at com.android.server.display.DisplayPowerController.updatePowerState(DisplayPowerController.java:1191)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at com.android.server.display.DisplayPowerController.access$700(DisplayPowerController.java:92)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at com.android.server.display.DisplayPowerController$DisplayControllerHandler.handleMessage(DisplayPowerController.java:2074)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:106)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.os.Looper.loop(Looper.java:223)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.os.HandlerThread.run(HandlerThread.java:67)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at com.android.server.ServiceThread.run(ServiceThread.java:44)04-08 15:41:38.802 12717 12758 E AndroidRuntime: Caused by: android.os.ServiceManager$ServiceNotFoundException: No service published for: wuxiaolong04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.os.ServiceManager.getServiceOrThrow(ServiceManager.java:153)04-08 15:41:38.802 12717 12758 E AndroidRuntime: at android.hardware.wuxiaolong.WuXiaolongManager.getInstance(WuXiaolongManager.java:40)04-08 15:41:38.802 12717 12758 E AndroidRuntime: ... 12 more

这里是缺少 SELinux 权限，可执行：