如何实现一个 System Services？

《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

 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

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

@Override

public String getSystemServiceName(Class<?> serviceClass) {

    return SystemServiceRegistry.getSystemServiceName(serviceClass);

}

继续跟 SystemServiceRegistry.getSystemServiceName。

SystemServiceRegistry#getSystemServiceName

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？

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

public void setTemporaryBrightness(float brightness) {

    mGlobal.setTemporaryBrightness(brightness);

}

mGlobal 是 DisplayManagerGlobal 对象。

DisplayManagerGlobal#setTemporaryBrightness

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。

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。

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：

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：

@startuml

title \n如何实现一个 System Services？\n

skinparam backgroundColor #EEEBDC

skinparam handwritten true

hide empty description

state 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.aidl

2.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.java

4.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 note

5.编写Manager类 --> 6.注册Manager

note right of 5.编写Manager类

  1.写成单例

  2.@Nullable注解

end note

5.编写Manager类:WuXiaolongManager.java

6.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:

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

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

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

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



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

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

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：

******************************

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

diff --git a/api/current.txt b/api/current.txt

index 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

diff --git a/non-updatable-api/current.txt b/non-updatable-api/current.txt

index 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：

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

编写 Manager 类需写成单例。

报错 3：

Missing nullability on method `getName` return [MissingNullability]

getName 方法加上@Nullable注解。

运行报错

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=1

04-08 15:41:38.802 12717 12758 E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: PowerManagerService

04-08 15:41:38.802 12717 12758 E AndroidRuntime: java.lang.IllegalStateException: android.os.ServiceManager$ServiceNotFoundException: No service published for: wuxiaolong

04-08 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: wuxiaolong

04-08 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：

adb shell

setenforce 0 （临时禁用掉SELinux）

getenforce  （得到结果为Permissive）

临时禁用掉 SELinux，功能就正常了，关于 SELinux 这里不说了，后面有机会写篇 SELinux 文章。

最后 Log 打印如下：

Line 832: 04-08 16:08:55.290 17649 17690 D wxl     : SystemServiceRegistry registerService

Line 833: 04-08 16:08:55.290 17649 17690 D wxl     : WuXiaolongManager getInstance

Line 835: 04-08 16:08:55.292 17649 17690 D wxl     : WuXiaolongManager getName

Line 836: 04-08 16:08:55.293 17649 17690 D wxl     : Name=WuXiaolong..

手写个 System Service 实践过后没那么简单，光 SELinux 权限够折腾半天了，这篇文章先就酱紫吧。

巴特西