如何实现一个 System Services？
Posted 吴小龙同學
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《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：
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.wuxiaolo以上是关于如何实现一个 System Services？的主要内容，如果未能解决你的问题，请参考以下文章
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