插件化框架解读之android系统服务实现原理(五)

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一、系统服务提供方式

1、我们平时最常见的系统服务使用方式

 WifiManager wifimanager = (WifiManager) getSystemService(Context.WIFI_SERVICE);
 wifimanager.disconnect();

这里我们通过Context的getSystemService(String name)方法,根据Context.WIFI_SERVICE标识获取到了WifiManager,然后调用disconnerct()这个api实现了下发wifi断开指令的功能,这里就是一个很经典的系统服务使用的例子了,getSystemService()通过我们传入的字符串标识返回我们需要的具体WifiManager实例,那么我们不妨从getSystemService()入手,看看google的工程师是如何做的。

2、getSystemService()方法

源码路径:xxx/framework/base/core/java/android/content/Context.java

 public abstract Object getSystemService(@ServiceName @NonNull String name);

Context中的getSystemService()方法是一个抽象方法,意料之中,他的具体实现是在ContextImpl中:
源码路径:xxx/framework/base/core/java/android/app/ContextImpl.java

@Override
 public Object getSystemService(String name) {
     return SystemServiceRegistry.getSystemService(this, name);
 }

这里要注意,在5.0之前是在ContextImpl中实现的,而5.0之后就去调SystemServiceRegistry中的getSystemService()方法:

源码路径:xxx/framework/base/core/java/android/app/SystemServiceRegistry.java

/**
 * Gets a system service from a given context.
 */
 public static Object getSystemService(ContextImpl ctx, String name) {
     ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
     return fetcher != null ? fetcher.getService(ctx) : null;
 }

这里我们看到,通过我们传入的标识Context.WIFI_SERVICE从SYSTEM_SERVICE_FETCHERS中获取了ServiceFetcher对象,并通过ServiceFetcher的getService()方法返回我们所需WifiManager对象,那么SYSTEM_SERVICE_FETCHERS是什么? ServiceFetcher又是什么,我们看下下面的代码:
源码路径:xxx/framework/base/core/java/android/app/SystemServiceRegistry.java

 private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new HashMap<String, ServiceFetcher<?>>();
 
    /**
     * Base interface for classes that fetch services.
     * These objects must only be created during static initialization.
     */
    static abstract interface ServiceFetcher<T> {
        T getService(ContextImpl ctx);
    }
 
    /**
     * Override this class when the system service constructor needs a
     * ContextImpl and should be cached and retained by that context.
     */
    static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
        private final int mCacheIndex;
 
        public CachedServiceFetcher() {
            mCacheIndex = sServiceCacheSize++;
        }
 
        @Override
        @SuppressWarnings("unchecked")
        public final T getService(ContextImpl ctx) {
            final Object[] cache = ctx.mServiceCache;
            synchronized (cache) {
                // Fetch or create the service.
                Object service = cache[mCacheIndex];
                if (service == null) {
                    service = createService(ctx);
                    cache[mCacheIndex] = service;
                }
                return (T)service;
            }
        }
        public abstract T createService(ContextImpl ctx);
    }

看到SYSTEM_SERVICE_FETCHERS的定义我们发现它其实是一个HashMap,而ServiceFetcher是一个接口,而CachedServiceFetcher就是它的其中一个实现,我们看到当调用个getService()方法的时候,先去mServiceCache这个数组中查找,如果取不到再调用createService()这个抽象方法创建,至于createService()的具体实现,请往下看。。。

3、WifiManager如何的创建

既然SYSTEM_SERVICE_FETCHERS是一个Map我们不妨找找他是在什么地方被添加数据的:

源码路径:xxx/framework/base/core/java/android/app/SystemServiceRegistry.java

/**
 * Statically registers a system service with the context.
 * This method must be called during static initialization only.
 */
private static <T> void registerService(String serviceName, Class<T> serviceClass,
            ServiceFetcher<T> serviceFetcher) {
    SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
    SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}

我们发现SYSTEM_SERVICE_FETCHERS中的数据是通过registerService()方法添加的,继续查看SystemServiceRegistry.java文件你会发现好多类似下面的方法在静态代码块中被调用:

registerService(Context.WIFI_SERVICE, WifiManager.class,
                new CachedServiceFetcher<WifiManager>() {
    @Override
    public WifiManager createService(ContextImpl ctx) {
         IBinder b = ServiceManager.getService(Context.WIFI_SERVICE);
         IWifiManager service = IWifiManager.Stub.asInterface(b);
         return new WifiManager(ctx.getOuterContext(), service);
}});

在这段代码中,我们看到createService()这个抽象方法被实现,到目前为止,我们已经明确了系统是如何一步步的给我们提供WifiManager这个对象的了,至于WifiManager到底做了什么,我们继续往后看!

二、系统服务的创建以及管理

1、WifiManager

既然我们拿到手的最后是WifiManager,那么我们先看看WifiManager什么东西,他又是如何工作的,就以我们刚开始的时候调用的disconnect()方法为例:

public boolean disconnect() {
    try {
        mService.disconnect();
        return true;
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }
}

在代码中我们看到,程序最终是走了mService.disconnect();的,那么mService又是什么呢?

public WifiManager(Context context, IWifiManager service, Looper looper) {
    mContext = context;
    mService = service;
    mLooper = looper;
    mTargetSdkVersion = context.getApplicationInfo().targetSdkVersion;
}

到此我们看到其实mService是IWifiManager,搜索源代码发现,其实它是一个IWifiManager.aidl文件,既然是aidl那么就肯定有具体的实现,至于它的具体实现是什么我们现在肯定是不知道的,我们需要一步步分析mService的来源,最终确定其具体的实现类是什么,我们继续往下走。。。
源码路径:xxx/frameworks/base/wifi/java/android/net/wifi/IWifiManager.aidl

2、IWifiManager service = IWifiManager.Stub.asInterface(b)

既然IWifiManager是通过IWifiManager.Stub.asInterface(b)返回的,那么我们不妨分析分析这段代码;看到这句代码的时候,相信很多朋友都觉得很熟悉吧,根据AIDL的特殊性,正常情况下编译后会根据IWifiManager.aidl这个文件生成一个IWifiManager.java的接口,而这个接口中维护了Stub怎么一个内部类,我们在AS上创建一个AIDL编译生成一个Stub分析分析:

public interface IWifiManager extends android.os.IInterface {
    /**
     * Local-side IPC implementation stub class.
     */
    public static abstract class Stub extends android.os.Binder implements android.discovery
            .light.IWifiManager {
        private static final java.lang.String DESCRIPTOR = "android.discovery.light.IWifiManager";
 
        /**
         * Construct the stub at attach it to the interface.
         */
        public Stub() {
            this.attachInterface(this, DESCRIPTOR);
        }
 
        /**
         * Cast an IBinder object into an android.discovery.light.IWifiManager interface,
         * generating a proxy if needed.
         */
        public static android.discovery.light.IWifiManager asInterface(android.os.IBinder obj) {
            if ((obj == null)) {
                return null;
            }
            android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
            if (((iin != null) && (iin instanceof android.discovery.light.IWifiManager))) {
                return ((android.discovery.light.IWifiManager) iin);
            }
            return new android.discovery.light.IWifiManager.Stub.Proxy(obj);
        }
       //此处省略N行代码============================================
}

在代码中我们可以看到asInterface(android.os.IBinder obj)通过obj.queryLocalInterface(DESCRIPTOR)返回了一个IInterface,而在Binder中queryLocalInterface()方法的实现如下:

 public IInterface queryLocalInterface(String descriptor) {
    if (mDescriptor.equals(descriptor)) {
         return mOwner;
    }
    return null;
}

这里首先对传入的descriptor进行判断,然后返回mOwner对象,他们在Binder中的定义以及赋值如下:

private IInterface mOwner;
private String mDescriptor;
public void attachInterface(IInterface owner, String descriptor) {
   mOwner = owner;
   mDescriptor = descriptor;
}

看到attachInterface()方法朋友们应该想起点什么了吧?没想其没关系,看看下面的代码:

private static final java.lang.String DESCRIPTOR = "android.discovery.light.IMyAidlInterface";
 
public Stub() {
      this.attachInterface(this, DESCRIPTOR);
}

这里我们看到其实当我们创建Stub实例的时候,通过构造方法把this以及包名为保存到mOwner,mDescriptor两个属性中,当我们调用asInterface(IBinder obj)方法的时候,根据mDescriptor标识进行判断,如果匹配就返回mOwner,返回的其实就是obj本身,这种是最正常的情况了,我们暂时以这种情况为准!

好了,扯远了,回到我们之前的代码:

registerService(Context.WIFI_SERVICE, WifiManager.class,
                new CachedServiceFetcher<WifiManager>() {
    @Override
    public WifiManager createService(ContextImpl ctx) {
         IBinder b = ServiceManager.getService(Context.WIFI_SERVICE);
         IWifiManager service = IWifiManager.Stub.asInterface(b);
         return new WifiManager(ctx.getOuterContext(), service);
}});

按照我们上面的分析,正常情况下,其实IWifiManager.Stub.asInterface(b)返回的其实就是b本身,那么接下来我们要确定b这个IBinder对象是什么即可:

3、那么接下来我们分析ServiceManager.getService(Context.WIFI_SERVICE)

源码路径:xxx/frameworks/base/core/java/android/os/ServiceManager.java

/**
 * Returns a reference to a service with the given name.
 * 
 * @param name the name of the service to get
 * @return a reference to the service, or <code>null</code> if the service doesn't exist
 */
public static IBinder getService(String name) {
    try {
        IBinder service = sCache.get(name);
        if (service != null) {
            return service;
        } else {
            return getIServiceManager().getService(name);
        }
    } catch (RemoteException e) {
        Log.e(TAG, "error in getService", e);
    }
    return null;
}

我们看到程序首先通过sCache这个HasMap中去取,如果不存在就通过getIServiceManager().getService(name)去取,而getIServiceManager()返回的是一个IServiceManager接口,他的具体实现是在ServiceManagerNative#ServiceManagerProxy这个内部类中的,看下源码:
源码路径:xxx/frameworks/base/core/java/android/os/ServiceManagerNative#ServiceManagerProxy

 public IBinder getService(String name) throws RemoteException {
     Parcel data = Parcel.obtain();
     Parcel reply = Parcel.obtain();
     data.writeInterfaceToken(IServiceManager.descriptor);
     data.writeString(name);
     mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0);
     IBinder binder = reply.readStrongBinder();
     reply.recycle();
     data.recycle();
     return binder;
}

至此,我们就知道了ServiceManager.getService(Context.WIFI_SERVICE)中返回的IBinder是怎么来的了,这里的Parcel大家先不用关心,这个东西只不过是一个容器而已,我们只要知道这里是通过Context.WIFI_SERVICE这个字符串标识获取到了对应的IBinder对象即可,但是直到此处,我们依然没能知道这个IBinder对象到底是什么,这个时候别急,既然是容器,能取出来东西自然也要能存东西,取出来的时候不知道是什么,存进去的时候总该知道吧,继续往下走。。。

4、ServiceManager.addService()

细心的朋友应该已经发现了ServiceManager中还有另一个方法addService(),这个方法就是我们创建服务之后注册到系统去管理的方法,他的调用是在SystemServer.java中,那我们去SystemServer找找看。。。找了半天是否发现没有在SystemServer中找不到哪个是队Wifi服务进行注册的?别急,其实他是通过了WifiService这个东西进行注册的!

private static final String WIFI_SERVICE_CLASS = "com.android.server.wifi.WifiService";
 
    /**
     * Starts a miscellaneous grab bag of stuff that has yet to be refactored
     * and organized.
     */
    private void startOtherServices() {
               //省略很多。。。
               mSystemServiceManager.startService(WIFI_SERVICE_CLASS);
               //省略很多。。。
     }

这里我们看到 mSystemServiceManager.startService(WIFI_SERVICE_CLASS)怎么一句代码,查看其定义发现mSystemServiceManager是SystemServiceManager:
源码路径: xxx/frameworks/base/services/core/java/com/android/server/SystemServiceManager.java

 * Starts a service by class name.
 *
 * @return The service instance.
 */
 @SuppressWarnings("unchecked")
 public SystemService startService(String className) {
        final Class<SystemService> serviceClass;
        try {
            serviceClass = (Class<SystemService>)Class.forName(className);
        } catch (ClassNotFoundException ex) {
            Slog.i(TAG, "Starting " + className);
            throw new RuntimeException("Failed to create service " + className
                    + ": service class not found, usually indicates that the caller should "
                    + "have called PackageManager.hasSystemFeature() to check whether the "
                    + "feature is available on this device before trying to start the "
                    + "services that implement it", ex);
        }
        return startService(serviceClass);
}

这里我们看到,通过反射的方式返回了WifiService的Class对象,然后作为参数传给了startService(serviceClass):

 /**
     * Creates and starts a system service. The class must be a subclass of
     * {@link com.android.server.SystemService}.
     *
     * @param serviceClass A Java class that implements the SystemService interface.
     * @return The service instance, never null.
     * @throws RuntimeException if the service fails to start.
     */
    @SuppressWarnings("unchecked")
    public <T extends SystemService> T startService(Class<T> serviceClass) {
        final String name = serviceClass.getName();
        Slog.i(TAG, "Starting " + name);
 
        // Create the service.
        if (!SystemService.class.isAssignableFrom(serviceClass)) {
            throw new RuntimeException("Failed to create " + name
                    + ": service must extend " + SystemService.class.getName());
        }
        final T service;
        try {
            Constructor<T> constructor = serviceClass.getConstructor(Context.class);
            service = constructor.newInstance(mContext);
        } catch (InstantiationException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service could not be instantiated", ex);
        } catch (IllegalAccessException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service must have a public constructor with a Context argument", ex);
        } catch (NoSuchMethodException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service must have a public constructor with a Context argument", ex);
        } catch (InvocationTargetException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service constructor threw an exception", ex);
        }
 
        // Register it.
        mServices.add(service);
 
        // Start it.
        try {
            service.onStart();
        } catch (RuntimeException ex) {
            throw new RuntimeException("Failed to start service " + name
                    + ": onStart threw an exception", ex);
        }
        return service;
    }

我们发现,代码通过service = constructor.newInstance(mContext)方法创建了WifiService的实例,并且调用了service.onStart();方法,继续查看WifiService的源码:

源码路径:xxx/frameworks/opt/net/wifi/service/java/com/android/server/wifi/WifiService.java

package com.android.server.wifi;
import android.content.Context;
import android.net.wifi.ScanResult;
import android.net.wifi.WifiScanner;
import android.util.Log;
import com.android.server.SystemService;
import java.util.List;
 
public final class WifiService extends SystemService {
    private static final String TAG = "WifiService";
    final WifiServiceImpl mImpl;
 
    public WifiService(Context context) {
        super(context);
        mImpl = new WifiServiceImpl(context);
    }
 
    @Override
    public void onStart() {
        Log.i(TAG, "Registering " + Context.WIFI_SERVICE);
        publishBinderService(Context.WIFI_SERVICE, mImpl);
    }
 
    @Override
    public void onBootPhase(int phase) {
        if (phase == SystemService.PHASE_SYSTEM_SERVICES_READY) {
            mImpl.checkAndStartWifi();
        }
    }
}

这里,我们看到了在WifiService的构造方法中创建了WifiServiceImpl实例,并且通过onStart()方法,将WifiServiceImpl实例mImpl传入到 publishBinderService(Context.WIFI_SERVICE, mImpl)中,我们继续到SystemService看看:
源码路径:xxx/frameworks/base/services/core/java/com/android/server/SystemService.java

/**
 * Publish the service so it is accessible to other services and apps.
 */
protected final void publishBinderService(String name, IBinder service,
    boolean allowIsolated) {
     ServiceManager.addService(name, service, allowIsolated);
}

历尽千辛万苦,终于看到我们想要找的东西了,原来publishBinderService()方法通过ServiceManager.addService(name, service, allowIsolated)将WifiServiceImpl注册到了ServiceManager中,我们看下注册的方式:

源码路径:xxx/frameworks/base/core/java/android/os/ServiceManagerNative#ServiceManagerProxy

public void addService(String name, IBinder service, boolean allowIsolated)
            throws RemoteException {
   Parcel data = Parcel.obtain();
   Parcel reply = Parcel.obtain();
   data.writeInterfaceToken(IServiceManager.descriptor);
   data.writeString(name);
   data.writeStrongBinder(service);
   data.writeInt(allowIsolated ? 1 : 0);
   mRemote.transact(ADD_SERVICE_TRANSACTION, data, reply, 0);
   reply.recycle();
   data.recycle();
}

这个保存方式跟我们getService()的方式是一致的!

至此,我们算是找到总算找到了IWifiManager.aidl的具体实现了,就是WifiServiceImpl,那么也就意味着我们的这整个WIfi的系统服务的接口定义、实现、注册、访问等流程都有两个一个大致的了解!

5、抱着怀疑的态度,我们看下WifiServiceImpl中是否真的实现了disconnect()方法:

源码路径:xxx/frameworks/opt/net/wifi/service/java/com/android/server/wifi/WifiServiceImpl.java

    /**
     * see {@link android.net.wifi.WifiManager#disconnect()}
     */
    public void disconnect() {
        enforceChangePermission();
        mWifiStateMachine.disconnectCommand();
    }

ok,验证通过!

三、总结

这整个Wifi服务是如何被创建以及注册到系统管理,然后如何向上层提供接口进行调用的过程看起来比较复杂,但是其实系统服务里面多数的实现方式都差不多是这样,只要理解了其中一个,其它的就相差不是很大,更重要的是我们去了解怎么一个过程,对我们以后进行framework层的开发还是有非常大的帮助的,比如说我们如何添加一个自己的系统服务。那么接下来我们队这整个流程进行简单的总结。

1、创建相关的AIDL文件IWifiManager.aidl,并定义相关功能方法。

2、创建WifiServiceImpl.java并实现IWifiManager.Stub,并在Contex中定义服务的唯一标识字符串。

3、创建创建WifiManager.java内部维护了通过ServiceManager获取到的WifiServiceImpl实例。

4、在SystemServiceRegistry.java的静态太代码块中通过registerService()方法创建WifiManager实例,并保存到SYSTEM_SERVICE_FETCHERS这个map中,这里要强调一点的是,由于WifiManager是在静态代码块中被创建,所以是单例的。

5、在SystemServer.java中通过SystemServiceManager创建WifiService实例,并调用其onStart()方法。

6、在WifiService的构造方法中创建WifiServiceImpl实例。

7、最终通过SystemService的publishBinderService()方法调用 ServiceManager.addService(name, service, allowIsolated);将WifiServiceImpl注册到ServiceManager中。
原文链接https://blog.csdn.net/monalisatearr/article/details/80801926

阿里P7移动互联网架构师进阶视频(每日更新中)免费学习请点击:https://space.bilibili.com/474380680