ServiceManager（Java层）源码分析_java 分层sevice,manager工程代码-优快云博客

文章详细解析了在Android系统中如何通过Context获取AudioManager服务的过程，涉及到的关键步骤包括Context的getSystemService方法、SystemServiceRegistry、ServiceFetcher、AudioManager的getService以及ServiceManager。在这一过程中，涉及到Binder机制，包括Java层和Native层的交互，如何通过BinderProxy和BpBinder建立服务连接。

从上篇文章中我们分析了Native层的ServiceManager处理流程，系统给应用层调用的话会提供Java层接口，接下来我们分析这个情景。我们通常可以通过下面代码来获取AudioManager，本文基于aosp13进行分析：

context.getSystemService(Context.AUDIO_SERVICE) as AudioManager

1.Context.getSystemService()

// frameworks/base/core/java/android/app/ContextImpl.java
public Object getSystemService(String name) {
   
   
    return SystemServiceRegistry.getSystemService(this, name);
}

2.SystemServiceRegistry.getSystemService()

// frameworks/base/core/java/android/app/SystemServiceRegistry.java
// 缓存服务
private static final Map<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new ArrayMap<String, ServiceFetcher<?>>();
// 获取服务
public static Object getSystemService(ContextImpl ctx, String name) {
   
   
    if (name == null) {
   
   
        return null;
    }
    final ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
    // 部分log打印省略
    final Object ret = fetcher.getService(ctx);
    return ret;
}

// 静态代码块中注册服务
static {
   
   
   registerService(Context.AUDIO_SERVICE, AudioManager.class, new CachedServiceFetcher<AudioManager>() {
   
   
    @Override
       public AudioManager createService(ContextImpl ctx) {
   
   
       return new AudioManager(ctx);
    }});
}
// 到这里可以知道，获取到了AudioManager对象

3.AudioManager.getService()


// /frameworks/base/media/java/android/media/AudioManager.java

// aidl很熟悉的代码，创建了IAudioService，但是多了个ServiceManager是啥，这个很关键，我们接着看
static IAudioService getService()
{
   
   
    if (sService != null) {
   
   
        return sService;
    }
    IBinder b = ServiceManager.getService(Context.AUDIO_SERVICE);
    sService = IAudioService.Stub.asInterface(b);
    return sService;
}

4.ServiceManager.getService()

// /frameworks/base/core/java/android/os/ServiceManager.java
// 获取Binder服务
public static IBinder getService(String name) {
   
   
    try {
   
   
        IBinder service = sCache.get(name);
        if (service != null) {
   
   
            return service;
        } else {
   
   
            return Binder.allowBlocking(rawGetService(name)); // Binder.allowBlocking可以忽略
        }
    } catch (RemoteException e) {
   
   
        Log