Android系统中提供了很多Service,如剪切板服务，AMS服务等.很有必要一个app是如何获得这些service的.
app中如何获取Android系统中提供的service
app是通过context来获取的.
例如获取AMS:
ActivityManager am = (ActivityManager)getSystemService(Context.ACTIVITY_SERVICE);
可以看出，上面getSystemService 之后是ActivityManager,实际获取的是AMS
getSystemService()方法是Activity类中定义.
android/frameworks/base/core/java/android/app/Activity.java
@Override
public Object getSystemService(@ServiceName @NonNull String name) {
if (getBaseContext() == null) {
throw new IllegalStateException(
"System services not available to Activities before onCreate()");
}
if (WINDOW_SERVICE.equals(name)) {
return mWindowManager;
} else if (SEARCH_SERVICE.equals(name)) {
ensureSearchManager();
return mSearchManager;
}
return super.getSystemService(name);
}
public class Activity extends ContextThemeWrapper{} ---->ContextThemeWrapper
public class ContextThemeWrapper extends ContextWrapper{} ---->ContextWrapper
public class ContextWrapper extends Context {}----------------------->Context
ActivityThread中的performLaunchActivity（）方法中，实例化了ContextImpl，然后调用activity的attach（）方法传递给mBase的，具体流程可以跟一下startActivity的流程
最后可以知道getSystemService 最后调用的是ContextImpl 中的getSystemService。
public Object getSystemService(String name) {
return SystemServiceRegistry.getSystemService(this, name);
}
frameworks/base/core/java/android/app/SystemServiceRegistry.java
SystemServiceRegistry.getSystemService
/**
* 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;
}
SYSTEM_SERVICE_FETCHERS:
final class SystemServiceRegistry {
private static final String TAG = "SystemServiceRegistry";
// Service registry information.
// This information is never changed once static initialization has completed.
private static final HashMap, String> SYSTEM_SERVICE_NAMES =
new HashMap, String>();
private static final HashMap> SYSTEM_SERVICE_FETCHERS =
new HashMap>(); //----> 可以看出是HashMap
private static int sServiceCacheSize;
}
ServiceFetcher: 看看这个是什么
/**
* Base interface for classes that fetch services.
* These objects must only be created during static initialization.
*/
static abstract interface ServiceFetcher {
T getService(ContextImpl ctx);
}
总结： 有上面的可以看出，所有service 都记录在hashmap 中，name 是key ,value是ServiceFetcher
ServiceFetcher 最后返回一定是ActivityManager，因为（ActivityManager am = (ActivityManager)getSystemService(Context.ACTIVITY_SERVICE);）
下面看看SYSTEM_SERVICE_FETCHERS
在SystemServiceRegistry类中只有registerService()这个方法操作了SYSTEM_SERVICE_FETCHERS:
private static void registerService(String serviceName, Class serviceClass,
ServiceFetcher serviceFetcher) {
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}
registerService是private 方法：查看SystemServiceRegistry 类中的调用关系
final class SystemServiceRegistry {
..........
registerService(Context.ACTIVITY_SERVICE, ActivityManager.class,
new CachedServiceFetcher() {
@Override
public ActivityManager createService(ContextImpl ctx) {
return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
}});
...............
}
SystemServiceRegistry静态代码块中调用了registerService()方法.
也就是说当SystemServiceRegistry类初始化的时候,就会在其静态代码块中执行registerService()方法,填充SYSTEM_SERVICE_FETCHERS这个hashmap.
分析下面new CachedServiceFetcher
registerService(Context.ACTIVITY_SERVICE, ActivityManager.class,
new CachedServiceFetcher() {
@Override
public ActivityManager createService(ContextImpl ctx) {
return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
}});
registerService: 参数1，参数2，参数3 匿名内部类
参数1：Context.ACTIVITY_SERVICE-> public static final String ACTIVITY_SERVICE = "activity";
参数2：AMS代理类:ActivityManager
private static
ServiceFetcher
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);-->serviceClass传递
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);---->serviceFetcher传递
}
参数3：CachedServiceFetcher
frameworks/base/core/java/android/app/SystemServiceRegistry.java
static abstract class CachedServiceFetcher implements ServiceFetcher {
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);--->如果service 不存在， 进行create
cache[mCacheIndex] = service;
}
return (T)service;
}
}
public abstract T createService(ContextImpl ctx); ----->抽象，子类要实现
}
对于AMS来说,其T为ActivityManager.
CachedServiceFetcher这个模块类很简单,就是实现了ServiceFetcher接口中getService()方法.
定义了一个抽象方法:T createService().那么其子类要负责实现这个创建Service的方法.
最终通过registerService()方法,第一个参数作为key将第三个参数存入了SYSTEM_SERVICE_FETCHERS这个hashmap中. 参考下面
(SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher))
缓存机制
SystemServiceRegistry::CachedServiceFetcher:getService
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;
}
}
当调用这createService()方法创建对象之后,为了下次能快速获取,所以将其缓存起来了.
frameworks/base/core/java/android/app/ContextImpl.java
// The system service cache for the system services that are cached per-ContextImpl.
final Object[] mServiceCache = SystemServiceRegistry.createServiceCache();
可以看出app 中进行了缓存mServiceCache，
Object service = cache[mCacheIndex]; 没有理解mCacheIndex 如何与service 对应起来，如果有两个service 如何进行index
有人这样写：
下次在获取的时候,直接从context的缓存中拿.其中索引mCacheIndex在创建CachedServiceFetcher子类对象的时候的初始化.和其在SystemServiceRegistry静态代码块中注册的顺序一致.
REF:
https://www.jianshu.com/p/c89a8ad9708c