Android中服务管理相关流程getSystemService

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我们经常会使用context来获取服务实例，完成某些特定的功能。

辣么他的大致是什么样的流程呢？下面跟踪一下source code。代码基于Android 5.1.1

一、相关的类文件：

framework/base/core/java/android/app/ContextImpl.java

二、相关流程

this.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ;

    @Override
    public Object getSystemService(String name) {
        ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
        return fetcher == null ? null : fetcher.getService(this);
    }

从上面getSystemService方法中看到，首先是从SYSTEM_SERVICE_MAP中获取ServiceFetcher对象，然后调用ServiceFetcher对象的getService方法获取管理的service.

这里的SYSTEM_SERVICE_MAP 是一个HashMap，键值对的方式存储service相关信息。

    private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP =
            new HashMap<String, ServiceFetcher>();

SYSTEM_SERVICE_MAP 是static 因此在加载ContextImpl.java这个类的时候，就初始化了。

下面用的是一个静态初始化块将service添加到对应的SYSTEM_SERVICE_MAP 中：

static {
        registerService(ACCESSIBILITY_SERVICE, new ServiceFetcher() {
                public Object getService(ContextImpl ctx) {
                    return AccessibilityManager.getInstance(ctx);
                }});
 
        registerService(CAPTIONING_SERVICE, new ServiceFetcher() {
                public Object getService(ContextImpl ctx) {
                    return new CaptioningManager(ctx);
                }});
 
        registerService(ACCOUNT_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    IBinder b = ServiceManager.getService(ACCOUNT_SERVICE);
                    IAccountManager service = IAccountManager.Stub.asInterface(b);
                    return new AccountManager(ctx, service);
                }});
 
        registerService(ACTIVITY_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
                }});
 
        registerService(ALARM_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    IBinder b = ServiceManager.getService(ALARM_SERVICE);
                    IAlarmManager service = IAlarmManager.Stub.asInterface(b);
                    return new AlarmManager(service, ctx);
                }});
 
        registerService(AUDIO_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new AudioManager(ctx);
                }});
 
        registerService(MEDIA_ROUTER_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new MediaRouter(ctx);
                }});
 
        registerService(BLUETOOTH_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new BluetoothManager(ctx);
                }});
 
        registerService(HDMI_CONTROL_SERVICE, new StaticServiceFetcher() {
                public Object createStaticService() {
                    IBinder b = ServiceManager.getService(HDMI_CONTROL_SERVICE);
                    return new HdmiControlManager(IHdmiControlService.Stub.asInterface(b));
                }});
 
        registerService(CLIPBOARD_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new ClipboardManager(ctx.getOuterContext(),
                            ctx.mMainThread.getHandler());
                }});
 
        registerService(CONNECTIVITY_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    IBinder b = ServiceManager.getService(CONNECTIVITY_SERVICE);
                    return new ConnectivityManager(IConnectivityManager.Stub.asInterface(b));
                }});
 
        registerService(COUNTRY_DETECTOR, new StaticServiceFetcher() {
                public Object createStaticService() {
                    IBinder b = ServiceManager.getService(COUNTRY_DETECTOR);
                    return new CountryDetector(ICountryDetector.Stub.asInterface(b));
                }});
 
        registerService(DEVICE_POLICY_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return DevicePolicyManager.create(ctx, ctx.mMainThread.getHandler());
                }});
 
        registerService(DOWNLOAD_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new DownloadManager(ctx.getContentResolver(), ctx.getPackageName());
                }});
 
        registerService(BATTERY_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new BatteryManager();
                }});
 
        registerService(NFC_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return new NfcManager(ctx);
                }});
 
        registerService(DROPBOX_SERVICE, new StaticServiceFetcher() {
                public Object createStaticService() {
                    return createDropBoxManager();
                }});
 
        registerService(INPUT_SERVICE, new StaticServiceFetcher() {
                public Object createStaticService() {
                    return InputManager.getInstance();
                }});
 
        registerService(DISPLAY_SERVICE, new ServiceFetcher() {
                @Override
                public Object createService(ContextImpl ctx) {
                    return new DisplayManager(ctx.getOuterContext());
                }});
 
        registerService(INPUT_METHOD_SERVICE, new StaticServiceFetcher() {
                public Object createStaticService() {
                    return InputMethodManager.getInstance();
                }});
 
        registerService(TEXT_SERVICES_MANAGER_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return TextServicesManager.getInstance();
                }});
 
        registerService(KEYGUARD_SERVICE, new ServiceFetcher() {
                public Object getService(ContextImpl ctx) {
                    // TODO: why isn't this caching it?  It wasn't
                    // before, so I'm preserving the old behavior and
                    // using getService(), instead of createService()
                    // which would do the caching.
                    return new KeyguardManager();
                }});
 
        registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());
                }});
 
        registerService(LOCATION_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    IBinder b = ServiceManager.getService(LOCATION_SERVICE);
                    return new LocationManager(ctx, ILocationManager.Stub.asInterface(b));
                }});
 
        registerService(NETWORK_POLICY_SERVICE, new ServiceFetcher() {
            @Override
            public Object createService(ContextImpl ctx) {
                return new NetworkPolicyManager(INetworkPolicyManager.Stub.asInterface(
                        ServiceManager.getService(NETWORK_POLICY_SERVICE)));
            }
        });

三、关系结构

这里先来研究一下ServiceFetcher这个内部类

/**
     * Override this class when the system service constructor needs a
     * ContextImpl.  Else, use StaticServiceFetcher below.
     */
    /*package*/ static class ServiceFetcher {
        int mContextCacheIndex = -1;
 
        /**
         * Main entrypoint; only override if you don't need caching.
         */
        public Object getService(ContextImpl ctx) {
            //先从缓存中获取，如果没有则创建，并缓存起来。
            ArrayList<Object> cache = ctx.mServiceCache;
            Object service;
            synchronized (cache) {
                if (cache.size() == 0) {
                    //这段代码是第一次调用的时候会执行，sNextPerContextServiceCacheIndex =0 貌似没什么意义啊？！
                    // Initialize the cache vector on first access.
                    // At this point sNextPerContextServiceCacheIndex
                    // is the number of potential services that are
                    // cached per-Context.
                    for (int i = 0; i < sNextPerContextServiceCacheIndex; i++) {
                        cache.add(null);
                    }
                } else {
                    service = cache.get(mContextCacheIndex);//以当前ServiceFetcher对象的mContextCacheIndex获取service
                    if (service != null) {
                        return service;
                    }
                }
              //如果缓存没有则创建service实例，缓存起来并返回。
                service = createService(ctx); //crateService方法会被重写，从registerService方法中可以看到
                cache.set(mContextCacheIndex, service);
                return service;
            }
        }
 
        /**
         * Override this to create a new per-Context instance of the
         * service.  getService() will handle locking and caching.
         */
        public Object createService(ContextImpl ctx) {
            throw new RuntimeException("Not implemented");//从这里可以看出该方法必须被重写
        }
    }

ServiceFetcher类中，mContextCacheIndex是存储在list中的index，index的值是在registerService的时候赋值的。

mServiceCache 属于ContextImpl的全局变量，应该是用于缓存service的，当我们在程序中使用到了某个service，它就会缓存创建出来的这个service，以供其他的使用、。

从getService的方法逻辑我们就可以看出来。

另外，大家可以看到在static静态初始化块中，、，ServiceFetcher并不是全部都重写了createService方法，而是直接重写了getService返回了service

实例，也就是说不缓存该service，为啥呢？具体的原因在代码后面有注释。google大佬说以前都没有缓存，现在也不缓存.....

        registerService(KEYGUARD_SERVICE, new ServiceFetcher() {
                public Object getService(ContextImpl ctx) {
                    // TODO: why isn't this caching it?  It wasn't
                    // before, so I'm preserving the old behavior and
                    // using getService(), instead of createService()
                    // which would do the caching.
                    return new KeyguardManager();
                }});

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