android 源码中的单例模式

在Android系统中，我们会经常通过Context获取系统级别的服务，如windoiwsManagerService、ActivityManagerService等，更常用的是一个LayoutInflate的类，这些服务会在合适的时候以单例的形式在注册在系统中，在我们需要的时候就通过Context的getSystemServie获取。我们以LayoutInflate为例来说明，平时我们使用LayoutInflate较为常见的地方是在ListView的getView方法中。

@Overridepublic View getView(int position,View convertView,ViewGroup parent){<span style="white-space:pre"></span>View itemView = null;

<span style="white-space:pre"></span>if(convertView == null){

<span style="white-space:pre"></span>itemView = LayoutInflate,from(mContext).inflate(mLayoutId,null);

<span style="white-space:pre"></span>//代码省略

<span style="white-space:pre"></span>}else{

<span style="white-space:pre"></span>//代码省略

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>return itemView;}

通常我们使用LayoutInflate.from(context)来获取LayoutInflate服务，下面看看LayoutInflate.from(context)  的实现：

public static LayoutInflate from(Context context){<span style="white-space:pre"></span>LayoutInflater layoutinflater = (LayoutInflater)context.getSystemService(Context.Layout_Inflater_service);

<span style="white-space:pre"></span>if(layoutinflater == null){

<span style="white-space:pre"></span>throw new AsserionError("LayoutInflater not found.");

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>return layoutinflater;

}

可以看到from(context)函数内部调用的是Context类的getSystemService(string key)方法，我们跟踪到Context类看到，该类是抽象类；

public abstract class Context{

//省略

}

getView中使用的Context对象的具体实现是什么呢？其实在Application,Activity,Service中都会存在一个Context对象，即Context的总数为Activity个数＋Service个数＋1。而ListView通常都是显示在Activity中，那么我们就以Activity中的Context来分析。

我们知道，一个Activity的入口是main函数，在main 函数中创建一个新的ActivityThread对象，并且启动消息循环，创建新的Activity，新的Context对象，然后将Context对象传递给Activity。下面看看ActivityThread源代码。

</pre><pre name="code" class="java">public static void main(String[] args){

<span style="white-space:pre"></span>Process.setArgVo("<pre-initialized>");

<span style="white-space:pre"></span>//主线程消息循环

<span style="white-space:pre"></span>Looper.prepareMainLooper();

<span style="white-space:pre"></span>//创建ActivityThread对象<span style="white-space:pre"></span>

<span style="white-space:pre"></span>ActivityThread thread = new ActivityThread();<span style="white-space:pre"></span>

<span style="white-space:pre"></span>thread.attach(false);

<span style="white-space:pre"></span>if(sMainTHreadHandler == null){<span style="white-space:pre"></span>sMainthreadHandler = thread.getHandler();

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>AsyncTask.init();

<span style="white-space:pre"></span>///代码省略

<span style="white-space:pre"></span>Looper.loop();

}

</pre><pre name="code" class="java">private void attach(boolean system){

<span style="white-space:pre"></span>sTgreadLocal.get(this);

<span style="white-space:pre"></span>mSystenTgread = system;

<span style="white-space:pre"></span>//不是系统应用

<span style="white-space:pre"></span>if(!system){

<span style="white-space:pre"></span>ViewRootImpl.addFirestDrawHandler(new Runnable(){

<span style="white-space:pre"></span>public void run(){

<span style="white-space:pre"></span>ensureJitEnabled();

<span style="white-space:pre"></span>}<span style="white-space:pre"></span>

<span style="white-space:pre"></span>});

<span style="white-space:pre"></span>abdroid.ddm.DdmHandler.setAppName(<span style="font-family: Arial, Helvetica, sans-serif;">"<pre-initialized>",UserHandler.myUserId());</span>

<span style="white-space:pre"></span>RuntimeInit.<span style="font-family: Arial, Helvetica, sans-serif;">getApplicationObject</span><span style="font-family: Arial, Helvetica, sans-serif;">(mAppTHread.asBinder());</span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>  IActivityManager mgr = ActivityManagerNative.getDefault();</span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>try{</span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>//关联mAppThread</span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span><span style="white-space:pre"></span></span><span style="font-family: Arial, Helvetica, sans-serif;"></span>mgr.attachApplication(mAppThread);

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>}catch(RemoteException e){</span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>}</span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>}else｛</span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>｝</span>

}<span style="white-space:pre"></span>

在main方法中，我们创建一个个ActivityThread实例后，调用了attach函数，并且参数为false。在attach函数中，参数为false的情况下，会通过Binder机制与ActivityManagerService通信，并且最终屌用handlerLaunchActivity函数，我们看看该函数的实现：

private void handleLaunchActivity(ActivityClientRecord r,Intent customIntent){

<span style="white-space:pre"></span>Activity a = performLaunchActivity(r,customIntent);

<span style="white-space:pre"></span>//代码省略

}

private Activity performLaunchActivity(ActivityClientRecord r,Intent customIntent){

<pre name="code" class="java"><span style="white-space:pre"></span>//代码省略

Activity activity = null;

<span style="white-space:pre"></span>try{

<span style="white-space:pre"></span>java.lang.ClassLoader cl = r.packageInfo.getClassLoader();

<span style="white-space:pre"></span>activity = mInstrumentation.newActivity(cl,component.getClassName(),r.intent);

<span style="white-space:pre"></span><span style="font-family: Arial, Helvetica, sans-serif;">//代码省略</span>

<span style="white-space:pre"></span>}catch(Exception e){

<pre name="code" class="java"><span style="font-family: Arial, Helvetica, sans-serif;"><span style="white-space:pre"></span>//代码省略</span>

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>try{

<span style="white-space:pre"></span>//创建Application对象

<span style="white-space:pre"></span>Application app = r.packageInfp.makeApplication(false,mInstrumentation);

<span style="white-space:pre"></span>if(activity != null){

<span style="white-space:pre"></span>Context appContext = createBaseContextForActivity(r,activity);

<span style="white-space:pre"></span>CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());

<span style="white-space:pre"></span>Configuration config = new Contiguration(mCompatConfiguration);

<span style="white-space:pre"></span>activity.attach(appContext,this,getInstrumentation(),r.token,r.ident,app,r.intent,r.activityInfo,title,r.parent,r.embeddedId,r.lastNonConfigurationInstances,config);

<span style="white-space:pre"></span>mInstrumentation.callActivityOnCreate(activity,r.state);

<span style="white-space:pre"></span><span style="font-family: Arial, Helvetica, sans-serif;">//代码省略</span>

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>}catch(SuperNotCalledException e){

<span style="white-space:pre"></span>//代码省略

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>return activity；

}

</pre><pre name="code" class="java">private Context createBaseContextForActivity(ActivityClientRecord r,final Activity activity){

<span style="white-space:pre"></span>ContextImpl appContext = ContextImpl.createActivityContext(this,r.packageInfo,r.token);

<span style="white-space:pre"></span>appContext.setOuterContext(activity);

<span style="white-space:pre"></span>Context baseContext = appContext;

<span style="white-space:pre"></span>return baseContext;

}

通过上面的代码分析可以知道，Context的实现类为ContextImpl。我们继续跟踪ContextImpl类：

class ContextImpl extends Context{

<span style="white-space:pre"></span>//代码省略

<span style="white-space:pre"></span>//ServerFetcher 通过getService 获取服务对象

<span style="white-space:pre"></span>static class ServiceFetcher｛

<span style="white-space:pre"></span>int mContextCacheIndex = -1;

<span style="white-space:pre"></span>//获取系统服务

<span style="white-space:pre"></span>public Object getService(ContextImpl ctx){

<span style="white-space:pre"></span>ArrayKust<Object> cacge = ctx.mServiceCache;

<span style="white-space:pre"></span>Object service;

<span style="white-space:pre"></span>synchronized(cache){

<span style="white-space:pre"></span>if(cache.size() == 0){

<span style="white-space:pre"></span>for(int i=0;i<sNextPerContextServuceCacheIndex;i++){<span style="white-space:pre"></span>cache.add(null);

<span style="white-space:pre"></span>}<span style="white-space:pre"></span>

<span style="white-space:pre"></span>}else{

<span style="white-space:pre"></span>service = cache.get(mContextCacheIndex);

<span style="white-space:pre"></span>if(service != null){<span style="white-space:pre"></span>return sevice;

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>service = createService(ctx);

<span style="white-space:pre"></span>cache.set(mContextCacheIndex,service);

<span style="white-space:pre"></span>return service;

<span style="font-family: Arial, Helvetica, sans-serif;"></span><span style="font-family: Arial, Helvetica, sans-serif;">}</span><span style="font-family: Arial, Helvetica, sans-serif;"></span>

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>//子类腹泻该方法以创建服务对象

<span style="white-space:pre"></span>public Object createService(ContextImpl ctx){

<span style="white-space:pre"></span>throw new RuntimeException("not implemented");

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>｝

</pre><pre name="code" class="java"><span style="white-space:pre"></span>private static final HashMap<String,ServiceFetcher> SYSTEM_SERVICE_MAP = new HashMap<String,ServiceFetcher>();

<span style="white-space:pre"></span>private static int sNextPerZContextServiceCacheIndex = 0;

<span style="white-space:pre"></span>private static void registerService(String seviceName,ServiceFetcher fetcher){

<span style="white-space:pre"></span>if(!(fetcher instance of ServiceFetcher){

<span style="white-space:pre"></span>fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>SYSTEM_SERVICE_MAP.put(serviceName,fetcher);

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>

<span style="white-space:pre"></span>static{

<span style="white-space:pre"></span>registerService(LAYOUT_INFLATER_SERVICE,new ServuceFetcher(){

<span style="white-space:pre"></span>public Object createServie(ContextImpl ct){

<span style="white-space:pre"></span>return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());

<span style="white-space:pre"></span>}

<span style="white-space:pre"></span>});

<span style="white-space:pre"></span>}

</pre><pre name="code" class="java"><span style="white-space:pre"></span>@Override

<span style="white-space:pre"></span>public Object getSystemService(String name){

<span style="white-space:pre"></span>ServiceFetcher fetcher = SYSTEM_SERVUCE_MAP.get(name);

<span style="white-space:pre"></span>return fetcher == null?null : fetcher.getService(this);

</pre><pre name="code" class="java"><span style="white-space:pre"></span>}

<span style="white-space:pre"></span>//代码省略

}

</pre><span style="white-space:pre"></span>从ContextImpl类的部分代码中可以看到，在虚拟机第一次夹在该类时会注册各种ServiceFetcher，其中就包括了LayoutInflater Srevice。将这些服务以键值对的形式存储在一个HashMap中，用户使用时只需要根据Key来获取到对应的ServiceFetcher，然后通过ServiceFetcher的get Service函数来获取具体的服务对象。当第一次获取时，会调用createService函数创建服务对象，然后将该对象缓存到一个列表中，下次再取时直接从缓存中获取，避免重复创建对象，从而达到单例的效果。这种创建系统服务单例的模式减少了资源的消耗<p></p><p></p><pre name="code" class="java">