Android中的Context详解

/** 
 * Common implementation of Context API, which provides the base 
 * context object for Activity and other application components. 
 */  
class ContextImpl extends Context{  
    //所有Application程序公用一个mPackageInfo对象  
    /*package*/ ActivityThread.PackageInfo mPackageInfo;  
      
    @Override  
    public Object getSystemService(String name){  
        ...  
        else if (ACTIVITY_SERVICE.equals(name)) {  
            return getActivityManager();  
        }   
        else if (INPUT_METHOD_SERVICE.equals(name)) {  
            return InputMethodManager.getInstance(this);  
        }  
    }   
    @Override  
    public void startActivity(Intent intent) {  
        ...  
        //开始启动一个Activity  
        mMainThread.getInstrumentation().execStartActivity(  
            getOuterContext(), mMainThread.getApplicationThread(), null, null, intent, -1);  
    }  
}  

public class ContextWrapper extends Context {  
    Context mBase;  //该属性指向一个ContextIml实例，一般在创建Application、Service、Activity时赋值  
      
    //创建Application、Service、Activity，会调用该方法给mBase属性赋值  
    protected void attachBaseContext(Context base) {  
        if (mBase != null) {  
            throw new IllegalStateException("Base context already set");  
        }  
        mBase = base;  
    }  
    @Override  
    public void startActivity(Intent intent) {  
        mBase.startActivity(intent);  //调用mBase实例方法  
    }  
}  

public class ContextThemeWrapper extends ContextWrapper {  
     //该属性指向一个ContextIml实例，一般在创建Application、Service、Activity时赋值  
       
     private Context mBase;  
    //mBase赋值方式同样有一下两种  
     public ContextThemeWrapper(Context base, int themeres) {  
            super(base);  
            mBase = base;  
            mThemeResource = themeres;  
     }  
  
     @Override  
     protected void attachBaseContext(Context newBase) {  
            super.attachBaseContext(newBase);  
            mBase = newBase;  
     }  
}  

public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) {  
    ...  
    try {
//创建Application时同时创建的ContextIml实例  
private final void handleBindApplication(AppBindData data){  
    ...  
    ///创建Application对象  
    Application app = data.info.makeApplication(data.restrictedBackupMode, null);  
    ...  
}  
  
  
        java.lang.ClassLoader cl = getClassLoader();  
        ContextImpl appContext = new ContextImpl();    //创建一个ContextImpl对象实例  
        appContext.init(this, null, mActivityThread);  //初始化该ContextIml实例的相关属性  
        ///新建一个Application对象   
        app = mActivityThread.mInstrumentation.newApplication(  
                cl, appClass, appContext);  
       appContext.setOuterContext(app);  //将该Application实例传递给该ContextImpl实例           
    }   
    ...  
}   

//创建一个Activity实例时同时创建ContextIml实例  
private final void handleLaunchActivity(ActivityRecord r, Intent customIntent) {  
    ...  
    Activity a = performLaunchActivity(r, customIntent);  //启动一个Activity  
}  
private final Activity performLaunchActivity(ActivityRecord r, Intent customIntent) {  
    ...  
    Activity activity = null;  
    try {  
        //创建一个Activity对象实例  
        java.lang.ClassLoader cl = r.packageInfo.getClassLoader();  
        activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);  
    }  
    if (activity != null) {  
        ContextImpl appContext = new ContextImpl();      //创建一个Activity实例  
        appContext.init(r.packageInfo, r.token, this);   //初始化该ContextIml实例的相关属性  
        appContext.setOuterContext(activity);            //将该Activity信息传递给该ContextImpl实例  
        ...  
    }  
    ...      
}  

//创建一个Service实例时同时创建ContextIml实例  
private final void handleCreateService(CreateServiceData data){  
    ...  
    //创建一个Service实例  
    Service service = null;  
    try {  
        java.lang.ClassLoader cl = packageInfo.getClassLoader();  
        service = (Service) cl.loadClass(data.info.name).newInstance();  
    } catch (Exception e) {  
    }  
    ...  
    ContextImpl context = new ContextImpl(); //创建一个ContextImpl对象实例  
    context.init(packageInfo, null, this);   //初始化该ContextIml实例的相关属性  
    //获得我们之前创建的Application对象信息  
    Application app = packageInfo.makeApplication(false, mInstrumentation);  
    //将该Service信息传递给该ContextImpl实例  
    context.setOuterContext(service);  
    ...  
}  

//Context类静态数据集合，以键值对保存了所有读取该xml文件后所形成的数据集合  
private static final HashMap<File, SharedPreferencesImpl> sSharedPrefs =   
       new HashMap<File, SharedPreferencesImpl>();   
  
@Override  
public SharedPreferences getSharedPreferences(String name, int mode){  
     //其所对应的SharedPreferencesImpl对象 ，该对象已一个HashMap集合保存了我们对该文件序列化结果  
     SharedPreferencesImpl sp;    
     File f = getSharedPrefsFile(name);  //该包下是否存在对应的文件，不存在就新建一个  
     synchronized (sSharedPrefs) {       //是否已经读取过该文件，是就直接返回该SharedPreferences对象  
         sp = sSharedPrefs.get(f);  
         if (sp != null && !sp.hasFileChanged()) {  
             //Log.i(TAG, "Returning existing prefs " + name + ": " + sp);  
             return sp;  
         }  
     }  
     //以下为序列化该xml文件，同时将数据写到map集合中       
     Map map = null;  
     if (f.exists() && f.canRead()) {  
         try {  
             str = new FileInputStream(f);  
             map = XmlUtils.readMapXml(str);  
             str.close();  
         }   
         ...  
     }  
       
     synchronized (sSharedPrefs) {  
         if (sp != null) {  
             //Log.i(TAG, "Updating existing prefs " + name + " " + sp + ": " + map);  
             sp.replace(map);   //更新数据集合  
         } else {  
             sp = sSharedPrefs.get(f);  
             if (sp == null) {    
                 //新建一个SharedPreferencesImpl对象，并且设置其相关属性  
                 sp = new SharedPreferencesImpl(f, mode, map);    
                 sSharedPrefs.put(f, sp);  
             }  
         }  
         return sp;  
     }  
}  

//soga，这种形式我们在分析Context ContextIml时接触过   
//SharedPreferences只是一种接口，其真正实现类是SharedPreferencesImpl类  
private static final class SharedPreferencesImpl implements SharedPreferences{  
     private Map mMap;  //保存了该文件序列化结果后的操作， 键值对形式  
       
     //通过key值获取对应的value值  
     public String getString(String key, String defValue) {  
         synchronized (this) {  
             String v = (String)mMap.get(key);  
             return v != null ? v : defValue;  
         }  
     }  
     ...  
     //获得该SharedPreferencesImpl对象对应的Edito类，对数据进行操作  
     public final class EditorImpl implements Editor {  
         private final Map<String, Object> mModified = Maps.newHashMap(); //保存了对键值变化的集合  
     }  
}