public abstract class Context { 
   ... 
   public abstract Object getSystemService(String name); //獲得系統(tǒng)級服務(wù) 
   public abstract void startActivity(Intent intent);   //通過一個Intent啟動Activity 
   public abstract ComponentName startService(Intent service); //啟動Service 
   //根據(jù)文件名得到SharedPreferences對象 
   public abstract SharedPreferences getSharedPreferences(String name,int mode); 
   ... 
} 

/** 
 * Common implementation of Context API, which provides the base 
 * context object for Activity and other application components. 
 */ 
class ContextImpl extends Context{ 
  //所有Application程序公用一個mPackageInfo對象 
  /*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) { 
    ... 
    //開始啟動一個Activity 
    mMainThread.getInstrumentation().execStartActivity( 
      getOuterContext(), mMainThread.getApplicationThread(), null, null, intent, -1); 
  } 
} 

public class ContextWrapper extends Context { 
  Context mBase; //該屬性指向一個ContextIml實例，一般在創(chuàng)建Application、Service、Activity時賦值 
   
  //創(chuàng)建Application、Service、Activity，會調(diào)用該方法給mBase屬性賦值 
  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); //調(diào)用mBase實例方法 
  } 
} 

public class ContextThemeWrapper extends ContextWrapper { 
   //該屬性指向一個ContextIml實例，一般在創(chuàng)建Application、Service、Activity時賦值 
    
   private Context mBase; 
  //mBase賦值方式同樣有一下兩種 
   public ContextThemeWrapper(Context base, int themeres) { 
      super(base); 
      mBase = base; 
      mThemeResource = themeres; 
   } 
 
   @Override 
   protected void attachBaseContext(Context newBase) { 
      super.attachBaseContext(newBase); 
      mBase = newBase; 
   } 
} 

//創(chuàng)建Application時同時創(chuàng)建的ContextIml實例 
private final void handleBindApplication(AppBindData data){ 
  ... 
  ///創(chuàng)建Application對象 
  Application app = data.info.makeApplication(data.restrictedBackupMode, null); 
  ... 
} 
 
public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) { 
  ... 
  try { 
    java.lang.ClassLoader cl = getClassLoader(); 
    ContextImpl appContext = new ContextImpl();  //創(chuàng)建一個ContextImpl對象實例 
    appContext.init(this, null, mActivityThread); //初始化該ContextIml實例的相關(guān)屬性 
    ///新建一個Application對象  
    app = mActivityThread.mInstrumentation.newApplication( 
        cl, appClass, appContext); 
    appContext.setOuterContext(app); //將該Application實例傳遞給該ContextImpl實例      
  }  
  ... 
} 

2、創(chuàng)建Activity對象的時機 
通過startActivity()或startActivityForResult()請求啟動一個Activity時，如果系統(tǒng)檢測需要新建一個Activity對象時，就會回調(diào)handleLaunchActivity()方法，該方法繼而調(diào)用performLaunchActivity()方法，去創(chuàng)建一個Activity實例，并且回調(diào)onCreate()，onStart()方法等， 函數(shù)都位于 ActivityThread.java類 ，如下：

//創(chuàng)建一個Activity實例時同時創(chuàng)建ContextIml實例 
private final void handleLaunchActivity(ActivityRecord r, Intent customIntent) { 
  ... 
  Activity a = performLaunchActivity(r, customIntent); //啟動一個Activity 
} 
private final Activity performLaunchActivity(ActivityRecord r, Intent customIntent) { 
  ... 
  Activity activity = null; 
  try { 
    //創(chuàng)建一個Activity對象實例 
    java.lang.ClassLoader cl = r.packageInfo.getClassLoader(); 
    activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent); 
  } 
  if (activity != null) { 
    ContextImpl appContext = new ContextImpl();   //創(chuàng)建一個Activity實例 
    appContext.init(r.packageInfo, r.token, this);  //初始化該ContextIml實例的相關(guān)屬性 
    appContext.setOuterContext(activity);      //將該Activity信息傳遞給該ContextImpl實例 
    ... 
  } 
  ...   
} 

//創(chuàng)建一個Service實例時同時創(chuàng)建ContextIml實例 
private final void handleCreateService(CreateServiceData data){ 
  ... 
  //創(chuàng)建一個Service實例 
  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(); //創(chuàng)建一個ContextImpl對象實例 
  context.init(packageInfo, null, this);  //初始化該ContextIml實例的相關(guān)屬性 
  //獲得我們之前創(chuàng)建的Application對象信息 
  Application app = packageInfo.makeApplication(false, mInstrumentation); 
  //將該Service信息傳遞給該ContextImpl實例 
  context.setOuterContext(service); 
  ... 
} 

//Context類靜態(tài)數(shù)據(jù)集合，以鍵值對保存了所有讀取該xml文件后所形成的數(shù)據(jù)集合 
private static final HashMap<File, SharedPreferencesImpl> sSharedPrefs =  
    new HashMap<File, SharedPreferencesImpl>();  
 
@Override 
public SharedPreferences getSharedPreferences(String name, int mode){ 
   //其所對應(yīng)的SharedPreferencesImpl對象 ，該對象已一個HashMap集合保存了我們對該文件序列化結(jié)果 
   SharedPreferencesImpl sp;  
   File f = getSharedPrefsFile(name); //該包下是否存在對應(yīng)的文件，不存在就新建一個 
   synchronized (sSharedPrefs) {    //是否已經(jīng)讀取過該文件，是就直接返回該SharedPreferences對象 
     sp = sSharedPrefs.get(f); 
     if (sp != null && !sp.hasFileChanged()) { 
       //Log.i(TAG, "Returning existing prefs " + name + ": " + sp); 
       return sp; 
     } 
   } 
   //以下為序列化該xml文件，同時將數(shù)據(jù)寫到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);  //更新數(shù)據(jù)集合 
     } else { 
       sp = sSharedPrefs.get(f); 
       if (sp == null) {  
         //新建一個SharedPreferencesImpl對象，并且設(shè)置其相關(guān)屬性 
         sp = new SharedPreferencesImpl(f, mode, map);  
         sSharedPrefs.put(f, sp); 
       } 
     } 
     return sp; 
   } 
} 

//soga，這種形式我們在分析Context ContextIml時接觸過  
//SharedPreferences只是一種接口，其真正實現(xiàn)類是SharedPreferencesImpl類 
private static final class SharedPreferencesImpl implements SharedPreferences{ 
   private Map mMap; //保存了該文件序列化結(jié)果后的操作， 鍵值對形式 
    
   //通過key值獲取對應(yīng)的value值 
   public String getString(String key, String defValue) { 
     synchronized (this) { 
       String v = (String)mMap.get(key); 
       return v != null ? v : defValue; 
     } 
   } 
   ... 
   //獲得該SharedPreferencesImpl對象對應(yīng)的Edito類，對數(shù)據(jù)進行操作 
   public final class EditorImpl implements Editor { 
     private final Map<String, Object> mModified = Maps.newHashMap(); //保存了對鍵值變化的集合 
   } 
} 

import android.app.Application;

public class ContextUtil extends Application {
  private static ContextUtil instance;

  public static ContextUtil getInstance() {
    return instance;
  }

  @Override
  public void onCreate() {
    // TODO Auto-generated method stub
    super.onCreate();
    instance = this;
  }
}

protected void onCreate(Bundle state) {
    super.onCreate(state);

    TextView label = new TextView(this); //傳遞context給view control
    label.setText("Leaks are bad");

    setContentView(label);
}

public class myactivity extends Activity {
    private static Drawable sBackground;
    protected void onCreate(Bundle state) {
        super.onCreate(state);

        TextView label = new TextView(this);
        label.setText("Leaks are bad");

        if (sBackground == null) {
            sBackground = getDrawable(R.drawable.large_bitmap);
        }
    label.setBackgroundDrawable(sBackground);//drawable attached to a view

    setContentView(label);
    }
}

package chroya.demo; 
 
import android.app.Activity; 
import android.os.Bundle; 
import android.util.Log; 
 
class Main extends Activity { 
   
  @Override 
  public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
  } 
   
  public void print(String msg) { 
    Log.d("Main", "msg:"+ msg); 
  } 
} 

package chroya.demo; 
 
import android.app.Activity; 
import android.os.Bundle; 
import android.util.Log; 
 
class Main extends Activity { 
   
  @Override 
  public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
  } 
   
  public void print(String msg) { 
    Log.d("Main", "msg:"+ msg); 
  } 
} 

Context c = createPackageContext("chroya.demo", Context.CONTEXT_INCLUDE_CODE | Context.CONTEXT_IGNORE_SECURITY); 
//載入這個類 
Class clazz = c.getClassLoader().loadClass("chroya.demo.Main"); 
//新建一個實例 
Object owner = clazz.newInstance(); 
//獲取print方法，傳入?yún)?shù)并執(zhí)行 
Object obj = clazz.getMethod("print", String.class).invoke(owner,"Hello"); 

Context c = createPackageContext("chroya.demo", Context.CONTEXT_INCLUDE_CODE | Context.CONTEXT_IGNORE_SECURITY); 
//載入這個類 
Class clazz = c.getClassLoader().loadClass("chroya.demo.Main"); 
//新建一個實例 
Object owner = clazz.newInstance(); 
//獲取print方法，傳入?yún)?shù)并執(zhí)行 
Object obj = clazz.getMethod("print", String.class).invoke(owner, "Hello");