详解Android中的Context抽象类

public abstract class Context { 
   ... 
   public abstract Object getSystemService(String name); //获得系统级服务 
   public abstract void startActivity(Intent intent);   //通过一个Intent启动Activity 
   public abstract ComponentName startService(Intent service); //启动Service 
   //根据文件名得到SharedPreferences对象 
   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; 
   } 
} 

//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); 
  ... 
} 
 
public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) { 
  ... 
  try { 
    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      
  }  
  ... 
} 

2、创建Activity对象的时机 
通过startActivity()或startActivityForResult()请求启动一个Activity时，如果系统检测需要新建一个Activity对象时，就会回调handleLaunchActivity()方法，该方法继而调用performLaunchActivity()方法，去创建一个Activity实例，并且回调onCreate()，onStart()方法等， 函数都位于 ActivityThread.java类 ，如下：

//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 
   } 
} 

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); //传递context给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); 
//载入这个类 
Class clazz = c.getClassLoader().loadClass("chroya.demo.Main"); 
//新建一个实例 
Object owner = clazz.newInstance(); 
//获取print方法，传入参数并执行 
Object obj = clazz.getMethod("print", String.class).invoke(owner,"Hello"); 

Context c = createPackageContext("chroya.demo", Context.CONTEXT_INCLUDE_CODE | Context.CONTEXT_IGNORE_SECURITY); 
//载入这个类 
Class clazz = c.getClassLoader().loadClass("chroya.demo.Main"); 
//新建一个实例 
Object owner = clazz.newInstance(); 
//获取print方法，传入参数并执行 
Object obj = clazz.getMethod("print", String.class).invoke(owner, "Hello");