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 sSharedPrefs = new HashMap(); @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 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");