Android 中的CreatePackageContext()

/**
     * Return a new Context object for the given application name.  This
     * Context is the same as what the named application gets when it is
     * launched, containing the same resources and class loader.  Each call to
     * this method returns a new instance of a Context object; Context objects
     * are not shared, however they share common state (Resources, ClassLoader,
     * etc) so the Context instance itself is fairly lightweight.
     *
     * <p>Throws {@link PackageManager.NameNotFoundException} if there is no
     * application with the given package name.
     *
     * <p>Throws {@link java.lang.SecurityException} if the Context requested
     * can not be loaded into the caller's process for security reasons (see
     * {@link #CONTEXT_INCLUDE_CODE} for more information}.
     *
     * @param packageName Name of the application's package.
     * @param flags Option flags, one of {@link #CONTEXT_INCLUDE_CODE}
     *              or {@link #CONTEXT_IGNORE_SECURITY}.
     *
     * @return A Context for the application.
     *
     * @throws java.lang.SecurityException
     * @throws PackageManager.NameNotFoundException if there is no application with
     * the given package name
     */
    
    Override
    public Context createPackageContext(String packageName, int flags)
        throws PackageManager.NameNotFoundException {
        if (packageName.equals("system") || packageName.equals("android")) {
            final ContextImpl context = new ContextImpl(mMainThread.getSystemContext());
            context.mBasePackageName = mBasePackageName;
            return context;
        }


        LoadedApk pi =
            mMainThread.getPackageInfo(packageName, mResources.getCompatibilityInfo(), flags);
        if (pi != null) {
            ContextImpl c = new ContextImpl();
            c.mRestricted = (flags & CONTEXT_RESTRICTED) == CONTEXT_RESTRICTED;
            c.init(pi, null, mMainThread, mResources, mBasePackageName);
            if (c.mResources != null) {
                return c;
            }
        }


        // Should be a better exception.
        throw new PackageManager.NameNotFoundException(
            "Application package " + packageName + " not found");
    }

    主要作用是:创建其它程序的Context,通过创建的这个Context，就可以访问该软件包的资源，甚至可以执行其它软件包的代码。
    
    使用：
    

   try {  
        Context c = createPackageContext("com.dolphin.demo", Context.CONTEXT_INCLUDE_CODE | Context.CONTEXT_IGNORE_SECURITY);  
    } catch (SecurityException e) {  
        e.printStackTrace();  
    } catch (Exception e) {  
        e.printStackTrace();  
    }

    通常一个软件是不能创建其它程序的Context的，除非它们拥有相同的用户ID与签名。用户ID是一个字符串标识，在程序的AndroidManifest.xml文件的manifest标签中指定，格式为android:shareUserId="**"。安装在设备中的每一个apk程序，Android系统会给其分配一个单独的用户空间,其中android:shareUserId就是对应一个Linux用户ID，并且为它创建一个沙箱，以防止与其它应用程序产生影响。用户ID 在应用程序被安装到设备中时分配。通过SharedUserid,拥有同一个Userid的多个APK可以配置成运行在同一个进程中，所以默认就是可以互相访问任意数据，也可以配置成运行在不同的进程中, 同时可以访问其APK的数据目录下的资源(图片，数据库和文件），就像访问本程序的数据一样。

    
   经常的用途：
   如经常一些应用提供的换皮肤功能，实现方法大致有两种：
   1）把需要替换的资源图片打包好之后，放在客户端指定的目录下面，切换皮肤，查找相应的资源时，直接切换资源查找的路径到该打包文件中对应的资源路径即可。
   2）把需要替换的资源，放在一个空的android 应用程序的drawable-**目录下面即可，编译，运行该apk，查找资源时，就需要用到createPackageContext获取对应包名的Context实例，然后通过context来查找对应的资源。查找时，也有两种方式：
   2.1）通过ID来查找资源图片；
       

 Context context;  
        try {  
            context = createPackageContext("com.dolphin.demo", Context.CONTEXT_INCLUDE_CODE  
                    | Context.CONTEXT_IGNORE_SECURITY);  
            txvA.setText(context.getResources().getText(R.string.message));  
        } catch (NameNotFoundException e) {  
            e.printStackTrace();  
        } 

        
   2.2）通过资源Name，反推出资源ID，然后查找对应的资源图片；
   

 /***  
     * @param clazz 目标资源的R.java  
     * @param className R.java的内部类，如layout,string,drawable...  
     * @param name 资源名称  
     * @return  
     */  
    private int getResourseIdByName(Class clazz, String className, String name) {  
        int id = 0;  
        try {  
  
            Class[] classes = clazz.getClasses(); // 获取R.java里的所有静态内部类  
            Class desireClass = null;  
  
            for (int i = 0; i < classes.length; i++) {  
                if (classes[i].getName().split("\\$")[1].equals(className)) {
                    desireClass = classes[i];  
                    break;  
                }  
            }  
            if (desireClass != null)  
                id = desireClass.getField(name).getInt(desireClass);
        } catch (IllegalArgumentException e) {  
            e.printStackTrace();  
        } catch (SecurityException e) {  
            e.printStackTrace();  
        } catch (IllegalAccessException e) {  
            e.printStackTrace();  
        } catch (NoSuchFieldException e) {  
            e.printStackTrace();  
        }  
  
        return id;  
    }  
   
   
    Context context;  
    try {  
        context = createPackageContext("com.dolphin.demo", Context.CONTEXT_INCLUDE_CODE  
                | Context.CONTEXT_IGNORE_SECURITY);  
        Class cls = context.getClassLoader().loadClass("com.dolphin.demo.R"); // 获得目标apk的R类  
        txvA.setText(context.getResources().getText(getResourseIdByName(cls, "string", "message")));  
    } catch (NameNotFoundException e) {  
        e.printStackTrace();  
    } catch (ClassNotFoundException e) {  
        e.printStackTrace();  
    }