Base class for maintaining global application state. You can provide your own implementation by creating a subclass and specifying the fully-qualified name of this subclass as the "android:name" attribute in your AndroidManifest.xml‘s <application> tag. The Application class, or your subclass of the Application class, is instantiated before any other class when the process for your application/package is created.
用来维护全局应用状态的基类，你可以通过创建子类并在AndroidManifest.xml的<application>标签下的 "android:name" 属性指定全限定类名来提供你自己的实现。Application类或者你自己创建的Application的子类，会在你的application/package的进程创建之前的任何类之前实例化。
Note: There is normally no need to subclass Application. In most situations, static singletons can provide the same functionality in a more modular way. If your singleton needs a global context (for example to register broadcast receivers), include Context.getApplicationContext() as a Context argument when invoking your singleton‘s getInstance() method.
注意：通常不太需要实现Application子类。在大多数情况下，静态单例类会更模块化地提供相同的功能。如果你的单例类需要全局的context（例如：要注册broadcast receiver），当调用你单例类的getInstance()方法时，包含Context.getApplicationContext()作为Context。
原文：http://www.cnblogs.com/newjeremy/p/6658627.html