android Application类的详细介绍

一.先对它的整体概念解释： 
在android源码中对他的描述是; 

        * Base class for those who need to maintain global application state. You can 
        * provide your own implementation by specifying its name in your 
        * AndroidManifest.xml's <application> tag, which will cause that class 
        * to be instantiated for you when the process for your application/package is 
        * created. 

   SDK中的描述：Application类是为了那些需要保存全局变量设计的基本类，你可以在AndroidManifest.xml的<application>标签中进行自己的实现，这样的结果是：当你的       application或者包被建立的时候将引起那个类被建立。 

理解：就是说application是用来保存全局变量的，并且是在package创建的时候就跟着存在了。所以当我们需要创建全局变量的时候，不需 要再像j2se那样需要创建public权限的static变量，而直接在application中去实现。只需要调用Context的getApplicationContext或者Activity的getApplication方法来获得一个application对象，再做出相应 的处理。 



例如Launcher模块中;它自己就写了个application，在AndroidManifest.xml中将它进行了设置： 
<application 
        android:name="com.android.launcher2.LauncherApplication" 
对于他的设置可以参考这个模块。 


二.里面的方法进行说明： 

      onCreate(); 

                          /** 
                             * Called when the application is starting, before any other application 
                             * objects have been created.  Implementations should be as quick as 
                             * possible (for example using lazy initialization of state) since the time 
                             * spent in this function directly impacts the performance of starting the 
                             * first activity, service, or receiver in a process. 
                             * If you override this method, be sure to call super.onCreate(). 
                          */ 

                        这个函数是当我们的应用开始之时就被调用了，比应用中的其他对象创建的早，这个实现尽可能的快一点，因为这个时间直接影响到我们第一个activity/service 

                        /receiver。如果你要重写这个方法必须调用super.onCreate(). 

      onTerminate(): 

                         /** 
                            * This method is for use in emulated process environments.  It will 
                            * never be called on a production Android device, where processes are 
                            * removed by simply killing them; no user code (including this callback) 
                            * is executed when doing so. 
                         */ 

                        这个函数是模拟一个过程环境，在真机中永远也不会被调用。