一、概述

Application源码介绍，如下所示：

/** * 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  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. * * 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 * {@link android.content.Context#getApplicationContext() Context.getApplicationContext()} * as a {@link android.content.Context} argument when invoking your singleton's * getInstance() method. * 
 */

概述： 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的标签中通过"android:name"属性进行使用，自定义的application作为基类会影响该程序中每个类的建立，只需要调用Context的getApplicationContext或者Activity的getApplication方法来获得一个application对象，就能做出相应的处理。

二、程序生命周期

1、onCreate():程序创建

/** * Called when the application is starting, before any activity, service, * or receiver objects (excluding content providers) 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(). */@CallSuperpublic void onCreate() {}

介绍：这个函数是当应用开始之时就被调用了，比其他对象创建的早，这个实现要尽可能的快一点，因为创建时间直接在进程中影响到我们第一个activity/service或者receiver。如果你要重写这个方法必须调用super.onCreate()。

2、onTerminate():程序终止

/** * 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. */@CallSuperpublic void onTerminate() {}

介绍：应用程序对象终止时调用。但不保证一定被调用，当程序是被内核终止以便为其他应用程序释放资源，那么将不会提醒，即不调用应用程序的对象的onTerminate方法而直接终止当前进程。

3、onLowMemory():程序低内存

/** * This is called when the overall system is running low on memory, and * actively running processes should trim their memory usage.  While * the exact point at which this will be called is not defined, generally * it will happen when all background process have been killed. * That is, before reaching the point of killing processes hosting * service and foreground UI that we would like to avoid killing. */void onLowMemory();

介绍：当后台程序已经终止资源还匮乏时会调用这个方法。好的应用程序一般会在这个方法里面释放一些不必要的资源来应付当后台程序已经终止，前台应用程序内存还不够时的情况。

4、onTrimMemory()：程序内存清理

/** * Called when the operating system has determined that it is a good * time for a process to trim unneeded memory from its process.  This will * happen for example when it goes in the background and there is not enough * memory to keep as many background processes running as desired.  You * should never compare to exact values of the level, since new intermediate * values may be added -- you will typically want to compare if the value * is greater or equal to a level you are interested in. */void onTrimMemory(@TrimMemoryLevel int level);

介绍：当系统回调确认该进程是时候回收不必要的内存了，这将例如发生在后台时,没有足够的内存保持尽可能多的后台进程运行一样。一般发生在点击Home键、Task任务菜单键时被执行。

三、实例演示

*记得把自定义的MyApplication类在AndroidManifest.xml中Application标签中设置到Android:name属性中。

public class MyApplication extends Application {    @Override    public void onCreate() {        // 程序创建的时候执行        Log.d(TAG, "onCreate");        super.onCreate();    }    @Override    public void onTerminate() {        // 程序终止的时候执行        Log.d(TAG, "onTerminate");        super.onTerminate();    }    @Override    public void onLowMemory() {        // 低内存的时候执行        Log.d(TAG, "onLowMemory");        super.onLowMemory();    }    @Override    public void onTrimMemory(int level) {        // 程序在内存清理的时候执行        Log.d(TAG, "onTrimMemory");        super.onTrimMemory(level);    }    @Override    public void onConfigurationChanged(Configuration newConfig) {        Log.d(TAG, "onConfigurationChanged");        super.onConfigurationChanged(newConfig);    }}

//打开oneActivity  01-19 15:16:27.142: D/CARLOZ - Application(28202):   onCreate   //创建  01-19 15:16:27.172: D/CARLOZ - oneActivity(28202):   onCreate  01-19 15:16:27.232: D/CARLOZ - oneActivity3(28202):  onStart  01-19 15:16:27.232: D/CARLOZ - oneActivity3(28202):  onResume  //跳转twoActivity  01-19 15:16:27.272: D/CARLOZ -  oneActivity(28202):  onPause  01-19 15:16:27.272: D/CARLOZ -  twoActivity(28202):  onCreate  01-19 15:16:27.322: D/CARLOZ - twoActivity3(28202): onStart  01-19 15:16:27.342: D/CARLOZ - twoActivity3(28202): onResume  01-19 15:16:27.352: D/CARLOZ - oneActivity3(28202):  onStop  01-19 15:16:27.362: D/CARLOZ - oneActivity3(28202):  onDestory  //返回oneActivity  01-19 15:16:27.272: D/CARLOZ -  twoActivity(28202):  onPause  01-19 15:16:27.272: D/CARLOZ -  oneActivity(28202):  onRestart  01-19 15:16:27.322: D/CARLOZ -  oneActivity3(28202): onStart  01-19 15:16:27.342: D/CARLOZ -  oneActivity3(28202): onResume  01-19 15:16:27.352: D/CARLOZ - twoActivity3(28202):  onStop  01-19 15:16:27.362: D/CARLOZ - twoActivity3(28202):  onDestory  //按HOME键退出应用程序  01-19 15:16:55.372: D/CARLOZ - oneActivity(28202):  onPause  01-19 15:16:55.942: D/CARLOZ - oneActivity(28202):  onStop  01-19 15:16:55.952: D/CARLOZ - Application(28202):  onTrimMemory  //回收内存  //重新打开oneActivity  01-19 15:17:20.962: D/CARLOZ - oneActivity(28202): onRestart  01-19 15:17:20.962: D/CARLOZ - oneActivity(28202): onStart  01-19 15:17:20.962: D/CARLOZ - oneActivity(28202): onResume  //按MENU键启动Recent TASK最近任务  01-19 15:17:28.972: D/CARLOZ - oneActivity(28202): onPause  01-19 15:17:28.992: D/CARLOZ - oneActivity(28202): onStop  01-19 15:17:29.022: D/CARLOZ - Application(28202): onTrimMemory //回收内存  //彻底关闭应用程序  01-19 15:17:31.542: D/CARLOZ - oneActivity(28202): onDestroy

四、共享数据

*新建一个MyApplication类继承Application。封装一个自动进行拆装箱。

public class MyApplication extends Application {        private String str = "大家一起享用这个蛋糕";    public String getStr() {        return str;    }    public void setStr(String str) {        this.str = str;    }}

*然后在Activity或其他组件中，调用Activity.getApplication方法或Context.getApplicationContext来获得一个application对象。在oneActivity中：

 textview.setText("共享数据one：" + application.getStr());

 application.setStr("oneActivity已经吃了一口");

在twoActivity中：

 textview.setText("共享数据two：" + application.getStr());

结果显示：

共享数据one：大家一起享用这个蛋糕共享数据two：oneActivity已经吃了一口

Android(安卓)源码分析Application的生命周期及共享数据详解