版本：1.0
日期：2014.3.28
版权：© 2014 kince 转载注明出处何为Application，有何用途？Google官方文档是这样说的：维护全局应用程序的状态，使用的时候继承于它创建一个子类即可，同时在AndroidManifest.xml文件的<application> 标签的name属性制定它的名字，也就是子类名称。当应用程序的包被创建的时候这个类就会被实例化，而且它的生命周期和应用程序是一样的。不过Google在下面的提示中也说道大多数情况下不需要使用这个类，因为如果应用程序使用了单例模式的话，效果和Application提供的事一样的，而且也更加容易控制、规范。
关于Application类有以下公开方法：
其中最常用的就是onCreaet()、onTerminate()以及onLowMemory()方法，onCreaet()它会在这个类实例化的时候调用（这个地方需要十分注意，它只会在程序第一次启动的时候调用，不是每次都调用，因为Application类会在创建包的时候初始化），onTerminate()会在app关闭的时候调用，onLowMemory()是在内存过低的情况下调用的。那onTerminate()是干什么用的呢？网上介绍说是在程序终结的时候调用，从下面的实例中也是这样用的。不过在Google的开发文档以及很多书籍对这个类的用法很少提及，但实际开发情况是如何的呢。不妨反编译几个常用的app一看究竟，因为涉及到一些因素就不方便说是哪个app，以代号表明。
1、app_1
先把代码贴出来，因为这个app的Application的实现比较简单，就先拿出来说。
import android.annotation.TargetApi;import android.app.Application;import android.content.Context;import android.os.Build.VERSION;import com.nostra13.universalimageloader.cache.disc.naming.Md5FileNameGenerator;import com.nostra13.universalimageloader.core.ImageLoader;import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;import com.nostra13.universalimageloader.core.ImageLoaderConfiguration.Builder;import com.nostra13.universalimageloader.core.assist.QueueProcessingType;import java.io.PrintStream;import org.apache.http.HttpVersion;import org.apache.http.client.HttpClient;import org.apache.http.conn.ClientConnectionManager;import org.apache.http.conn.scheme.PlainSocketFactory;import org.apache.http.conn.scheme.Scheme;import org.apache.http.conn.scheme.SchemeRegistry;import org.apache.http.conn.ssl.SSLSocketFactory;import org.apache.http.impl.client.DefaultHttpClient;import org.apache.http.impl.conn.tsccm.ThreadSafeClientConnManager;import org.apache.http.params.BasicHttpParams;import org.apache.http.params.HttpConnectionParams;import org.apache.http.params.HttpProtocolParams;public class MyApplication extends Application{ private HttpClient httpClient; public boolean isLogin = false; private HttpClient createHttpClient() { BasicHttpParams localBasicHttpParams = new BasicHttpParams(); HttpProtocolParams.setVersion(localBasicHttpParams, HttpVersion.HTTP_1_1); HttpProtocolParams.setContentCharset(localBasicHttpParams, "ISO-8859-1"); HttpProtocolParams.setUseExpectContinue(localBasicHttpParams, true); HttpConnectionParams.setConnectionTimeout(localBasicHttpParams, 20000); HttpConnectionParams.setSoTimeout(localBasicHttpParams, 20000); HttpConnectionParams.setSocketBufferSize(localBasicHttpParams, 8192); SchemeRegistry localSchemeRegistry = new SchemeRegistry(); localSchemeRegistry.register(new Scheme("http", PlainSocketFactory.getSocketFactory(), 80)); localSchemeRegistry.register(new Scheme("https", SSLSocketFactory.getSocketFactory(), 443)); return new DefaultHttpClient(new ThreadSafeClientConnManager(localBasicHttpParams, localSchemeRegistry), localBasicHttpParams); } public static void initImageLoader(Context paramContext) { ImageLoaderConfiguration localImageLoaderConfiguration = new ImageLoaderConfiguration.Builder(paramContext).threadPriority(3).denyCacheImageMultipleSizesInMemory().discCacheFileNameGenerator(new Md5FileNameGenerator()).tasksProcessingOrder(QueueProcessingType.LIFO).writeDebugLogs().build(); ImageLoader.getInstance().init(localImageLoaderConfiguration); } private void shutdownHttpClient() { if ((this.httpClient != null) && (this.httpClient.getConnectionManager() != null)) { this.httpClient.getConnectionManager().shutdown(); } } public HttpClient getHttpClient() { return this.httpClient; } @TargetApi(9) public void onCreate() { super.onCreate(); this.httpClient = createHttpClient(); System.out.println(Build.VERSION.SDK_INT); initImageLoader(getApplicationContext()); } public void onLowMemory() { super.onLowMemory(); shutdownHttpClient(); } public void onTerminate() { super.onTerminate(); shutdownHttpClient(); } public static class Config { public static final boolean DEVELOPER_MODE; }} 从import导入的包就可以看出来，在这个类里面使用了强大的universalimageloader异步加载图片以及使用的apache的HttpClient。然后整体看一下这个类，发现它不仅重载了 onCreate()方法，还重载了onLowMemory()以及onTerminate()方法。onLowMemory()顾名思义就是在Android系统内存低的情况下调用的，可以在这个方法里面做一下释放内存的功能。 那就从onCreate()方法开始，里面代码如下：
this.httpClient = createHttpClient(); System.out.println(Build.VERSION.SDK_INT); initImageLoader(getApplicationContext());
第一个方法，创建httpClient 的对象，至于方法的具体细节就不介绍了，不是本文讨论重点，下同。第二个方法就是初始化universalimageloader了。总体来说这个app的Application的实现还是比较简单的，也比较容易理解。接下来介绍一个稍微复杂一点的。
2、app_2
import android.app.Application;import android.content.pm.ApplicationInfo;import android.content.pm.PackageManager;import android.os.Bundle;import android.os.Environment;import android.widget.Toast;import com.baidu.mapapi.BMapManager;import com.baidu.mapapi.MKGeneralListener;import com.baidu.mapapi.MKLocationManager;import java.io.File;public class App extends Application{ public static App app; public static boolean hideOtherExchanges = false; public static boolean hideUpdateNotice = false; public BMapManager mBMapMan = null; public String mStrKey = "4BFEB7A7F3E7CD4B0FBAE61BABD11F1E11D7EAC8"; public static void delete(File paramFile) { if (paramFile.isFile()) { paramFile.delete(); } while (!paramFile.isDirectory()) { return; } File[] arrayOfFile = paramFile.listFiles(); if ((arrayOfFile == null) || (arrayOfFile.length == 0)) { paramFile.delete(); return; } for (int i = 0;; i++) { if (i >= arrayOfFile.length) { paramFile.delete(); return; } delete(arrayOfFile[i]); } } public boolean init() { this.mBMapMan = new BMapManager(this); return this.mBMapMan.init(this.mStrKey, new MyGeneralListener()); } public void onCreate() { app = this; if (init()) { this.mBMapMan.getLocationManager().setNotifyInternal(10, 5); } super.onCreate(); try { localBundle = getPackageManager().getApplicationInfo(getPackageName(), 128).metaData; str2 = localBundle.getString("UMENG_CHANNEL"); arrayOfString1 = localBundle.getString("HIDE_OTHER_EXCHANGE_APK_CHANNEL").split(","); i = 0; } catch (Exception localException1) { for (;;) { Bundle localBundle; String str2; String[] arrayOfString1; int i; String[] arrayOfString2; int j; Logger.w(localException1); continue; i++; continue; label303: if (arrayOfString2[j].equals(str2)) { hideUpdateNotice = true; } else { j++; } } } if (i >= arrayOfString1.length) {} for (;;) { arrayOfString2 = localBundle.getString("HIDE_DOUGUO_UPDATE_NOTICE").split(","); j = 0; int k = arrayOfString2.length; if (j < k) { break label303; } WeiboHelper.setConfig(app); RenrenHelper.setConfig(app); DouguoWebAPI.setConfig(app); Logger.setConfig(app); Thread.setDefaultUncaughtExceptionHandler(new ExceptionHandler(getApplicationContext())); try { String str1 = Environment.getExternalStorageDirectory().getAbsolutePath() + "/douguo/" + getPackageName(); File localFile1 = new File(str1); File localFile2 = localFile1.getParentFile(); if (!localFile2.exists()) { localFile2.mkdir(); } if (!localFile1.exists()) { localFile1.mkdir(); } File localFile3 = new File(str1 + "/.nomedia"); if (!localFile3.exists()) { localFile3.createNewFile(); } return; } catch (Exception localException2) { Logger.w(localException2); } if (!arrayOfString1[i].equals(str2)) { break; } hideOtherExchanges = true; } } public void onLowMemory() { super.onLowMemory(); Logger.e("!!!!!!!!!!!!!!onLowMemory!!!!!!!!!!!!"); } public void onTerminate() { if (this.mBMapMan != null) { this.mBMapMan.destroy(); this.mBMapMan = null; } super.onTerminate(); new Thread() { public void run() { App.delete(new File(Environment.getExternalStorageDirectory() + "/douguo/yummyDiary/")); } }.start(); } static class MyGeneralListener implements MKGeneralListener { public void onGetNetworkState(int paramInt) { Toast.makeText(App.app.getApplicationContext(), "您的网络出错啦！", 1).show(); } public void onGetPermissionState(int paramInt) {} }} 这个类有两个自定义的方法，一个是delete(File paramFile)，用于删除指定文件，一个是 init()，用于初始化百度地图以及网络连接。进入onCreate()方法，首选是给变量app实例化。接着调用init()方法进行判断，如果在init()方法返回true，说明有网络连接正常并且已经初始化完毕百度地图，之后会不断发送tNotify通知。然后进入try catch方法里面，由于反编译的问题，有些语句反编译完毕后就是现在这个样子。至于如何解读，以后会介绍，现在不是重点。看一下，在try里面是初始化了友盟的一些东西，然后再catch里面声明了一些变量，并且根据某些条件创建或者删除指定的文件。最后就是onTerminate()方法，可以看到在这个方法里面它销毁了百度地图、删除了一些程序的文件等。
3、app_3
import android.app.ActivityManager;import android.app.ActivityManager.RunningAppProcessInfo;import android.app.Application;import android.content.Context;import android.os.Process;import com.baidu.android.silentupdate.SilentManager;import java.util.Iterator;import java.util.List;public class FrontiaApplication extends Application{ private static final String a = "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCYAFbG0oYmKgh6o7BhZIHf1njBpZXqyWBnYz2ip3Wp+s97OeA/pTe8xebuGJHwq4xbsGQrJWepIbUVrdjm6JRmdvuJhar7/hC/UNnUkJgYdYl10OZKlvcFFgK3V7XGBPplXldDnhbgscna3JG8U3025WSxZCP5vy/8cfxsEoVx5QIDAQAB"; public static void initFrontiaApplication(Context paramContext) { SilentManager.setKey("MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCYAFbG0oYmKgh6o7BhZIHf1njBpZXqyWBnYz2ip3Wp+s97OeA/pTe8xebuGJHwq4xbsGQrJWepIbUVrdjm6JRmdvuJhar7/hC/UNnUkJgYdYl10OZKlvcFFgK3V7XGBPplXldDnhbgscna3JG8U3025WSxZCP5vy/8cfxsEoVx5QIDAQAB"); SilentManager.enableRSA(false); SilentManager.loadLib(paramContext.getApplicationContext(), "frontia_plugin", "plugin-deploy.jar"); } public void onCreate() { super.onCreate(); ActivityManager localActivityManager = (ActivityManager)getSystemService("activity"); int i = Process.myPid(); Iterator localIterator = localActivityManager.getRunningAppProcesses().iterator(); while (localIterator.hasNext()) { ActivityManager.RunningAppProcessInfo localRunningAppProcessInfo = (ActivityManager.RunningAppProcessInfo)localIterator.next(); if ((localRunningAppProcessInfo.pid == i) && (!localRunningAppProcessInfo.processName.endsWith(getPackageName()))) { initFrontiaApplication(getApplicationContext()); } } }}import android.content.Context;import com.baidu.frontia.FrontiaApplication;import com.nostra13.universalimageloader.cache.disc.naming.Md5FileNameGenerator;import com.nostra13.universalimageloader.core.ImageLoader;import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;import com.nostra13.universalimageloader.core.ImageLoaderConfiguration.Builder;import com.nostra13.universalimageloader.core.assist.QueueProcessingType;public class UILApplication extends FrontiaApplication{ public static void initImageLoader(Context paramContext) { ImageLoaderConfiguration localImageLoaderConfiguration = new ImageLoaderConfiguration.Builder(paramContext).threadPriority(3).denyCacheImageMultipleSizesInMemory().discCacheFileNameGenerator(new Md5FileNameGenerator()).tasksProcessingOrder(QueueProcessingType.LIFO).build(); ImageLoader.getInstance().init(localImageLoaderConfiguration); } public void onCreate() { com.nostra13.universalimageloader.utils.StorageUtils.basePath = "Meishi"; com.nostra13.universalimageloader.utils.StorageUtils.childPath = "Temp"; com.nostra13.universalimageloader.utils.StorageUtils.childImgName = "imgs"; super.onCreate(); CrashHandler.getInstance().init(this); initImageLoader(getApplicationContext()); }} 这个app的Application的实现比较特殊一点，使用了两个类，在第一个类的onCreate()方法里面主要是先是获取所有的进程，然后迭代进行判断，如果满足条件则调用initFrontiaApplication()方法用来设置key。第二个类则用于初始化universalimageloader。其实觉得没有必要分开来操作，写在一个Application类里面即可。
4、app_4
import android.app.Application;import android.graphics.Bitmap.Config;import android.text.TextUtils;import android.webkit.CookieManager;import android.webkit.CookieSyncManager;import cn.jpush.android.api.BasicPushNotificationBuilder;import cn.jpush.android.api.JPushInterface;import com.baidu.location.BDLocation;import com.baidu.location.LocationClient;import com.baidu.location.LocationClientOption;import com.baidu.mapapi.BMapManager;import com.baidu.mapapi.MKGeneralListener;import com.nostra13.universalimageloader.cache.memory.impl.WeakMemoryCache;import com.nostra13.universalimageloader.core.DisplayImageOptions;import com.nostra13.universalimageloader.core.DisplayImageOptions.Builder;import com.nostra13.universalimageloader.core.ImageLoader;import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;import com.nostra13.universalimageloader.core.ImageLoaderConfiguration.Builder;import com.nostra13.universalimageloader.core.assist.ImageScaleType;import com.nostra13.universalimageloader.core.display.RoundedBitmapDisplayer;import java.util.Calendar;import java.util.Date;public class AzkApp extends Application{ public static final String BASEURL = Api.BASE_URL; public static DisplayImageOptions avatarOptions; public static DisplayImageOptions defaultOptions; private static AzkApp instance; private static boolean jpushInitied; public static DisplayImageOptions noStubOptions; private final String COOKIEURL = "http://aimeiwei.me"; public BDLocation bdLocation; private CookieManager cookieManager; private CookieSyncManager cookieSyncManager; public BMapManager mBMapMan; public LocationClient mLocationClient; public int mapCount; private String nickname = ""; private String password = ""; private String username = ""; private String getBaiduKey() { return AzkHelper.getMetaData(getApplicationContext(), "BAIDU_KEY"); } public static AzkApp getInstance() { return instance; } private void initCookieManager() { this.cookieSyncManager = CookieSyncManager.createInstance(this); this.cookieManager = CookieManager.getInstance(); this.cookieManager.setAcceptCookie(true); if (isLogin()) { String str = this.cookieManager.getCookie("http://aimeiwei.me"); if ((str == null) || (!str.contains("username")) || (!str.contains("password"))) { AzkHelper.showLog("初始化cookie"); setCookie(this.username, this.password); } } showCookie(); } private void initHttp() { AzkHelper.showLog("httpClient初始化"); Http.initHttp(AzkHelper.sAzkUserAgent); Http.setBaseUrl(Api.API_URL); if ((!TextUtils.isEmpty(this.username)) && (!TextUtils.isEmpty(this.password))) { Http.setUser(this.username, this.password); } } private void initImageLoader() { AzkHelper.showLog("ImageLoader图片加载器初始化"); long l1 = Runtime.getRuntime().maxMemory(); long l2 = l1 / 1024L / 1024L; AzkHelper.showLog("maxHeap: " + l2); int i = (int)l1 / 4; DisplayImageOptions.Builder localBuilder1 = new DisplayImageOptions.Builder(); DisplayImageOptions.Builder localBuilder2 = localBuilder1.showImageOnLoading(2130837835); Bitmap.Config localConfig; ImageScaleType localImageScaleType; label103: int j; label188: ImageLoaderConfiguration.Builder localBuilder6; if (l2 >= 256L) { localConfig = Bitmap.Config.ARGB_8888; DisplayImageOptions.Builder localBuilder3 = localBuilder2.bitmapConfig(localConfig); if (l2 < 256L) { break label283; } localImageScaleType = ImageScaleType.IN_SAMPLE_POWER_OF_2; localBuilder3.imageScaleType(localImageScaleType).cacheInMemory(true).cacheOnDisc(true); defaultOptions = localBuilder1.build(); DisplayImageOptions.Builder localBuilder4 = new DisplayImageOptions.Builder(); localBuilder4.cacheInMemory(true).cacheOnDisc(true); noStubOptions = localBuilder4.build(); ImageLoaderConfiguration.Builder localBuilder5 = new ImageLoaderConfiguration.Builder(getApplicationContext()).memoryCacheExtraOptions(Constants.IMAGE_SIZE, Constants.IMAGE_SIZE); if (l2 < 256L) { break label291; } j = 1; localBuilder6 = localBuilder5.threadPriority(5 - j); if (l2 < 256L) { break label297; } } label283: label291: label297: for (int k = 3;; k = 2) { ImageLoaderConfiguration localImageLoaderConfiguration = localBuilder6.threadPoolSize(k).memoryCacheSize(i).memoryCache(new WeakMemoryCache()).denyCacheImageMultipleSizesInMemory().defaultDisplayImageOptions(defaultOptions).build(); ImageLoader.getInstance().init(localImageLoaderConfiguration); avatarOptions = localBuilder1.displayer(new RoundedBitmapDisplayer(8)).build(); return; localConfig = Bitmap.Config.RGB_565; break; localImageScaleType = ImageScaleType.IN_SAMPLE_INT; break label103; j = 2; break label188; } } private void removeCookie() { this.cookieManager.removeSessionCookie(); this.cookieManager.removeAllCookie(); this.cookieSyncManager.sync(); } private void setCookie(String paramString1, String paramString2) { Calendar localCalendar = Calendar.getInstance(); localCalendar.set(2025, 11, 29); String str = localCalendar.getTime().toString(); this.cookieManager.setCookie("http://aimeiwei.me", "username=" + paramString1 + "; path=/; expires=" + str); this.cookieManager.setCookie("http://aimeiwei.me", "password=" + paramString2 + "; path=/; expires=" + str); this.cookieSyncManager.sync(); showCookie(); } private void setJpushAlias() { AzkHelper.showLog("jpush", "username: " + this.username); JPushInterface.setAliasAndTags(this, this.username, null); } private void showCookie() { String str = this.cookieManager.getCookie("http://aimeiwei.me"); AzkHelper.showLog("cookie:" + str); } public String getNickname() { return this.nickname; } public String getPassword() { return this.password; } public String getUsername() { return this.username; } public void initBMapManager() { if (this.mBMapMan == null) { this.mBMapMan = new BMapManager(this); String str = getBaiduKey(); AzkHelper.showLog("初始化BMapManager, BAIDU_KEY: " + str); this.mBMapMan.init(str, new MKGeneralListener() { public void onGetNetworkState(int paramAnonymousInt) { if (paramAnonymousInt == 2) { AzkHelper.showErrorLog("您的网络出错啦！"); } } public void onGetPermissionState(int paramAnonymousInt) { if (paramAnonymousInt == 300) { AzkHelper.showLog("请输入正确的百度授权Key"); } } }); } } public void initJpush() { if (jpushInitied) { return; } AzkHelper.showLog("极光推送初始化"); AzkHelper.showLog("jpush", "极光推送初始化"); JPushInterface.init(this); BasicPushNotificationBuilder localBasicPushNotificationBuilder = new BasicPushNotificationBuilder(this); localBasicPushNotificationBuilder.statusBarDrawable = 2130837777; JPushInterface.setPushNotificationBuilder(Integer.valueOf(1), localBasicPushNotificationBuilder); if (!TextUtils.isEmpty(this.username)) { setJpushAlias(); } jpushInitied = true; } public void initMap() { AzkHelper.showLog("百度地图初始化"); this.mLocationClient = new LocationClient(this); LocationClientOption localLocationClientOption = new LocationClientOption(); localLocationClientOption.setProdName(getString(2131296262)); localLocationClientOption.setOpenGps(true); localLocationClientOption.setPriority(2); localLocationClientOption.setAddrType("all"); localLocationClientOption.setCoorType("gcj02"); this.mLocationClient.setLocOption(localLocationClientOption); this.mLocationClient.setAK(getBaiduKey()); initBMapManager(); } public boolean isLogin() { boolean bool = true; if ((TextUtils.isEmpty(this.username)) || (TextUtils.isEmpty(this.password))) { bool = false; } return bool; } public boolean isYourself(User paramUser) { if ((TextUtils.isEmpty(this.username)) || (paramUser == null)) { return false; } return this.username.equals(paramUser.getId()); } public boolean isYourself(UserNew paramUserNew) { if ((TextUtils.isEmpty(this.username)) || (paramUserNew == null)) { return false; } return this.username.equals(paramUserNew.getId()); } public void onCreate() { AzkHelper.showLog("Application Aimeiwei onCreate"); super.onCreate(); instance = this; AzkHelper.prepareUserAgent(this); this.username = AzkHelper.getStringPref(this, "username"); this.password = AzkHelper.getStringPref(this, "password"); this.nickname = AzkHelper.getStringPref(this, "nickname"); AzkHelper.showLog("BuildConfig.DEBUG: false"); AzkHelper.closeLog(); initHttp(); initImageLoader(); initJpush(); initMap(); initCookieManager(); } public void onLowMemory() { AzkHelper.showLog("onLowMemory"); super.onLowMemory(); } public void onTerminate() { instance = null; if (this.mBMapMan != null) { this.mBMapMan.destroy(); this.mBMapMan = null; } super.onTerminate(); } public void removeUser() { Http.clearUser(); AzkHelper.removePref(this, "username"); AzkHelper.removePref(this, "password"); AzkHelper.removePref(this, "nickname"); this.username = ""; this.password = ""; this.nickname = ""; AccessTokenKeeper.clear(getApplicationContext()); removeCookie(); } public void removeUser2() { Http.clearUser(); AzkHelper.removePref(this, "username"); AzkHelper.removePref(this, "password"); AzkHelper.removePref(this, "nickname"); this.password = ""; this.nickname = ""; AccessTokenKeeper.clear(getApplicationContext()); AccessTokenKeeperForQQ.clear(getApplicationContext()); removeCookie(); } public void setNickname(String paramString) { AzkHelper.setStringPref(this, "nickname", paramString); this.nickname = paramString; } public void setPassword(String paramString) { AzkHelper.setStringPref(this, "password", paramString); this.password = paramString; } public void setUser(String paramString1, String paramString2) { setUserName(paramString1); setPassword(paramString2); Http.setUser(paramString1, paramString2); } public void setUser(String paramString1, String paramString2, String paramString3) { setNickname(paramString3); setUser(paramString1, paramString2); setCookie(paramString1, paramString2); } public void setUserName(String paramString) { AzkHelper.setStringPref(this, "username", paramString); this.username = paramString; setJpushAlias(); }} 这个app的实现相对来说还是比较复杂的，但是无非就是初始化一些信息。从import导包可以看到，使用了极光推送、百度地图以及universalimageloader，所以按照之前的分析经验之后一定会在onCreate()方法里面去初始化这些功能的。看一下onCreate()方法： public void onCreate() { AzkHelper.showLog("Application Aimeiwei onCreate"); super.onCreate(); instance = this; AzkHelper.prepareUserAgent(this); this.username = AzkHelper.getStringPref(this, "username"); this.password = AzkHelper.getStringPref(this, "password"); this.nickname = AzkHelper.getStringPref(this, "nickname"); AzkHelper.showLog("BuildConfig.DEBUG: false"); AzkHelper.closeLog(); initHttp(); initImageLoader(); initJpush(); initMap(); initCookieManager(); } 很清晰明了，从方法名称也可以看出来，就是初始化各种功能。至于具体方法的实现，不多做介绍，感兴趣可以自己去研究。 最后总结一下，上面例举了四个市场上的app程序的Application类的实现。从中可以发现，在Application类的实现类中，无非就是做一些初始化程序数据、检查运行环境、销毁数据等。
1、初始化程序数据：
常用的初始化的手段有创建程序所需要的目录，设置开发平台的key，初始化地图信息，初始化http或者其他和网络有关的东西，初始化推送等等。
2、检查运行环境
比如检查是否存在sd卡、判断网络连接状况、检查内存等等。
3、销毁数据
就是在onTerminate()方法里面销毁之前使用到的一些东西。说到onTerminate()这个方法，在官方文档中是这样说的，
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.
它是在模拟环境下调用的，在真机中永远不会被调用。那为什么那么多app的Application类都用到这个方法呢？下面写一个demo测试一下，看结果如何。
首先在真机测试，代码：
import android.app.Application;public class APP extends Application{ @Override public void onCreate() { // TODO Auto-generated method stub super.onCreate(); System. out.println("onCreate()" ); } @Override public void onLowMemory() { // TODO Auto-generated method stub super.onLowMemory(); System. out.println("onCreate()" ); } @Override public void onTerminate() { // TODO Auto-generated method stub super.onTerminate(); System. out.println("onCreate()" ); } } 结果：
打开程序的时候调用了ocCreate()方法，这个没有问题，下面退出程序看看。我直接按返回家退出，发现并没有调用onTerminate()方法。接下来使用模拟器看看结果如何：
运行程序，发现调用了ocCreate()方法，
退出程序看看，发现也没有调用。所以感觉onTerminate()这个方法还是不要使用为好，官方既然说了不会再真机中调用，那么应该不会有错。所以在很多app里面使用到了这个方法，有点不明觉厉，明白的朋友可以交流一下。