git中总结了5种方法：https://github.com/wenmingvs/AndroidProcess

方法一ActivityManager：

通常情况下，我们判断app是否在前台都是通过 ActivityManager。

/**
   * Activity是否在前台
   * @param context
   * @return
   */
  private boolean isOnForground(Context context){
    ActivityManager activityManager = (ActivityManager) context.getSystemService(ACTIVITY_SERVICE);
    List<ActivityManager.RunningAppProcessInfo> appProcessInfoList = activityManager.getRunningAppProcesses();
    if(appProcessInfoList == null){
      return false;
    }

    String packageName = context.getPackageName();
    for(ActivityManager.RunningAppProcessInfo processInfo : appProcessInfoList){
      if(processInfo.processName.equals(packageName) && processInfo.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND ){
        return true;
      }
    }
    return false;
  }

但是这个并不是最好的：

• 性能差，相当于遍历所有的进程来找前台的且包名符合的。
• 在有些手机上 不适用。

方法二ActivityLifecycleCallbacks：

Application可以通过 registerActivityLifecycleCallbacks 来注册 ActivityLifecycleCallbacks 接口，以实现对所有Activity生命周期的回调。

this.registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
      @Override
      public void onActivityCreated(Activity activity, Bundle savedInstanceState) {

      }

      @Override
      public void onActivityStarted(Activity activity) {

      }

      @Override
      public void onActivityResumed(Activity activity) {

      }

      @Override
      public void onActivityPaused(Activity activity) {

      }

      @Override
      public void onActivityStopped(Activity activity) {

      }

      @Override
      public void onActivitySaveInstanceState(Activity activity, Bundle outState) {

      }

      @Override
      public void onActivityDestroyed(Activity activity) {

      }
    });

每一个Activity的生命周期都会回调相应的方法。意思是Application能监听所有Activity的生命周期咯，是不是很牛x。 
这个功能可以用来做某种统计，或者弄成一个Activity链表可以知道前后Activity什么的，虽然目前还没遇到这种需求，当然了”判断APP是否在前台“这个功能算半个不过分吧。

生命周期
然后，为了达到 ”判断APP是否在前台“这个功能，我们需要先知道activity的生命周期。是不是很简单？ 
但是很多人不知道 如果ActivityA intent跳转到 ActivityB，这两个的生命周期是怎么切换的。 
公布答案： 
ActivityA.OnPause() —> AcitivityB.OnCreate() —> AcitivityB.OnStart() —> ActivityB.OnResume() —> ActivityA.OnStop() 
其实，也确实应该是这样嘛，你要切换新界面，总得等新界面显示出来了，才能去处理旧界面。总不能先把旧的处理掉，然后黑屏加载新界面吧。

判断APP是否在前台

在Application中继承 Application.ActivityLifecycleCallbacks ，

并在onCreate()方法中注册监听：registerActivityLifecycleCallbacks(this);

最后通过isForeground来判断即可。具体可查看下面代码：

public class BaseApplication extends MultiDexApplication implements Application.ActivityLifecycleCallbacks {

    private int activityCount;//activity的count数
    public static boolean isForeground;//是否在前台
    private static Context context;
    private static Resources resource;
    private String TAG = "----BaseApplication ";
    protected static String BASE_URL = "";

public static Resources getAppResources() {
    return resource;
}

@Override
public void onCreate() {
super.onCreate();
CrashHandler.getInstance().setContext(this);
context = getApplicationContext();
resource = context.getResources();
//全局管理Activity生命周期
registerActivityLifecycleCallbacks(this);
}

public static Context getContext() {
return context;
}

public static void setContext(Context context) {
BaseApplication.context = context;
}

public static boolean debug() {
return BuildConfig.DEBUG;
}

@Override
protected void attachBaseContext(Context base) {
    super.attachBaseContext(base);
    MultiDex.install(getApplicationContext());
}

@Override
public File getCacheDir() {
    if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
        File cacheDir = getExternalCacheDir();
        if (cacheDir != null && (cacheDir.exists() || cacheDir.mkdirs())) {
            return cacheDir;
        }
    }
    File cacheDir = super.getCacheDir();
    return cacheDir;
}

@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
    Log.e(TAG+"activityCount","-----onActivityCreated="+activityCount);
}


@Override
public void onActivityStarted(Activity activity) {
    activityCount++;
    Log.e(TAG+"activityCount","-------onActivityStarted="+activityCount);
}

@Override
public void onActivityResumed(Activity activity) {
Log.e(TAG+"activityCount","-------onActivityResumed="+activityCount);
isForeground();
}

@Override
public void onActivityPaused(Activity activity) {
    Log.e(TAG+"activityCount","-------onActivityPaused="+activityCount);
}

@Override
public void onActivityStopped(Activity activity) {
    activityCount--;
    Log.e(TAG+"activityCount","-------onActivityStopped="+activityCount);
    isForeground();
}

/**
* 判断是否在前台
*/
private void isForeground() {
    if (activityCount>0) {
       isForeground = true;
    }else {
    isForeground=false;
    }
    Log.e(TAG+"activityCount=",+activityCount+"-------isForeground="+isForeground);
}

@Override
public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
    Log.e(TAG+"activityCount","-------onActivitySaveInstanceState="+activityCount);
}

@Override
public void onActivityDestroyed(Activity activity) {
    Log.e(TAG+"activityCount","-------onActivityDestroyed="+activityCount);
}

}