你可以使用:

onRestart ()

重新开始和重新开始的区别。

在你的应用程序中添加回调并检查根活动，如下所示:

@Override
public void onCreate() {
    super.onCreate();
    registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
        @Override
        public void onActivityStopped(Activity activity) {
        }

        @Override
        public void onActivityStarted(Activity activity) {
        }

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

        @Override
        public void onActivityResumed(Activity activity) {
        }

        @Override
        public void onActivityPaused(Activity activity) {
        }

        @Override
        public void onActivityDestroyed(Activity activity) {
        }

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            if (activity.isTaskRoot() && !(activity instanceof YourSplashScreenActivity)) {
                Log.e(YourApp.TAG, "Reload defaults on restoring from background.");
                loadDefaults();
            }
        }
    });
}

正确答案

创建名为MyApp的类，如下所示:

public class MyApp implements Application.ActivityLifecycleCallbacks, ComponentCallbacks2 {

    private Context context;
    public void setContext(Context context)
    {
        this.context = context;
    }

    private boolean isInBackground = false;

    @Override
    public void onTrimMemory(final int level) {
        if (level == ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN) {


            isInBackground = true;
            Log.d("status = ","we are out");
        }
    }


    @Override
    public void onActivityCreated(Activity activity, Bundle bundle) {

    }

    @Override
    public void onActivityStarted(Activity activity) {

    }

    @Override
    public void onActivityResumed(Activity activity) {

        if(isInBackground){

            isInBackground = false;
            Log.d("status = ","we are in");
        }

    }

    @Override
    public void onActivityPaused(Activity activity) {

    }

    @Override
    public void onActivityStopped(Activity activity) {

    }

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

    }

    @Override
    public void onActivityDestroyed(Activity activity) {

    }

    @Override
    public void onConfigurationChanged(Configuration configuration) {

    }

    @Override
    public void onLowMemory() {

    }
}

然后，在你想要的任何地方(最好是在应用程序中启动的第一个活动)，添加下面的代码:

MyApp myApp = new MyApp();
registerComponentCallbacks(myApp);
getApplication().registerActivityLifecycleCallbacks(myApp);

完成了!现在当应用程序在后台时，我们得到日志状态:我们退出 当我们进入app，我们得到log status，我们是out

这似乎是Android中最复杂的问题之一，因为(在撰写本文时)Android没有iOS中等效的applicationDidEnterBackground()或applicationwillenter前台()回调。我使用了一个由@jenzz组合的AppState库。

[AppState]是一个基于RxJava的简单的响应式Android库，用于监控应用程序状态的变化。每当应用程序进入后台并返回前台时，它都会通知订阅者。

事实证明，这正是我所需要的，特别是因为我的应用程序有多个活动，所以简单地检查onStart()或onStop()对一个活动不会削减它。

首先，我将这些依赖项添加到gradle:

dependencies {
    compile 'com.jenzz.appstate:appstate:3.0.1'
    compile 'com.jenzz.appstate:adapter-rxjava2:3.0.1'
}

然后，将这些行添加到代码中适当的位置就很简单了:

//Note that this uses RxJava 2.x adapter. Check the referenced github site for other ways of using observable
Observable<AppState> appState = RxAppStateMonitor.monitor(myApplication);
//where myApplication is a subclass of android.app.Application
appState.subscribe(new Consumer<AppState>() {
    @Override
    public void accept(@io.reactivex.annotations.NonNull AppState appState) throws Exception {
        switch (appState) {
            case FOREGROUND:
                Log.i("info","App entered foreground");
                break;
            case BACKGROUND:
                Log.i("info","App entered background");
                break;
        }
    }
});

根据你订阅可观察对象的方式，你可能不得不取消订阅以避免内存泄漏。更多信息再次在github页面。

这些答案似乎不正确。当另一个活动开始和结束时，也会调用这些方法。你能做的是保留一个全局标志(是的，全局不好:)，并在每次启动一个新活动时将其设置为true。在每个活动的onCreate中将其设置为false。然后，在onPause中检查这个标志。如果为假，你的应用就会进入后台，或者被杀死。

我知道有点晚了，但我认为所有这些答案都有一些问题，而我这样做了，这是完美的。

创建一个活动生命周期回调，如下所示:

 class ActivityLifeCycle implements ActivityLifecycleCallbacks{

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {

    }

    @Override
    public void onActivityStarted(Activity activity) {

    }

    Activity lastActivity;
    @Override
    public void onActivityResumed(Activity activity) {
        //if (null == lastActivity || (activity != null && activity == lastActivity)) //use this condition instead if you want to be informed also when  app has been killed or started for the first time
        if (activity != null && activity == lastActivity) 
        {
            Toast.makeText(MyApp.this, "NOW!", Toast.LENGTH_LONG).show();
        }

        lastActivity = 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) {

    }
}

然后像下面这样在你的应用类上注册它:

public class MyApp extends Application {

@Override
public void onCreate() {
    super.onCreate();
    registerActivityLifecycleCallbacks(new ActivityLifeCycle());
}