从后台恢复活动后，DrawerLayout或TabLayout为空

Question

每当我最初创建活动时，一切都很顺利，因为我能够正确地扩展我的布局。然而，问题是，每当我切换到使用另一个应用程序，然后返回到我自己的应用程序时，我都会收到一个错误，说TabLayout为空，导致我的应用程序崩溃。如果我切换到另一个应用程序几分钟，我的DrawerLayout为空，也会导致我的应用程序崩溃。

如果我正确理解安卓生命周期，我会意识到当活动从后台到前台时，它会被重新创建，并且onCreate方法会被再次调用，如以下所述：安卓活动生命周期-所有这些方法都是用来做什么的？。然而，我不明白为什么当应用程序启动时，一切都很好，但当我进入后台时，TabLayout或DrawerLayout被设置为null。这是我的尝试：

news_feed.java：

private static DrawerLayout drawer;
private static TabLayout tabLayout;
public class news_feed extends BatchAppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.news_feed);
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//Listen for navigation events
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close){
public void onDrawerOpened(View view) {
super.onDrawerOpened(view);
}
/** Called when a drawer has settled in a completely closed state. */
public void onDrawerClosed(View view) {
super.onDrawerClosed(view);
}
};
System.out.println("news_feed: toolbar: " + toolbar);
System.out.println("news_feed: Drawer: " + drawer);
System.out.println("news_feed: toggle: " + toggle);
drawer.setDrawerListener(toggle);
toggle.syncState();
TabLayout tabLayout = (TabLayout) findViewById(R.id.tab_layout);
System.out.println("news_feed: Tab Layout: " + tabLayout);
tabLayout.addTab(tabLayout.newTab().setText("First"));
tabLayout.addTab(tabLayout.newTab().setText("Second"));
tabLayout.addTab(tabLayout.newTab().setText("Third"));
tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);
}
}

news_feed.xml：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools" android:id="@+id/drawer_layout"
android:layout_width="match_parent" android:layout_height="match_parent"
android:fitsSystemWindows="true" tools:openDrawer="start">
<include layout="@layout/app_bar_news_feed" android:layout_width="match_parent"
android:layout_height="match_parent" android:id="@+id/app_bar_main" />
<android.support.design.widget.NavigationView
android:id="@+id/nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
android:fitsSystemWindows="true">
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<include
layout="@layout/nav_header_friends_list"
android:id="@+id/navigation_header"/>
<ListView
android:id="@+id/friends_list"
android:layout_width="match_parent"
android:layout_height="wrap_content"></ListView>
</LinearLayout>
</android.support.design.widget.NavigationView>
</android.support.v4.widget.DrawerLayout>

app_bar_news_feed.xml：

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:fitsSystemWindows="true"
tools:context=".news_feed">
<RelativeLayout
android:id="@+id/main_layout"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.design.widget.AppBarLayout android:layout_height="wrap_content"
android:layout_width="match_parent" android:theme="@style/AppTheme.AppBarOverlay" android:id="@+id/app_bar">
<android.support.v7.widget.Toolbar android:id="@+id/toolbar"
android:layout_width="match_parent" android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary" app:popupTheme="@style/AppTheme.PopupOverlay" />
</android.support.design.widget.AppBarLayout>
<!-- <include layout="@layout/content_news_feed" /> -->
<android.support.design.widget.TabLayout
android:id="@+id/tab_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/app_bar"
android:background="?attr/colorPrimary"
android:elevation="6dp"
android:minHeight="?attr/actionBarSize"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"/>
<android.support.v4.view.ViewPager
android:id="@+id/pager"
android:layout_width="match_parent"
android:layout_height="fill_parent"
android:layout_below="@id/tab_layout"/>
</RelativeLayout>
<android.support.design.widget.FloatingActionButton android:id="@+id/fab"
android:layout_width="wrap_content" android:layout_height="wrap_content"
android:layout_gravity="bottom|end" android:layout_margin="@dimen/fab_margin"
android:src="@android:drawable/ic_dialog_email" />
</android.support.design.widget.CoordinatorLayout>

我确保findViewById都引用了正确的id，但我似乎不知道出了什么问题。奇怪的是：如果我从app -> another app -> app快速切换，那么TabLayout为空。如果我从app -> another app -> wait a few minutes -> app切换，则DrawerLayout为空。我不知道为什么会发生这种事！如有任何帮助，我们将不胜感激。

编辑：应用程序在我的活动的onCreate方法上崩溃。此外，我提供了一个stacktrace：如下所示：

03-16 09:31:21.116 25852-25852/com.test.tabs.tabs I/System.out:news_feed:toolbar:android.support.v7.widget.toolbar{380bcc25 V.E……ID 0,0-0,0#7f0d009c app:ID/toolbar}03-16 09:31:21.116 25852-25852/com.test.tabs.tabs I/System.out:news_feed:Drawer:android.support.v4.widget.DrawerLayout｛3ed761fa VFE……I.0,0-0,0#7f0d0095 app:id/Drawer_layout｝03-16 09:31:21.116 25852-25852/com.test.tabs.tabs I/System.out:news_feed:toggle:com.test.tabs.tabs.com.tabs.activity.news_feed$1@22eb13ab03-16 09:31:211.116 25852-25852/com.test.tabs.tabs I/System.out:news_feed:选项卡布局：null03-16 09:31:211.116 25852-25852/com.test.tabs.tabs E/AndroidRuntime：致命异常：main流程：com.test.tabs.tabs，PID:25852java.lang.RuntimeException：无法启动活动ComponentInfo｛com.test.tables.tabs/com.test.tables.com.tables.activity.new_feed｝：java.lang.NullPointerException：尝试在null对象引用上调用虚拟方法"android.support.design.widget.TabLayout$Tab android.ssupport.design.witget.TabLayout.newTab()"在android.app.ActivityThread.performLaunchActivity(ActivityThreads.java:2661)在android.app.ActivityThread.handleLaunchActivity(ActivityThreads.java:2726)在android.app.ActivityThread.access上$900(ActivityThreads.java:172)在android.app.ActivityThread$H.handleMessage(ActivityThreads.java:1421)在android.os.Handler.dispatchMessage(Handler.java:102)在android.os.Looper.loop(Looper.java:145)在android.app.ActivityThread.main(ActivityThreads.java：5835)位于java.lang.reflect.Method.ioke(本机方法)位于java.lang.reflect.Method.ioke(Method.java:372)网址：com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java：1399)网址：com.android.internal.os.ZygoteInit.main(ZygoteNit.java:1194)引起原因：java.lang.NullPointerException：试图在null对象引用上调用虚拟方法"android.support.design.widget.TabLayout$Tab android.ssupport.design.witget.TabLayout.newTab()"网址：com.test.tabs.tabs.com.tabs.activity.news_feed.onCreate(news_feed.java:174)在android.app.Activity.performCreate(Activity.java:6221)在android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)在android.app.ActivityThread.performLaunchActivity(ActivityThreads.java:2614)在android.app.ActivityThread.handleLaunchActivity(ActivityThreads.java:2726)在android.app.ActivityThread.access上$900(ActivityThreads.java:172)在android.app.ActivityThread$H.handleMessage(ActivityThreads.java:1421)在android.os.Handler.dispatchMessage(Handler.java:102)在android.os.Looper.loop(Looper.java:145)在android.app.ActivityThread.main(ActivityThreads.java：5835)位于java.lang.reflect.Method.ioke(本机方法)位于java.lang.reflect.Method.ioke(Method.java:372)网址：com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java：1399)网址：com.android.internal.os.ZygoteInit.main(ZygoteNit.java:1194)

编辑：包含的BatchAppCompatActivity类：

public class BatchAppCompatActivity extends AppCompatActivity {
@Override
protected void onStart()
{
super.onStart();
Batch.onStart(this);
}
@Override
protected void onStop()
{
Batch.onStop(this);
super.onStop();
}
@Override
protected void onDestroy()
{
Batch.onDestroy(this);
super.onDestroy();
}
@Override
protected void onNewIntent(Intent intent)
{
Batch.onNewIntent(this, intent);
super.onNewIntent(intent);
}
}

Answer 1

您对drawer和tabLayout变量的声明似乎有点奇怪。除非是错别字，否则我很惊讶它竟然有效。类似的东西

public class news_feed extends BatchAppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
private DrawerLayout drawer;
private TabLayout tabLayout;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.news_feed);
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//Listen for navigation events
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close){
public void onDrawerOpened(View view) {
super.onDrawerOpened(view);
}
/** Called when a drawer has settled in a completely closed state. */
public void onDrawerClosed(View view) {
super.onDrawerClosed(view);
}
};
System.out.println("news_feed: toolbar: " + toolbar);
System.out.println("news_feed: Drawer: " + drawer);
System.out.println("news_feed: toggle: " + toggle);
drawer.setDrawerListener(toggle);
toggle.syncState();
tabLayout = (TabLayout) findViewById(R.id.tab_layout);
System.out.println("news_feed: Tab Layout: " + tabLayout);
tabLayout.addTab(tabLayout.newTab().setText("First"));
tabLayout.addTab(tabLayout.newTab().setText("Second"));
tabLayout.addTab(tabLayout.newTab().setText("Third"));
tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);
}
}

假设工具栏是在类的其他地方定义的，应该可以工作。