【错误记录】Android 编译报错 ( Attempt to invoke virtual method ‘void xx.ActionBar.setTitle()‘ on a null obj )

韩曙亮

发布于 2024-04-20 08:47:08

68300

代码可运行

文章被收录于专栏：韩曙亮的移动开发专栏

运行总次数：0

代码可运行

一、报错信息

在 Android Studio 中 , 使用右键菜单 , 创建 " Bottom Navigation Activity " ,

创建完成后 , 启动该 Activity , 报如下错误 :

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: cn.zkhw.client, PID: 30022
    java.lang.RuntimeException: Unable to start activity ComponentInfo{cn.zkhw.client/cn.zkhw.client.BottomNavigationActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:4765)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4983)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:123)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:149)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:103)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:3059)
        at android.os.Handler.dispatchMessage(Handler.java:117)
        at android.os.Looper.loopOnce(Looper.java:205)
        at android.os.Looper.loop(Looper.java:293)
        at android.app.ActivityThread.loopProcess(ActivityThread.java:9934)
        at android.app.ActivityThread.main(ActivityThread.java:9923)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:586)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1240)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at androidx.navigation.ui.ActionBarOnDestinationChangedListener.setTitle(ActionBarOnDestinationChangedListener.java:48)
        at androidx.navigation.ui.AbstractAppBarOnDestinationChangedListener.onDestinationChanged(AbstractAppBarOnDestinationChangedListener.java:103)
        at androidx.navigation.NavController.addOnDestinationChangedListener(NavController.java:233)
        at androidx.navigation.ui.NavigationUI.setupActionBarWithNavController(NavigationUI.java:227)
        at androidx.navigation.ui.ActivityKt.setupActionBarWithNavController(Activity.kt:74)
        at cn.zkhw.client.BottomNavigationActivity.onCreate(BottomNavigationActivity.kt:32)
        at android.app.Activity.performCreate(Activity.java:8592)
        at android.app.Activity.performCreate(Activity.java:8565)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1344)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:4733)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4983) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:123) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:149) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:103) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:3059) 
        at android.os.Handler.dispatchMessage(Handler.java:117) 
        at android.os.Looper.loopOnce(Looper.java:205) 
        at android.os.Looper.loop(Looper.java:293) 
        at android.app.ActivityThread.loopProcess(ActivityThread.java:9934) 
        at android.app.ActivityThread.main(ActivityThread.java:9923) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:586) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1240)

二、问题分析

生成的布局文件代码如下 :

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="?attr/actionBarSize">

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/nav_view"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/bottom_nav_menu" />

    <fragment
        android:id="@+id/nav_host_fragment_activity_bottom_navigation"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@id/nav_view"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/mobile_navigation" />

</androidx.constraintlayout.widget.ConstraintLayout>

生成的主界面代码如下 :

import android.os.Bundle
import com.google.android.material.bottomnavigation.BottomNavigationView
import androidx.appcompat.app.AppCompatActivity
import androidx.navigation.findNavController
import androidx.navigation.ui.AppBarConfiguration
import androidx.navigation.ui.setupActionBarWithNavController
import androidx.navigation.ui.setupWithNavController
import cn.zkhw.client.databinding.ActivityBottomNavigationBinding

class BottomNavigationActivity : AppCompatActivity() {

    private lateinit var binding: ActivityBottomNavigationBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityBottomNavigationBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val navView: BottomNavigationView = binding.navView

        val navController = findNavController(R.id.nav_host_fragment_activity_bottom_navigation)
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        val appBarConfiguration = AppBarConfiguration(
            setOf(
                R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications
            )
        )
        setupActionBarWithNavController(navController, appBarConfiguration)
        navView.setupWithNavController(navController)
    }
}

报错的代码是

setupActionBarWithNavController(navController, appBarConfiguration)

报错信息 :

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at androidx.navigation.ui.ActionBarOnDestinationChangedListener.setTitle(ActionBarOnDestinationChangedListener.java:48)
        at androidx.navigation.ui.AbstractAppBarOnDestinationChangedListener.onDestinationChanged(AbstractAppBarOnDestinationChangedListener.java:103)
        at androidx.navigation.NavController.addOnDestinationChangedListener(NavController.java:233)
        at androidx.navigation.ui.NavigationUI.setupActionBarWithNavController(NavigationUI.java:227)
        at androidx.navigation.ui.ActivityKt.setupActionBarWithNavController(Activity.kt:74)
        at cn.zkhw.client.BottomNavigationActivity.onCreate(BottomNavigationActivity.kt:32)

分析上述报错信息 , 可以得知 , 尝试调用 androidx.appcompat.app.ActionBar 的 void setTitle() 方法 , 但是 androidx.appcompat.app.ActionBar 对象是空的 , 直接报空指针异常 ;

进入 setupActionBarWithNavController(navController, appBarConfiguration) 方法中 , 分析其代码 :

fun AppCompatActivity.setupActionBarWithNavController(
    navController: NavController,
    configuration: AppBarConfiguration = AppBarConfiguration(navController.graph)
) {
    NavigationUI.setupActionBarWithNavController(this, navController, configuration)
}

在上述代码中调用了 NavigationUI.setupActionBarWithNavController() 方法 , 这是 Navigation 中的常用方法 , 用于将 NavigationView 和 NavigationController 和 ActionBar 关联起来 ;

此处没有获取到 ActionBar 对象 , ActionBar 是页面顶部的标题栏 ;

在 Android Studio 中生成的代码 , 一般都是标准代码 , 不会出错 , 这里出现问题大概率是自己在项目中的其它设置导致的 ;

检查 AndroidManifest.xml 中的 Theme 主题设置 , 在 application 标签中 , 设置了 Theme.AppCompat.Light.NoActionBar 标题 , 这就是此时的报错原因 ,

    <application
        android:name=".MyApplication"
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:networkSecurityConfig="@xml/network_security_config"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.AppCompat.Light.NoActionBar"
        android:usesCleartextTraffic="true">

设置了 Theme.AppCompat.Light.NoActionBar 主题 , 肯定是没有 ActionBar 的 , 因此这里尝试调用 androidx.appcompat.app.ActionBar 的 setTitle() 方法直接报空指针异常 ;

三、解决方案

1、配置有标题的主题 - Theme.AppCompat.Light.DarkActionBar

为报错的 Activity 单独配置一个

Theme.AppCompat.Light.DarkActionBar

主题 , 这样该 Activity 就会有 ActionBar , 就不会报错了 ;

        <activity
            android:name=".NavigationActivity"
            android:theme="@style/Theme.AppCompat.Light.DarkActionBar"
            android:label="@string/title_activity_bottom_navigation"></activity>

执行效果如下图所示 :

2、不关联 Navigation 与 ActionBar

在上述代码中 , 调用了 NavigationUI.setupActionBarWithNavController 函数 , 关联了 Navigation 与 ActionBar , 这里可以调用其他的关联方法 ,

调用 NavigationUI.setupWithNavController() 方法 , 可以只关联 NavigationView 与 NavigationController , 不需要关联 ActionBar ,

此外上述代码与 navView.setupWithNavController(navController) 效果相同 , 这里可以注释掉 setupActionBarWithNavController(navController, appBarConfiguration) 代码 , 此时就不会因为找到 ActionBar 而报空指针异常 ;

注释掉上述代码后 , 效果如下 :

class BottomNavigationActivity : AppCompatActivity() {

    private lateinit var binding: ActivityBottomNavigationBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityBottomNavigationBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val navView: BottomNavigationView = binding.navView

        val navController = findNavController(R.id.nav_host_fragment_activity_bottom_navigation)
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        val appBarConfiguration = AppBarConfiguration(
            setOf(
                R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications
            )
        )

        //setupActionBarWithNavController(navController, appBarConfiguration)
        navView.setupWithNavController(navController)
    }
}

运行效果如下 :