url 打开期间浏览器活动中的空异常

Question

我有一个只有浏览器视图的活动。每次应用程序尝试打开 url 时，我都会收到此异常：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.webkit.WebView.loadUrl(java.lang.String)' on a null object reference

指的是这一行：

mWebView.loadUrl("http://google.com");

评论这一行后，活动从另一个视图正常打开。这是尝试加载网页的代码：

 public class Browser extends AppCompatActivity {


 WebView mWebView;
/**
 * ATTENTION: This was auto-generated to implement the App Indexing API.
 * See https://g.co/AppIndexing/AndroidStudio for more information.
 */
private GoogleApiClient client;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
  setContentView(layout.activity_browser);

    mWebView = (WebView) findViewById(R.id.webView);


       mWebView.loadUrl("http://google.com");


    // ATTENTION: This was auto-generated to implement the App Indexing API.
    // See https://g.co/AppIndexing/AndroidStudio for more information.
    client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
}

Logcat：

java.lang.RuntimeException: Unable to start activity ComponentInfo{karolklimek.mpkdisplay2/karolklimek.mpkdisplay2.Browser}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.webkit.WebView.loadUrl(java.lang.String)' on a null object reference
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
                                                                             at android.app.ActivityThread.access$800(ActivityThread.java:144)
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
                                                                             at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                             at android.os.Looper.loop(Looper.java:135)
                                                                             at android.app.ActivityThread.main(ActivityThread.java:5221)
                                                                             at java.lang.reflect.Method.invoke(Native Method)
                                                                             at java.lang.reflect.Method.invoke(Method.java:372)
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
                                                                          Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.webkit.WebView.loadUrl(java.lang.String)' on a null object reference
                                                                             at karolklimek.mpkdisplay2.Browser.onCreate(Browser.java:38)
                                                                             at android.app.Activity.performCreate(Activity.java:5933)
                                                                             at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
                                                                             at android.app.ActivityThread.access$800(ActivityThread.java:144) 
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
                                                                             at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                             at android.os.Looper.loop(Looper.java:135) 
                                                                             at android.app.ActivityThread.main(ActivityThread.java:5221) 
                                                                             at java.lang.reflect.Method.invoke(Native Method) 
                                                                             at java.lang.reflect.Method.invoke(Method.java:372) 
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 

以及content_browser XML文件的内容：

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="karolklimek.mpkdisplay2.Browser"
    >
<WebView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/webView"
    android:layout_alignParentBottom="true"
    android:layout_alignParentEnd="true"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true" />

</RelativeLayout>

Answer 1

setContentView() 有误。您必须从 R.java 文件传递​​布局的整数值。因此，您必须通过 R.layout.activity_browser 而不是 layout.activity_browser。

应该是

setContentView(R.layout.activity_browser);

更新

在您的代码中，您将内容视图设置为 activity_browser，但根据您的问题，我了解到您在 content_browser.xml 文件中有 webview，因此您获得了 NPE。

解决该错误的最简单方法是将 webview 移动到您的 activity_browser.xml 或更改您的 setContentView 以加载 content_browser 而不是 activity_browser。

Answer 2

从代码中我们可以看到布局没有加载，因为 mWebView 对象没有初始化，你最终得到了 NPE。

代码应该是 setContentView(R.layout.activity_browser); 反而 setContentView(layout.activity_browser);