尝试在空对象引用上调用虚拟方法 'android.content.Context android.content.Context.getApplicationContext()'

Question

我正在用webview构建一个应用程序。当我试图重新加载网络视图时，我遇到了一个错误。

错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:117)
at in.bongtech.apps.bongtechlite.MainActivity.reLoad(MainActivity.java:118)
at in.bongtech.apps.bongtechlite.HandleWebview$1.onClick(HandleWebview.java:30)
at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:163)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:157)
at android.app.ActivityThread.main(ActivityThread.java:5613)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:774)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:652)

主要活动.java

public class MainActivity extends AppCompatActivity {
private WebView content;
private String url;
String appCachePath;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
appCachePath = this.getCacheDir().getAbsolutePath();
content = (WebView) findViewById(R.id.content);
content.setWebViewClient(new HandleWebview());
content.loadUrl("https://www.example.com");
WebSettings webViewSettings = content.getSettings();
webViewSettings.setJavaScriptEnabled(true);
webViewSettings.setDomStorageEnabled(true);
webViewSettings.setAppCacheEnabled(true);
webViewSettings.setAppCachePath(appCachePath);
url = content.getUrl();
}
public void reLoad(){
content.reload();
}}

处理Webview.java

public class HandleWebview extends WebViewClient {
MainActivity activity = new MainActivity();
@Override
public boolean shouldOverrideUrlLoading(WebView view, String url) {
if (Uri.parse(url).getHost().contains("www.example.com")) {
// This is my website, so do not override; let my WebView load the page
return false;
}
// Otherwise, the link is not for a page on my site, so launch another Activity that handles URLs
Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(url));
view.getContext().startActivity(intent);
return true;
}
public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
AlertDialog.Builder builder = new AlertDialog.Builder(view.getContext());
builder.setMessage(R.string.errorMessage).setTitle("Error");
builder.setPositiveButton("Retry", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int id) {
activity.reLoad();
}
});
builder.setNegativeButton("Dismiss", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
dialog.cancel();
}
});
builder.create();
builder.show();
}}

当我按下警报对话框上的RETRY按钮时，我会收到上面提到的错误。我从HandleWebview.java文件的警报中调用MainActivity的reLoad()方法。我知道AlertDialog.Builder=new AlertDialog.Bilter(view.getContext())；正在产生此错误。但不知道如何解决错误。请帮忙。请告诉我如何从基本Java类中的对话的积极响应中调用MainActivity中的方法。

Answer 1

MainActivity activity = new MainActivity();

永远不要自己实例化活动。它们对你想要的任何活动都没有好处，比如用作上下文。

相反，您可以将MainActivity引用作为参数传递给HandleWebview。