java.lang.NullPointerException：Android 尝试调用虚拟方法错误

Posted 2023-05-07

【中文标题】java.lang.NullPointerException：Android 尝试调用虚拟方法错误

【英文标题】：java.lang.NullPointerException: Android Attempt to invoke virtual method error

【发布时间】：2018-10-23 01:52:51

【问题描述】：

我的 android webview 片段应用程序崩溃了，在我添加了一个从一个片段转到另一个片段的按钮后，控制台上的错误是：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.ahmed.stories, PID: 11846
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.ahmed.stories.MainActivity.setUrl(java.lang.String)' on a null object reference at com.ahmed.stories.Saying$3.onPageStarted(Saying.java:127)
at com.android.webview.chromium.WebViewContentsClientAdapter.onPageStarted(WebViewContentsClientAdapter.java:517)
at org.chromium.android_webview.AwContentsClientCallbackHelper$MyHandler.handleMessage(AwContentsClientCallbackHelper.java:144)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:207)
at android.app.ActivityThread.main(ActivityThread.java:5728)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)

它说问题出在((MainActivity) getActivity()).setUrl(url); 的第 127 行，但我找不到任何问题，我是 android 并正在努力学习，也许你们可以帮忙。

MainActivity

package com.ahmed.stories;

import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.customtabs.CustomTabsIntent;
import android.support.design.widget.BottomNavigationView;
import android.support.design.widget.NavigationView;
import android.support.v4.content.ContextCompat;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.CardView;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.WindowManager;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener, View.OnClickListener 
    FragmentManager fragmentManager;
    FragmentTransaction fragmentTransaction;
    TextView home_tv, stories, bookmarks, drafts, fakecheck, lost, leaving;
    CardView lyt_top_bar;
    View toolbar_line;
    BottomNavigationView navigation;

    BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
            = new BottomNavigationView.OnNavigationItemSelectedListener() 

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) 
            switch (item.getItemId()) 
                case R.id.navigation_home:
                    fragmentManager = getFragmentManager();
                    fragmentTransaction = fragmentManager.beginTransaction();
                    Home home = new Home();
                    fragmentTransaction.replace(R.id.fragmentContainer, home, "Home");
                    fragmentTransaction.commit();
                    return true;
                case R.id.navigation_search:
                    fragmentManager = getFragmentManager();
                    fragmentTransaction = fragmentManager.beginTransaction();
                    Search search = new Search();
                    fragmentTransaction.replace(R.id.fragmentContainer, search, "Home");
                    fragmentTransaction.commit();
                    return true;
                case R.id.navigation_edit:
                    fragmentManager = getFragmentManager();
                    fragmentTransaction = fragmentManager.beginTransaction();
                    Edit edit = new Edit();
                    fragmentTransaction.replace(R.id.fragmentContainer, edit, "Home");
                    fragmentTransaction.commit();
                    return true;

                case R.id.navigation_message:
                    fragmentManager = getFragmentManager();
                    fragmentTransaction = fragmentManager.beginTransaction();
                    Notification notification = new Notification();
                    fragmentTransaction.replace(R.id.fragmentContainer, notification, "Home");
                    fragmentTransaction.commit();
                    return true;


                case R.id.navigation_profile:
                    fragmentManager = getFragmentManager();
                    fragmentTransaction = fragmentManager.beginTransaction();
                    Profile profile = new Profile();
                    fragmentTransaction.replace(R.id.fragmentContainer, profile, "Home");
                    fragmentTransaction.commit();
                    return true;

            
            return false;
        
    ;

    @Override
    protected void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main3);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);


        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
         navigation = (BottomNavigationView) findViewById(R.id.navigation);
        BottomNavigationViewHelper.disableShiftMode(navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
        toolbar.setNavigationIcon(getResources().getDrawable(R.drawable.navigation));

        bookmarks = findViewById(R.id.bookmarks);
        stories = findViewById(R.id.stories);
        home_tv = findViewById(R.id.home_tv);
        lyt_top_bar = findViewById(R.id.lyt_top_bar);
        toolbar_line = findViewById(R.id.toolbar_line);

        stories.setOnClickListener(this);
        home_tv.setOnClickListener(this);
        bookmarks.setOnClickListener(this);

        fragmentManager = getFragmentManager();
        fragmentTransaction = fragmentManager.beginTransaction();
        Home home = new Home();
        fragmentTransaction.replace(R.id.fragmentContainer, home, "Home");
        fragmentTransaction.commit();

    

    @Override
    public void onBackPressed() 
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) 
            drawer.closeDrawer(GravityCompat.START);
         else 
            super.onBackPressed();
        
    

    @Override
    public boolean onCreateOptionsMenu(Menu menu) 
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main3, menu);
        return true;
    

    @Override
    public boolean onOptionsItemSelected(MenuItem item) 
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) 
            shareTextUrl();
            return true;
        

        return super.onOptionsItemSelected(item);
    

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) 
        // Handle navigation view item clicks here.
        int id = item.getItemId();
        showSystemUI();
        if (id == R.id.nav_camera) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Profile profile = new Profile();
            fragmentTransaction.replace(R.id.fragmentContainer, profile, "Home");
            fragmentTransaction.commit();
            // Handle the camera action
         else if (id == R.id.nav_gallery) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Edit edit = new Edit();
            fragmentTransaction.replace(R.id.fragmentContainer, edit, "Home");
            fragmentTransaction.commit();

         else if (id == R.id.nav_slideshow) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Drafts drafts = new Drafts();
            fragmentTransaction.replace(R.id.fragmentContainer, drafts, "Home");
            fragmentTransaction.commit();

         else if (id == R.id.nav_manage) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Fakecheck fakecheck = new Fakecheck();
            fragmentTransaction.replace(R.id.fragmentContainer, fakecheck, "Home");
            fragmentTransaction.commit();

         else if (id == R.id.nav_lost) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Lost lost = new Lost();
            fragmentTransaction.replace(R.id.fragmentContainer, lost, "Home");
            fragmentTransaction.commit();


         else if (id == R.id.nav_send) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Leaving leaving = new Leaving();
            fragmentTransaction.replace(R.id.fragmentContainer, leaving, "Home");
            fragmentTransaction.commit();


        

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    

    private void hideSystemUI() 

        lyt_top_bar.setVisibility(View.GONE);
         navigation.setVisibility(View.GONE);
        toolbar_line.setVisibility(View.GONE);

    

    public void showSystemUI() 
        lyt_top_bar.setVisibility(View.VISIBLE);
        navigation.setVisibility(View.VISIBLE);
        toolbar_line.setVisibility(View.VISIBLE);

//        View decorView = getWindow().getDecorView();
//        decorView.setSystemUiVisibility(
//                View.SYSTEM_UI_FLAG_LAYOUT_STABLE
//        );
    

    @Override
    public void onClick(View v) 
        showSystemUI();
        if (v == home_tv) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Saying saying = new Saying();
            fragmentTransaction.replace(R.id.fragmentContainer, saying, "Home");
            fragmentTransaction.commit();
            saying.hideProgress();
            hideSystemUI();
        

        if (v == bookmarks) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Bookmarks bookmarks = new Bookmarks();
            fragmentTransaction.replace(R.id.fragmentContainer, bookmarks, "Home");
            fragmentTransaction.commit();
        

        if (v == drafts) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Drafts drafts = new Drafts();
            fragmentTransaction.replace(R.id.fragmentContainer, drafts, "Home");
            fragmentTransaction.commit();
        

        if (v == fakecheck) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Fakecheck fakecheck = new Fakecheck();
            fragmentTransaction.replace(R.id.fragmentContainer, fakecheck, "Home");
            fragmentTransaction.commit();
        

        if (v == lost) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Lost lost = new Lost();
            fragmentTransaction.replace(R.id.fragmentContainer, lost, "Home");
            fragmentTransaction.commit();
        

        if (v == leaving) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Leaving leaving = new Leaving();
            fragmentTransaction.replace(R.id.fragmentContainer, leaving, "Home");
            fragmentTransaction.commit();
        

        if (v == stories) 
            fragmentManager = getFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            Trending trending = new Trending();
            fragmentTransaction.replace(R.id.fragmentContainer, trending, "Home");
            fragmentTransaction.commit();
        
    
    String urlOpen="";
    public void setUrl(String url)
        this.urlOpen = url;
    
    public void shareTextUrl()
        Intent share = new Intent(android.content.Intent.ACTION_SEND);
        share.setType("text/plain");
        share.addFlags(Intent.FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET);

        // Add data to the intent, the receiving app will decide
        // what to do with it.
        share.putExtra(Intent.EXTRA_SUBJECT, "Title Of The Post");
        share.putExtra(Intent.EXTRA_TEXT, urlOpen);

        startActivity(Intent.createChooser(share, "We Only share with the people we care!"));
    
    public void goToHome()
        showSystemUI();
        fragmentManager = getFragmentManager();
        fragmentTransaction = fragmentManager.beginTransaction();
        Home home = new Home();
        fragmentTransaction.replace(R.id.fragmentContainer, home, "Home");
        fragmentTransaction.commit();
    

说片段

package com.ahmed.stories;

import android.app.Fragment;
import android.content.Context;
import android.content.Intent;
import android.graphics.Bitmap;
import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.KeyEvent;
import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.webkit.WebChromeClient;
import android.webkit.WebResourceError;
import android.webkit.WebResourceRequest;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.ImageButton;
import android.widget.ProgressBar;

import java.io.File;

/**
 * Created by ahmed on 21/4/18.
 */

public class Saying extends BaseFragment 

    private String postUrl = "https://wewritesay.herokuapp.com";
    private ProgressBar progressBar;
    WebView home_web;
    ImageButton btn_home;
    private float m_downX;

    //
    public Saying() 
        // Required empty public constructor
    

    @Override
    public void hideProgress() 
        if (progressBar != null)
            progressBar.setVisibility(View.GONE);
    

    //    https://my-medium-clone.herokuapp.com/top-stories
    @Override
    public void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);

    

    @Override
    public void onResume() 
        super.onResume();
        ((AppCompatActivity) getActivity()).getSupportActionBar().hide();
    

    @Override
    public void onStop() 
        super.onStop();
        ((AppCompatActivity) getActivity()).getSupportActionBar().show();

    

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) 
        // Inflate the layout for this fragment

        View view = inflater.inflate(R.layout.fragment_saying, container, false);
        home_web = (WebView) view.findViewById(R.id.home_web);
        btn_home = view.findViewById(R.id.btn_home);
        btn_home.setOnClickListener(new View.OnClickListener() 
            @Override
            public void onClick(View view) 
                ((MainActivity) getActivity()).goToHome();
            
        );
        WebSettings settings = home_web.getSettings();
        settings.setjavascriptEnabled(true);
        home_web.getSettings().setRenderPriority(WebSettings.RenderPriority.HIGH);
        home_web.getSettings().setAppCacheEnabled(true);
        settings.setDomStorageEnabled(true);
        settings.setLayoutAlgorithm(WebSettings.LayoutAlgorithm.NARROW_COLUMNS);
        settings.setUseWideViewPort(true);
        settings.setSavePassword(true);
        settings.setSaveFormData(true);
        settings.setEnableSmoothTransition(true);
        progressBar = (ProgressBar) view.findViewById(R.id.progressBar);
//        home_web.getSettings().setJavaScriptEnabled(true);
        home_web.loadUrl(postUrl);
        home_web.setOnKeyListener(new View.OnKeyListener() 
            public boolean onKey(View v, int keyCode, KeyEvent event) 
                if ((keyCode == KeyEvent.KEYCODE_BACK) && home_web.canGoBack()) 
                    home_web.goBack();
                    return true;
                
                return false;
            
        );
        initWebView();
//        home_web.setHorizontalScrollBarEnabled(false);
        return view;
    

    private class MyWebChromeClient extends WebChromeClient 
        Context context;

        public MyWebChromeClient(Context context) 
            super();
            this.context = context;
        


    

    private void initWebView() 
        home_web.setWebChromeClient(new Saying.MyWebChromeClient(getActivity()));
        home_web.setWebViewClient(new WebViewClient() 
            @Override
            public void onPageStarted(WebView view, String url, Bitmap favicon) 
                super.onPageStarted(view, url, favicon);
                ((MainActivity) getActivity()).setUrl(url);
                progressBar.setVisibility(View.VISIBLE);
//                invalidateOptionsMenu();
            

            @Override
            public boolean shouldOverrideUrlLoading(WebView view, String url) 
                home_web.loadUrl(url);
                return true;
            

            @Override
            public void onPageFinished(WebView view, String url) 
                super.onPageFinished(view, url);
                progressBar.setVisibility(View.GONE);
//                invalidateOptionsMenu();
            

            @Override
            public void onReceivedError(WebView view, WebResourceRequest request, WebResourceError error) 
                super.onReceivedError(view, request, error);
                progressBar.setVisibility(View.GONE);
//                invalidateOptionsMenu();
            
        );
        home_web.clearCache(true);
        home_web.clearHistory();
        home_web.getSettings().setJavaScriptEnabled(true);
        home_web.setHorizontalScrollBarEnabled(false);
        home_web.setOnTouchListener(new View.OnTouchListener() 
            public boolean onTouch(View v, MotionEvent event) 

                if (event.getPointerCount() > 1) 
                    //Multi touch detected
                    return true;
                

                switch (event.getAction()) 
                    case MotionEvent.ACTION_DOWN: 
                        // save the x
                        m_downX = event.getX();
                    
                    break;

                    case MotionEvent.ACTION_MOVE:
                    case MotionEvent.ACTION_CANCEL:
                    case MotionEvent.ACTION_UP: 
                        // set x so that it doesn't move
                        event.setLocation(m_downX, event.getY());
                    
                    break;
                

                return false;
            
        );
    

【问题讨论】：

What is a NullPointerException, and how do I fix it?的可能重复

getActivity()==null.

【参考方案1】：

试试下面：

@Override
public void onAttach(Activity activity) 
super.onAttach(activity);
    mActivity = activity;

然后为了防止泄漏：

@Override
public void onDetach() 
    super.onDetach();
    mContext = null;

现在使用 MainActivity 做：

((MainActivity)mContext).setUrl(url);

【讨论】：

嘿@HaroldSer 我对android 真的很陌生，并试图自学，我不明白你做了什么，请你详细说明。这将是一个很大的帮助谢谢。

onAttach() 我们只是获取对片段附加到的活动的引用（稍后我们使用它来调用方法 setUrl(url)。现在，因为我们要确保没有内存泄漏（片段附加到的活动的引用在分离后仍然存在）我们必须在 onDetach() 上设置活动的引用 null。请参阅片段生命周期。