java - Android DrawerLayout 与 NullPointerException

Question

我遇到了这个 NullPointerException 问题。我搜索过类似的帖子并尝试了所有这些，但没有一个有效。好像我错过了一些东西，但我找不到它是什么。请帮忙。

这是调试输出:

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: project_name, PID: 4101
              java.lang.RuntimeException: Unable to start activity ComponentInfo{project_name/project_name.activity.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.support.design.widget.NavigationView.getHeaderView(int)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                  at android.app.ActivityThread.-wrap11(ActivityThread.java)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:5417)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.support.design.widget.NavigationView.getHeaderView(int)' on a null object reference
                  at project_name.activity.HomeActivity.onCreate(HomeActivity.java:93)
                  at android.app.Activity.performCreate(Activity.java:6237)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                  at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:148) 
                  at android.app.ActivityThread.main(ActivityThread.java:5417) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

我的 Activity :

package project_name.activity;

import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.NavigationView;
import android.support.design.widget.Snackbar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
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.Toolbar;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import com.bumptech.glide.Glide;
import com.bumptech.glide.load.engine.DiskCacheStrategy;

import project_name.activity.R;
import project_name.activity.fragment.DashboardFragment;
import project_name.activity.fragment.PhotosFragment;
import project_name.activity.other.CircleTransform;

public class HomeActivity extends AppCompatActivity {

    private NavigationView navigationView;
    private DrawerLayout drawer;
    private View navHeader;
    private ImageView imgNavHeaderBg, imgProfile;
    private TextView txtName, txtWebsite;
    private Toolbar toolbar;
    private FloatingActionButton fab;

    // urls to load navigation header background image
    // and profile image
    private static final String urlNavHeaderBg = "http://api.androidhive.info/images/nav-menu-header-bg.jpg";
    private static final String urlProfileImg = "https://lh3.googleusercontent.com/eCtE_G34M9ygdkmOpYvCag1vBARCmZwnVS6rS5t4JLzJ6QgQSBquM0nuTsCpLhYbKljoyS-txg";

    // index to identify current nav menu item
    public static int navItemIndex = 0;

    // tags used to attach the fragments
    private static final String TAG_HOME = "home";
    private static final String TAG_PHOTOS = "photos";
    private static final String TAG_MOVIES = "movies";
    private static final String TAG_NOTIFICATIONS = "notifications";
    private static final String TAG_SETTINGS = "settings";
    public static String CURRENT_TAG = TAG_HOME;

    // toolbar titles respected to selected nav menu item
    private String[] activityTitles;

    // flag to load home fragment when user presses back key
    private boolean shouldLoadHomeFragOnBackPress = true;
    private Handler mHandler;


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

        mHandler = new Handler();

        drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        navigationView = (NavigationView) findViewById(R.id.nav_view);
        fab = (FloatingActionButton) findViewById(R.id.fab);

        // Workarounds with Null Pointer Exception
//        View headerLayout =
//                navigationView.inflateHeaderView(R.layout.nav_header_home);
//        navigationView = (NavigationView) headerLayout.findViewById(R.id.nav_view);
//        navHeader = navigationView.inflateHeaderView(R.layout.nav_header_home);
//        navHeader = LayoutInflater.from(this).inflate(R.layout.nav_header_home, null);
//        navigationView.addHeaderView(navHeader);
//        TextView text = (TextView) navHeader.findViewById(R.id.textView);
//        text.setText("HELLO");
//        navHeader = navigationView.inflateHeaderView(R.layout.nav_header_home);

        // Navigation view header
        navHeader = navigationView.getHeaderView(0);
        txtName = (TextView) navHeader.findViewById(R.id.name);
        txtWebsite = (TextView) navHeader.findViewById(R.id.website);
        imgNavHeaderBg = (ImageView) navHeader.findViewById(R.id.img_header_bg);
        imgProfile = (ImageView) navHeader.findViewById(R.id.img_profile);

        // load toolbar titles from string resources
        activityTitles = getResources().getStringArray(R.array.nav_item_activity_titles);

        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        // load nav menu header data
        loadNavHeader();

        // initializing navigation menu
        setUpNavigationView();

        if (savedInstanceState == null) {
            navItemIndex = 0;
            CURRENT_TAG = TAG_HOME;
            loadHomeFragment();
        }
    }

    /***
     * Load navigation menu header information
     * like background image, profile image
     * name, website, notifications action view (dot)
     */
    private void loadNavHeader() {
        // name, website
        txtName.setText("My Name");
        txtWebsite.setText("project_name");

        // loading header background image
        Glide.with(this).load(urlNavHeaderBg)
                .crossFade()
                .diskCacheStrategy(DiskCacheStrategy.ALL)
                .into(imgNavHeaderBg);

        // Loading profile image
        Glide.with(this).load(urlProfileImg)
                .crossFade()
                .thumbnail(0.5f)
                .bitmapTransform(new CircleTransform(this))
                .diskCacheStrategy(DiskCacheStrategy.ALL)
                .into(imgProfile);

        // showing dot next to notifications label
        navigationView.getMenu().getItem(3).setActionView(R.layout.menu_dot);
    }

    /***
     * Returns respected fragment that user
     * selected from navigation menu
     */
    private void loadHomeFragment() {
        // selecting appropriate nav menu item
        selectNavMenu();

        // set toolbar title
        setToolbarTitle();

        // if user select the current navigation menu again, don't do anything
        // just close the navigation drawer
        if (getSupportFragmentManager().findFragmentByTag(CURRENT_TAG) != null) {
            drawer.closeDrawers();

            // show or hide the fab button
            toggleFab();
            return;
        }

        // Sometimes, when fragment has huge data, screen seems hanging
        // when switching between navigation menus
        // So using runnable, the fragment is loaded with cross fade effect
        // This effect can be seen in GMail app
        Runnable mPendingRunnable = new Runnable() {
            @Override
            public void run() {
                // update the main content by replacing fragments
                Fragment fragment = getHomeFragment();
                FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
                fragmentTransaction.setCustomAnimations(android.R.anim.fade_in,
                        android.R.anim.fade_out);
                fragmentTransaction.replace(R.id.frame, fragment, CURRENT_TAG);
                fragmentTransaction.commitAllowingStateLoss();
            }
        };

        // If mPendingRunnable is not null, then add to the message queue
        if (mPendingRunnable != null) {
            mHandler.post(mPendingRunnable);
        }

        // show or hide the fab button
        toggleFab();

        //Closing drawer on item click
        drawer.closeDrawers();

        // refresh toolbar menu
        invalidateOptionsMenu();
    }

    private Fragment getHomeFragment() {
        switch (navItemIndex) {
            case 0:
                // home
                DashboardFragment homeFragment = new DashboardFragment();
                return homeFragment;
            case 1:
                // photos
                PhotosFragment photosFragment = new PhotosFragment();
                return photosFragment;
            case 2:
                // movies fragment
                DashboardFragment moviesFragment = new DashboardFragment();
                return moviesFragment;
            case 3:
                // notifications fragment
                DashboardFragment notificationsFragment = new DashboardFragment();
                return notificationsFragment;

            case 4:
                // settings fragment
                DashboardFragment settingsFragment = new DashboardFragment();
                return settingsFragment;
            default:
                return new DashboardFragment();
        }
    }

    private void setToolbarTitle() {
        getSupportActionBar().setTitle(activityTitles[navItemIndex]);
    }

    private void selectNavMenu() {
        navigationView.getMenu().getItem(navItemIndex).setChecked(true);
    }

    private void setUpNavigationView() {
        //Setting Navigation View Item Selected Listener to handle the item click of the navigation menu
        navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {

            // This method will trigger on item Click of navigation menu
            @Override
            public boolean onNavigationItemSelected(MenuItem menuItem) {

                //Check to see which item was being clicked and perform appropriate action
                switch (menuItem.getItemId()) {
                    //Replacing the main content with ContentFragment Which is our Inbox View;
                    case R.id.nav_home:
                        navItemIndex = 0;
                        CURRENT_TAG = TAG_HOME;
                        break;
                    case R.id.nav_photos:
                        navItemIndex = 1;
                        CURRENT_TAG = TAG_PHOTOS;
                        break;
                    case R.id.nav_movies:
                        navItemIndex = 2;
                        CURRENT_TAG = TAG_MOVIES;
                        break;
                    case R.id.nav_notifications:
                        navItemIndex = 3;
                        CURRENT_TAG = TAG_NOTIFICATIONS;
                        break;
                    case R.id.nav_settings:
                        navItemIndex = 4;
                        CURRENT_TAG = TAG_SETTINGS;
                        break;
                    case R.id.nav_about_us:
                        // launch new intent instead of loading fragment
                        startActivity(new Intent(HomeActivity.this, MainActivity.class));
                        drawer.closeDrawers();
                        return true;
                    case R.id.nav_privacy_policy:
                        // launch new intent instead of loading fragment
                        startActivity(new Intent(HomeActivity.this, MainActivity.class));
                        drawer.closeDrawers();
                        return true;
                    default:
                        navItemIndex = 0;
                }

                //Checking if the item is in checked state or not, if not make it in checked state
                if (menuItem.isChecked()) {
                    menuItem.setChecked(false);
                } else {
                    menuItem.setChecked(true);
                }
                menuItem.setChecked(true);

                loadHomeFragment();

                return true;
            }
        });


        ActionBarDrawerToggle actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.openDrawer, R.string.closeDrawer) {

            @Override
            public void onDrawerClosed(View drawerView) {
                // Code here will be triggered once the drawer closes as we dont want anything to happen so we leave this blank
                super.onDrawerClosed(drawerView);
            }

            @Override
            public void onDrawerOpened(View drawerView) {
                // Code here will be triggered once the drawer open as we dont want anything to happen so we leave this blank
                super.onDrawerOpened(drawerView);
            }
        };

        //Setting the actionbarToggle to drawer layout
        drawer.setDrawerListener(actionBarDrawerToggle);

        //calling sync state is necessary or else your hamburger icon wont show up
        actionBarDrawerToggle.syncState();
    }

    @Override
    public void onBackPressed() {
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawers();
            return;
        }

        // This code loads home fragment when back key is pressed
        // when user is in other fragment than home
        if (shouldLoadHomeFragOnBackPress) {
            // checking if user is on other navigation menu
            // rather than home
            if (navItemIndex != 0) {
                navItemIndex = 0;
                CURRENT_TAG = TAG_HOME;
                loadHomeFragment();
                return;
            }
        }

        super.onBackPressed();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.

        // show menu only when home fragment is selected
        if (navItemIndex == 0) {
            getMenuInflater().inflate(R.menu.home, menu);
        }

        // when fragment is notifications, load the menu created for notifications
        if (navItemIndex == 3) {
            getMenuInflater().inflate(R.menu.notifications, 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_logout) {
            Toast.makeText(getApplicationContext(), "Logout user!", Toast.LENGTH_LONG).show();
            return true;
        }

        // user is in notifications fragment
        // and selected 'Mark all as Read'
        if (id == R.id.action_mark_all_read) {
            Toast.makeText(getApplicationContext(), "All notifications marked as read!", Toast.LENGTH_LONG).show();
        }

        // user is in notifications fragment
        // and selected 'Clear All'
        if (id == R.id.action_clear_notifications) {
            Toast.makeText(getApplicationContext(), "Clear all notifications!", Toast.LENGTH_LONG).show();
        }

        return super.onOptionsItemSelected(item);
    }

    // show or hide the fab
    private void toggleFab() {
        if (navItemIndex == 0)
            fab.show();
        else
            fab.hide();
    }
}

我认为其他 Activity 和 fragment 无关紧要。所以我会给你布局文件。这是抽屉:

<?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_home"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <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"
        app:headerLayout="@layout/nav_header_home"
        app:menu="@menu/activity_home_drawer" />

</android.support.v4.widget.DrawerLayout>

抽屉本身的标题:

<?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"
    android:id="@+id/view_container"
    android:layout_width="match_parent"
    android:layout_height="@dimen/nav_header_height"
    android:gravity="bottom"
    android:orientation="vertical"
    android:theme="@style/ThemeOverlay.AppCompat.Dark">

    <ImageView
        android:id="@+id/img_header_bg"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scaleType="fitXY"
        android:src="@drawable/favicon" />

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:orientation="vertical"
        android:padding="@dimen/activity_horizontal_margin">

        <ImageView
            android:id="@+id/img_profile"
            android:layout_width="@dimen/profile_width"
            android:layout_height="@dimen/profile_height"
            android:paddingTop="@dimen/nav_header_vertical_spacing"
            app:srcCompat="@android:drawable/sym_def_app_icon" />

        <TextView
            android:id="@+id/name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:paddingTop="@dimen/nav_header_vertical_spacing"
            android:textAppearance="@style/TextAppearance.AppCompat.Body1" />

        <TextView
            android:id="@+id/website"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />

    </LinearLayout>
</RelativeLayout>

请帮忙，我现在真的迷路了。

Answer 1

当您调用 findViewById() 时，仅搜索您的 Activity 布局，并且您的 navigationView 位于 DrawerLayout 中。

这会导致 navigationView 为 null，并且在 null 对象上调用 getHeaderView(0) 会导致 NullPointerException。

尝试切换到:

drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
navigationView = (NavigationView) drawer.findViewById(R.id.nav_view);