java - 尝试结束 session 时应用程序崩溃..空对象引用

Question

我正在尝试为我正在开发的应用程序实现一个注销按钮。当我从导航菜单中单击注销时 session.logoutUser();被调用，并在我的 IDE 中导致以下错误；

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ravi.myapplication, PID: 32694
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.ravi.myapplication.SessionHandler.logoutUser()' on a null object reference
    at com.example.ravi.myapplication.BaseActivity$1.onNavigationItemSelected(BaseActivity.java:57)
    at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:154)
    at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
    at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
    at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
    at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:342)
    at android.view.View.performClick(View.java:6597)
    at android.view.View.performClickInternal(View.java:6574)
    at android.view.View.access$3100(View.java:778)
    at android.view.View$PerformClick.run(View.java:25885)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

当我在基本 Activity 类开始时调用 session 类时，我收到一条关于从未分配的私有(private)字段的警告。如果有人能解释为什么会发生这种情况，我们将非常感激!类的代码如下；

BaseActivity.java

package com.example.ravi.myapplication;

import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.design.widget.NavigationView;
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.MenuItem;
import android.widget.Spinner;

public class BaseActivity extends AppCompatActivity {
private SessionHandler session;
DrawerLayout drawerLayout;
ActionBarDrawerToggle actionBarDrawerToggle;
Toolbar toolbar;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_base);
    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);

    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawerLayout.setDrawerListener(actionBarDrawerToggle);



    navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(MenuItem item) {
            final String appPackageName = getPackageName();

            switch (item.getItemId()) {

                case R.id.nav_dashboard:
                    Intent dash = new Intent(getApplicationContext(), MainActivity.class);
                    startActivity(dash);
//                        finish();
                    drawerLayout.closeDrawers();
                    break;


                case R.id.nav_about_us:
                    Intent anIntent = new Intent(getApplicationContext(), AboutUS.class);
                    startActivity(anIntent);
//                        finish();
                    drawerLayout.closeDrawers();
                    break;

                case R.id.nav_logout:
                    session.logoutUser();
                    Intent i = new Intent(getApplicationContext(), LoginActivity.class);
                    startActivity(i);
                    break;


            }
            return false;
        }
    });

}

@Override
protected void onPostCreate(Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);

    actionBarDrawerToggle.syncState();
}

@Override
public void finish() {
    super.finish();
    overridePendingTransitionExit();
}

@Override
public void startActivity(Intent intent) {
    super.startActivity(intent);
    overridePendingTransitionEnter();
}

/**
 * Overrides the pending Activity transition by performing the "Enter" animation.
 */
protected void overridePendingTransitionEnter() {
    overridePendingTransition(R.anim.slide_from_right, R.anim.slide_to_left);
}

/**
 * Overrides the pending Activity transition by performing the "Exit" animation.
 */
protected void overridePendingTransitionExit() {
    overridePendingTransition(R.anim.slide_from_left, R.anim.slide_to_right);
}
}

SessionHandler.java

package com.example.ravi.myapplication;

import android.content.Context;
import android.content.SharedPreferences;

import java.util.Date;

public class SessionHandler {
private static final String PREF_NAME = "UserSession";
private static final String KEY_USERNAME = "username";
private static final String KEY_EXPIRES = "expires";
private static final String KEY_FULL_NAME = "full_name";
private static final String KEY_EMPTY = "";
private Context mContext;
private SharedPreferences.Editor mEditor;
private SharedPreferences mPreferences;

public SessionHandler(Context mContext) {
    this.mContext = mContext;
    mPreferences = mContext.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
    this.mEditor = mPreferences.edit();
}

/**
 * Logs in the user by saving user details and setting session
 */
public void loginUser(String username, String fullName) {
    mEditor.putString(KEY_USERNAME, username);
    mEditor.putString(KEY_FULL_NAME, fullName);
    Date date = new Date();

    //Set user session for next 7 days
    long millis = date.getTime() + (7 * 24 * 60 * 60 * 1000);
    mEditor.putLong(KEY_EXPIRES, millis);
    mEditor.commit();
}

/**
 * Checks whether user is logged in
 */
public boolean isLoggedIn() {
    Date currentDate = new Date();

    long millis = mPreferences.getLong(KEY_EXPIRES, 0);

    /* If shared preferences does not have a value
     then user is not logged in
     */
    if (millis == 0) {
        return false;
    }
    Date expiryDate = new Date(millis);

    /* Check if session is expired by comparing
    current date and Session expiry date
    */
    return currentDate.before(expiryDate);
}

/**
 * Fetches and returns user details
 */
public User getUserDetails() {
    //Check if user is logged in first
    if (!isLoggedIn()) {
        return null;
    }
    User user = new User();
    user.setUsername(mPreferences.getString(KEY_USERNAME, KEY_EMPTY));
    user.setFullName(mPreferences.getString(KEY_FULL_NAME, KEY_EMPTY));
    user.setSessionExpiryDate(new Date(mPreferences.getLong(KEY_EXPIRES, 0)));

    return user;
}

/**
 * Logs out user by clearing the session
 */
public void logoutUser(){
    mEditor.clear();
    mEditor.commit();
}

}

Answer 1

到 BaseActivity 类中的 session 字段未分配任何值，因此为 null