java.lang.NullPointerException jdbc大错误

我是java编程新手，因此我有一个无法解决的大错误：

以下是异常的堆栈跟踪：

java.lang.NullPointerException
    at com.smy.gome.MySQL.checkUser(MySQL.java:56)
    at com.smy.gome.LogReg$1.actionPerformed(LogReg.java:43)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

我在MySQL.java的代码：56

以下是Java代码：

ResultSet user = statement.executeQuery("SELECT * FROM tadzio WHERE nick='benas12' AND pass='benas'");

MySQL.java：

package com.smy.gome;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class MySQL {

    static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
    static final String DB_URL = "jdbc:mysql://localhost/root";

    static final String USER = "root";
    static final String PASS = "benux1";

    public static Connection conn = null;
    public static Statement statement = null;
    public static ResultSet results = null;

    public synchronized static void Connection(){
        try{
            Class.forName("com.mysql.jdbc.Driver").newInstance();
            conn = DriverManager.getConnection("jdbc:mysql://localhost/root", "root", "benux1");
        }catch(Exception e) {
            e.printStackTrace();
        }
    }

    public synchronized static void Destroy(){
        try {
            statement.close();
            conn.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public synchronized static ResultSet query(String s) throws SQLException {
        try {
            if (s.toLowerCase().startsWith("select")) {
                ResultSet rs = statement.executeQuery(s);
                return rs;
            } else {
                statement.executeUpdate(s);
            }
            return null;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    public static int checkUser() {
        try {
            conn.createStatement();
            ResultSet user = statement.executeQuery("SELECT * FROM tadzio WHERE nick='benas12' AND pass='benas'");
            user.next();
            int count = user.getRow();
            return count;
        } catch(Exception e) {
            e.printStackTrace();
            return -1;
        }
    }

    public static String getVersion(){

        return "tadis";
    }

}

---

感谢您帮助我

您没有分配

语句

。将代码更改为以下内容：

public static int checkUser() {
    try {
        statement = conn.createStatement();
        ResultSet user = statement.executeQuery("SELECT * FROM tadzio WHERE nick='benas12' AND pass='benas'");
        user.next();
        int count = user.getRow();
        return count;
    } catch(Exception e) {
        e.printStackTrace();
        return -1;
    }
}

---

它是由

语句

为

null

要使整个代码按预期工作，请使用以下代码，因为

语句

在不同的位置使用

package com.smy.gome;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class MySQL {

    static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
    static final String DB_URL = "jdbc:mysql://localhost/root";

    static final String USER = "root";
    static final String PASS = "benux1";

    public static Connection conn = null;
    public static Statement statement = null;
    public static ResultSet results = null;

    public synchronized static void Connection(){
        try{
            Class.forName("com.mysql.jdbc.Driver").newInstance();
            conn = DriverManager.getConnection("jdbc:mysql://localhost/root", "root", "benux1");
            statement = conn.createStatement();
        }catch(Exception e) {
            e.printStackTrace();
        }
    }

    public synchronized static void Destroy(){
        try {
            statement.close();
            conn.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public synchronized static ResultSet query(String s) throws SQLException {
        try {
            if (s.toLowerCase().startsWith("select")) {
                ResultSet rs = statement.executeQuery(s);
                return rs;
            } else {
                statement.executeUpdate(s);
            }
            return null;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    public static int checkUser() {
        try {
            ResultSet user = statement.executeQuery("SELECT * FROM tadzio WHERE nick='benas12' AND pass='benas'");
            user.next();
            int count = user.getRow();
            return count;
        } catch(Exception e) {
            e.printStackTrace();
            return -1;
        }
    }

    public static String getVersion(){

        return "tadis";
    }

}

---

你是否在第56行之前创建了一条语句。。i、 你有像CreateStatement（）或prepareStatement（）这样的行吗？用这个堆栈跟踪发布一些代码。这将很有帮助：）变量语句可能未初始化。请确保“statement”对象“not null”。获取连接对象，通过连接创建statement对象，然后使用statement对象进行查询。您需要添加相关代码，以便人们真正帮助您。另外，请在这里或谷歌搜索其他人面临的类似问题。我相信NullPointerException本身会有很多问题和答案。这段代码对我不起作用。你需要更具体一点。@JurgenCamilleri他正在其他地方使用

语句@Smygolas使用我答案中的代码。希望它能起作用。如果出现其他错误，请共享堆栈跟踪
conn.CreateStatement（）应该从
checkUser（）
中删除。@JurgenCamilleri谢谢你指出，我同意！！请根据粘滞痕迹和行号编辑。另外，在运行之前，您是否再次编译了MySQL.java文件？@Smygolas up vote if working:）今天将是我在stackoverflow的日子（贪图名誉和分数）