我在使用 MySQL 时遇到此错误，并且不知道如何修复它。自从我从Connection.execute[whatever]()切换, 至 PreparedStatement ，我一直收到此错误，但似乎无法修复。

空指针异常:

guild info space
[09:51:56 WARN]: java.lang.NullPointerException
[09:51:56 WARN]:    at net.aspace.guilds.Guild.getGuild(Guild.java:56)
[09:51:56 WARN]:    at net.aspace.guilds.command.GuildsInfoCommand.execute(GuildsInfoCommand.java:28)
[09:51:56 WARN]:    at net.aspace.command.CommandNode.onCommand(CommandNode.java:64)
[09:51:56 WARN]:    at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
[09:51:56 WARN]:    at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:140)
[09:51:56 WARN]:    at org.bukkit.craftbukkit.v1_10_R1.CraftServer.dispatchCommand(CraftServer.java:646)
[09:51:56 WARN]:    at org.bukkit.craftbukkit.v1_10_R1.CraftServer.dispatchServerCommand(CraftServer.java:632)
[09:51:56 WARN]:    at net.minecraft.server.v1_10_R1.DedicatedServer.aL(DedicatedServer.java:438)
[09:51:56 WARN]:    at net.minecraft.server.v1_10_R1.DedicatedServer.D(DedicatedServer.java:401)
[09:51:56 WARN]:    at net.minecraft.server.v1_10_R1.MinecraftServer.C(MinecraftServer.java:668)
[09:51:56 WARN]:    at net.minecraft.server.v1_10_R1.MinecraftServer.run(MinecraftServer.java:567)
[09:51:56 WARN]:    at java.lang.Thread.run(Thread.java:745)
[09:51:56 INFO]: Guild space does not exist!

参数索引超出范围异常:

java.sql.SQLException: Parameter index out of range (1 > number of parameters, which is 0).
[11:11:37 WARN]:    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:963)
[11:11:37 WARN]:    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:896)
[11:11:37 WARN]:    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:885)
[11:11:37 WARN]:    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:860)
[11:11:37 WARN]:    at com.mysql.jdbc.PreparedStatement.checkBounds(PreparedStatement.java:3321)
[11:11:37 WARN]:    at com.mysql.jdbc.PreparedStatement.setInternal(PreparedStatement.java:3306)
[11:11:37 WARN]:    at com.mysql.jdbc.PreparedStatement.setString(PreparedStatement.java:4021)
[11:11:37 WARN]:    at net.aspace.guilds.Guild.getGuild(Guild.java:57)
[11:11:37 WARN]:    at net.aspace.guilds.command.GuildsInfoCommand.execute(GuildsInfoCommand.java:28)

这是 Guild类:

package net.aspace.guilds;

import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Logger;

import net.aspace.chat.Message;
import net.aspace.guilds.GuildsPlugin;
import net.aspace.guilds.util.MySQL;
import net.aspace.util.SimpleCallback;

import org.bukkit.entity.Player;
import org.bukkit.plugin.Plugin;
import org.bukkit.scheduler.BukkitRunnable;
import org.bukkit.scheduler.BukkitTask;

public class Guild {

    public String name;
    public List<String> members;
    public int maxPlayers = 3;

    public Guild(String name) {
    }

    public Guild(String name, List<String> members, int maxPlayers) {
        this.name = name;
        this.members = members;
        this.maxPlayers = maxPlayers;
    }

    public void addPlayer(final Player p) {
        final String gName = this.name;
        new BukkitRunnable() {

            public void run() {
                try {
                    PreparedStatement ps = MySQL.getConnection()
                            .prepareStatement("UPDATE PlayerData SET GUILD='?' WHERE PLAYERNAME='?'");
                    ps.setString(1, gName);
                    ps.setString(2, p.getName());
                    ps.executeUpdate();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }.runTaskAsynchronously((Plugin) GuildsPlugin.getInstance());
    }

    public static void getGuild(final String name, final SimpleCallback<Guild> callback) {
        try {
            PreparedStatement ps = MySQL.getConnection().prepareStatement("SELECT * FROM Guilds WHERE GUILD_NAME='?'");
            ps.setString(1, name);
            ArrayList<String> players = new ArrayList<String>();

            ResultSet set = ps.executeQuery();
            if (set.next()) {
                String gName = set.getString("GUILD_NAME");
                int maxMembers = set.getInt("MAX_MEMBERS");
                try {
                    PreparedStatement ps2 = MySQL.getConnection()
                            .prepareStatement("SELECT * FROM PlayerData WHERE GUILD = '?'");
                    ps2.setString(1, name);
                    ResultSet result = ps2.executeQuery();
                    GuildsPlugin.getInstance().getLogger().info("select");
                    while (result.next()) {
                        players.add(result.getString("PLAYERNAME"));
                    }
                    result.close();
                    callback.execute(new Guild(gName, players, maxMembers));
                } catch (SQLException e) {
                    e.printStackTrace();
                }
                set.close();

            }
        } catch (SQLException e) {
            e.printStackTrace();
            callback.execute(null);
        } catch (NullPointerException e) {
            e.printStackTrace();
            callback.execute(null);
        }
    }

    public static void createGuild(final Player p, final String name) {
        new BukkitRunnable() {

            @Override
            public void run() {
                try {
                PreparedStatement ps = MySQL.getConnection().prepareStatement("SELECT * FROM PlayerData WHERE PLAYER_NAME = '?'");
                ps.setString(1,p.getName());



                    //Get player data
                    ResultSet result = ps.executeQuery();
                    if (result.first()) {
                        //Check if player is in a guild
                        if (result.getString("GUILD") == null) {
                            // TODO Check if guild exists
                            try {
                                //Add guild
                                PreparedStatement ps2 = MySQL.getConnection().prepareStatement("INSERT INTO Guilds (GUILD_NAME,MAX_MEMBERS) VALUES ('?',10)");
                                ps2.setString(1,name);
                                ps2.executeUpdate();
                                //Add player to guild
                                PreparedStatement ps3 = MySQL.getConnection().prepareStatement("UPDATE PlayerData SET GUILD='?'"
                                        + "WHERE PLAYER_NAME='?'");
                                ps3.setString(1,name);
                                ps3.setString(2,p.getName());
                                p.sendMessage(Message.fromString(String.format("&eYou have created and joined the guild &b%s&e!",name)));
                            } catch (SQLException e) {
                                e.printStackTrace();
                            }
                        } else {
                            //Player is in a guild
                            p.sendMessage(Message.fromString("&cYou are already in a guild!"));
                        }
                    }
                } catch (SQLException e1) {
                    e1.printStackTrace();
                }
            }
        }.runTaskAsynchronously(GuildsPlugin.getInstance());
    }

}

这是 GuildInfoCommand类:

package net.aspace.guilds.command;

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

import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.plugin.Plugin;

import net.aspace.chat.Message;
import net.aspace.command.CommandNode;
import net.aspace.guilds.Guild;
import net.aspace.guilds.GuildsPlugin;
import net.aspace.guilds.util.GuildsPlayer;
import net.aspace.util.SimpleCallback;

public class GuildsInfoCommand<E extends Plugin> extends CommandNode<E> {

    public GuildsInfoCommand(E plugin) {
        super(plugin);
        // TODO Auto-generated constructor stub
    }

    @Override
    public boolean execute(CommandSender sender, String... args) {
        if (args.length >= 1) {
            Guild.getGuild(args[0], new SimpleCallback<Guild>() {

                @Override
                public void execute(Guild guild) {
                    if (guild != null) {

                        sender.sendMessage(Message.fromString("&b&lGUILD INFO"));
                        sender.sendMessage(Message.fromString("&eGuild name: " + guild.name));
                        sender.sendMessage(Message.fromString("&eGuild maxmembers: " + guild.maxPlayers));
                        sender.sendMessage(
                                Message.fromString("&eGuild members &7(" + Integer.toString(guild.members.size()) + "/"
                                        + Integer.toString(guild.maxPlayers) + ")&e: "));
                        guild.members.forEach(s -> {
                            sender.sendMessage(Message.fromString("&7- " + s));
                        });
                        sender.sendMessage(Message.fromString("&b&lGUILD INFO"));

                    } else {
                        sender.sendMessage(Message.fromString(String.format("&cGuild &e%s&c does not exist!", args[0])));
                    }

                }
            });

        } else {
            if (sender instanceof Player) {
                Player player = (Player) sender;
                GuildsPlayer player2 = GuildsPlayer.getPlayer(player);
                player2.inGuild(new SimpleCallback<Boolean>() {

                    @Override
                    public void execute(Boolean response) {
                        if (response) {
                            Guild.getGuild(args[0], new SimpleCallback<Guild>() {

                                @Override
                                public void execute(Guild guild) {
                                    if (guild != null) {
                                        sender.sendMessage(Message.fromString("&b&lGUILD INFO"));
                                        sender.sendMessage(Message.fromString("&eGuild name: &b" + guild.name));
                                        sender.sendMessage(Message.fromString("&eMember Limit: &b" + guild.maxPlayers));
                                        sender.sendMessage(Message.fromString(
                                                "&eGuild members &7(" + Integer.toString(guild.members.size()) + "/"
                                                        + Integer.toString(guild.maxPlayers) + ")&e: "));
                                        guild.members.forEach(s -> {
                                            sender.sendMessage(Message.fromString("&7- " + s));
                                        });
                                        sender.sendMessage(Message.fromString("&b&lGUILD INFO"));

                                    } else {
                                        sender.sendMessage(Message.fromString("&cYou are not in a guild!"));
                                    }
                                }

                            });
                        } else {

                        }
                    }
                });
            } else {
                sender.sendMessage(Message.fromString("&cYou must specify a guild name!"));
            }
        }

        return true;
    }

    @Override
    public String getName() {
        // TODO Auto-generated method stub
        return "info";
    }

}

MySQL 的代码类:

package net.aspace.guilds.util;

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

import org.bukkit.Bukkit;
import org.bukkit.command.ConsoleCommandSender;

public class MySQL {

    public static String host = "****";
    public static String port = "****";
    public static String database = "****";
    public static String username = "****";
    public static String password = "****";
    public static Connection con;

    static ConsoleCommandSender console = Bukkit.getConsoleSender();

    // connect
    public static void connect() {
        if (!isConnected()) {
            try {
                con = DriverManager.getConnection("jdbc:mysql://" + host + ":" + port + "/" + database, username, password);
                console.sendMessage("\247c[\2476Minepedia-System\247c] \247bMySQL-Verbindung wurde aufgebaut!");
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // disconnect
    public static void disconnect() {
        if (isConnected()) {
            try {
                con.close();
                console.sendMessage("\247c[\2476Minepedia-System\247c]\247bMySQL-Verbindung wurde geschlossen!");
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // isConnected
    public static boolean isConnected() {
        return (con == null ? false : true);
    }

    // getConnection
    public static Connection getConnection() {
        return con;
    }
}

最佳答案

在尝试调用 MySQL.getConnection() 之前，您需要先调用 MySQL.connect() 方法。

当你在没有 MySQL.connect() 方法的情况下调用 MySQL.getConnection() 方法时，你不会创建连接，结果得到了 NullPointerException 尝试调用 MySQL.getConnection().prepareStatement...

您需要删除 ' 字符，因为 SQL 解析器将它们解释为字符串值并省略占位符 ?。

例如:

PreparedStatement ps = 
    MySQL.getConnection().prepareStatement("SELECT * FROM Guilds WHERE GUILD_NAME=?");

关于java - 使用 MySQL 和 PreparedStatement 的 NullPointerException 和参数索引超出范围异常，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/40745236/