我在使用MySQL时遇到此错误，无法弄清楚如何修复它。自从我从Connection.execute[whatever]()切换到PreparedStatement后，我一直收到此错误，但似乎无法解决此问题。
的NullPointerException：
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)
以下是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());
}
}
以下是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类的代码：
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;
}
}
答案 0 :(得分：3)
在尝试调用MySQL.connect()之前，需要调用MySQL.getConnection()方法。
当您使用MySQL.getConnection()方法调用MySQL.connect()方法时，您不会创建连接，并且在尝试调用NullPointerException时获得MySQL.getConnection().prepareStatement...
您需要删除'个字符，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=?");