Java mySQL 查询

Question

基本上我只是检查 mySQL 数据库中是否存在某个玩家，但它不断抛出错误等。我不知道该怎么做。（这都是在 bukkit api 中完成的，但问题只是 SQL）

玩家登录后会调用databaseFindPlayer方法查看数据库中是否存在

我只需要查看玩家是否存在于数据库中

 @EventHandler
    public void onPlayerLogin(PlayerJoinEvent e) throws SQLException {
        getLogger().info("Player joining.... + TEST");
        if (databaseFindPlayer(e.getPlayer().getName()) == false) {
            getLogger().info("Player does not have data file.. Generating one now");
            databaseCreatePlayer(e.getPlayer().getName());
            databaseFindPlayer(e.getPlayer().getName()); //For debugging
        }
    }





  public boolean databaseFindPlayer(String name) throws SQLException {
            getLogger().info("finding players");
            PreparedStatement preparedStatement = c.prepareStatement("show columns from players where field = '" + name + "';");
            preparedStatement.setString(1, name);

            ResultSet resultSet = preparedStatement.executeQuery();

            if (resultSet.next()) {
                System.out.println("Player exists!");
                return true;
            } else {
                System.out.println("Player doesn't exist!");
                return false;
            }


    }

（所有的 getLogger() 调用都是为了调试）

编辑：

Error message
[18:14:17 ERROR]: Could not pass event PlayerJoinEvent to CustomLeaderboard v0.1

org.bukkit.event.EventException
        at org.bukkit.plugin.java.JavaPluginLoader.execute(JavaPluginLoader.ja
va:294) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.jav
a:62) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.j
ava:501) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.j
ava:486) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.PlayerList.c(PlayerList.java:251) [craft
bukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.PlayerList.a(PlayerList.java:138) [craft
bukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.LoginListener.c(LoginListener.java:76) [
craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.LoginListener.a(LoginListener.java:42) [
craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.NetworkManager.a(NetworkManager.java:160
) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.ServerConnection.c(SourceFile:134) [craf
tbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.MinecraftServer.v(MinecraftServer.java:6
67) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.DedicatedServer.v(DedicatedServer.java:2
58) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.MinecraftServer.u(MinecraftServer.java:5
58) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.MinecraftServer.run(MinecraftServer.java
:469) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at net.minecraft.server.v1_7_R4.ThreadServerApplication.run(SourceFile:6
28) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
Caused by: java.sql.SQLException: Parameter index out of range (1 > number of pa
rameters, which is 0).
        at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1073) ~[craf
tbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:987) ~[craft
bukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:982) ~[craft
bukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:927) ~[craft
bukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at com.mysql.jdbc.PreparedStatement.checkBounds(PreparedStatement.java:3
709) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at com.mysql.jdbc.PreparedStatement.setInternal(PreparedStatement.java:3
693) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at com.mysql.jdbc.PreparedStatement.setString(PreparedStatement.java:454
4) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        at com.mcaletheia.customLeaderboard.CustomLeaderboard.databaseFindPlayer
(CustomLeaderboard.java:88) ~[?:?]
        at com.mcaletheia.customLeaderboard.CustomLeaderboard.onPlayerLogin(Cust
omLeaderboard.java:69) ~[?:?]
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0
_25]
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0
_25]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1
.8.0_25]
        at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_25]
        at org.bukkit.plugin.java.JavaPluginLoader.execute(JavaPluginLoader.ja
va:292) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-24-g07d4558-b3116jnks]
        ... 14 more

Answer 1

异常准确地告诉了哪里出了问题

Caused by: java.sql.SQLException: Parameter index out of range (1 > number of parameters, which is 0).

回顾一下您使用

声明语句的代码

PreparedStatement preparedStatement = c.prepareStatement("show columns from players where field = '" + name + "';");
preparedStatement.setString(1, name);

但是您没有提供可以绑定您的值的参数

相反，应该是

PreparedStatement preparedStatement = c.prepareStatement("show columns from players where field = ?");
preparedStatement.setString(1, name);

有关详细信息，请参阅 Using Prepared Statements

Answer 2

您在这里混淆了 PreparedStatement class 的意图。

PreparedStatement preparedStatement = c.prepareStatement("show columns from players where field = '" + name + "';");
preparedStatement.setString(1, name);

说 setString(1, name)，就是说 "Set the first replacement in the query to whatever is in the field called name"。由于您的查询没有任何替换，因此它正在抱怨。（参数索引超出范围）。它知道你有零个参数，而你告诉它你有一个参数。

试试这个（注意，我还删除了您查询末尾的分号）：

PreparedStatement preparedStatement = c.prepareStatement("show columns from players where field = ?");
preparedStatement.setString(1, name);

解决此问题的另一种方法是根本不设置 name 参数，但通过手动插入一个可能被污染的值，您将面临 SQL 注入攻击。坚持使用上面概述的 ? 替换。

Java mySQL 查询

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