为什么MySQL数据库返回空的ResultSet，尽管数据应该是正确的

Question

上下文：我的数据库中有两个不同的表。第一个是actionblock（Java中的actionblock)，第二个是location（Java中的PermanentLocation）。ActionBlock类需要一个位置才能工作，因此为了让代码工作，它首先加载ActionBlock数据，其中包含所需的位置，然后代码加载所需的位置。（是的，我知道下面的代码将垃圾邮件加载位置查询）

问题：使用与ActionBlock加载程序相同的DB连接加载该位置会给出空的ResultSet

我所尝试的：

null

/**
 * Loads the blocks from the database. All registerClass calls must be called before this
 */
public void loadBlocksFromDatabase() {
    Connection conn = null;
    try {
        conn = KaranteeniCore.getDatabaseConnector().openConnection();
        PreparedStatement stmt = conn.prepareStatement(
                "SELECT uuid,permission,classtype FROM actionblock WHERE serverID = ?;");
        stmt.setString(1, KaranteeniCore.getServerIdentificator());
        ResultSet set = stmt.executeQuery();

        // loop all the blocks from the database
        while(set.next()) {
            String uuidStr = set.getString(1);
            String permission = set.getString(2);
            String classType = set.getString(3);

            UUID uuid = UUID.fromString(uuidStr);

            // check if loaded block is a block or a sign
            for(Class<? extends ActionBlock> clazz : subBlockClasses)
            if(clazz.getName().equals(classType)) {
                // load the location of the block
                PermanentLocation loc = PermanentLocation.loadLocation(conn, uuid);
                if(loc == null) {
                    Bukkit.getLogger().log(Level.WARNING, "Could not load location " + uuid.toString() + ". Has it been deleted from database?");
                    continue;
                }

                Block block = loc.getLocation().getBlock();
                try {
                    // try to create a new class out of this block
                    if(permission != null) {
                        Constructor<? extends ActionBlock> cstr = clazz.getConstructor(Block.class, UUID.class, String.class);
                        ActionBlock aBlock = cstr.newInstance(block, uuid, permission);
                        aBlock.onLoad();
                        registerBlock(aBlock); // register created class
                    } else {
                        Constructor<? extends ActionBlock> cstr = clazz.getConstructor(Block.class, UUID.class);
                        ActionBlock aBlock = cstr.newInstance(block, uuid);
                        aBlock.onLoad();
                        registerBlock(aBlock); // register created class
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }

            // load all signs
            for(Class<? extends ActionSign> clazz : subSignClasses)
            if(clazz.getName().equals(classType)) {
                // load the location of the block
                Block block = PermanentLocation.loadLocation(uuid).getLocation().getBlock();
                try {
                    // try to create a new class out of this block
                    if(permission != null) {
                        Constructor<? extends ActionSign> cstr = clazz.getConstructor(Block.class, UUID.class, String.class);
                        ActionBlock aBlock = cstr.newInstance(block, uuid, permission);
                        aBlock.onLoad();
                        registerBlock(aBlock); // register created class
                    } else {
                        Constructor<? extends ActionSign> cstr = clazz.getConstructor(Block.class, UUID.class);
                        ActionBlock aBlock = cstr.newInstance(block, uuid);
                        aBlock.onLoad();
                        registerBlock(aBlock); // register created class
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    } catch(SQLException e) {
        e.printStackTrace();
        // ignored
    } finally {
        try {
            if(conn != null)
                conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

/**
 * Loads a permanent location from database using the given uuid
 * @param conn Connection to the database
 * @param uuid uuid of the location
 * @return location loaded or null if none found
 */
public static PermanentLocation loadLocation(Connection conn, UUID uuid) {
    PermanentLocation location = null;
    try {
        PreparedStatement st = conn.prepareStatement("SELECT world, x, y, z, pitch, yaw FROM location WHERE id = ? AND serverID = ?;");
        st.setString(1, uuid.toString());
        st.setString(2, KaranteeniPlugin.getServerIdentificator());
        ResultSet set = st.executeQuery();
        if(set.next()) {
            String w = set.getString(1);
            double x = set.getDouble(2);
            double y = set.getDouble(3);
            double z = set.getDouble(4);
            float pitch = set.getFloat(5);
            float yaw = set.getFloat(6);

            World world = Bukkit.getWorld(w);
            if(world == null)
                return null;
            else
                Bukkit.getLogger().log(Level.SEVERE, "Could not find world " + w);

            Location loc = new Location(world, x,y,z,pitch,yaw);

            location = new PermanentLocation(uuid, loc);
        }
        else
            location = null;
    } catch(SQLException e) {
        Bukkit.getLogger().log(Level.SEVERE, e.getMessage());
    }

    return location;
}

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编辑：重新启动数据库或系统不影响结果

Answer 1

我想问题只是错字，请换行

  "SELECT uuid,permission,classtype FROM actionblock WHERE serverID = ?;");

与

"SELECT uuid,permission,classtype FROM actionblock WHERE serverID = ?");

希望我能有所帮助；