conn.prepareStatement(sql) 上的 java.lang.NullPointerException答案

【问题标题】：java.lang.NullPointerException on conn.prepareStatement(sql)conn.prepareStatement(sql) 上的 java.lang.NullPointerException
【发布时间】：2018-09-21 02:02:57
【问题描述】：

我正在开发一个使用带有 glassfish 的 postgresql 的项目。当我执行 Statement PreparedStatement pstmt = conn.prepareStatement(sql) 时，它会给我 java.lang.NullPointerException。

这是我的代码：

    String sql = "DELETE FROM blocklist WHERE ? >= time + ?;";
    
    try(Connection conn = this.connect(); 
            PreparedStatement pstmt = conn.prepareStatement(sql))
    {
        pstmt.setLong(1, System.currentTimeMillis());
        pstmt.setLong(2, blocktime);
        pstmt.executeUpdate();
    }
    catch(SQLException ex)
    {
        Logger.getLogger(RecordManager.class.getName()).log(Level.SEVERE, null, ex);
    }

connect() 函数在这里被调用 Connection conn = this.connect();:

private Connection connect()
{
    Connection con = null;
    try
    {
        con = DriverManager.getConnection(Config.dburl, Config.dbusername, Config.dbpassword);
    }
    catch(SQLException ex)
    {
        Logger.getLogger(RecordManager.class.getName()).log(Level.SEVERE, null, ex);
    }
    return con;
}

这里是异常日志：

Severe:   Exception in thread "AWT-EventQueue-0"
Severe:   java.lang.NullPointerException
    at urrsm.sng.RecordManager.unblockrun(RecordManager.java:91)
    at urrsm.sng.WebScocketEnd$1.actionPerformed(WebScocketEnd.java:45)
    at javax.swing.Timer.fireActionPerformed(Timer.java:313)
    at javax.swing.Timer$DoPostEvent.run(Timer.java:245)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

【问题讨论】：

Jacob G. 我知道 NullPointerException。我只想知道为什么会在这里发生？您可能会看到我的代码在这里没有设置为 null。

标签： java postgresql glassfish war

【解决方案1】：

无法使用预准备语句在查询中绑定实际的列名。只能绑定文字值。尝试将您的删除查询改写如下：

String sql = "DELETE FROM blocklist WHERE time <= ?;";

try (Connection conn = this.connect(); 
        PreparedStatement pstmt = conn.prepareStatement(sql)) {
    pstmt.setLong(1, System.currentTimeMillis() - blocktime);
    pstmt.executeUpdate();
}
catch (SQLException ex) {
    Logger.getLogger(RecordManager.class.getName()).log(Level.SEVERE, null, ex);
}

【讨论】：