Question

当我运行程序时，有一个与createCarMaker关联的空指针异常。我检查了数据库连接，看起来一切都还好。我可以请一些帮助排除故障。

CarDBHelper(){
    try 
    { 
        //Establish a driver
        Class.forName("com.mysql.jdbc.Driver").newInstance(); 

        //Connect to the database
        conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
    } 
    catch (Exception e) 
    { 
        System.err.println("Unable to load MySQL driver."); 
        e.printStackTrace(); 
    } 

}
/**
 * Creates the tables carMaker and Vehicle with various parameters.
 * 
 */
public void createTables(){
    try {
        Statement stmt=conn.createStatement();
        String createcarMaker = "CREATE TABLE carMaker " +
            "(makerID INT AUTO_INCREMENT NOT NULL, " +
            " makerName VARCHAR(50), " +
            " PRIMARY KEY ( makerID ))";

        statement.executeUpdate(createcarMaker);

        String createVehicle = "CREATE TABLE Vehicle " +
            "(id INT AUTO_INCREMENT NOT NULL, " +
            " year INT, " +
            " make INT, " +
            " model VARCHAR(50), " +
            " FOREIGN KEY (make) REFERENCES carMaker(makerID)," +
            " PRIMARY KEY ( id ))";

        statement.executeUpdate(createVehicle);

    } catch (SQLException e) {
        e.printStackTrace();
    }

Answer 1

我认为你在＆＃39;陈述中使用了错误的声明变量声明＆＃39;和＆＃39; stmt＆＃39;。

此外，请将堆栈跟踪放在此处。

Answer 2

您正在使用未声明的变量。你声明：

Statement stmt=conn.createStatement();

但是参考：

statement.executeUpdate(createcarMaker);

stmt！=声明。

创建表时JDBC Null指针异常

2 个答案: