• 1.要获取自增长键的值 需要在获取操作对象时声明一个参数 Statement.RETURN_GENERATED_KEYS
  PreparedStatement preparedStatement = conn.prepareStatement(sql,Statement.RETURN_GENERATED_KEYS);

• 2.当数据插入成功后，就可以取出这个自增长键的值
  //获取自增长键的结果集
  ResultSet generatedKeys = preparedStatement.getGeneratedKeys();
  while (generatedKeys.next()){
  keyValue = generatedKeys.getInt(1);
  }

• 3.你在其他表中就可以使用这个自增长键的值

public class JDBCDemo5 {
    public static void main(String[] args) throws SQLException, ClassNotFoundException {
        //获取连接对象
        Connection conn = JDBCUtils.getConnection();
        String sql="insert into user values(?,?)";
        //我想要获取到自增长键的值，需要再加一个参数，声明一下
       // Statement.RETURN_GENERATED_KEYS 加上这个常量
        PreparedStatement preparedStatement = conn.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS);
        //给字段设置null值
        preparedStatement.setNull(1, Types.NULL);
        preparedStatement.setString(2,"李某某");
        int i = preparedStatement.executeUpdate();
        int keyValue=0;
        if(i>0){
            System.out.println("插入成功");
            //获取自增长键的结果集
            ResultSet generatedKeys = preparedStatement.getGeneratedKeys();
            while (generatedKeys.next()){
                keyValue = generatedKeys.getInt(1);
            }
        }else{
            System.out.println("插入失败");
        }

        System.out.println(keyValue);
        //在订单表里面给沈某某2下张订单
        String sql2="insert into orders values(?,?,?)";
        PreparedStatement preparedStatement2 = conn.prepareStatement(sql2);
        preparedStatement2.setNull(1,Types.NULL);
        preparedStatement2.setDouble(2,2000);
        preparedStatement2.setInt(3,keyValue); //外键--->主表中主键的值
        //执行
        preparedStatement2.executeUpdate();

        //释放资源
        JDBCUtils.close(conn,preparedStatement);
        preparedStatement2.close();
    }
}