回滚的意义---JDBC事务回滚探究

JDBC手动事务提交回滚的常见写法一直是rollback写在commit的catch之后:

try{
   conn.setAutoCommit(false);
　　ps.executeUpdate();

　　ps.executeUpdate();

　　conn.commit();

}catch(Exception e){
      conn.rollback();
}

但是,这种回滚是没有意义的:

一旦commit前出错,　　就不提交了,　　回滚无用

一旦commit了,　　　　说明没错,　　　不用回滚

找到一篇和我观点相同的文章:

<JDBC 事务的回滚 提交>

我以为无需回滚,即使真要回滚,需要将commit写在主要业务的try...catch之后,一旦主要业务中途出错,回滚.

表:JDBC事务3种写法回滚比较

import java.sql.*;

public class TestJDBC {

    public static final String URL = "jdbc:mysql://127.0.0.1/test";
    public static final String USER_NAME = "root";
    public static final String USER_PWD = "root";

    private static Connection conn = null;
    private static PreparedStatement ps;

    private static void connOpen() throws SQLException {
        conn = DriverManager.getConnection(URL, USER_NAME, USER_PWD);
    }

    private static void connClose() throws SQLException {
        if (ps != null) {
            ps.close();
            ps = null;
        }
        if (conn != null) {
            conn.close();
            conn = null;
        }
    }

    private static void testRollBack() throws SQLException {
        connOpen();
        try {

            conn.setAutoCommit(false);

            String strSQL = "INSERT INTO customer(uname,pwd) VALUES(?,'1')";
            ps = conn.prepareStatement(strSQL);

            // 插入一条数据
            ps.setString(1, "悟空");
            ps.executeUpdate();

            // 出异常
            if (true) {
                throw new SQLException();
            }

            // 再插入一条数据
            ps.setString(1, "八戒");
            ps.executeUpdate();

            // conn.commit();
        } catch (Exception e) {
            System.out.println("异常!");
            try {
                conn.rollback();
            } catch (SQLException e1) {
                e1.printStackTrace();
            }
        }
        try {
            conn.commit();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        connClose();

    }

    private static void selectAll() throws SQLException {
        connOpen();
        ps = conn.prepareStatement("select * from customer");
        ResultSet rs = ps.executeQuery();
        while (rs.next()) {
            System.out.print(rs.getString(2) + " : ");
            System.out.println(rs.getString(1));
        }
        connClose();
    }

    public static void main(String[] args) throws SQLException {
        testRollBack();
        selectAll();
    }
}