对order表的两种查询操作的对比，针对sql字段名不等于java属性名（仍然是查询的结果是：一行多列）

对order表的两种查询操作的对比（仍然是查询的结果是：一行多列）
注意：order是mysql中的一个关键字
这里故意mysql的字段名和javabean的属性名不一样
其中一些通用的类和属性配置文件和上一篇文章一样

使用getMetaData

1、新建表order

新建javabean，

注意：导包一定要导sql下的包

java.sql.Date下的包

order类的代码：

package myself_test.entity;

import java.sql.Date;

public class Order {
    private Integer orderId;
    private String orderName;
    private Date orderDate;

    public Order() {
    }

    public Order(Integer orderId, String orderName, Date orderDate) {
        this.orderId = orderId;
        this.orderName = orderName;
        this.orderDate = orderDate;
    }

    public Integer getOrderId() {
        return orderId;
    }

    public void setOrderId(Integer orderId) {
        this.orderId = orderId;
    }

    public String getOrderName() {
        return orderName;
    }

    public void setOrderName(String orderName) {
        this.orderName = orderName;
    }

    public Date getOrderDate() {
        return orderDate;
    }

    public void setOrderDate(Date orderDate) {
        this.orderDate = orderDate;
    }

    @Override
    public String toString() {
        return "Order{" +
                "orderId=" + orderId +
                ", orderName='" + orderName + '\'' +
                ", orderDate=" + orderDate +
                '}';
    }
}

普通查询的代码：

//普通查询Order表的操作
    @Test
    public void queryOrder() {
        Connection connection = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            connection = JDBCUtils.getConnection();
            String sql="select order_id,order_name,order_date from `order` where order_id=?";
            ps = connection.prepareStatement(sql);
            ps.setObject(1,1);
            rs = ps.executeQuery();
            if(rs.next()) {
                int orderID = rs.getInt(1);
                String order_name = rs.getString(2);
                Date date = (Date) rs.getObject(3);
                Order order = new Order(orderID, order_name, date);
                System.out.println(order);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            JDBCUtils.closeResource(connection,ps,rs);
        }
    }

结果：

通用查询的代码：

注意这里的order表的字段名和java中order类的属性名是不一样的！

会出现两个问题

第一个问题：

错误显示：
com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'order where order_id=1' at line 1
  at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
  at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
  at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
  at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
  at com.mysql.jdbc.Util.handleNewInstance(Util.java:400)
  at com.mysql.jdbc.Util.getInstance(Util.java:383)
  at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:980)
  at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3847)
  at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3783)
  at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2447)
  at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2594)
  at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2545)
  at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1901)
  at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:2002)
  at myself_test.commonControl.Common.queryOrderCommon(Common.java:69)
  at myself_test.utils_test.JDBCTest.testCommonQueryOrder(JDBCTest.java:41)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
  at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
  at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
  at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
  at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
null

这个错误的原因是因为order表，其中order就是关键字

String sql=“select order_id,order_name,order_date from order where order_id=?”;

这句话不应该这样写：

而应该在order这里加上飘号（esc下面的那个键）

String sql=“select order_id,order_name,order_date from ​​order​​ where order_id=?”;

但是加上之后，又会报错：

java.lang.NoSuchFieldException: order_id
  at java.lang.Class.getDeclaredField(Class.java:2070)
  at myself_test.commonControl.Common.queryOrderCommon(Common.java:77)
  at myself_test.utils_test.JDBCTest.testCommonQueryOrder(JDBCTest.java:41)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
  at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
  at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
  at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
  at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Order{orderId=null, orderName='null', orderDate=null}

这个错误出现的原因就是java类order的属性名和mysql表的字段名不一致导致的。

代码改进：
有两个地方需要改进：
1、写sql的地方需要添加别名，而且切记：

别名一定要和java的属性名一致

String sql=“select order_id orderId,order_name orderName,order_date orderDate from ​​order​​ where order_id=?”;

2、不要使用getColumnName（）方法，这个方法仍然返回的是表的真实列名，应该使用getColumnLabel（）

其他的地方不变

运行结果截图：

全部代码：

//通用查询的操作
    public static Order queryOrderCommon(String sql,Object... args)  {
        Connection connection = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        Order order= null;
        try {
            connection = JDBCUtils.getConnection();
            ps = connection.prepareStatement(sql);
            for(int i=0;i<args.length;i++){
                ps.setObject(i+1,args[i]);
            }
            rs = ps.executeQuery();
            order = new Order();
            if(rs.next()) {
                ResultSetMetaData rsmd = rs.getMetaData();
                int columnCount = rsmd.getColumnCount();
                for (int i = 0; i < columnCount; i++) {
//                    String columnName = rsmd.getColumnName(i + 1);
                    String columnLabel = rsmd.getColumnLabel(i + 1);
                    Object objectValue = rs.getObject(i + 1);
                    Field field = Order.class.getDeclaredField(columnLabel);
                    field.setAccessible(true);
                    field.set(order,objectValue);
                }
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } finally {
            JDBCUtils.closeResource(connection,ps,rs);
        }
        return order;

    }

测试代码：
public void testCommonQueryOrder(){
        String sql="select order_id orderId," +
                "order_name orderName,order_date orderDate from `order` where order_id=?";
        Order order = Common.queryOrderCommon(sql, 1);
        System.out.println(order);
    }

总结：以后全部使用getColumnLabel()，这个方法会好点，不管你有没有在sql中起别名。