接口回调，多表查询

`三表查询
Topic类
public class Topic {

private int tid;
private String title;
private String context;
private Date ptime;
private int uid;
private int bid;

private User user;
private Board board;

public User getUser() {
    return user;
}
public void setUser(User user) {
    this.user = user;
}
public Board getBoard() {
    return board;
}
public void setBoard(Board board) {
    this.board = board;
}
public int getTid() {
    return tid;
}
public void setTid(int tid) {
    this.tid = tid;
}
public String getTitle() {
    return title;
}
public void setTitle(String title) {
    this.title = title;
}
public String getContext() {
    return context;
}
public void setContext(String context) {
    this.context = context;
}
public Date getPtime() {
    return ptime;
}
public void setPtime(Date ptime) {
    this.ptime = ptime;
}
public int getUid() {
    return uid;
}
public void setUid(int uid) {
    this.uid = uid;
}
public int getBid() {
    return bid;
}
public void setBid(int bid) {
    this.bid = bid;
}

Board类
public class Board {

private int bid;

private String bname;

public int getBid() {
    return bid;
}

public void setBid(int bid) {
    this.bid = bid;
}

public String getBname() {
    return bname;
}

public void setBname(String bname) {
    this.bname = bname;
}

User类

public class User {

private int uid;
private String uname;
private String upass;
//用户状态，0表示正常，1表示禁用
private int state;
//用户标识，0表示管理员，1表示用户
private int flag;


public int getUid() {
    return uid;
}
public void setUid(int uid) {
    this.uid = uid;
}
public String getUname() {
    return uname;
}
public void setUname(String uname) {
    this.uname = uname;
}
public String getUpass() {
    return upass;
}
public void setUpass(String upass) {
    this.upass = upass;
}
public int getState() {
    return state;
}
public void setState(int state) {
    this.state = state;
}
public int getFlag() {
    return flag;
}
public void setFlag(int flag) {
    this.flag = flag;
}

创建接口RowMapper

/**
 *    结果集一行和对象的映射器
 * @author Admin
 *
 * @param <E>
 */

public interface RowMapper<E> {
    /**
     *   结果集的一行映射为一个对象
     * @param rs  已经打开的结果集
     * @return  映射好的对象
     */
    E oneRowMapper(ResultSet rs);

实现接口类

public class TopicRowMapper implements RowMapper<Topic>{
@Override
public Topic oneRowMapper(ResultSet rs) {

    try {
        Topic topic = new Topic();
        topic.setTid(rs.getInt("tid"));
        topic.setTitle(rs.getString("title"));
        topic.setContext(rs.getString("context"));
        topic.setPtime(rs.getDate("ptime"));
        User user = new User();
        user.setUname(rs.getString("uname"));
        topic.setUser(user);
        Board board = new Board();
        board.setBname(rs.getString("bname"));
        topic.setBoard(board);
        return topic;
    } catch (SQLException e) {
        e.printStackTrace();
    }


    return null;
}

测试类

public class TopicTest {
public static void main(String[] args) throws Exception {
    TopicDao topicDao =new TopicDao();
    //查询全部
    List<Topic> list =topicDao.list("select * from topic t inner join user u on t.uid=u.uid "
            + "inner join board b on b.bid=t.bid", new TopicRowMapper());
    for(Topic topic:list) {
        System.out.println(topic.getTid()+"\t"+topic.getTitle()+"\t"+topic.getContext()+
                "\t"+topic.getPtime()+"\t"+topic.getUser().getUname()+"\t"+topic.getBoard().getBname());
    }

    Topic topic = new Topic();
    //查询一个
    topic = topicDao.get("select * from topic t inner join user u on t.uid=u.uid 
      inner join board b on b.bid=t.bid where tid=?", new TopicRowMapper(),4);
    System.out.println(topic.getTid()+"\t"+topic.getTitle()+"\t"+topic.getContext()+
    "\t"+topic.getPtime()+"\t"+topic.getUser().getUname()+"\t"+topic.getBoard().getBname());
}