mybatis的关联查询

Blog.xml配置：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis.test.IBlogOperation">
    <resultMap id="userResultMap" type="User">
        <id property="id" column="user_id"  />
        <result property="userName" column="user_userName"  />
        <result property="userAge" column="user_userAge"  />
        <result property="userAddress" column="user_userAddress"  />
    </resultMap>
    <resultMap id="articleResultMap" type="Article">
        <id property="id" column="article_id" />
        <result property="title" column="article_title" />
        <result property="content" column="article_content" />
        <association property="user" javaType="User" resultMap="userResultMap"/>  
    </resultMap>
    <resultMap id="blogResultMap" type="Blog">
        <id property="id" column="blog_id" />
        <result property="title" column="blog_title" />
        <!-- 将article list属性映射到collection -->
        <collection property="articles" ofType="Article" resultMap="articleResultMap"/>
    </resultMap>
    <!-- select语句 -->
    <select id="getBlogByID" parameterType="int" resultMap="blogResultMap">
       select user.id user_id,user.userName user_userName,user.userAddress user_userAddress,
       article.id article_id,article.title article_title,article.content article_content, 
       blog.id blog_id, blog.title blog_title
       from user,article,blog 
       where user.id=article.userid and blog.id=article.blogid and blog.id=#{id}
    </select>
</mapper>

IArticleOperation定义：

package com.mybatis.test;
import java.util.List;
public interface IArticleOperation {
    public List<Article> getUserArticles(int userID);
}

IBlogOperation定义：

package com.mybatis.test;
public interface IBlogOperation {
    Blog getBlogByID(int id);
}

Test类：

package com.mybatis.test;
import java.io.Reader;
import java.util.List;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
public class Test {
    private static SqlSessionFactory sqlSessionFactory;
    private static Reader reader;
    static {
        try {
            //通过配置文件初始化sqlSessionFactory
            reader = Resources.getResourceAsReader("Configuration.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    public static SqlSessionFactory getSession() {
        return sqlSessionFactory;
    }
    public void getUserByID(int userID) {
        SqlSession session = sqlSessionFactory.openSession();
        try {
            IUserOperation userOperation = session
                    .getMapper(IUserOperation.class);
            User user = userOperation.selectUserByID(userID);
            if (user != null) {
                System.out.println(user.getId() + ":" + user.getUserName()
                        + ":" + user.getUserAddress());
            }
        } finally {
            session.close();
        }
    }
    public void getUserList(String userName) {
        SqlSession session = sqlSessionFactory.openSession();
        try {
            IUserOperation userOperation = session
                    .getMapper(IUserOperation.class);
            List<User> users = userOperation.selectUsersByName(userName);
            for (User user : users) {
                System.out.println(user.getId() + ":" + user.getUserName()
                        + ":" + user.getUserAddress());
            }
        } finally {
            session.close();
        }
    }
    /**
     * 增加后要commit
     */
    public void addUser() {
        User user = new User();
        user.setUserAddress("place");
        user.setUserName("test_add");
        user.setUserAge(30);
        SqlSession session = sqlSessionFactory.openSession();
        try {
            IUserOperation userOperation = session
                    .getMapper(IUserOperation.class);
            userOperation.addUser(user);
            session.commit();
            System.out.println("新增用户ID：" + user.getId());
        } finally {
            session.close();
        }
    }
    /**
     * 修改后要commit
     */
    public void updateUser() {
        SqlSession session = sqlSessionFactory.openSession();
        try {
            IUserOperation userOperation = session
                    .getMapper(IUserOperation.class);
            User user = userOperation.selectUserByID(1);
            if (user != null) {
                user.setUserAddress("A new place");
                userOperation.updateUser(user);
                session.commit();
            }
        } finally {
            session.close();
        }
    }
    /**
     * 删除后要commit.
     * 
     * @param id
     */
    public void deleteUser(int id) {
        SqlSession session = sqlSessionFactory.openSession();
        try {
            IUserOperation userOperation = session
                    .getMapper(IUserOperation.class);
            userOperation.deleteUser(id);
            session.commit();
        } finally {
            session.close();
        }
    }
    public void getUserArticles(int userid) {
        SqlSession session = sqlSessionFactory.openSession();
        try {
            IArticleOperation articleOperation = session
                    .getMapper(IArticleOperation.class);
            List<Article> articles = articleOperation.getUserArticles(userid);
            for (Article article : articles) {
                System.out.println(article.getTitle() + ":"
                        + article.getContent() + "用户名："
                        + article.getUser().getUserName() + "用户地址："
                        + article.getUser().getUserAddress());
            }
        } finally {
            session.close();
        }
    }
    public void getBlogArticles(int blogid) {
        SqlSession session = sqlSessionFactory.openSession();
        try {
            IBlogOperation blogOperation = session
                    .getMapper(IBlogOperation.class);
            Blog blog = blogOperation.getBlogByID(blogid);
            System.out.println(blog.getTitle() + ":");
            List<Article> articles = blog.getArticles();
            for (Article article : articles) {
                System.out.println(article.getTitle() + ":"
                        + article.getContent() + "用户名："
                        + article.getUser().getUserName() + "用户地址："
                        + article.getUser().getUserAddress());
                /*System.out.println(article.getTitle() + ":"
                        + article.getContent());*/
            }
        } finally {
            session.close();
        }
    }
    public static void main(String[] args) {
        try {
            Test test = new Test();
            // test.getUserByID(1);
            // test.getUserList("test1");
            // test.addUser();
            // test.updateUser();
            // test.deleteUser(6);
            //test.getUserArticles(1);
            test.getBlogArticles(1);
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}

Mybatis的“N+1查询问题”

示例：

<resultMap id=”blogResult” type=”Blog”>
        <association property="author" column="blog_author_id"
            javaType="Author" select=”selectAuthor” />
    </resultMap>
    <select id=”selectBlog” parameterType=”int” resultMap=”blogResult”>
        SELECT * FROM BLOG WHERE ID = #{id}
    </select>
    <select id=”selectAuthor” parameterType=”int” resultType="Author">
        SELECT * FROM AUTHOR WHERE ID = #{id}
    </select>

有两个查询语句：一个来加载博客，另外一个来加载作者，而且博客的结果映射描述了“selectAuthor”语句应该被用来加载它的 author 属性。

其他所有的属性将会被自动加载，假设它们的列和属性名相匹配。

这种方式很简单，但是对于大型数据集合和列表将不会表现很好。问题就是我们熟知的“N+1 查询问题”。概括地讲，N+1 查询问题可以是这样引起的：

你执行了一个单独的 SQL 语句来获取结果列表（就是“+1”）。

对返回的每条记录，你执行了一个查询语句来为每个加载细节（就是“N”）。

这个问题会导致成百上千的 SQL 语句被执行。这通常不是期望的。

可以采用关联的嵌套结果来解决这个问题：

<resultMap id="blogResult" type="Blog">
        <id property=”id” column="blog_id" />
        <result property="title" column="blog_title" />
        <association property="author" column="blog_author_id"
            javaType="Author" resultMap=”authorResult” />
    </resultMap>
    <resultMap id="authorResult" type="Author">
        <id property="id" column="author_id" />
        <result property="username" column="author_username" />
        <result property="password" column="author_password" />
        <result property="email" column="author_email" />
        <result property="bio" column="author_bio" />
    </resultMap>

resultMap 这是结果映射的 ID，可以映射关联的嵌套结果到一个合适的对象图中。这是一种替代方法来调用另外一个查询语句。