mybatis学习一之入门示例

一、什么是MyBatis

• mybatis是对jdbc的封装
• 将sql语句放在映射文件中(xml)
• 自动将输入参数映射到sql语句的动态参数上
• 自动将sql语句执行的结果映射成java对象

Mybatis工作原理

 

二、入门示例

创建mavn项目：

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.zy</groupId>
    <artifactId>mybatis-maven</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.3</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>

        <!--log4j4日志-->
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-core</artifactId>
            <version>2.12.1</version>
        </dependency>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-api</artifactId>
            <version>2.12.1</version>
        </dependency>

        <!--不需要显式的去写实体类的get set tostring方法-->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.10</version>
        </dependency>
    </dependencies>
    <!--这个必须配置，否则找不到配置文件-->
    <build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.xml</include>
                    <include>**/*.properties</include>
                </includes>
                <filtering>false</filtering>
            </resource>
        </resources>
    </build>
</project>

配置文件：

mysql.properties:

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis
jdbc.username=root
jdbc.password=*******

log4j2.xml:

<?xml version="1.0" encoding="UTF-8"?>
<configuration status="OFF">
    <appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
        </Console>
        <!--  <File name="File" fileName="../logs/test.log" append="true">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %class{36} %L %M - %msg%xEx%n"/>
        </File>  -->
    </appenders>

    <loggers>
        <logger name="com.lfy.mapping" level="debug" additivity="false">
            <appender-ref ref="Console"/>
        </logger>
        <!--  <logger name="log4j.logger.java.sql.Statement" level="info" additivity="false">
            <appender-ref ref="File"/>
        </logger>
        <Logger name="com.lfy.mapping" level="debug" additivity="false">
            <AppenderRef ref="File"/>
        </Logger>  -->

        <root level="DEBUG">
            <appender-ref ref="Console"/>
            <!--  <appender-ref ref="File"/> -->
        </root>
    </loggers>
</configuration>

mybatis-config.xml:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!--引入mysql的配置文件-->
    <properties resource="mysql.properties" />
    <!--日志配置-->
    <settings>
        <setting name="logImpl" value="LOG4J2"/>
    </settings>
    <typeAliases>
        <!--这种定义方式只能定义一种-->
        <!--<typeAlias type="com.zy.entity.Blog" alias="Blog" />-->
        <!--该包下的所有实体类都可以用类名作为别名-->
        <package name="com.zy.entity" />
    </typeAliases>
    <!--定义数据源-->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--这种方式，如果增减一个mapper文件，都需要增加一个配置-->
        <!--<mapper resource="com/zy/mapper/BlogMapper.xml"/>-->
        <!--可以通过下面的方式,扫描整个包中的mapper文件-->
        <package name="com.zy.mapper" />
    </mappers>
</configuration>

代码：

定义pojo:

package com.zy.entity;

import lombok.Builder;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

@Getter
@Setter
@ToString
@Builder(toBuilder = true)
public class Blog {
    private Integer id;
    private String title;
    private int authorId;
    private String state;
    private Boolean featured;
    private String style;
}

Mapper文件：

<?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">
<!--namespace一般是接口路径-->
<mapper namespace="com.huawei.mapper.BlogMapper">
    <select id="selectBlog" resultType="Blog">
        select * from Blog where id = #{id}
    </select>

    <select id="insert" parameterType="Blog">
        insert into Blog(id, title, author_id, state, featured, style)
        values
        (#{id}, #{title}, #{authorId}, #{state}, #{featured}, #{style});
    </select>
</mapper>

定义接口：

package com.zy.mapper;

import com.huawei.entity.Blog;

public interface BlogMapper {
    //接口中的方法名必须和mapper中sql语句的id相同
    Blog selectBlog(Integer id);

    //插入
    void insert(Blog blog);
}

对应的类型：

定义DAO：

import com.huawei.entity.Blog;
import com.huawei.mapper.BlogMapper;
import com.huawei.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

public class BlogDao {
    private SqlSession sqlSession = MyBatisUtil.getSqlSession();
    private BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);

    public Blog queryBlogById(Integer id) {
        return mapper.selectBlog(id);
    }
    public void insert(Blog blog) {
        try {
            mapper.insert(blog);
            sqlSession.commit();
        } finally {
            if (sqlSession != null) {
                sqlSession.close();
            }
        }

    }
}

工具类的编写：

package com.zy.util;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.Reader;

public class MyBatisUtil {
    private static SqlSessionFactory sqlSessionFactory = null;
    static {
        try {
            Reader reader = Resources.getResourceAsReader("mybatis/mybatis-config.xml");
            sqlSessionFactory  = new SqlSessionFactoryBuilder().build(reader);
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
    public MyBatisUtil() {

    }

    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
    }
}

编写测试类：

package com.zy.mapper;

import com.huawei.dao.BlogDao;
import com.huawei.entity.Blog;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.junit.Test;

public class BlogMapperTest {
    private static final Logger logger = LogManager.getLogger(BlogMapperTest.class);

    @Test
    public void testSelectBlog() {
        Blog blog = new BlogDao().queryBlogById(1);
        System.out.println(blog);
        logger.info("blog is:{}", blog);

    }

    @Test
    public void testInsertBlog() {
        Blog build = Blog.builder()
                .authorId(1)
                .state("ACTIVE")
                .style("333")
                .featured(false)
                .title("三国演义")
                .build();
        new BlogDao().insert(build);
    }
}

运行结果：

 

问题：

1.为什么代码中的sqlSession需要关闭，但是获取配置文件的Reader不需要关闭呢？？？

看源码：

查看build()方法：

    public SqlSessionFactory build(Reader reader) {
        return this.build((Reader)reader, (String)null, (Properties)null);
    }

    public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
        SqlSessionFactory var5;
        try {
            XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
            var5 = this.build(parser.parse());
        } catch (Exception var14) {
            throw ExceptionFactory.wrapException("Error building SqlSession.", var14);
        } finally {
            ErrorContext.instance().reset();

            try {
                reader.close();
            } catch (IOException var13) {
            }

        }

        return var5;
    }

我们发现，reader已经被关闭，因此不需要我们关闭

2.为什么插入数据时，需要手动提交呢？？？难道不能自动提交吗？？？

查看SqlSession的创建过程：

sqlSession = sqlSessionFactory.openSession();

进入DefaultSqlSessionFactory：

    public SqlSession openSession() {
        return this.openSessionFromDataSource(this.configuration.getDefaultExecutorType(), (TransactionIsolationLevel)null, false);
    }

我们发现最后一个参数时false，这个参数时干嘛呢？？？继续往下看

 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;

        DefaultSqlSession var8;
        try {
            Environment environment = this.configuration.getEnvironment();
            TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
            tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
            Executor executor = this.configuration.newExecutor(tx, execType);
            var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);
        } catch (Exception var12) {
            this.closeTransaction(tx);
            throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
        } finally {
            ErrorContext.instance().reset();
        }

        return var8;
    }

这个参数，其实就是autoCommit的参数，默认是false，表示不能自动提交，必须手动提交。

那么有没有支持传值的函数呢？？？

    public SqlSession openSession(boolean autoCommit) {
        return this.sqlSessionFactory.openSession(autoCommit);
    }

显然是存在这样函数的，我们可以自己传参，来设置是否支持自动提交。

我的工具类可以改造为一个抽象类：

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.Reader;

public abstract class MyBatisUtil {
    private static SqlSessionFactory sqlSessionFactory = null;
    private static SqlSession sqlSession = null;
    static {
        try {
            Reader reader = Resources.getResourceAsReader("mybatis/mybatis-config.xml");
            sqlSessionFactory  = new SqlSessionFactoryBuilder().build(reader);
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
    public MyBatisUtil() {

    }

    public static SqlSession getSqlSession() {
        sqlSession = sqlSessionFactory.openSession(true);
        return sqlSession;
    }

    public <T> T getMapper(Class<T> typeClass)  {
        SqlSession sqlSession = getSqlSession();
        return sqlSession.getMapper(typeClass);
    }

    public void close() {
        if (sqlSession != null) {
            sqlSession.close();
        }
    }
}

所有的Dao类都可以继承这个工具类，直接去获取Mapper，然后操作即可：

public class BlogDao extends MyBatisUtil {
    private BlogMapper mapper = getMapper(BlogMapper.class);

    public Blog queryBlogById(Integer id) {
        return mapper.selectBlog(id);
    }
    public void insert(Blog blog) {
        mapper.insert(blog);
        this.close();
    }
}