MyBatis-Spring 执行SQL语句的流程

1. 从SqlSessionDaoSupport开始

通常我们使用MyBatis会让自己的DAO继承SqlSessionDaoSupport,那么SqlSessionDaoSupport是如何运作的呢,下面是SqlSessionDaoSupport的源代码

/*
 *    Copyright 2010 The myBatis Team
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.mybatis.spring.support;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionTemplate;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.dao.support.DaoSupport;
import org.springframework.util.Assert;

/**
 * Convenient super class for MyBatis SqlSession data access objects.
 * It gives you access to the template which can then be used to execute SQL methods.
 * <p>
 * This class needs a SqlSessionTemplate or a SqlSessionFactory.
 * If both are set the SqlSessionFactory will be ignored.
 *
 * @see #setSqlSessionFactory
 * @see #setSqlSessionTemplate
 * @see SqlSessionTemplate
 * @version $Id: SqlSessionDaoSupport.java 3266 2010-11-22 06:56:51Z simone.tripodi $
 */
public abstract class SqlSessionDaoSupport extends DaoSupport {

    // 这个SqlSession就是我们平时用来执行SQL和事务的一次会话
    private SqlSession sqlSession;

    private boolean externalSqlSession;

    // 可以看到以下两个Autowired的set方法,实际上他们的功能都是设置sqlSession的实例
    // 区别在于一个是通过传入sqlSessionFactory然后包装成SqlSessionTemplate
    // 另一个直接传入SqlSessionTemplate赋值给sqlSession
    @Autowired(required = false)
    public final void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
        if (!this.externalSqlSession) {
            this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
        }
    }

    @Autowired(required = false)
    public final void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {
        this.sqlSession = sqlSessionTemplate;
        this.externalSqlSession = true;
    }

    /**
     * Users should use this method to get a SqlSession to call its statement methods
     * This is SqlSession is managed by spring. Users should not commit/rollback/close it
     * because it will be automatically done.
     *
     * @return Spring managed thread safe SqlSession 
     */
    public final SqlSession getSqlSession() {
        return this.sqlSession;
    }

    /**
     * {@inheritDoc}
     */
    protected void checkDaoConfig() {
        Assert.notNull(this.sqlSession, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
    }

}

2.SqlSessionDaoSupport中的SqlSession产生

先提一提既然我们是用dao去继承这个SqlSessionDaoSupport,观察我们的dao的配置

    <bean id="userDao" class="dao.UserDao">
        <!--<property name="sqlSessionFactory" ref="sqlSessionFactory" />-->
        <property name="sqlSessionTemplate" ref="sqlSession" />
    </bean>

    <bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate" >
        <!-- 第一个参数是 sqlSessionFactory -->
        <constructor-arg index="0" ref="sqlSessionFactory"/>
        <!-- 第二个参数是 ExecutorType -->
        <constructor-arg index="1" ref="BATCH"/>
    </bean>

    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <!-- 指定数据源 -->
        <property name="dataSource" ref="dataSource" />
        <!-- 指定MyBatis配置文件 -->
        <property name="configLocation" value="classpath:mybatis-config.xml" />
        <!-- 导入Mapper -->
        <property name="mapperLocations" value="classpath*:mappers/*.xml" />
    </bean>

    <!-- datasource -->
    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource">
        <property name="driverClassName" value="com.mysql.jdbc.Driver" />
        <property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatistest?characterEncoding=utf8" />
        <property name="username" value="root" />
        <property name="password" value="root" />
    </bean>

观察我们的dao的配置可以发现我们可以配置sqlSessionFactory和sqlSessionTemplate,实际上我们配置 sqlSessionTemplate的话就是能多配置一个ExecutorType参数(这个参数在MyBatis的Settings参数里也可以找 到,叫做defaultExecutorType,参数配置详细介绍见 http://mybatis.github.io/mybatis-3/configuration.html),这个参数的选项有三个:

SIMPLE: 普通SQL执行器,不会使用预解析和批量处理

REUSE: 重用prepared statement

BATCH: 不但重用prepared statement,而且能执行批量处理,如

public void insertUsers(User[] users) {
   for (User user : users) {
     sqlSession.insert("org.mybatis.spring.sample.mapper.UserMapper.insertUser", user);
   }
 }

3. 分析sqlSessionTemplate的构造

查看sqlSessionTemplate的源代码,关键部分如下

    /**
     * Constructs a Spring managed {@link SqlSession} with the given
     * {@link SqlSessionFactory} and {@link ExecutorType}.
     * A custom {@link SQLExceptionTranslator} can be provided as an 
     * argument so any {@link PersistenceException} thrown by MyBatis
     * can be custom translated to a {@link RuntimeException}
     * The {@link SQLExceptionTranslator} can also be null and thus no
     * exception translation will be done and MyBatis exceptions will be 
     * thrown
     *
     * @param sqlSessionFactory
     * @param executorType
     * @param exceptionTranslator
     */
    public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
            PersistenceExceptionTranslator exceptionTranslator) {

        Assert.notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
        Assert.notNull(executorType, "Property 'executorType' is required");

        this.sqlSessionFactory = sqlSessionFactory;
        this.executorType = executorType;
        this.exceptionTranslator = exceptionTranslator;
        this.sqlSessionProxy = (SqlSession) Proxy.newProxyInstance(
                SqlSessionFactory.class.getClassLoader(),
                new Class[] { SqlSession.class }, 
                new SqlSessionInterceptor());
    }

它的功能如下

1) 设置sqlSessionFactory,查看上面的xml的配置,就知道sqlSessionFactory这个东西其实就是用来指定 datasource,读取mybatis配置(environment,settings种种),还有读取mybatis的各个sql语句的 mapper的东西

2) 设置executor type,上面已经介绍过了

3) 设置exception translator,这个东西是用来将jdbc的异常转换成spring的sql异常的东西(因为jdbc的异常很单一,无法详细的表达出错时错误到底 是什么,所以spring自己写了一套更好理解的异常,这是题外话),这个东西保持默认就可以了,所以我们在配置sqlSessionTemplate的 bean的时候并没有配置这个参数,如果没有配置,则构造方法会使用一个默认的(这是一个重载方法,另外还有一个构造方法是不需要设置这个参数的,那个构 造方法调用了这个构造方法,其中execeptionTranslator就是传了一个默认的进来)

4) 对SqlSessionInteceptor()设置了一个代理,从这个动态代理的构造函数参数我们就能看出来这个东西是一个SqlSession

其中,这个SqlSessionInterceptor是SqlSessionTemplate一个内部类,他返回了一个SqlSession关键代码如下

            final SqlSession sqlSession = SqlSessionUtils.getSqlSession(
                    SqlSessionTemplate.this.sqlSessionFactory,
                    SqlSessionTemplate.this.executorType);

很简单,其实就是用sqlSessionFactory和executorType生成了一个sqlSession

接下来进入一段七弯八拐的调用,过程如下

SqlSessionUtils.getSqlSession() -> SessionFactory.openSession(executorType, conn) -> DefaultSessionFactory.openSession(ExecutorType execType) ->Configuration.newExecutor(tx, execType, autoCommit) -> return new DefaultSqlSession(configuration, executor)

其中的关键部分是newExecutor(),这玩意就是生成SQL语句执行器的地方,生成的代码如下:

  public Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    // 这里就是根据ExecutorType创建Executor的地方
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    // 这句就是判断setting里的cacheEnabled参数的地方
    // 所以设置了cacheEnabled参数后就会被包装成缓存Executor
    if (cacheEnabled) {
      executor = new CachingExecutor(executor, autoCommit);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

主要做的事情就是 1) 根据executorType生成合适的executor 2) 更具cacheEnabled参数包装executor 



至此, SqlSessionTemplate中的sqlSessionProxy的executor终于生成出来,以后我们使用dao中的session来执行sql相关的操作用的就都是这个SqlSessionTemplate中的sqlSessionProxy

最后,画个图总结一下


也就是说,我们其实使用的是SqlSessionTemplate在做各种数据库操作,这个东西读取了我们的datasource和mybatisconfig,用它的Executor去执行我们Mapper里的sql语句来获取查询结果