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
