构造方法

SqlSessionTemplate的构造方法源代码如下，其他构造方法，都是在此构造方法上进行的重载。

    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 SqlSessionTemplate.SqlSessionInterceptor());
    }

sqlSessionProxy

从构造方法中能够看出sqlSessionProxy 是SqlSessionTemplate执行CRUD操作时的实际执行者。所以重点解析下sqlSessionProxy的实例化过程。不难看出，sqlSessionProxy实例化使用了java的动态代理设计模式。传入的参数如下：

• ClassLoader是SqlSessionFactory；
• 接口是SqlSession；
• 调用处理程序是SqlSessionTemplate的内部类SqlSessionInterceptor。

接下来看看SqlSessionInterceptor都做了什么。

SqlSessionInterceptor

    private class SqlSessionInterceptor implements InvocationHandler {
        private SqlSessionInterceptor() {
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);

            Object unwrapped;
            try {
                Object result = method.invoke(sqlSession, args);
                if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
                    sqlSession.commit(true);
                }

                unwrapped = result;
            } catch (Throwable var11) {
                unwrapped = ExceptionUtil.unwrapThrowable(var11);
                if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
                    Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);
                    if (translated != null) {
                        unwrapped = translated;
                    }
                }

                throw (Throwable)unwrapped;
            } finally {
                SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
            }

            return unwrapped;
        }
    }

SqlSessionInterceptor的构造方法并没有任何参数，即没有指明SqlSession的实际实现类。这是因为SqlSession的实际实现类在invoke()中进行了实例化。即SqlSessionUtils的getSqlSession()方法参与生成SqlSession的实际实现类。

SqlSessionUtils getSqlSession

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
        Assert.notNull(sessionFactory, "No SqlSessionFactory specified");
        Assert.notNull(executorType, "No ExecutorType specified");
        SqlSessionHolder holder = (SqlSessionHolder)TransactionSynchronizationManager.getResource(sessionFactory);
        if (holder != null && holder.isSynchronizedWithTransaction()) {
            if (holder.getExecutorType() != executorType) {
                throw new TransientDataAccessResourceException("Cannot change the ExecutorType when there is an existing transaction");
            } else {
                ...
                return holder.getSqlSession();
            }
        } else {
            if (logger.isDebugEnabled()) {
                logger.debug("Creating a new SqlSession");
            }

            SqlSession session = sessionFactory.openSession(executorType);
            ...          
            return session;
        }
    }

getSqlSession方法分为两部分。

1. 判断SqlSessionHolder在内存中是否存在，如果存在则直接从内存中取出，通过sqlSessionholder.getSqlSession()获取SqlSession的实际实现类。
2. 如果不存在，则先创建SqlSession，然后再创建SqlSessionHolder，最后放入内存方便下次直接使用。

下面我们看看是如何生成SqlSession的。

SqlSession session = sessionFactory.openSession(executorType);

在通过sessionFactory创建SqlSession时。要搞清楚sessionFactory是在application.xml配置Mybatis时传入的。即：

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
  <property name="dataSource" ref="dataSource" />
</bean>

需要注意的是SqlSessionFactoryBean实现了Spring的FactoryBean接口。这意味着由Spring最终创建的 bean并不是SqlSessionFactoryBean本身，而是工厂类（SqlSessionFactoryBean）的getObject()方法的返回结果。这种情况下，Spring 将会在应用启动时为你创建SqlSessionFactory并使用sqlSessionFactory这个名字存储起来。关于FactoryBean这里不再详细解释。

查看SqlSessionFactoryBean的getObject()方法，然后顺着内部方法一层一层查看。最终发现发现返回的SqlSessionFactory其实是DefaultSqlSessionFactory类。路径：afterPropertiesSet方法 -->buildSqlSessionFactory方法 --> build()方法。代码路径如下：

SqlSessionFactoryBean

    public SqlSessionFactory getObject() throws Exception {
        if (this.sqlSessionFactory == null) {
            this.afterPropertiesSet();
        }

        return this.sqlSessionFactory;
    }

afterPropertiesSet

    public void afterPropertiesSet() throws Exception {
        Assert.notNull(this.dataSource, "Property 'dataSource' is required");
        Assert.notNull(this.sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
        this.sqlSessionFactory = this.buildSqlSessionFactory();
    }

buildSqlSessionFactory

    protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
        XMLConfigBuilder xmlConfigBuilder = null;
        Configuration configuration;
        ...
        ...
        return this.sqlSessionFactoryBuilder.build(configuration);
    }

SqlSessionFactoryBuilder

    public SqlSessionFactory build(Configuration config) {
        return new DefaultSqlSessionFactory(config);
    }

既然传入的sessionFactory参数是DefaultSqlSessionFactory，那我们就可以查看DefaultSqlSessionFactory的openSession方法了。即sqlSessionProxy是DefaultSqlSession实例。

DefaultSqlSessionFactory

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

    ...

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

        DefaultSqlSession var8;
        try {
            ...
            var8 = new DefaultSqlSession(this.configuration, executor);
        } catch (Exception var12) {
            this.closeTransaction(tx);
            throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
        } finally {
            ErrorContext.instance().reset();
        }

        return var8;
    }

总结

SqlSessionTemplate的实例化过程主要利用了java的动态代理设计模式和FactoryBean<T>自定义工厂这两个知识点。

• SqlSessionTemplate 是代理类；
• SqlSession 是接口；
• DefaultSqlSession 是实际实现类；