Spring+Mybatis 每次请求数据库，为什么都会创建一个SqlSesssion?

先看一段代码：

@Service
public class TblEmployeeServiceimpl implements TblEmployeeService {
    @Autowired
    private TblEmployeeMapper tblEmployeeMapper;
    @Override
    public TblEmployeePO select(Integer id) {
        TblEmployeePO tblEmployeePO=new  TblEmployeePO();
        tblEmployeePO = tblEmployeeMapper.selectByPrimaryKey(id);
        return tblEmployeePO;
    }
}
//jmeter测试： https://jingyan.baidu.com/article/eb9f7b6d52079d869364e825.html

每请求一次数据库就创建一个SqlSession，这是为啥呢？

容器启动时：
Spring调用MapperFactoryBean.getObject()来生成Dao代理MapperyProxy分成如下两步：getSqlSession()和getMapper(this.mapperInterface)

public T getObject() throws Exception {
return this.getSqlSession().getMapper(this.mapperInterface);
}
看看这个getSqlSession方法

SqlSessionDaoSupport类：
public SqlSession getSqlSession() {
return this.sqlSessionTemplate;
}
//发现sqlSessionTemplate的值由createSqlSessionTemplate获得
public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
if (this.sqlSessionTemplate == null || sqlSessionFactory != this.sqlSessionTemplate.getSqlSessionFactory()) {
this.sqlSessionTemplate = this.createSqlSessionTemplate(sqlSessionFactory);
}
}
//直接new了一个SqlSessionTemplate
protected SqlSessionTemplate createSqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
return new SqlSessionTemplate(sqlSessionFactory);
}
SqlSessionTemplate类：一直跟进SqlSessionTemplate的构造方法
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());
}

到最后，发现SqlSessionTemplate，有一个属性SqlSession，是一个SqlSession的代理对象，SqlSessionTemplate本身也是一个SqlSession。并且在new SqlSessionTemplate的时候，通过构造函数给sqlSessionProxy 创建了一个sqlsession代理对象。

public class SqlSessionTemplate implements SqlSession, DisposableBean {
private final SqlSessionFactory sqlSessionFactory;
private final ExecutorType executorType;
private final SqlSession sqlSessionProxy;
private final PersistenceExceptionTranslator exceptionTranslator;

现在来看看创建sqlsession代理对象用到的拦截器SqlSessionInterceptor的invoke方法

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) {
SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw (Throwable)unwrapped;
} finally {
if (sqlSession != null) {
SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
return unwrapped;
}

过程：

进入请求，先生成一个新的sqlSession，为本次db操作做准备；
通过反射调用请求进来的方法，将 sqlSession 回调，进行复杂查询及结果映射；
如果需要立即提交事务，do it；
如果出现异常，包装异常信息，重新抛出；
操作完成后，关闭本次session；
看第一步：SqlSessionUtils.getSqlSession，生成一个新的sqlSession

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);
SqlSession session = sessionHolder(executorType, holder);
if (session != null) {
return session;
} else {
LOGGER.debug(() -> {
return “Creating a new SqlSession”;
});
//创建一个sqlsession
session = sessionFactory.openSession(executorType);
registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
return session;
}
}

看到了熟悉的日志 “Creating a new SqlSession”,同时还创建了一个sqlsession。

回到SqlSessionTemplate ：可以看到SqlSessionTemplate的增删改查实际上用的是sqlsession代理类。而使用代理类执行增删改查，实际上就是执行代理类的invoke方法，而sqlsession代理类的invoke方法的第一步就是新建一个SqlSession。这下总算明白啦。

https://www.cnblogs.com/yougewe/p/10072740.html
https://blog.csdn.net/mayongzhan_csdn/article/details/78481987
https://www.xttblog.com/?p=4018