spring中的mybatis的sqlSession是如何做到线程隔离的？-马育民老师

摘自：
https://www.cnblogs.com/yougewe/p/10072740.html

项目中常常使用mybatis配合spring进行数据库操作，但是我们知道，数据的操作是要求做到线程安全的，而且按照原来的jdbc的使用方式，每次操作完成之后都要将连接关闭，但是实际使用中我们并没有这么干。

更让人疑惑的点是，spring中默认使用单例形式来加载bean，而往往我们也不会改变这种默认，所以，是所有线程共享数据连接？

让我们来看看真相！

自然是要个栗子的：

我们来看下spring中配置mybatis数据库操作bean(使用 druid 连接池)：

```
    <bean id="dataSource" class="com.alibaba.druid.pool.DruidDataSource">
        <property name="url" value="${jdbc.url}" />
        <property name="driverClassName" value="${jdbc.driver}" />
        <property name="username" value="${jdbc.username}" />
        <property name="password" value="${jdbc.password}" />
    </bean>
    
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dataSource" />
        <property name="configLocation" value="classpath:mybatis-config.xml" />
    </bean>
    
    
    <bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate">
        <constructor-arg index="0" ref="sqlSessionFactory" />
    </bean>
    
    
    <bean name="transactionManager"
          class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
        <property name="dataSource" ref="dataSource"></property>
    </bean>
```

而在java代码中使用则是使用依赖注入直接使用 @resource sqlSession, 如下:

```
    @Resource
    private SqlSessionTemplate sqlSession;
    
    @Override
    public User getUser(Map<String, String> cond) {
        // 此句执行db查询
        User result = sqlSession.selectOne(NAME_SPACE
                + ".getUser", cond);
        return result;
    }
```
　　这个sqlSession就是直接去操作数据库了看起来是这样，是在bean初始化的时候依赖注入的！

所以，难道每次进入该操作的时候，sqlSession 的实例都会变化吗？答案是否定的。

那么，肯定就是往下使用的时候才发生的变化呗！

再往下走，可以看到，调用了一个代理来进行具体的查询！
```
  // org/mybatis/spring/SqlSessionTemplate.selectOne()
  public <T> T selectOne(String statement, Object parameter) {
    return this.sqlSessionProxy.<T> selectOne(statement, parameter);
  }
```
　　为啥要用代理呢？自己直接查不就行了吗？其实，用代理是有好处的，那就可以可以进行另外的包装！

代理是怎么生成的呢？其实只要看一下 SqlSessionTemplate 的构造方法就知道了！

```
  /**
   * Constructs a Spring managed {@code SqlSession} with the given
   * {@code SqlSessionFactory} and {@code ExecutorType}.
   * A custom {@code SQLExceptionTranslator} can be provided as an
   * argument so any {@code PersistenceException} thrown by MyBatis
   * can be custom translated to a {@code RuntimeException}
   * The {@code 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) {

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

this.sqlSessionFactory = sqlSessionFactory;
    this.executorType = executorType;
    this.exceptionTranslator = exceptionTranslator;
    // 生成代理 SqlSessionInterceptor 为 InvocationHandler
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
  }
```

从上面的代码，看不到细节，但是，大致还是知道代理的具体实现了！即使用 SqlSessionInterceptor 去处理具体查询逻辑！

我们来看下 SqlSessionInterceptor  的实现！

```
  /**
   * Proxy needed to route MyBatis method calls to the proper SqlSession got
   * from Spring's Transaction Manager
   * It also unwraps exceptions thrown by {@code Method#invoke(Object, Object...)} to
   * pass a {@code PersistenceException} to the {@code PersistenceExceptionTranslator}.
   */
  private class SqlSessionInterceptor implements InvocationHandler {
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // force commit even on non-dirty sessions because some databases require
          // a commit/rollback before calling close()
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) {
        Throwable unwrapped = unwrapThrowable(t);
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
          // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
          sqlSession = null;
          Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
          if (translated != null) {
            unwrapped = translated;
          }
        }
        throw unwrapped;
      } finally {
        if (sqlSession != null) {
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }
```

SqlSessionInterceptor 是 SqlSessionTemplate 的内部类，目的只有一个，就是处理多个 session 的db操作！

所有请求都被 invoke() 拦截,从而做相应处理：

1. 进入请求，先生成一个新的sqlSession，为本次db操作做准备；

2. 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；

3. 如果需要立即提交事务，do it；

4. 如果出现异常，包装异常信息，重新抛出；

5. 操作完成后，关闭本次session；

到这里，其实我们好像已经明白了，其实外面的 sqlSession 单例，并不会影响具体的db操作控制，所以不用担心session的线程安全问题！

不过，还有个点值得考虑下，如果我一次请求里有多次数据库操作，难道我真的要创建多个sqlSession或者说数据库连接？不会吧！

如果这个问题得不到解决，可能你并不真正了解session的定义了！

所以我们需要继续看一下 session 到底是怎么获取的？

getSqlSession() 方法是在 SqlSessionUtils 中实现的！如下：

```
  /**
   * Gets an SqlSession from Spring Transaction Manager or creates a new one if needed.
   * Tries to get a SqlSession out of current transaction. If there is not any, it creates a new one.
   * Then, it synchronizes the SqlSession with the transaction if Spring TX is active and
   * <code>SpringManagedTransactionFactory</code> is configured as a transaction manager.
   *
   * @param sessionFactory a MyBatis {@code SqlSessionFactory} to create new sessions
   * @param executorType The executor type of the SqlSession to create
   * @param exceptionTranslator Optional. Translates SqlSession.commit() exceptions to Spring exceptions.
   * @throws TransientDataAccessResourceException if a transaction is active and the
   *             {@code SqlSessionFactory} is not using a {@code SpringManagedTransactionFactory}
   * @see SpringManagedTransactionFactory
   */
  public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {

notNull(sessionFactory, "No SqlSessionFactory specified");
    notNull(executorType, "No ExecutorType specified");

SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);

// 如果已经有holder，则直接返回，复用连接
    if (holder != null && holder.isSynchronizedWithTransaction()) {
      if (holder.getExecutorType() != executorType) {
        throw new TransientDataAccessResourceException("Cannot change the ExecutorType when there is an existing transaction");
      }

holder.requested();

if (logger.isDebugEnabled()) {
        logger.debug("Fetched SqlSession [" + holder.getSqlSession() + "] from current transaction");
      }

return holder.getSqlSession();
    }

if (logger.isDebugEnabled()) {
      logger.debug("Creating a new SqlSession");
    }

SqlSession session = sessionFactory.openSession(executorType);

// Register session holder if synchronization is active (i.e. a Spring TX is active)
    //
    // Note: The DataSource used by the Environment should be synchronized with the
    // transaction either through DataSourceTxMgr or another tx synchronization.
    // Further assume that if an exception is thrown, whatever started the transaction will
    // handle closing / rolling back the Connection associated with the SqlSession.
    if (TransactionSynchronizationManager.isSynchronizationActive()) {
      Environment environment = sessionFactory.getConfiguration().getEnvironment();

if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
        if (logger.isDebugEnabled()) {
          logger.debug("Registering transaction synchronization for SqlSession [" + session + "]");
        }

holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
        TransactionSynchronizationManager.bindResource(sessionFactory, holder);
        TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
        holder.setSynchronizedWithTransaction(true);
        holder.requested();
      } else {
        if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {
          if (logger.isDebugEnabled()) {
            logger.debug("SqlSession [" + session + "] was not registered for synchronization because DataSource is not transactional");
          }
        } else {
          throw new TransientDataAccessResourceException(
              "SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
        }
      }
    } else {
      if (logger.isDebugEnabled()) {
        logger.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");
      }
    }

return session;
  }
```
如上获取 sqlSession 逻辑，主要分两种情况！

1. 如果存在holder，则返回原有的sqlSession，到于这个holder我们稍后再说；
　　2. 如果没有，则创建一个新连接！

所以，看起来情况还不是太糟，至少有复用的概念了！

那么问题来了，复用？如何做到线程安全？所以我们要看下 SqlSessionHolder 的实现了！

获取holder是通过 TransactionSynchronizationManager.getResource(sessionFactory); 获取的：

```
    public static Object getResource(Object key) {
        Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        // 实际获取
        Object value = doGetResource(actualKey);
        if (value != null && logger.isTraceEnabled()) {
            logger.trace("Retrieved value [" + value + "] for key [" + actualKey + "] bound to thread [" +
                    Thread.currentThread().getName() + "]");
        }
        return value;
    }
    
    private static Object doGetResource(Object actualKey) {
        Map<Object, Object> map = resources.get();
        if (map == null) {
            return null;
        }
        Object value = map.get(actualKey);
        // Transparently remove ResourceHolder that was marked as void...
        if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) {
            map.remove(actualKey);
            // Remove entire ThreadLocal if empty...
            if (map.isEmpty()) {
                resources.remove();
            }
            value = null;
        }
        return value;
    }
```
　　咱们忽略对 key 的处理，实际是直接调用 doGetResource() 获取holder.
　　而 doGetResource() 中，则使用了 resources 来保存具体的 kv。 resources 明显是个共享变量，但是看起来这里没有任何的加锁操作！这是为何？
　　只要看一下 resources 的定义就知道了，其实现为 ThreadLocal, 所以是线程安全了！
```
    private static final ThreadLocal<Map<Object, Object>> resources =
            new NamedThreadLocal<Map<Object, Object>>("Transactional resources");
```
　　在新的请求进来时，自然是没有值的，所以直接返回null.而后续进入，则获取缓存返回！

而对于没有获取到 holder 的情况，则需要重新创建一个 session 了！

这里获取session由DefaultSqlSessionFactory 进行创建！如下：

```
  // org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession()
  public SqlSession openSession(ExecutorType execType) {
    return openSessionFromDataSource(execType, null, false);
  }
  
  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      // SpringManagedTransactionFactory
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      final Executor executor = configuration.newExecutor(tx, execType);
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
```
创建 session 几件事:
　　1. 根据环境配置，开启一个新事务，该事务管理器会负责后续jdbc连接管理工作；
　　2. 根据事务创建一个 Executor，备用；
　　3. 用DefaultSqlSession 将 executor 包装后返回，用于后续真正的db操作；

至此，真正的 sqlSession 已经创建成功！返回后，就可以真正使用了！

等等，创建的session好像并没有保存，那么还是那个问题，每个sql都会创建一个 sqlSession ？ 好吧，是这样的！前面的holder，只是用于存在事务操作的连接！（holder的理解出了偏差哦）

但是有一点，这里虽然创建了多个 sqlSession 实例，但是并不意味着有多个db连接，具体使用db连接时，则一般会会使用连接池来进行优化！如前面提到的 druid 就是个不错的选择！

真实的jdbc连接获取，是在进行真正的 query 时，才进行调用 getConnection() 进行接入！

具体则是在 doQuery() 时，进行st的组装时调用的 ，如下：

```
    // SimpleExecutor.prepareStatement()
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    // 获取 jdbc 连接，返回 java.sql.Connection
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection);
    handler.parameterize(stmt);
    return stmt;
  }
  
  // 调用 BaseExecutor.getConnection()
  protected Connection getConnection(Log statementLog) throws SQLException {
    // SpringManagedTransaction 管理 connection
    Connection connection = transaction.getConnection();
    if (statementLog.isDebugEnabled()) {
      return ConnectionLogger.newInstance(connection, statementLog, queryStack);
    } else {
      return connection;
    }
  }
```

通过前面通过事务管理工厂创建的 SpringManagedTransaction 进行 connection 获取！一个事务管理器只会存在一次获取数据库连接的操作！

```
  public Connection getConnection() throws SQLException {
    if (this.connection == null) {
      openConnection();
    }
    return this.connection;
  }
  
  // 而 SpringManagedTransaction 又将connection交由 DataSourceUtils 进行管理！
  // org/springframework/jdbc/datasource/DataSourceUtils
    public static Connection getConnection(DataSource dataSource) throws CannotGetJdbcConnectionException {
        try {
            // 真正的连接获取
            return doGetConnection(dataSource);
        }
        catch (SQLException ex) {
            throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex);
        }
    }

/**
     * Actually obtain a JDBC Connection from the given DataSource.
     * Same as {@link #getConnection}, but throwing the original SQLException.
     * <p>Is aware of a corresponding Connection bound to the current thread, for example
     * when using {@link DataSourceTransactionManager}. Will bind a Connection to the thread
     * if transaction synchronization is active (e.g. if in a JTA transaction).
     * <p>Directly accessed by {@link TransactionAwareDataSourceProxy}.
     * @param dataSource the DataSource to obtain Connections from
     * @return a JDBC Connection from the given DataSource
     * @throws SQLException if thrown by JDBC methods
     * @see #doReleaseConnection
     */
    public static Connection doGetConnection(DataSource dataSource) throws SQLException {
        Assert.notNull(dataSource, "No DataSource specified");

ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
        if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
            conHolder.requested();
            if (!conHolder.hasConnection()) {
                logger.debug("Fetching resumed JDBC Connection from DataSource");
                conHolder.setConnection(dataSource.getConnection());
            }
            return conHolder.getConnection();
        }
        // Else we either got no holder or an empty thread-bound holder here.

logger.debug("Fetching JDBC Connection from DataSource");
        // 通过接入的dataSource进行连接获取，这里将会是最终的jdbc连接
        Connection con = dataSource.getConnection();

if (TransactionSynchronizationManager.isSynchronizationActive()) {
            logger.debug("Registering transaction synchronization for JDBC Connection");
            // Use same Connection for further JDBC actions within the transaction.
            // Thread-bound object will get removed by synchronization at transaction completion.
            ConnectionHolder holderToUse = conHolder;
            if (holderToUse == null) {
                holderToUse = new ConnectionHolder(con);
            }
            else {
                holderToUse.setConnection(con);
            }
            holderToUse.requested();
            TransactionSynchronizationManager.registerSynchronization(
                    new ConnectionSynchronization(holderToUse, dataSource));
            holderToUse.setSynchronizedWithTransaction(true);
            if (holderToUse != conHolder) {
                TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
            }
        }

return con;
    }
```

上面的实现主要做三件事：
　　1. 再次确认，是否存在事务处理，holder是否存在，如果有则复用；
　　2. 如果没有，那再从数据源处获取连接；
　　3. 获取新连接成功后，检查如果存在事务，则将新获取的连接放入holder中保存起来，以备下次使用；

获取jdbc连接后，就可以真正发起execute()查询了。

数据库连接的疑问算是解答了！我们发现，外部的框架并没有多少为我们节省db连接的动作！而是把最终 getConnection() 交给 datasource 数据源！

而真正解决我们连接复用的问题的，是像 Druid 这样的连接池组件！所以，咱们可以单独来看这些中间件了！

原文出处：http://malaoshi.top/show_1IX78ZGZJPYs.html