获取SqlSession对象

具体操作

openSession()

openSessionFromDataSource()

获取一些信息，创建事物管理器tx

newExecutor()

根据Executor在全局配置中的类型，创建出SimpleExecutor/ReuseExecutor/BatchExecutor

如果有二级缓存配置开启，创建CachingExecutor(executor)

使用每一个拦截器重新包装executor并返回

创建DefaultSqlSession，包含Configuration和Executor

返回DefaultSqlSession

源码分析

1. `openSession()`

SqlSession openSession = sqlSessionFactory.openSession();

2. `openSessionFromDataSource()`

从configuration中拿到默认的Executor类型。Executor类型的类型有三种：SIMPLE/REUSE/BATCH。

@Override  public SqlSession openSession() {       return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);  }

3. 获取一些信息，创建事物管理器`tx`

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {       Transaction tx = null;    try {       //从全局配置中获取环境等信息      final Environment environment = configuration.getEnvironment();      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);      //创建事务      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();    }  }

4. `newExecutor()`

传入的参数为事务管理器和Executor的类型，根据传入的类型来创建Executor，Executor是一个接口，是用来增删改查的。

final Executor executor = configuration.newExecutor(tx, execType);

5. 根据`Executor`在全局配置中的类型，创建出`SimpleExecutor/ReuseExecutor/BatchExecutor`

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {       executorType = executorType == null ? defaultExecutorType : executorType;    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;    Executor executor;    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);    }    //看全局配置中是否配置了二级缓存，如果有用CachingExecutor包装    if (cacheEnabled) {         executor = new CachingExecutor(executor);    }    //使用拦截器包装executor    executor = (Executor) interceptorChain.pluginAll(executor);    return executor;  }

6. 如果有二级缓存配置开启，创建`CachingExecutor(executor)`

对二级缓存的具体操作：

public CachingExecutor(Executor delegate) {       this.delegate = delegate;    delegate.setExecutorWrapper(this);  }

7. 使用每一个拦截器重新包装`executor`并返回

pluginAll具体实现：拿到所有的拦截器，调用每一个拦截器的plugin方法。

public Object pluginAll(Object target) {       for (Interceptor interceptor : interceptors) {         target = interceptor.plugin(target);    }    return target;  }