MyBatis运行原理(二) : 获取SqlSession的流程、getMapper获取接口代理对象流程 (源码分析)

二、获取SqlSession的流程

1、进入到openSession()方法, 因为sqlSessionFactory在运行时的对象为DefaultSqlSessionFactory, 所以通过动态绑定机制调用的openSession为DefaultSqlSessionFactory的

@Override public SqlSession openSession(boolean autoCommit) { // configuration.getDefaultExecutorType() 为默认的SIMPLE类型的执行器 return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, autoCommit); } 

2、进入到DefaultSqlSessionFactory类的openSessionFromDataSource方法中

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { // 获取环境信息 final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); // 创建事务 tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); // 创建Executor执行器对象 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(); } } 

3、进入Configuration类的newExecutor(tx, execType)方法

public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; // 此处根据全局配置文件中配置的ExecutorType的类型, 来创建不同的XxxExecutor if (ExecutorType.BATCH == executorType) { executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) { executor = new ReuseExecutor(this, transaction); } else { // 默认是SIMPLE类型, 所以创建SimpleExecutor对象 executor = new SimpleExecutor(this, transaction); } // 如果有二级缓存, 就会创建CachingExecutor对象 /*
    	该对象内部调用的查询方法对象, 实际还是传进去的executor
    */ if (cacheEnabled) { executor = new CachingExecutor(executor); } executor = (Executor) interceptorChain.pluginAll(executor); return executor; } public enum ExecutorType { // BATCH为批量执行器 SIMPLE, REUSE, BATCH } 

Executor : 每一个SqlSession都会拥有一个Executor对象，这个对象负责增删改查的具体操作，我们可以简单的将它理解为JDBC中Statement的封装版。

4、进入CachingExecutor看一下

public class CachingExecutor implements Executor { private final Executor delegate; private final TransactionalCacheManager tcm = new TransactionalCacheManager(); public CachingExecutor(Executor delegate) { this.delegate = delegate; delegate.setExecutorWrapper(this); } // 省略... @Override public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, boundSql); @SuppressWarnings("unchecked") List<E> list = (List<E>) tcm.getObject(cache, key); if (list == null) { // 使用的仍然是传进来的executor list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116 } return list; } } // 使用的仍然是通过构造器传进来的executor return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); } 

5、newExecutor方法最终返回的executor对象都要走InterceptorChain类的pluginAll方法, 来将executor对象通过每个拦截器包装一下, 并返回最终的executor

executor = (Executor) interceptorChain.pluginAll(executor);

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

6、最终将这个包装好的executor对象, 传入到DefaultSqlSession中 7、最终调用的openSession()方法, 会返回一个DefaultSqlSession对象 1-7时序图如下:

总结: 返回的SqlSession的实现类DefaultSqlSession对象, 它包含了Executor和Configuration。Executor会在这一步被创建

三、getMapper获取接口的代理对象

1、进入DefaultSqlSession类的getMapper(Class class)方法

// 进入到DefaultSqlSession的getMapper方法 @Override public <T> T getMapper(Class<T> type) { // 调用configuration的getMapper return configuration.getMapper(type, this); } // 进入到Configuration的getMapper方法 public <T> T getMapper(Class<T> type, SqlSession sqlSession) { return mapperRegistry.getMapper(type, sqlSession); } 

2、进入到MapperRegistry类的getMapper方法

public <T> T getMapper(Class<T> type, SqlSession sqlSession) { // 直接从knownMappers拿到type的MapperProxy对象 final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MapperRegistry."); } try { return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } } 

3、进入mapperProxyFactory.newInstance(sqlSession)方法

// MapperProxyFactory类的方法 public T newInstance(SqlSession sqlSession) { // 创建了MapperProxy对象 // public class MapperProxyimplements InvocationHandler, Serializable {} // 所以MapperProxy final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); } // 进入newInstance(MapperProxy proxy)方法 // 创建MapperProxy代理对象 protected T newInstance(MapperProxy<T> mapperProxy) { return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); } 

4、最终就创建了UserMapper接口的代理对象

时序图如下: