正文 SqlSession是mybatis的核心接口之一，是myabtis接口层的主要组成部分，对外提供了mybatis常用的api。myabtis提供了两个SqlSesion接口的实现，常用的实现类是DefaultSqlSession，它相当于一个数
正文
SqlSession是mybatis的核心接口之一，是myabtis接口层的主要组成部分，对外提供了mybatis常用的api。myabtis提供了两个SqlSesion接口的实现，常用的实现类是DefaultSqlSession，它相当于一个数据库连接对象，在一个SqlSession中可以执行多条SQL语句。
创建SqlSession
前面的两篇文章已经得到了SqlSessionFactory，那么SqlSession将由SqlSessionFactory进行创建。
SqlSession sqlSession=sqlSessionFactory.openSession();我们就来看看这个SqlSessionFactory的 openSession方法是如何创建SqlSession对象的。根据上面的分析，这里的SqlSessionFactory类型对象其实是一个DefaultSqlSessionFactory对象，因此，需要到DefaultSqlSessionFactory类中去看openSession方法。
public class DefaultSqlSessionFactory implements SqlSessionFactory { @Override public SqlSession openSession() { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); } }调用了openSessionFromDataSource方法，并且第一个参数获取了默认的执行器类型，第二个参数为null，第三个参数为false，看看这个默认的执行器类型是啥
public class Configuration { protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE; public ExecutorType getDefaultExecutorType() { return defaultExecutorType; } } public enum ExecutorType { SIMPLE, REUSE, BATCH }默认的执行器类型SIMPLE，我们跟进openSessionFromDataSource方法
public class DefaultSqlSessionFactory implements SqlSessionFactory { private final Configuration configuration; /** * ExecutorType 指定Executor的类型，分为三种：SIMPLE, REUSE, BATCH，默认使用的是SIMPLE * TransactionIsolationLevel 指定事务隔离级别，使用null,则表示使用数据库默认的事务隔离界别 * autoCommit 是否自动提交，传过来的参数为false，表示不自动提交 */ private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { // 获取配置中的环境信息，包括了数据源信息、事务等 final Environment environment = configuration.getEnvironment(); // 创建事务工厂 final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); // 创建事务，配置事务属性 tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); // 创建Executor，即执行器 // 它是真正用来Java和数据库交互操作的类，后面会展开说。 final Executor executor = configuration.newExecutor(tx, execType); // 创建DefaultSqlSession对象返回，其实现了SqlSession接口 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(); } } }主要包含以下几个步骤：
- 1、首先从configuration获取Environment对象，里面主要包含了DataSource和TransactionFactory对象。
- 2、创建TransactionFactory
- 3、创建Transaction
- 4、从configuration获取Executor
- 5、构造DefaultSqlSession对象
我们先来看看常规的environment配置
//配置environment环境 <environments default="development"> <environment id="development"> /** 事务配置 type= JDBC、MANAGED * 1.JDBC:这个配置直接简单使用了JDBC的提交和回滚设置。它依赖于从数据源得到的连接来管理事务范围。 * 2.MANAGED:这个配置几乎没做什么。它从来不提交或回滚一个连接。 */ <transactionManager type="JDBC" /> /** 数据源类型：type = UNPOOLED、POOLED、JNDI * 1.UNPOOLED：这个数据源的实现是每次被请求时简单打开和关闭连接。 * 2.POOLED：这是JDBC连接对象的数据源连接池的实现。 * 3.JNDI：这个数据源的实现是为了使用如Spring或应用服务器这类的容器 */ <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://localhost:3306/xhm" /> <property name="username" value="root" /> <property name="password" value="root" /> //默认连接事务隔离级别 <property name="defaultTransactionIsolationLevel" value=""/> </dataSource> </environment> </environments>还记得前面文章是怎么解析environments的吗，Mybatis源码分析——创建SqlSessionFactory(Configuration的创建过程)，我们简单的回顾一下
private void environmentsElement(XNode context) throws Exception { if (context != null) { if (environment == null) { // 获取 default 属性 environment = context.getStringAttribute("default"); } for (XNode child : context.getChildren()) { // 获取 id 属性 String id = child.getStringAttribute("id"); /* * 检测当前 environment 节点的 id 与其父节点 environments 的属性 default * 内容是否一致，一致则返回 true，否则返回 false * 将其default属性值与子元素environment的id属性值相等的子元素设置为当前使用的Environment对象 */ if (isSpecifiedEnvironment(id)) { // 将environment中的transactionManager标签转换为TransactionFactory对象 TransactionFactory txFactory = transactionManagerElement(child.eval（"transactionManager")); // 将environment中的dataSource标签转换为DataSourceFactory对象 DataSourceFactory dsFactory = dataSourceElement(child.eval（"dataSource")); // 创建 DataSource 对象 DataSource dataSource = dsFactory.getDataSource(); Environment.Builder environmentBuilder = new Environment.Builder(id) .transactionFactory(txFactory) .dataSource(dataSource); // 构建 Environment 对象，并设置到 configuration 中 configuration.setEnvironment(environmentBuilder.build()); } } } } private TransactionFactory transactionManagerElement(XNode context) throws Exception { if (context != null) { String type = context.getStringAttribute("type"); Properties props = context.getChildrenAsProperties(); //通过别名获取Class,并实例化 TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance(); factory.setProperties(props); return factory; } throw new BuilderException("Environment declaration requires a TransactionFactory."); } private DataSourceFactory dataSourceElement(XNode context) throws Exception { if (context != null) { String type = context.getStringAttribute("type"); //通过别名获取Class,并实例化 Properties props = context.getChildrenAsProperties(); DataSourceFactory factory = (DataSourceFactory) resolveClass(type).newInstance(); factory.setProperties(props); return factory; } throw new BuilderException("Environment declaration requires a DataSourceFactory."); }获取TransactionFactory
我们的environment配置中transactionManager type="JDBC"和dataSource type="POOLED"，则生成的transactionManager为JdbcTransactionFactory，DataSourceFactory为PooledDataSourceFactory
我们回到openSessionFromDataSource，接着看看getTransactionFactoryFromEnvironment方法
private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) { if (environment == null || environment.getTransactionFactory() == null) { return new ManagedTransactionFactory(); } return environment.getTransactionFactory(); }创建Transaction
很明显 environment.getTransactionFactory() 就是JdbcTransactionFactory，看看这个工厂是如何创建Transaction的
public final class Environment { private final TransactionFactory transactionFactory; private final DataSource dataSource; public TransactionFactory getTransactionFactory() { return this.transactionFactory; } } public class JdbcTransactionFactory implements TransactionFactory { @Override public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) { return new JdbcTransaction(ds, level, autoCommit); } }直接通过工厂方法创建了一个JdbcTransaction对象，并传参DataSource ，事务隔离级别null，自动提交false三个参数，我们来看看JdbcTransaction
public class JdbcTransaction implements Transaction { //数据库连接对象 protected Connection connection; //数据库DataSource protected DataSource dataSource; //数据库隔离级别 protected TransactionIsolationLevel level; // MEMO: We are aware of the typo. See #941 //是否自动提交 protected boolean autoCommmit; public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) { //设置dataSource和隔离级别，是否自动提交属性 //这里隔离级别传过来的是null,表示使用数据库默认隔离级别，自动提交为false，表示不自动提交 dataSource = ds; level = desiredLevel; autoCommmit = desiredAutoCommit; } @Override public Connection getConnection() throws SQLException { if (connection == null) { openConnection(); } return connection; } //提交功能是通过Connection去完成的 @Override public void commit() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Committing JDBC Connection [" + connection + "]"); } connection.commit(); } } //回滚功能是通过Connection去完成的 @Override public void rollback() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Rolling back JDBC Connection [" + connection + "]"); } connection.rollback(); } } //关闭功能是通过Connection去完成的 @Override public void close() throws SQLException { if (connection != null) { resetAutoCommit(); if (log.isDebugEnabled()) { log.debug("Closing JDBC Connection [" + connection + "]"); } connection.close(); } } //获取连接是通过dataSource来完成的 protected void openConnection() throws SQLException { if (log.isDebugEnabled()) { log.debug("Opening JDBC Connection"); } connection = dataSource.getConnection(); if (level != null) { connection.setTransactionIsolation(level.getLevel()); } setDesiredAutoCommit(autoCommmit); } }JdbcTransaction主要维护了一个默认autoCommit为false的Connection对象，对事物的提交，回滚，关闭等都是接见通过Connection完成的。
创建Executor
public class Configuration { protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE; public Executor newExecutor(Transaction transaction) { return newExecutor(transaction, defaultExecutorType); } //创建一个执行器，默认是SIMPLE public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; //根据executorType来创建相应的执行器,Configuration默认是SIMPLE if (ExecutorType.BATCH == executorType) { executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) { executor = new ReuseExecutor(this, transaction); } else { //创建SimpleExecutor实例，并且包含Configuration和transaction属性 executor = new SimpleExecutor(this, transaction); } //如果要求缓存，生成另一种CachingExecutor,装饰者模式,默认都是返回CachingExecutor /** * 二级缓存开关配置示例 * <settings> * <setting name="cacheEnabled" value="true"/> * </settings> */ if (cacheEnabled) { //CachingExecutor使用装饰器模式，将executor的功能添加上了二级缓存的功能，二级缓存会单独文章来讲 executor = new CachingExecutor(executor); } //此处调用插件,通过插件可以改变Executor行为，此处我们后面单独文章讲 executor = (Executor) interceptorChain.pluginAll(executor); return executor; } }executor包含了Configuration和刚刚创建的Transaction，默认的执行器为SimpleExecutor，如果开启了二级缓存(默认开启)，则CachingExecutor会包装SimpleExecutor，然后依次调用拦截器的plugin方法返回一个被代理过的Executor对象。
CachingExecutor 对象里面包含了刚创建的SimpleExecutor，后面文章我们会及具体讲这个类
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); } //......略 }构造DefaultSqlSession对象
new DefaultSqlSession(this.configuration, executor, autoCommit);传参configuration和刚生成的executor，我们来简单看看
public class DefaultSqlSession implements SqlSession { /** * mybatis全局配置新 */ private final Configuration configuration; /** * SQL执行器 */ private final Executor executor; /** * 是否自动提交 */ private final boolean autoCommit; private boolean dirty; private List<Cursor<?>> cursorList; public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) { this.configuration = configuration; this.executor = executor; this.dirty = false; this.autoCommit = autoCommit; } public DefaultSqlSession(Configuration configuration, Executor executor) { this(configuration, executor, false); } @Override public <T> T selectOne(String statement) { return this.<T>selectOne(statement, null); } @Override public <T> T selectOne(String statement, Object parameter) { // Popular vote was to return null on 0 results and throw exception on too many. List<T> list = this.<T>selectList(statement, parameter); if (list.size() == 1) { return list.get(0); } else if (list.size() > 1) { throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size()); } else { return null; } } @Override public <K, V> Map<K, V> selectMap(String statement, String mapKey) { return this.selectMap(statement, null, mapKey, RowBounds.DEFAULT); } @Override public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey) { return this.selectMap(statement, parameter, mapKey, RowBounds.DEFAULT); } @Override public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds) { final List<? extends V> list = selectList(statement, parameter, rowBounds); final DefaultMapResultHandler<K, V> mapResultHandler = new DefaultMapResultHandler<K, V>(mapKey, configuration.getObjectFactory(), configuration.getObjectWrapperFactory(), configuration.getReflectorFactory()); final DefaultResultContext<V> context = new DefaultResultContext<V>(); for (V o : list) { context.nextResultObject(o); mapResultHandler.handleResult(context); } return mapResultHandler.getMappedResults(); } @Override public <T> Cursor<T> selectCursor(String statement) { return selectCursor(statement, null); } @Override public <T> Cursor<T> selectCursor(String statement, Object parameter) { return selectCursor(statement, parameter, RowBounds.DEFAULT); } @Override public <T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds) { try { MappedStatement ms = configuration.getMappedStatement(statement); Cursor<T> cursor = executor.queryCursor(ms, wrapCollection(parameter), rowBounds); registerCursor(cursor); return cursor; } catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } @Override public <E> List<E> selectList(String statement) { return this.selectList(statement, null); } @Override public <E> List<E> selectList(String statement, Object parameter) { return this.selectList(statement, parameter, RowBounds.DEFAULT); } @Override public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { MappedStatement ms = configuration.getMappedStatement(statement); return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); } catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } @Override public void select(String statement, Object parameter, ResultHandler handler) { select(statement, parameter, RowBounds.DEFAULT, handler); } @Override public void select(String statement, ResultHandler handler) { select(statement, null, RowBounds.DEFAULT, handler); } @Override public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) { try { MappedStatement ms = configuration.getMappedStatement(statement); executor.query(ms, wrapCollection(parameter), rowBounds, handler); } catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } @Override public int insert(String statement) { return insert(statement, null); } @Override public int insert(String statement, Object parameter) { return update(statement, parameter); } @Override public int update(String statement) { return update(statement, null); } @Override public int update(String statement, Object parameter) { try { dirty = true; MappedStatement ms = configuration.getMappedStatement(statement); return executor.update(ms, wrapCollection(parameter)); } catch (Exception e) { throw ExceptionFactory.wrapException("Error updating database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } @Override public int delete(String statement) { return update(statement, null); } @Override public int delete(String statement, Object parameter) { return update(statement, parameter); } @Override public void commit() { commit(false); } @Override public void commit(boolean force) { try { executor.commit(isCommitOrRollbackRequired(force)); dirty = false; } catch (Exception e) { throw ExceptionFactory.wrapException("Error committing transaction. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } @Override public void rollback() { rollback(false); } @Override public void rollback(boolean force) { try { executor.rollback(isCommitOrRollbackRequired(force)); dirty = false; } catch (Exception e) { throw ExceptionFactory.wrapException("Error rolling back transaction. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } @Override public List<BatchResult> flushStatements() { try { return executor.flushStatements(); } catch (Exception e) { throw ExceptionFactory.wrapException("Error flushing statements. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } @Override public void close() { try { executor.close(isCommitOrRollbackRequired(false)); closeCursors(); dirty = false; } finally { ErrorContext.instance().reset(); } } private void closeCursors() { if (cursorList != null && cursorList.size() != 0) { for (Cursor<?> cursor : cursorList) { try { cursor.close(); } catch (IOException e) { throw ExceptionFactory.wrapException("Error closing cursor. Cause: " + e, e); } } cursorList.clear(); } } @Override public Configuration getConfiguration() { return configuration; } @Override public <T> T getMapper(Class<T> type) { return configuration.<T>getMapper(type, this); } @Override public Connection getConnection() { try { return executor.getTransaction().getConnection(); } catch (SQLException e) { throw ExceptionFactory.wrapException("Error getting a new connection. Cause: " + e, e); } } @Override public void clearCache() { executor.clearLocalCache(); } private <T> void registerCursor(Cursor<T> cursor) { if (cursorList == null) { cursorList = new ArrayList<Cursor<?>>(); } cursorList.add(cursor); } private boolean isCommitOrRollbackRequired(boolean force) { return (!autoCommit && dirty) || force; } private Object wrapCollection(final Object object) { if (object instanceof Collection) { StrictMap<Object> map = new StrictMap<Object>(); map.put("collection", object); if (object instanceof List) { map.put("list", object); } return map; } else if (object != null && object.getClass().isArray()) { StrictMap<Object> map = new StrictMap<Object>(); map.put("array", object); return map; } return object; } public static class StrictMap<V> extends HashMap<String, V> { private static final long serialVersionUID = -5741767162221585340L; @Override public V get(Object key) { if (!super.containsKey(key)) { throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + this.keySet()); } return super.get(key); } } }SqlSession的所有查询接口最后都归结位Exector的方法调用。后面文章我们来分析其调用流程。
参考： https://www.cnblogs.com/java-chen-hao/p/11743506.html