SqlSession是mybatis的核心接口之一，是myabtis接口層的主要組成部分，對外提供了mybatis常用的api。myabtis提供了兩個SqlSesion接口的實現，常用的實現類是DefaultSqlSession。它相當於一個數據庫連接對象，在一個SqlSession中可以執行多條SQL語句。
創建SqlSession
前面的兩篇文章我們已經得到了SqlSessionFactory，那么SqlSession將由SqlSessionFactory進行創建。
SqlSession sqlSession=sqlSessionFactory.openSession();
我們就來看看這個SqlSessionFactory的 openSession方法是如何創建SqlSession對象的。根據上面的分析，這里的SqlSessionFactory類型對象其實是一個DefaultSqlSessionFactory對象，因此，需要到DefaultSqlSessionFactory類中去看openSession方法。
@Override public SqlSession openSession() { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); }
調用了openSessionFromDataSource方法，並且第一個參數獲取了默認的執行器類型，第二個參數為null,第三個參數為false,看看這個默認的執行器類型是啥
默認的執行器類型SIMPLE，我們跟進openSessionFromDataSource方法
/** * ExecutorType 指定Executor的類型，分為三種：SIMPLE, REUSE, BATCH，默認使用的是SIMPLE * TransactionIsolationLevel 指定事務隔離級別，使用null,則表示使用數據庫默認的事務隔離界別 * autoCommit 是否自動提交，傳過來的參數為false，表示不自動提交 */ private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { // 獲取配置中的環境信息，包括了數據源信息、事務等 final Environment environment = configuration.getEnvironment(); // 創建事務工廠 final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); // 創建事務，配置事務屬性 tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); // 創建Executor，即執行器 // 它是真正用來Java和數據庫交互操作的類，后面會展開說。 final Executor executor = configuration.newExecutor(tx, execType); // 創建DefaultSqlSession對象返回，其實現了SqlSession接口 return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }
主要包含以下幾個步驟：
- 首先從configuration獲取Environment對象，里面主要包含了DataSource和TransactionFactory對象
- 創建TransactionFactory
- 創建Transaction
- 從configuration獲取Executor
- 構造DefaultSqlSession對象
我們先來看看常規的environment配置
//配置environment環境 <environments default="development"> <environment id="development"> /** 事務配置 type= JDBC、MANAGED * 1.JDBC:這個配置直接簡單使用了JDBC的提交和回滾設置。它依賴於從數據源得到的連接來管理事務范圍。 * 2.MANAGED:這個配置幾乎沒做什么。它從來不提交或回滾一個連接。 */ <transactionManager type="JDBC" /> /** 數據源類型：type = UNPOOLED、POOLED、JNDI * 1.UNPOOLED：這個數據源的實現是每次被請求時簡單打開和關閉連接。 * 2.POOLED：這是JDBC連接對象的數據源連接池的實現。 * 3.JNDI：這個數據源的實現是為了使用如Spring或應用服務器這類的容器 */ <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" /> //默認連接事務隔離級別 <property name="defaultTransactionIsolationLevel" value=""/> </dataSource> </environment> </environments>
還記得前面文章是怎么解析environments的嗎，Mybaits 源碼解析 （二）----- 根據配置文件創建SqlSessionFactory（Configuration的創建過程），我們簡單的回顧一下
private void environmentsElement(XNode context) throws Exception { if (context != null) { if (environment == null) { // 獲取 default 屬性 environment = context.getStringAttribute("default"); } for (XNode child : context.getChildren()) { // 獲取 id 屬性 String id = child.getStringAttribute("id"); /* * 檢測當前 environment 節點的 id 與其父節點 environments 的屬性 default * 內容是否一致，一致則返回 true，否則返回 false * 將其default屬性值與子元素environment的id屬性值相等的子元素設置為當前使用的Environment對象 */ if (isSpecifiedEnvironment(id)) { // 將environment中的transactionManager標簽轉換為TransactionFactory對象 TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager")); // 將environment中的dataSource標簽轉換為DataSourceFactory對象 DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource")); // 創建 DataSource 對象 DataSource dataSource = dsFactory.getDataSource(); Environment.Builder environmentBuilder = new Environment.Builder(id) .transactionFactory(txFactory) .dataSource(dataSource); // 構建 Environment 對象，並設置到 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(); //通過別名獲取Class,並實例化 TransactionFactory factory = (TransactionFactory)this.resolveClass(type).newInstance(); factory.setProperties(props); return factory; } else { throw new BuilderException("Environment declaration requires a TransactionFactory."); } } private DataSourceFactory dataSourceElement(XNode context) throws Exception { if (context != null) { String type = context.getStringAttribute("type"); //通過別名獲取Class,並實例化 Properties props = context.getChildrenAsProperties(); DataSourceFactory factory = (DataSourceFactory)this.resolveClass(type).newInstance(); factory.setProperties(props); return factory; } else { throw new BuilderException("Environment declaration requires a DataSourceFactory."); } }
獲取TransactionFactory
我們的environment配置中transactionManager type="JDBC"和dataSource type="POOLED"，則生成的transactionManager為JdbcTransactionFactory，DataSourceFactory為PooledDataSourceFactory
我們回到openSessionFromDataSource，接着看看getTransactionFactoryFromEnvironment方法
private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) { return (TransactionFactory)(environment != null && environment.getTransactionFactory() != null ? environment.getTransactionFactory() : new ManagedTransactionFactory()); }
創建Transaction
很明顯 environment.getTransactionFactory() 就是JdbcTransactionFactory，看看這個工廠是如何創建Transaction的
public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) { return new JdbcTransaction(ds, level, autoCommit); }
直接通過工廠方法創建了一個JdbcTransaction對象，並傳參DataSource ，事務隔離級別null，自動提交false三個參數，我們來看看JdbcTransaction
public class JdbcTransaction implements Transaction { //數據庫連接對象 protected Connection connection; //數據庫DataSource protected DataSource dataSource; //數據庫隔離級別 protected TransactionIsolationLevel level; //是否自動提交 protected boolean autoCommmit; public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) { //設置dataSource和隔離級別，是否自動提交屬性 //這里隔離級別傳過來的是null,表示使用數據庫默認隔離級別，自動提交為false，表示不自動提交 this.dataSource = ds; this.level = desiredLevel; this.autoCommmit = desiredAutoCommit; } public Connection getConnection() throws SQLException { if (this.connection == null) { this.openConnection(); } return this.connection; } //提交功能是通過Connection去完成的 public void commit() throws SQLException { if (this.connection != null && !this.connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Committing JDBC Connection [" + this.connection + "]"); } this.connection.commit(); } } //回滾功能是通過Connection去完成的 public void rollback() throws SQLException { if (this.connection != null && !this.connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Rolling back JDBC Connection [" + this.connection + "]"); } this.connection.rollback(); } } //關閉功能是通過Connection去完成的 public void close() throws SQLException { if (this.connection != null) { this.resetAutoCommit(); if (log.isDebugEnabled()) { log.debug("Closing JDBC Connection [" + this.connection + "]"); } this.connection.close(); } } //獲取連接是通過dataSource來完成的 protected void openConnection() throws SQLException { if (log.isDebugEnabled()) { log.debug("Opening JDBC Connection"); } this.connection = this.dataSource.getConnection(); if (this.level != null) { this.connection.setTransactionIsolation(this.level.getLevel()); } this.setDesiredAutoCommit(this.autoCommmit); } }
JdbcTransaction主要維護了一個默認autoCommit為false的Connection對象，對事物的提交，回滾，關閉等都是接見通過Connection完成的。
創建Executor
//創建一個執行器，默認是SIMPLE public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; //根據executorType來創建相應的執行器,Configuration默認是SIMPLE if (ExecutorType.BATCH == executorType) { executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) { executor = new ReuseExecutor(this, transaction); } else { //創建SimpleExecutor實例，並且包含Configuration和transaction屬性 executor = new SimpleExecutor(this, transaction); } //如果要求緩存，生成另一種CachingExecutor,裝飾者模式,默認都是返回CachingExecutor /** * 二級緩存開關配置示例 * <settings> * <setting name="cacheEnabled" value="true"/> * </settings> */ if (cacheEnabled) { //CachingExecutor使用裝飾器模式，將executor的功能添加上了二級緩存的功能，二級緩存會單獨文章來講 executor = new CachingExecutor(executor); } //此處調用插件,通過插件可以改變Executor行為，此處我們后面單獨文章講 executor = (Executor) interceptorChain.pluginAll(executor); return executor; }
executor包含了Configuration和剛剛創建的Transaction，默認的執行器為SimpleExecutor，如果開啟了二級緩存(默認開啟)，則CachingExecutor會包裝SimpleExecutor，然后依次調用攔截器的plugin方法返回一個被代理過的Executor對象。
CachingExecutor 對象里面包含了剛創建的SimpleExecutor，后面文章我們會及具體講這個類
public class CachingExecutor implements Executor { private Executor delegate; private TransactionalCacheManager tcm = new TransactionalCacheManager(); public CachingExecutor(Executor delegate) { this.delegate = delegate; delegate.setExecutorWrapper(this); } //略 }
構造DefaultSqlSession對象
new DefaultSqlSession(this.configuration, executor, autoCommit);
傳參configuration和剛生成的executor，我們來簡單看看
public class DefaultSqlSession implements SqlSession { /** * mybatis全局配置新 */ private final Configuration configuration; /** * SQL執行器 */ private final Executor executor; /** * 是否自動提交 */ private final boolean autoCommit; private List<Cursor<?>> cursorList; public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) { this.configuration = configuration; this.executor = executor; this.dirty = false; this.autoCommit = autoCommit; } @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 <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(); } } //略....update等方法 }
SqlSession的所有查詢接口最后都歸結位Exector的方法調用。后面文章我們來分析其調用流程