MyBatis
作为目前最常用的持久层框架之一,分析其源码,对我们的使用过程中可更好的运用它。本系列基于 mybatis-3.4.6
进行分析。
MyBatis 的初始化工作就是解析主配置文件,映射配置文件以及注解信息。然后保存在 org.apache.ibatis.session.Configuration
,供后期执行数据请求的相关调用。 Configuration
里有大量配置信息,在后面每涉及到一个相关配置,会进行详细的分析。
public static void main(String[] args) throws IOException { // 获取配置文件 Reader reader = Resources.getResourceAsReader("mybatis-config.xml"); // 通过 SqlSessionFactoryBuilder 构建 sqlSession 工厂 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader); // 获取 sqlSession 实例 SqlSession sqlSession = sqlSessionFactory.openSession(); reader.close(); sqlSession.close(); } 复制代码
SqlSessionFactoryBuilder 的 build()
是Mybatis启动的初始化入口,使用builder模式加载配置文件。
通过查看该类,使用方法重载,有以下9个方法:
方法重载最终实现处理的方法源码如下:
public SqlSessionFactory build(Reader reader, String environment, Properties properties) { try { // 实例化 XMLConfigBuilder,用于读取配置文件信息 XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties); // 解析配置信息,保存到 Configuration return build(parser.parse()); } catch (Exception e) { throw ExceptionFactory.wrapException("Error building SqlSession.", e); } finally { ErrorContext.instance().reset(); try { reader.close(); } catch (IOException e) { // Intentionally ignore. Prefer previous error. } } } 复制代码
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) { try { XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties); return build(parser.parse()); } catch (Exception e) { throw ExceptionFactory.wrapException("Error building SqlSession.", e); } finally { ErrorContext.instance().reset(); try { inputStream.close(); } catch (IOException e) { // Intentionally ignore. Prefer previous error. } } } 复制代码
通过 SqlSessionFactoryBuilder 中 XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties)
, 分析 XMLConfigBuilder实例化过程。
该类中有四个变量:
private boolean parsed; private final XPathParser parser; private String environment; private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory(); 复制代码
true
为已解析过。 SqlSessionFactoryBuilder
中的 environment
Reflector
对象,一个类对应一个 Reflector
。因为参数处理、结果映射等操作时,会涉及大量的反射操作。 DefaultReflectorFactory
实现类比较简单,这里不再进行讲解。 XMLConfigBuilder构建函数实现:
public XMLConfigBuilder(Reader reader, String environment, Properties props) { this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props); } 复制代码
实例化 XPathParser
对象
首先实例化 XPathParser
对象,里面定义了5个变量:
private final Document document; private boolean validation; private EntityResolver entityResolver; private Properties variables; private XPath xpath; 复制代码
XPathParser
对象构造函数有:
函数里面都处理了两件事:
public XPathParser(Reader reader, boolean validation, Properties variables, EntityResolver entityResolver) { commonConstructor(validation, variables, entityResolver); this.document = createDocument(new InputSource(reader)); } 复制代码
XPath
对象,用于对XML文件节点的操作。 private void commonConstructor(boolean validation, Properties variables, EntityResolver entityResolver) { this.validation = validation; this.entityResolver = entityResolver; this.variables = variables; // 创建Xpath对象,用于对XML文件节点的操作 XPathFactory factory = XPathFactory.newInstance(); this.xpath = factory.newXPath(); } 复制代码
Document
对象并赋值到 document
变量, 这里属于Document创建的操作,不再详细讲述,不懂可以点击这里查看API private Document createDocument(InputSource inputSource) { // important: this must only be called AFTER common constructor try { // 实例化 DocumentBuilderFactory 对象,用于创建 DocumentBuilder 对象 DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); // 是否校验文档 factory.setValidating(validation); // 设置 DocumentBuilderFactory 的配置 factory.setNamespaceAware(false); factory.setIgnoringComments(true); factory.setIgnoringElementContentWhitespace(false); factory.setCoalescing(false); factory.setExpandEntityReferences(true); // 创建 DocumentBuilder DocumentBuilder builder = factory.newDocumentBuilder(); builder.setEntityResolver(entityResolver); builder.setErrorHandler(new ErrorHandler() { @Override public void error(SAXParseException exception) throws SAXException { throw exception; } @Override public void fatalError(SAXParseException exception) throws SAXException { throw exception; } @Override public void warning(SAXParseException exception) throws SAXException { } }); // 加载文件 return builder.parse(inputSource); } catch (Exception e) { throw new BuilderException("Error creating document instance. Cause: " + e, e); } } 复制代码
XMLConfigBuilder
构造函数赋值
private XMLConfigBuilder(XPathParser parser, String environment, Properties props) { super(new Configuration()); ErrorContext.instance().resource("SQL Mapper Configuration"); this.configuration.setVariables(props); this.parsed = false; this.environment = environment; this.parser = parser; } 复制代码
BaseBuilder
的值。 XPathParser
赋值给 parser
。
最后返回 XMLConfigBuilder
对象。
通过 XMLConfigBuilder.parse()
解析配置信息,保存至 Configuration
。解析详解在后面文章中进行分析。
public Configuration parse() { // 是否解析过配置文件 if (parsed) { throw new BuilderException("Each XMLConfigBuilder can only be used once."); } // 标志解析过,定义为 true parsed = true; // 解析 configuration 节点中的信息 parseConfiguration(parser.evalNode("/configuration")); return configuration; } 复制代码
DefaultSqlSessionFactory
实现了 SqlSessionFactory
接口。
通过上面解析得到的 Configuration
,调用 SqlSessionFactoryBuilder.build(Configuration config)
创建一个 DefaultSqlSessionFactory
。
public SqlSessionFactory build(Configuration config) { return new DefaultSqlSessionFactory(config); } 复制代码
实例化 DefaultSqlSessionFactory
的过程,就是将 Configuration
传递给 DefaultSqlSessionFactory
成员变量 configuration
。
public DefaultSqlSessionFactory(Configuration configuration) { this.configuration = configuration; } 复制代码
通过调用 SqlSessionFactory.openSession()
创建 SqlSession
。
public interface SqlSessionFactory { // 默认创建 SqlSession openSession(); SqlSession openSession(boolean autoCommit); SqlSession openSession(Connection connection); SqlSession openSession(TransactionIsolationLevel level); SqlSession openSession(ExecutorType execType); SqlSession openSession(ExecutorType execType, boolean autoCommit); SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level); SqlSession openSession(ExecutorType execType, Connection connection); Configuration getConfiguration(); } 复制代码
SIMPLE
(不做特殊处理), REUSE
(复用预处理语句), BATCH
(会批量执行)
因为上面 DefaultSqlSessionFactory
实现了 SqlSessionFactory
接口,所以进入到 DefaultSqlSessionFactory
查看 openSession()
。
public SqlSession openSession() { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); } 复制代码
openSession()
方法最终实现代码如下:
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { // 获取configuration中的加载环境 final Environment environment = configuration.getEnvironment(); // 获取事务工厂 final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); // 创建一个事务 tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); // 生成一个处理器,事务保存在处理器 BaseExecutor 中 final Executor executor = configuration.newExecutor(tx, execType); // 实例化一个 DefaultSqlSession,DefaultSqlSession实现了SqlSession接口 return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { // 异常情况下关闭事务 closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { // 重置错误实例上下文 ErrorContext.instance().reset(); } } 复制代码
生成处理器 Configuration.newExecutor(Transaction transaction, ExecutorType executorType)
:
public Executor newExecutor(Transaction transaction, ExecutorType executorType) { // 默认为 ExecutorType.SIMPLE 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); } if (cacheEnabled) { executor = new CachingExecutor(executor); } executor = (Executor) interceptorChain.pluginAll(executor); return executor; } 复制代码
以 ExecutorType.SIMPLE
为例, BatchExecutor
, ReuseExecutor
同理:
至此,mybatis的启动流程大致简单的介绍到这里,对mybatis的启动初始化有个大致了解。接下将会针对单独模块进行详细分析。