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1.前言 Mybatis是当前最流行的持久层框架之一，其官方使用手册详见：http://www./mybatis-3/zh/index.html。 使用Mybatis主要分为以下几个步骤： 1) 添加mybatis依赖到pom文件（maven项目）或jar包到项目中； 2) 添加mybatis-config.xml配置文件，包含的配置信息与配置方式详见使用手册； 3) 解析配置文件并创建Configuration对象configuration； 4) 使用configuration创建SqlSessionFactory对象； 5) 通过sqlSession获取mapper实例，并调用mapper接口中方法与DB交换。 下面就对上述步骤涉及到的主要源码进行分析讲解。 2.构造SqlSessionFactory实例 通过SqlSessionFactoryBuilder构造应用级别SqlSessionFactory实例，核心代码如下： // inputStream,mybatis-config.xml配置文件的输入流 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. } } } public SqlSessionFactory build(Configuration config) { return new DefaultSqlSessionFactory(config); }
构造过程如下： 1）创建XMLConfigBuilder对象parser，同时创建了Configuration对象configuration； 2）调用parser.parser()解析mybatis配置文件，保持配置信息到configuration中； 3）创建SqlSessionFactory实例，即DefaultSqlSessionFactory对象。
3.解析mybatis配置文件 解析mybatis配置文件核心代码如下： public Configuration parse() { if (parsed) { throw new BuilderException("Each XMLConfigBuilder can only be used once."); } parsed = true; parseConfiguration(parser.evalNode("/configuration")); return configuration; }
private void parseConfiguration(XNode root) { try { propertiesElement(root.evalNode("properties")); Properties settings = settingsAsProperties(root.evalNode("settings")); loadCustomVfs(settings); typeAliasesElement(root.evalNode("typeAliases")); pluginElement(root.evalNode("plugins")); objectFactoryElement(root.evalNode("objectFactory")); objectWrapperFactoryElement(root.evalNode("objectWrapperFactory")); reflectorFactoryElement(root.evalNode("reflectorFactory")); settingsElement(settings); // read it after objectFactory and objectWrapperFactory issue #631 environmentsElement(root.evalNode("environments")); databaseIdProviderElement(root.evalNode("databaseIdProvider")); typeHandlerElement(root.evalNode("typeHandlers")); mapperElement(root.evalNode("mappers")); } catch (Exception e) { throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); } }
parseConfiguration方法依次解析mybatis配置文件中各元素，具体解析过程如下所示： propertiesElement，解析元素properties，保存在variables中； typeAliasesElement，解析元素typeAliases，保存在typeAliasRegistry中； pluginElement，解析插元素plugins，保存在interceptorChain中； objectFactoryElement，解析元素objectFactory，保存在objectFactory中； objectWrapperFactoryElement，解析元素objectWrapperFactory，保存在objectWrapperFactory中； reflectorFactoryElement，解析元素reflectorFactory，保存在reflectorFactory中； settingsElement，解析元素settings，保存在configuration属性中； environmentsElement，解析元素environments，保存在environment中； databaseIdProviderElement，解析元素databaseIdProvider，保存在databaseId中； typeHandlerElement，解析元素typeHandlers，保存在typeHandlerRegistry中； mapperElement，解析元素mappers，保存在mapperRegistry中。
4.解析mappers元素 4.1 mappers配置方式 mappers有四种配置方式，如下所示： 详细配置详见：http://www./mybatis-3/configuration.html#mappers。
4.2解析mappers核心代码 private void mapperElement(XNode parent) throws Exception { if (parent != null) { for (XNode child : parent.getChildren()) { if ("package".equals(child.getName())) { // 对应配置方式4，查找属性name指定包下所有的接口类型，注册到mapperRegistry String mapperPackage = child.getStringAttribute("name"); configuration.addMappers(mapperPackage); } else { String resource = child.getStringAttribute("resource"); String url = child.getStringAttribute("url"); String mapperClass = child.getStringAttribute("class"); if (resource != null && url == null && mapperClass == null) { // 对应配置方式1 ErrorContext.instance().resource(resource); InputStream inputStream =Resources.getResourceAsStream(resource); XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments()); mapperParser.parse(); } else if (resource == null && url != null && mapperClass == null) { // 对应配置方式2 ErrorContext.instance().resource(url); InputStream inputStream = Resources.getUrlAsStream(url); XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments()); mapperParser.parse(); } else if (resource == null && url == null && mapperClass != null) { // 对应配置方式3 Class mapperInterface = Resources.classForName(mapperClass); configuration.addMapper(mapperInterface); } else { throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one."); } } } } } mappers的前两种配置解析步骤： 1) 创建XMLMapperBuilder对象mapperParser，调用mapperParser.parse()解析*mapper.xml文件； 2) 以mapper配置文件的namespace属性值为type，创建type对应的MapperProxyFactory mappers的后两种配置解析步骤： 1) 以mapper接口类型为type，创建type对应的MapperProxyFactory 2) 创建XMLMapperBuilder对象mapperParser，调用mapperParser.parse()解析*mapper.xml文件。
4.3 MapperRegistry MapperRegistry是mybatis中用于注册mapper带configuration的核心类，其源码如下： public class MapperRegistry { private final Configuration config; private final Map public MapperRegistry(Configuration config) { this.config = config; }
// 当执行sqlSession.getMapper(Class // 取出type对应的MapperProxyFactory对象，返回mapper的代理对象mapperProxy @SuppressWarnings("unchecked") public final MapperProxyFactory 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); } }
public if (type.isInterface()) { if (hasMapper(type)) { throw new BindingException("Type " + type + " is already known to the MapperRegistry."); } boolean loadCompleted = false; try { // 以mapper类型为参数创建mapper的代理工厂对象，并将其保存， knownMappers.put(type, new MapperProxyFactory MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type); parser.parse(); loadCompleted = true; } finally { if (!loadCompleted) { knownMappers.remove(type); } } } }
...... }
5.MapperProxyFactory MapperProxyFactory是MapperProxy的工厂类，其核心源码如下： public class MapperProxyFactory private final Class
// 构造函数接收mapper接口类型，用于创建mapper的代理对象mapperProxy public MapperProxyFactory(Class this.mapperInterface = mapperInterface; }
@SuppressWarnings("unchecked") protected T newInstance(MapperProxy return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); }
// 该方法实际上就是创建mapperProxy的工厂方法 public T newInstance(SqlSession sqlSession) { final MapperProxy return newInstance(mapperProxy); }
...... }
6.MapperProxy Mapper接口的jdk动态代理类，调用mapper接口方法会执行代理类invoke方法。 核心代码如下： public class MapperProxy
private final SqlSession sqlSession; private final Class private final Map
public MapperProxy(SqlSession sqlSession, Class this.sqlSession = sqlSession; this.mapperInterface = mapperInterface; this.methodCache = methodCache; }
@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, args); } else if (isDefaultMethod(method)) { return invokeDefaultMethod(proxy, method, args); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } final MapperMethod mapperMethod = cachedMapperMethod(method); // 执行下面的方法，会调用sqlSession的insert、update、delete、 // select等方法，进而调用executor执行sql等操作 return mapperMethod.execute(sqlSession, args); }
...... }
7.执行mapper接口方法 SqlSession session = sqlSessionFactory.openSession(true); try { // mapper实际上是BlogMapper接口的代理对象mapperProxy BlogMapper mapper = session.getMapper(BlogMapper.class); // 实际上执行的时MapperProxy的invoke方法 Blog blog = mapper.selectBlog(101); } finally { session.close(); }
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