spring-mybatis源码解析

本文主要围绕三个问题展开？

1.spirng是怎么和mybatis关联起来的？
2.xml和mapper是如何解析的
3.mapper中的方法是怎么和xml中的方法关联起来的？

Spirng是怎么和mybatis关联起来的

在基本的 MyBatis 中，session 工厂可以使用 SqlSessionFactoryBuilder 来创建。而在MyBatis-Spring 中,则使用 SqlSessionFactoryBean 来替代。

spring-mybatis包中有一个类SqlSessionFactoryBean

SqlSessionFactoryBean 实现了 Spring 的 FactoryBean 接口这就说明了由 Spring 最终创建的 bean 不是 SqlSessionFactoryBean 本身,。而是工厂类的 getObject()返回的方法的结果。

  @Overridepublic SqlSessionFactory getObject() throws Exception {if (this.sqlSessionFactory == null) {// sqlSessionFactory默认为空，直接走afterPropertiesSet()方法// 实际不是这样的，由于SqlSessionFactoryBean实现了InitializingBean，则再该bean生成之后，// 会直接调用afterPropertiesSet()方法，来创建sqlSessionFactory，故sqlSessionFactory应该// 是已经被创建好的afterPropertiesSet();}return this.sqlSessionFactory;}

如果sqlSessionFactory为空的话就调用afterPropertiesSet();方法，该方法是重写InitializingBean的方法，在bean初始化的时候就会执行。

  @Overridepublic void afterPropertiesSet() throws Exception {notNull(dataSource, "Property 'dataSource' is required");notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),"Property 'configuration' and 'configLocation' can not specified with together");this.sqlSessionFactory = buildSqlSessionFactory();}

做了一些环境前置的判断。然后执行buildSqlSessionFactory();构建sqlSessionFactory

核心内容是对于 mapperLocations 的解析，如下代码

  protected SqlSessionFactory buildSqlSessionFactory() throws Exception {final Configuration targetConfiguration;XMLConfigBuilder xmlConfigBuilder = null;//....省略代码/*** mapperLocations里面就是xml文件* 把xml文件进行解析（重要）*/if (this.mapperLocations != null) {if (this.mapperLocations.length == 0) {LOGGER.warn(() -> "Property 'mapperLocations' was specified but matching resources are not found.");} else {for (Resource mapperLocation : this.mapperLocations) {if (mapperLocation == null) {continue;}try {XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());//解析mapper文件xmlMapperBuilder.parse();} catch (Exception e) {throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);} finally {ErrorContext.instance().reset();}LOGGER.debug(() -> "Parsed mapper file: '" + mapperLocation + "'");}}} else {LOGGER.debug(() -> "Property 'mapperLocations' was not specified.");}return this.sqlSessionFactoryBuilder.build(targetConfiguration);}

这里主要就是构建一个Configuration对象，在最后利用这个对象去构建一个sqlSessionFactory。
this.sqlSessionFactoryBuilder.build(targetConfiguration)

SqlSessionFactoryBuilder支持不同的方式去创建SqlSessionFactory。（SqlSessionFactoryBuilder是mybatis源码里面的类了）

如通过配置文件，如：mybatis-config.xml，以前ssm那一套
通过IO流
通过Configuration参数

  public SqlSessionFactory build(Configuration config) {return new DefaultSqlSessionFactory(config);}

xml和mapper是如何解析的

进入xmlMapperBuilder.parse();

public void parse() {// 1.如果 resource 没被加载过才进行加载if (!configuration.isResourceLoaded(resource)) {// 1.1 解析 mapper 文件configurationElement(parser.evalNode("/mapper"));// 1.2 将 resource 添加到已加载列表configuration.addLoadedResource(resource);// 1.3 绑定 namespace 的 mapperbindMapperForNamespace();}parsePendingResultMaps();parsePendingCacheRefs();parsePendingStatements();
}

configurationElement

//context就是一个mapper.xml的内容
private void configurationElement(XNode context) {try {// 1.获取namespace属性String namespace = context.getStringAttribute("namespace");if (namespace == null || namespace.isEmpty()) {throw new BuilderException("Mapper's namespace cannot be empty");}// 2.设置currentNamespace属性builderAssistant.setCurrentNamespace(namespace);// 3.解析parameterMap、resultMap、sql等节点cacheRefElement(context.evalNode("cache-ref"));cacheElement(context.evalNode("cache"));parameterMapElement(context.evalNodes("/mapper/parameterMap"));resultMapElements(context.evalNodes("/mapper/resultMap"));sqlElement(context.evalNodes("/mapper/sql"));// 4.解析增删改查节点，封装成 StatementbuildStatementFromContext(context.evalNodes("select|insert|update|delete"));} catch (Exception e) {throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);}
}

private void buildStatementFromContext(List<XNode> list) {if (configuration.getDatabaseId() != null) {buildStatementFromContext(list, configuration.getDatabaseId());}// 解析增删改查节点，封装成StatementbuildStatementFromContext(list, null);
}

这里的每个Xnode就是对应一个方法

解析节点构建成一个MappedStatement对象，最后放到Configuration中的Map<String, MappedStatement> mappedStatements变量中

private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {for (XNode context : list) {// 1.构建XMLStatementBuilderfinal XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);try {// 2.解析节点statementParser.parseStatementNode();} catch (IncompleteElementException e) {configuration.addIncompleteStatement(statementParser);}}
}

一次put会放入两个值，key不同但是value是一样的。MappedStatement就是一个sql方法。所以通过这两种key都可以找到对应的sql。

bindMapperForNamespace

通过之前解析xml得到的namespace获取到对应的Class
放到Map<Class<?>, MapperProxyFactory<?>> knownMappers变量中

private void bindMapperForNamespace() {// 找出当前的命名空间String namespace = builderAssistant.getCurrentNamespace();if (namespace != null) {Class<?> boundType = null;try {// 1.找到对应的mapper类boundType = Resources.classForName(namespace);} catch (ClassNotFoundException e) {}if (boundType != null && !configuration.hasMapper(boundType)) {// 2.boundType不为空，并且configuration还没有添加boundType，// 则将namespace添加到已加载列表，将boundType添加到knownMappers缓存configuration.addLoadedResource("namespace:" + namespace);configuration.addMapper(boundType);}}
}public <T> void addMapper(Class<T> type) {mapperRegistry.addMapper(type);
}public <T> void addMapper(Class<T> type) {if (type.isInterface()) {if (hasMapper(type)) {throw new BindingException("Type " + type + " is already known to the MapperRegistry.");}boolean loadCompleted = false;try {// 放到knownMappers缓存中去，value是一个MapperProxyFactory的代理knownMappers.put(type, new MapperProxyFactory<>(type));MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);//解析mapper类parser.parse();loadCompleted = true;} finally {if (!loadCompleted) {knownMappers.remove(type);}}}
}

解析mapper类

public void parse() {// Class对象的唯一标识，如：// 类似"interface com.xxx.mapper.xxxMapper"String resource = type.toString();// 如果当前Class对象已经解析过，则不在解析if (!configuration.isResourceLoaded(resource)) {// 加载并解析指定的xml配置文件，Class所在的包对应文件路径，Class类名对应文件名称，如：// com.xxx.mapper.xxxMapper类对应的配置文件为com/xxx/mapper/xxxMapper.xmlloadXmlResource();// 把Class对应的标识添加到已加载的资源列表中configuration.addLoadedResource(resource);// 设置当前namespace为接口Class的全限定名assistant.setCurrentNamespace(type.getName());// 解析@CacheNamespace注解对应mapper.xml配置文件中的<cache>元素parseCache();// 解析缓存引用，会覆盖之前解析的缓存对象parseCacheRef();// 获取mapper接口所有方法，解析方法上的注解Method[] methods = type.getMethods();// 遍历所有获取到的方法for (Method method : methods) {try {// 主要是针对注解的 例如：方法上的@Select// 解析一个方法生成对应的MapperedStatement对象// 并添加到配置对象中parseStatement(method);} catch (IncompleteElementException e) {configuration.addIncompleteMethod(new MethodResolver(this, method));}}}// 解析挂起的方法parsePendingMethods();}

mapper中的方法是怎么和xml中的方法关联起来的

在上篇文章说到用@MapperScan扫描到的bean，BeanDefinition中的beanClass会被替换成MapperFactoryBean.class

我们来看一看MapperFactoryBean的类图

SqlSessionDaoSupport:提供SqlSessionTemplate模版,这个变量会在初始化mapper bean的时候属性填充的时候设置进去
DaoSupport：实现了InitializingBean，具体方法有子类实现，做一些检查，初始化工作

MapperFactoryBean也是一个FactoryBean，也就是说spirng真正返回的是getObject()中的bean。

  @Overridepublic T getObject() throws Exception {return getSqlSession().getMapper(this.mapperInterface);}

使用mapper来执行方法的流程

首先就要先获取到mapper,这个mapper是一个被代理的对象

SqlSessionTemplate

  @Overridepublic <T> T getMapper(Class<T> type) {return getConfiguration().getMapper(type, this);}

Configuration中的getMapper方法。mapper是哪里来的呢？

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {return mapperRegistry.getMapper(type, sqlSession);}

mapper是从knownMappers变量中获取，也就是上面在bindMapperForNamespace方法中就放进入了

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {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);}}

最后调用MapperProxyFactory里面的newInstance方法

  protected T newInstance(MapperProxy<T> mapperProxy) {return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);}public T newInstance(SqlSession sqlSession) {final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);return newInstance(mapperProxy);}

MapperProxy实现了InvocationHandler，是基于jdk动态代理的。里面的invoke方法。最后执行的是MapperMethod中的execute方法。

// MapperProxy.invoke()
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {// Object的方法执行if (Object.class.equals(method.getDeclaringClass())) {try {return method.invoke(this, args);} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}}// 获取MapperMethodfinal MapperMethod mapperMethod = cachedMapperMethod(method);// 真正的处理在这里return mapperMethod.execute(sqlSession, args);
}private MapperMethod cachedMapperMethod(Method method) {MapperMethod mapperMethod = methodCache.get(method);if (mapperMethod == null) {mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());methodCache.put(method, mapperMethod);}return mapperMethod;
}

MapperMethod.execute()方法执行

// MapperMethod.execute()
public Object execute(SqlSession sqlSession, Object[] args) {Object result;// 具体的增删改查操作，都有具体的执行，if (SqlCommandType.INSERT == command.getType()) {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));} else if (SqlCommandType.UPDATE == command.getType()) {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));} else if (SqlCommandType.DELETE == command.getType()) {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));// 在这里我们主要看本例中的查询操作} else if (SqlCommandType.SELECT == command.getType()) {if (method.returnsVoid() && method.hasResultHandler()) {executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {result = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {result = executeForCursor(sqlSession, args);} else {// 本例就返回一个结果值，看这里// 封装参数值，最后还是交给SqlSession来处理Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);}} else if (SqlCommandType.FLUSH == command.getType()) {result = sqlSession.flushStatements();} else {throw new BindingException("Unknown execution method for: " + command.getName());}if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {throw new BindingException("Mapper method '" + command.getName() + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");}return result;
}

比如说这样一个方法。通过断点看看

    @Autowiredprivate DemoMapper demoMapper;@Testpublic void test() {Demo demo1 = demoMapper.selectByPrimaryKey(1L);System.out.println(demo1.toString());}

我们可以很清楚的看到demoMapper就是一个被代理的类。

经过SqlSessionTemplate-selectOne->DefaultSqlSession-selectOne最后到selectList中

就是从Map<String, MappedStatement> mappedStatements这个变量中去获取的，而这个变量中的值在解析xml文件的时候已经放进去了。

我们都知道在mapper里面写的方法名称，在xml里面的id要一致才可以。Configuration中的mappedStatements是一个map。把xml里面的方法解析后放到这里面。然后这样就把mapper和xml对应起来了。

总结

通过xml的namespace和mapper接口的类路径一致绑定
然后通过在mapper里面写的方法名称和xml中的id一致这样就把xml和mapper绑定起来了