Mybatis源码分析之（三）mapper接口底层原理（为什么不用写方法体就能访问到数据库）

mybatis是怎么拿sqlSession

在上一篇的时候，我们的SqlSessionFactoryBuilder已经从xml文件中解析出了Configuration并且返回了sessionFactory。

然后我们要从session;中拿到sqlSession

public class DefaultSqlSessionFactory implements SqlSessionFactory {private final Configuration configuration;@Overridepublic SqlSession openSession() {//默认情况下ExecutorType是ExecutorType.SIMPLE类型的return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);}private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {Transaction tx = null;try {//获取配置的环境信息final Environment environment = configuration.getEnvironment();//获取environment中的TransactionFactoryfinal TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);//生成Transactiontx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);//生成Executor（重要，之后的查询都得靠它）final Executor executor = configuration.newExecutor(tx, execType);//返回DefaultSqlSessionreturn 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();}}
}//DefaultSqlSession类的组成，其实新建的时候就只是把他的字段赋值而已
public class DefaultSqlSession implements SqlSession {private Configuration configuration;private Executor executor;private 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;}

上面分析的是到拿到sqlSession为止，重点其实不是在上面这里，因为到上面为止，其实主要的功能只是将配置的信息解析成我们要的类，然后进行初始化赋值。

Mapper的实现原理

下面我们从SqlSession中拿到mapper，并执行方法其实才是，你感觉到mybatis框架开始帮我们做事的开始。

    public static void main(String[] args) {//拿到SqlSessionSqlSession sqlsession = MybatisUtil.getSqlsession();//拿mapperTDemoMapper mapper = sqlsession.getMapper(TDemoMapper.class);//调用mapper的方法List<TDemo> all = mapper.getAll();for (TDemo item : all)System.out.println(item);}

因为我们在项目中的TDemoMapper只是一个接口，并没有实现这个接口方法，但是为什么我们在调用这个接口方法的时候就可以得到返回结果呢？mybatis究竟做了什么？

首先我们回到之前解析Mapper的语句

mapperElement(root.evalNode("mappers"));private void mapperElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {if ("package".equals(child.getName())) {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) { //根据resource解析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) {//根据url 解析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) {//根据mapperClass解析//首先通过mapperClass的路径，生成mapperClass的接口类Class<?> mapperInterface = Resources.classForName(mapperClass);//降mapperClass加入到configuration中去configuration.addMapper(mapperInterface);} else {throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");}}}}}//Configuration类下public <T> void addMapper(Class<T> type) {mapperRegistry.addMapper(type);}//MapperRegistry类下
public class MapperRegistry {private final Configuration config;private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();//最终调用这个方法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 {//将接口包装成MapperProxyFactory类放入knownMappers中（knownMappers就是存放我们的mapper接口的）knownMappers.put(type, new MapperProxyFactory<T>(type));// It's important that the type is added before the parser is run// otherwise the binding may automatically be attempted by the// mapper parser. If the type is already known, it won't try.//通过这个builder来解析mapper的statement。（把mapper和mapper.xml文件相关联，方法名与xml中的id相关联，为了之后调用的时候能找到的语句）MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);//开始解析parser.parse();loadCompleted = true;} finally {if (!loadCompleted) {knownMappers.remove(type);}}}}
}//MapperAnnotationBuilder类中public void parse() {String resource = type.toString();if (!configuration.isResourceLoaded(resource)) {//通过xml文件解析loadXmlResource();configuration.addLoadedResource(resource);assistant.setCurrentNamespace(type.getName());parseCache();parseCacheRef();//获得接口的方法（为了获取方法上的注解，通过注解的方式来让方法于sql语句相关联）Method[] methods = type.getMethods();for (Method method : methods) {try {// issue #237if (!method.isBridge()) {//具体的解析过程parseStatement(method);}} catch (IncompleteElementException e) {configuration.addIncompleteMethod(new MethodResolver(this, method));}}}parsePendingMethods();}

具体的调用过程就不细跟了，无非就是获取节点，获取属性值，（或者是获取方法，然后获取注解信息），巴拉巴拉……然后设置到configuration中。
上面要注意的点是，若既配置xml又配置注解的情况下，注解会覆盖xml，原因非常简单，源码中注解解析在xml解析后面，然后覆盖的情况是，他们有相同的namespace+id。
然后我们继续我们的主线任务，就是mapper的设计架构。从上面我们可以知道，configuration中有一个MapperRegistry类型的字段mapperRegistry，其中有一个字段叫knownMappers，knownMappers里面存着key为接口类型，值为MapperProxyFactory的。

//我们在调用TDemoMapper mapper = sqlsession.getMapper(TDemoMapper.class);的时候
//先调用DefaultSqlSession类下的@Overridepublic <T> T getMapper(Class<T> type) {return configuration.<T>getMapper(type, this);}
//然后调用Configuration类下的public <T> T getMapper(Class<T> type, SqlSession sqlSession) {return mapperRegistry.getMapper(type, sqlSession);}//最后调用MapperRegistry类下的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 {//实际生成是这段代码，通过mapperProxyFactory来生成实例对象return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}//实际调用类
public class MapperProxyFactory<T> {private final Class<T> mapperInterface;private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();public T newInstance(SqlSession sqlSession) {//实例化一个代理类final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);//通过这个函数实例化return newInstance(mapperProxy);}protected T newInstance(MapperProxy<T> mapperProxy) {//动态代理的基本操作（说明最终实现方式是动态代理）return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);}
}public class MapperProxy<T> implements InvocationHandler, Serializable {private static final long serialVersionUID = -6424540398559729838L;private final SqlSession sqlSession;private final Class<T> mapperInterface;private final Map<Method, MapperMethod> methodCache;public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {this.sqlSession = sqlSession;this.mapperInterface = mapperInterface;this.methodCache = methodCache;}//动态代理中最重要的方法invoke@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {if (Object.class.equals(method.getDeclaringClass())) {//如果是Object中的方法就不走下面的代理了，直接执行（比如toString，hashCode）return method.invoke(this, args);} else if (isDefaultMethod(method)) {//如果不是静态方法而且不是抽象方法，则不增强方法return invokeDefaultMethod(proxy, method, args);}} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}//实际我们的mapper接口的方法走的逻辑就是下面这2条final MapperMethod mapperMethod = cachedMapperMethod(method);return mapperMethod.execute(sqlSession, args);}
}

总结

我们通过sqlSession获得mapper方法，而sqlSession从configuration中的mapperRegistry中获取MapperProxyFactory对象，在通过MapperProxyFactory对象的newInstance方法得到MapperProxy的动态代理实例对象。

我们使用的mapper其实是通过MapperProxy动态代理，在运行时候生成的一个新的对象进行方法增强的，里面的接口方法都会通过下面2个语句进行数据库的操作，以及后续对数据的处理

    final MapperMethod mapperMethod = cachedMapperMethod(method);return mapperMethod.execute(sqlSession, args);12

这两条语句其实包含对访问数据库对象的创建，访问数据库到得到数据库返回数据后的处理等内容，非常复杂，本篇就到此为止。