type interface is not known to（聊聊Mybatis的binding模块）

为什么我们在使用Mybatis的时候只需要写接口和xml文件就能执行sql呢？这就是Mybatis的binding模块需要做的事情了，今天我们分析一下Mybatis的binding模块，binding包下的类主要有四个MapperRegistry、MapperProxyFactory、MapperProxy和MapperMethod

MapperRegistry是个注册类，它的knownMappers集合保存着Mapper接口和MapperProxyFactory实例的映射关系

它的addMapper()方法就是添加映射关系的方法：

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 { knownMappers.put(type, new MapperProxyFactory<>(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. MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type); parser.parse(); loadCompleted = true; } finally { if (!loadCompleted) { knownMappers.remove(type); } } } }

当传入的type是个接口并knownMappers中没有，就把type和对应的MapperProxyFactory实例放入knownMappers中

然后在执行sql的时候，Mybatis会调用MapperRegistry.getMapper()方法

@SuppressWarnings("unchecked") 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); } }

1. 先从knownMappers集合中找到对应的MapperProxyFactory实例
2. 然后调用newInstance()方法
3. 映射代理工厂类MapperProxyFactory
4. MapperProxyFactory的newInstance()方法：

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

生成mapperInterface接口 的代理对象的实例，代理类是MapperProxy，它实现了InvocationHandler，利用jdk动态代理生成代理对象

看一下它重写的invoke()方法：

MapperProxy的invoke()方法：

@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 { return cachedInvoker(method).invoke(proxy, method, args, sqlSession); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } }

不是Object的类会被拦截，拦截调用cachedInvoker()方法，其他执行invoke()方法，

private MapperMethodInvoker cachedInvoker(Method method) throws Throwable { try { return MapUtil.computeIfAbsent(methodCache, method, m -> { if (m.isDefault()) { try { if (privateLookupInMethod == null) { return new DefaultMethodInvoker(getMethodHandleJava8(method)); } else { return new DefaultMethodInvoker(getMethodHandleJava9(method)); } } catch (IllegalAccessException | InstantiationException | InvocationTargetException | NoSuchMethodException e) { throw new RuntimeException(e); } } else { return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration())); } }); } catch (RuntimeException re) { Throwable cause = re.getCause(); throw cause == null ? re : cause; } }

1. 从methodCache中获取对应的MapperMethodInvoker
2. 如果缓存没有，如果是方法是default方法就创建DefaultMethodInvoker对象，否则创建PlainMethodInvoker对象
3. 默认方法调用类DefaultMethodInvoker
4. 对于DefaultMethodInvoker类通过MethodHandle完成调用

private static class DefaultMethodInvoker implements MapperMethodInvoker { private final MethodHandle methodHandle; public DefaultMethodInvoker(MethodHandle methodHandle) { super(); this.methodHandle = methodHandle; } @Override public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable { return methodHandle.bindTo(proxy).invokeWithArguments(args); } }

对于普通方法对应的类PlainMethodInvoker通过MapperMethod来完成调用，

private static class PlainMethodInvoker implements MapperMethodInvoker { private final MapperMethod mapperMethod; public PlainMethodInvoker(MapperMethod mapperMethod) { super(); this.mapperMethod = mapperMethod; } @Override public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable { return mapperMethod.execute(sqlSession, args); } }总结

这篇文章主要介绍了聊聊Mybatis的binding模块，它的MapperRegistry保存着Mapper接口和MapperProxyFactory实例的映射关系，而MapperProxyFactory是一个的工厂，生成的是Mapper接口的代理类，MapperProxy实现InvocationHandler，重写invoke()方法进行拦截处理，根据方法判断是不是default类型创建不同的MethodInvoker类，然后调用MapperMethod执行sql，下篇文章我们将介绍MapperMethod类