思考问题：Mapper是定义的接口，如何完成代码实现的？
其实在业务代码使用的Mapper并不是自己所建立的Mapper接口，而是通过MyBatis的
MapperProxyFactory.newInstance(sqlSession);动态代理生成的MapperProxy的代理对象。
/**
* 映射器代理工厂
*/
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return mapperInterface;
}
public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//用JDK自带的动态代理生成映射器
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
思考问题：MapperProxy代理对象又是什么呢？
想了解MapperProxy，需要先了解Java的动态代理技术。
MapperProxy通过重写invoke方法，完成Mapper接口的示例化。
/**
* 映射器代理，代理模式
*
*/
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;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//代理以后，所有Mapper的方法调用时，都会调用这个invoke方法
//并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是Object中通用的方法（toString、hashCode等）无需执行
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//这里优化了，去缓存中找MapperMethod
final MapperMethod mapperMethod = cachedMapperMethod(method);
//执行
return mapperMethod.execute(sqlSession, args);
}
//去缓存中找MapperMethod
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
//找不到才去new
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
}
