MyBatis框架的使用及源码分析(七) MapperProxy，MapperProxyFactory

从上文<MyBatis框架中Mapper映射配置的使用及原理解析(六) MapperRegistry> 中我们知道DefaultSqlSession的getMapper方法，最后是通过MapperRegistry对象获得Mapper实例：

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
  final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
  if (mapperProxyFactory == null) {  //说明这个Mapper接口没有注册
    throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
  }
  try {  //生成一个MapperProxy对象  
    return mapperProxyFactory.newInstance(sqlSession);
  } catch (Exception e) {
    throw new BindingException("Error getting mapper instance. Cause: " + e, e);
  }
}  

从代码中我们看到试图从一个叫knownMappers的变量取出MapperProxyFactory。

我们看看这个knownMapper在MapperRegistry中的定义：

private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();  

有getMapper方法，那么必然后addMapper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<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.
      MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
      parser.parse();
      loadCompleted = true;
    } finally {
      if (!loadCompleted) {
        knownMappers.remove(type);
      }
    }
  }
}  

在<MyBatis框架中Mapper映射配置的使用及原理解析(四) 解析Mapper接口映射xml文件> 文章中，解析玩xml后，绑定命名空间bindMapperForNamespace()方法

//绑定到命名空间
private void bindMapperForNamespace() {
  String namespace = builderAssistant.getCurrentNamespace();
  if (namespace != null) {
    Class<?> boundType = null;
    try {
      boundType = Resources.classForName(namespace);
    } catch (ClassNotFoundException e) {
      //ignore, bound type is not required
    }
    if (boundType != null) {
      if (!configuration.hasMapper(boundType)) {
        // Spring may not know the real resource name so we set a flag
        // to prevent loading again this resource from the mapper interface
        // look at MapperAnnotationBuilder#loadXmlResource
        configuration.addLoadedResource("namespace:" + namespace);
        configuration.addMapper(boundType);
      }
    }
  }
}

我们看到

configuration.addMapper(boundType);

正是调用MapperRegistry.addMapper方法

  public <T> void addMapper(Class<T> type) {
    mapperRegistry.addMapper(type);
  }

我们在回过头来看getMapper是如何获得Mapper对象的：

1.先获取MapperProxyFactory

2.再调用MapperProxyFactory对象的newInstance方法获得Mapper。

我们看MapperProxyFactory代码：

public T newInstance(SqlSession sqlSession) {  
  //创建一个Mapperxy对象，这个方法实现了JDK动态代理中的InvocationHandler接口
  final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
  return newInstance(mapperProxy);
}  

protected T newInstance(MapperProxy<T> mapperProxy) {  
  //mapperInterface，说明Mapper接口被代理了，这样子返回的对象就是Mapper接口的子类，方法被调用时会被mapperProxy拦截,也就是执行mapperProxy.invoke()方法 
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
} 

这里就是返回的一个代理类实例MapperProxy。

package org.apache.ibatis.binding;

import java.io.Serializable;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Map;

import org.apache.ibatis.reflection.ExceptionUtil;
import org.apache.ibatis.session.SqlSession;

/**
 * @author Clinton Begin
 * @author Eduardo Macarron
 */
public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  //Mapper接口 
  private final Class<T> mapperInterface;

/*
     * Mapper接口中的每个方法都会生成一个MapperMethod对象, methodCache维护着他们的对应关系
     */  
    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;
  }

  //这里会拦截Mapper接口的所有方法 
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) { //如果是Object中定义的方法，直接执行。如toString(),hashCode()等
      try {
        return method.invoke(this, args);//
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);  //其他Mapper接口定义的方法交由mapperMethod来执行
    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;
  }

}

要使用Java的动态代理就必须得实现InvocationHandler接口:

@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);
  return mapperMethod.execute(sqlSession, args);
}  

首先判断代理对象是一个接口还是一个类，显然我们没有对mapper接口进行任何实现，那么它将执行

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

生成一个MapperMethod对象，接着调用其execute方法，把sqlSession和参数传递进去，执行Mapper方法。