Mybatis源码解析之mapper接口的代理模式

Mybatis源码解析之核心类分析
Mybatis源码解析之初始化分析
Mybatis源码解析之执行流程解析
Mybatis源码解析之数据库连接和连接池
Mybatis源码解析之事务管理
Mybatis源码解析之缓存机制（一）：一级缓存
Mybatis源码解析之缓存机制（二）：二级缓存
Mybatis源码解析之插件机制

一、简介

在mybatis中执行sql时有两种方式，一种是基于statementId，也就是直接调用SqlSession的方法，如sqlSession.update(“statementId”);
还有一种方法是基于java接口，也是日常开发中最常用的方式。
mapper接口中的每个方法都可以喝mapper xml中的一条sql语句对应，我们可以直接通过调用接口方法的方式进行sql执行。因为mybatis会为mapper接口通过jdk动态代理的方法生成接口的实现类，本篇文章将针对mapper接口的代理展开分析。
以下是mapper接口的代理模式的核心组件的类图。

二、MapperRegistry

MapperRegistry通过Map结构的属性knownMappers中维护着mybatis中所有的mapper接口。

1. MapperRegistry#addMapper(class)

当开发者在配置文件中配置了通过mappers节点的子节点mapper配置了mapper接口时，会调用configuation#addMapper(Class)记录mapper接口，而configuration又委托了MapperRegistry#addMapper(class)处理逻辑。

public  void addMapper(Class 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);
      }
    }
  }
}

可以看到，MapperRegistry以mapper接口的类型为key值，将接口类型封装成MapperProxyFactory作为value值放入knownMappers。

2. MapperRegistry#getMapper(Class, SqlSession)

该方法基于SqlSession参数向外提供了对应类型的mapper接口的对象。

public  T getMapper(Class type, SqlSession sqlSession) {
  final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);
  }
}

根据mapper接口类型找到对应的MapperProxyFactory时，调用其newInstance方法得到对应的对象返回。

三、MapperProxyFactory

public class MapperProxyFactory {

  private final Class mapperInterface;
  private final Map methodCache = new ConcurrentHashMap();

  public MapperProxyFactory(Class mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class getMapperInterface() {
    return mapperInterface;
  }

  public Map getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }
}

如果对jdk动态代理有一定了解，很容易就能看出来MapperProxyFactory的newInstance方法是很典型的生成代理对象的方式。

四、MapperProxy

MapperProxy作为InvocationHandler的实现类，是jdk动态代理模式的核心。

1. MapperProxy#invoke(Object, Method, Object[])

@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);
}

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;
}

对于Object方法和default方法，执行原方法逻辑即可。
对于对于sql语句的方法，交给MapperMethod 处理。

五、MapperMethod

Mapper类负责去处理mapper接口中的sql方法。

public Object execute(SqlSession sqlSession, Object[] args) {
	 Object result;
	 switch (command.getType()) {
	   case INSERT: {
	 	Object param = method.convertArgsToSqlCommandParam(args);
	     result = rowCountResult(sqlSession.insert(command.getName(), param));
	     break;
	   }
	   case UPDATE: {
	     Object param = method.convertArgsToSqlCommandParam(args);
	     result = rowCountResult(sqlSession.update(command.getName(), param));
	     break;
	   }
	   case DELETE: {
	     Object param = method.convertArgsToSqlCommandParam(args);
	     result = rowCountResult(sqlSession.delete(command.getName(), param));
	     break;
	   }
	   case SELECT:
	     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 {
	       Object param = method.convertArgsToSqlCommandParam(args);
	       result = sqlSession.selectOne(command.getName(), param);
	     }
	     break;
	   case FLUSH:
	     result = sqlSession.flushStatements();
	     break;
	   default:
	     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;
}

MapperMethod还有一个静态内部类MethodSignature用作mapper方法的标签,SqlCommand用作sql命令。
可以看到，根据sql命令的类型（insert|update|delete|select|flush）和返回类型分别调用SqlSession的不同方法，然后对insert|update|delete方法的返回值做适配。

2. MapperMethod#rowCountResult(int)

private Object rowCountResult(int rowCount) {
 final Object result;
  if (method.returnsVoid()) {
    result = null;
  } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
    result = rowCount;
  } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
    result = (long)rowCount;
  } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
    result = rowCount > 0;
  } else {
    throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
  }
  return result;
}

insert|update|delete方法方法的返回值就是sql命令的匹配行数，而在mapper方法中支持Integer、Long、Boolean和void类型的返回，因此做简单适配。