Mybatis源码分析:MapperMethod中内部静态类MethodSignature的作用

MethodSignature分析

　　MethodSignature为MapperMethod类提供了三个作用,获取待执行方法中的参数和@Param注解标注的参数名,获取标注有@MapKey的参数(@Mapkey作用在后续会讲到),方法的返回类型,获取SELECT操作时必要的标志位。以下是MethodSignature的所有属性。

• private final boolean returnsMany;//是否多值查询
• private final boolean returnsMap;//是否map查询
• private final boolean returnsVoid;//是否void查询
• private final boolean returnsCursor;//是否游标查询
• private final Class<?> returnType; //返回类型
• private final String mapKey;//获取mapKey的值
• private final Integer resultHandlerIndex;
• private final Integer rowBoundsIndex;
• private final ParamNameResolver paramNameResolver; //参数解析器

　　观察下列的代码,构造器首先获取返回类型,这个值通过类型参数解析器来获取入参mapperInterface本身及所有的父类和所有的接口来查找所要返回的类型,Mybatis提供了三种类型实现ParameterizedType,TypeVariable,GenericArrayType。通过返回的类型值来判定后续的SElECT将要做哪种查询,void查询,Map查询，游标查询,多值查询,这里就会给相关的属性进行打标,在mapperMethod执行exeute()方法时便会进行判断。构造器还会检查待执行的Mapper方法是否标注有@Mapkey的注解,如果有这意味着返回的结果类型是一个Map。

   public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {
        //解析返回的类型 ParameterizedType,TypeVariable,GenericArrayType
      Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
      if (resolvedReturnType instanceof Class<?>) {
        this.returnType = (Class<?>) resolvedReturnType;
      } else if (resolvedReturnType instanceof ParameterizedType) {
        this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();
      } else {
        this.returnType = method.getReturnType();
      }
      this.returnsVoid = void.class.equals(this.returnType);
      //是否是集合或者数组类型
      this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
      //判断是否是游标类型
      this.returnsCursor = Cursor.class.equals(this.returnType);
      //获取MapKey注解
      this.mapKey = getMapKey(method);
      //判断mapkey是否不为NULL
      this.returnsMap = this.mapKey != null;
      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
      //初始化参数名解析器
      this.paramNameResolver = new ParamNameResolver(configuration, method);
    }