SpringBoot（二）HandlerMethodArgumentResolver

如下：开发过程中，你可能会使用RequestParam注解，设置required = true，告诉前端这个参数是必传的，
但是用过的基本了解，通常会因为前端传了空字符串，导致校验通过，实用性不佳，
原生的注解基本不能满足实际开发需求，所以你需要自己写一个。

@ResponseBody
@RequestMapping("/data")
public Map<String, Object> data(
        @RequestParam(required = true) String b) {
    return new HashMap<>();
}

HandlerMethodArgumentResolver

此时你需要写一个切面，HandlerMethodArgumentResolver，直译是，处理函数参数的分解器，实现这个接口也很简单，

supportsParameter（支持参数）：可以设置一些标志，表示你这个分解器可以处理这些参数，返回ture才执行resolveArgument()函数
resolveArgument（分解实参）：处理实参的具体方法

import org.springframework.core.MethodParameter;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.ModelAndViewContainer;

/**
 * Created by 12614 on 2018/5/11.
 */
public class TestArgumentResolver implements HandlerMethodArgumentResolver {
    @Override
    public boolean supportsParameter(MethodParameter methodParameter) {
        return true;
    }

    @Override
    public Object resolveArgument(MethodParameter methodParameter,
                                  ModelAndViewContainer modelAndViewContainer,
                                  NativeWebRequest nativeWebRequest,
                                  WebDataBinderFactory webDataBinderFactory) throws Exception {
        System.out.println("TestArgumentResolver:resolveArgument");
        return null;
    }
}

Spring配置

<mvc:annotation-driven>
      <mvc:argument-resolvers>
        <bean class="xxxxx全类名"/>
      </mvc:argument-resolvers>
</mvc:annotation-driven>

SpringBoot配置

既然要零配置，就不要违背设计初衷刻意去启用Xml配置，在WebMvcConfigurerAdapter添加HandlerMethodArgumentResolver就好了 ，

import org.springframework.context.annotation.Configuration;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;

import java.util.List;

/**
 * Created by 12614 on 2018/5/11.
 */
@Configuration
public class ApplicationConfigurer extends WebMvcConfigurerAdapter {
    @Override
    public void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {
        super.addArgumentResolvers(argumentResolvers);
        argumentResolvers.add(new TestArgumentResolver());
    }
}

简单的案例

自定义注解

import java.lang.annotation.*;

/**
 * Created by 12614 on 2018/5/11.
 */
@Documented
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface Params {
    @AliasFor("name")
    String value() default "";

    @AliasFor("value")
    String name() default "";

    boolean required() default true;

    boolean notEmpty() default true;

    int maxLength() default Integer.MAX_VALUE;

    String defaultValue() default "
		
		
ue000ue001ue002
				
";
}

参数分解器

未包含全部代码

import org.springframework.core.MethodParameter;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.ModelAndViewContainer;

import javax.servlet.http.HttpServletRequest;

/**
 * Created by 12614 on 2018/5/11.
 */
public class TestArgumentResolver implements HandlerMethodArgumentResolver {
    @Override
    public boolean supportsParameter(MethodParameter methodParameter) {
        //如果函数包含我们的自定义注解，那就走resolveArgument()函数
        return methodParameter.hasParameterAnnotation(Params.class);
    }

    @Override
    public Object resolveArgument(MethodParameter methodParameter,
                                  ModelAndViewContainer modelAndViewContainer,
                                  NativeWebRequest nativeWebRequest,
                                  WebDataBinderFactory webDataBinderFactory) throws Exception {
        System.out.println("TestArgumentResolver:resolveArgument");
        Params params = methodParameter.getParameterAnnotation(Params.class);
        String paramName = params.name();
        if (paramName == null) {
            paramName = methodParameter.getParameterName();
        }
        //简单的案例：如果客户端未传值，就设置默认值
        Object res = nativeWebRequest.getNativeRequest(HttpServletRequest.class).getParameter(paramName);
        return res == null ? params.defaultValue() : res;
    }
}

使用场景

    @ResponseBody
    @RequestMapping("/data")
    public Map<String, Object> data(
            @Params(notEmpty = true,maxLength = 30, defaultValue = "aaa") String a) {
        System.out.println(a);
        return null;
    }