第二十九讲-ControllerAdvice之ResponseBodyAdvice

第二十九讲-ControllerAdvice之ResponseBodyAdvice

本讲我们看一下ControllerAdvice的扩展功能-ResponseBodyAdvice

ResponseBodyAdvice本质上就是对消息转换的一个扩展点。这里呢，我们来分析一下ResponseBodyAdvice。

比如我现在有一个需求，就是将控制器中的某一个方法将返回的对象将来写入到响应体里，如下面的代码:

@RestController
    public static class MyController {
        public User user() {
            return new User("王五", 18);
        }
    }

    public static class User {
        private String name;
        private int age;

        public User(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }
    }

package com.cherry.a29;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.core.DefaultParameterNameDiscoverer;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.mock.web.MockHttpServletRequest;
import org.springframework.mock.web.MockHttpServletResponse;
import org.springframework.web.context.request.ServletWebRequest;
import org.springframework.web.method.ControllerAdviceBean;
import org.springframework.web.method.annotation.ExpressionValueMethodArgumentResolver;
import org.springframework.web.method.annotation.RequestHeaderMethodArgumentResolver;
import org.springframework.web.method.annotation.RequestParamMethodArgumentResolver;
import org.springframework.web.method.support.HandlerMethodArgumentResolverComposite;
import org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite;
import org.springframework.web.method.support.ModelAndViewContainer;
import org.springframework.web.servlet.mvc.method.annotation.*;

import java.nio.charset.StandardCharsets;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class A29 {

    // {"name":"王五","age":18}
    // {"code":xx, "msg":xx, data: {"name":"王五","age":18} }
    public static void main(String[] args) throws Exception {
        AnnotationConfigApplicationContext context =
                new AnnotationConfigApplicationContext(WebConfig.class);

        ServletInvocableHandlerMethod handlerMethod = new ServletInvocableHandlerMethod(
                context.getBean(WebConfig.MyController.class),
                WebConfig.MyController.class.getMethod("user")
        );
        handlerMethod.setDataBinderFactory(new ServletRequestDataBinderFactory(Collections.emptyList(), null));
        handlerMethod.setParameterNameDiscoverer(new DefaultParameterNameDiscoverer());
        handlerMethod.setHandlerMethodArgumentResolvers(getArgumentResolvers(context));
        handlerMethod.setHandlerMethodReturnValueHandlers(getReturnValueHandlers(context));

        MockHttpServletRequest request = new MockHttpServletRequest();
        MockHttpServletResponse response = new MockHttpServletResponse();
        ModelAndViewContainer container = new ModelAndViewContainer();
        handlerMethod.invokeAndHandle(new ServletWebRequest(request, response), container);

        System.out.println(new String(response.getContentAsByteArray(), StandardCharsets.UTF_8));

        /*
            学到了什么
                a. advice 之三, ResponseBodyAdvice 返回响应体前包装
         */
    }

    public static HandlerMethodArgumentResolverComposite getArgumentResolvers(AnnotationConfigApplicationContext context) {
        HandlerMethodArgumentResolverComposite composite = new HandlerMethodArgumentResolverComposite();
        composite.addResolvers(
                new RequestParamMethodArgumentResolver(context.getDefaultListableBeanFactory(), false),
                new PathVariableMethodArgumentResolver(),
                new RequestHeaderMethodArgumentResolver(context.getDefaultListableBeanFactory()),
                new ServletCookieValueMethodArgumentResolver(context.getDefaultListableBeanFactory()),
                new ExpressionValueMethodArgumentResolver(context.getDefaultListableBeanFactory()),
                new ServletRequestMethodArgumentResolver(),
                new ServletModelAttributeMethodProcessor(false),
                new RequestResponseBodyMethodProcessor(List.of(new MappingJackson2HttpMessageConverter())),
                new ServletModelAttributeMethodProcessor(true),
                new RequestParamMethodArgumentResolver(context.getDefaultListableBeanFactory(), true)
        );
        return composite;
    }

    public static HandlerMethodReturnValueHandlerComposite getReturnValueHandlers(AnnotationConfigApplicationContext context) {
        // 添加 advice
        List<ControllerAdviceBean> annotatedBeans = ControllerAdviceBean.findAnnotatedBeans(context);
        List<Object> collect = annotatedBeans.stream().filter(b -> ResponseBodyAdvice.class.isAssignableFrom(b.getBeanType()))
                .collect(Collectors.toList());

        HandlerMethodReturnValueHandlerComposite composite = new HandlerMethodReturnValueHandlerComposite();
        composite.addHandler(new ModelAndViewMethodReturnValueHandler());
        composite.addHandler(new ViewNameMethodReturnValueHandler());
        composite.addHandler(new ServletModelAttributeMethodProcessor(false));
        composite.addHandler(new HttpEntityMethodProcessor(List.of(new MappingJackson2HttpMessageConverter())));
        composite.addHandler(new HttpHeadersReturnValueHandler());
        composite.addHandler(new RequestResponseBodyMethodProcessor(List.of(new MappingJackson2HttpMessageConverter()), collect));
        composite.addHandler(new ServletModelAttributeMethodProcessor(true));
        return composite;
    }
}

测试结果如下:

{"name":"王五","age":18}

我们发现，响应结果成功将消息转为了JSON，但是呢，一般的程序，需要我们将结果做一个统一的响应格式，例如，JSON结果里有coed,message,data等信息。那么，现在我们的代码如何进行改写呢?

我们可以提供一个Result对象，将code, message, data封装成一个对象里，所有的控制器返回值都统一成Result。但是呢，我们发现，这样的工作量蛮大的，你得通知所有的成员按照这种规范来写。我们能不能用编程的方式来完成自动的转换呢？当然可以，我们可以通过@ControllerAdvice来实现:

@Configuration
public class WebConfig {

    // 对响应体做增强，注意只对加了@ResponseBody注解的类上才做响应体增强，否则不做增强
    @ControllerAdvice
    static class MyControllerAdvice implements ResponseBodyAdvice<Object> {
        // 满足条件才转换: 方法上加了@ResponseBody注解或者类上加了@ResponseBody注解或者类上加了@RestController注解才满足成立条件
        public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
            if (returnType.getMethodAnnotation(ResponseBody.class) != null ||
                AnnotationUtils.findAnnotation(returnType.getContainingClass(), ResponseBody.class) != null) {
//                returnType.getContainingClass().isAnnotationPresent(ResponseBody.class)) {
                return true;
            }
            return false;
        }


        // 真正的转换逻辑:将 User 或其它类型统一为 Result 类型
        public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
            // 如果返回结果的类型已经是Result类型了，就不需要转换了
            if (body instanceof Result) {
                return body;
            }
            // 如果返回的结果类型不是Result，那就封装成Result，然后返回封装后的Result对象
            return Result.ok(body);
        }
    }

    // @Controller
    // @ResponseBody
    @RestController
    public static class MyController {
        public User user() {
            return new User("王五", 18);
        }
    }

    public static class User {
        private String name;
        private int age;

        public User(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }
    }
}

其中Result的代码如下:

@JsonInclude(JsonInclude.Include.NON_NULL)
public class Result {
    private int code;
    private String msg;
    private Object data;

    public int getCode() {
        return code;
    }

    public void setCode(int code) {
        this.code = code;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

    @JsonCreator
    private Result(@JsonProperty("code") int code, @JsonProperty("data") Object data) {
        this.code = code;
        this.data = data;
    }

    private Result(int code, String msg) {
        this.code = code;
        this.msg = msg;
    }

    public static Result ok() {
        return new Result(200, null);
    }

    public static Result ok(Object data) {
        return new Result(200, data);
    }

    public static Result error(String msg) {
        return new Result(500, "服务器内部错误:" + msg);
    }
}

测试如下:

{"code":200,"data":{"name":"王五","age":18}}

我们发现响应体消息转换成功了!