RequestBodyAdvice和ResponseBodyAd

场景：

1. 前后端交互中，前端传给后端的是一个String类型的加密串，后端需要解析成Json报文，并反序列为对象。如何在不侵入业务代码的前提上来实现这个功能？
2. 例如需要解析报文中的某些字段，放入到ThreadLocal中，后续进行使用；
3. 填充一些公共字段，例如前后端交互过程中，uid不会信任前端报文传递的，而是信任cookie中解析出来的，需要set到对象中。
4. 对项目报文某些字段进行加密；
5. 填充项目报文公共字段；

这些都可以交由RequestBodyAdvice和ResponseBodyAdvice来实现：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface DecryptRequestBody {

}

import static java.nio.charset.StandardCharsets.UTF_8;

import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Type;
import java.nio.charset.Charset;
import java.util.Map;

import org.apache.commons.io.IOUtils;
import org.springframework.core.MethodParameter;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.servlet.mvc.method.annotation.RequestBodyAdviceAdapter;

import com.alibaba.fastjson.JSON;

@ControllerAdvice
public class DecryptRequestBodyAdvice extends RequestBodyAdviceAdapter {
    @Override
    public boolean supports(MethodParameter methodParameter, Type targetType,
            Class<? extends HttpMessageConverter<?>> converterType) {
        //当存在这个注解时，则进行解析
        return methodParameter.hasParameterAnnotation(DecryptRequestBody.class);
    }

    //报文解析前进行出来
    @Override
    public HttpInputMessage beforeBodyRead(HttpInputMessage inputMessage, MethodParameter parameter, Type targetType,
            Class<? extends HttpMessageConverter<?>> converterType) throws IOException {

        DecryptHttpInputMessage message = new DecryptHttpInputMessage(inputMessage);
        //todo message可以做各种处理，典型的是将加密串解析成Json，以适配RequestBody转成对象
        return message;
    }


    //报文解析后，可以做的工作
    @Override
    public Object afterBodyRead(Object body, HttpInputMessage inputMessage, MethodParameter parameter, Type targetType,
            Class<? extends HttpMessageConverter<?>> converterType) {

        //todo 例如强制为对象，来赋予公共值；例如读取出特定的值
        String jsonString = JSON.toJSONString(body);
        Map<String, Object> params = JSON.parseObject(jsonString);
        Object language = params.get("language");
        //...

        return body;
    }

    /**
     * 自定义一个HttpInputMessage用于存储新的信息
     */
    public static class DecryptHttpInputMessage implements HttpInputMessage {
        private final Charset charset = UTF_8;
        private HttpHeaders headers;
        private InputStream body;
        private String bodyData;

        public DecryptHttpInputMessage(HttpInputMessage inputMessage) throws IOException {
            headers = inputMessage.getHeaders();
            bodyData = IOUtils.toString(inputMessage.getBody(), charset);
            body = IOUtils.toInputStream(bodyData, charset);
        }

        @Override
        public InputStream getBody() throws IOException {
            return body;
        }

        @Override
        public HttpHeaders getHeaders() {
            return headers;
        }

        public String getBodyData() {
            return bodyData;
        }

        public void setBody(InputStream inputStream) {
            body = inputStream;
        }

        /**
         * 写入新的body流
         */
        public void setBody(String newBodyData) {
            body = IOUtils.toInputStream(newBodyData, charset);
        }

        public void setBodyData(String bodyDataStr) {
            bodyData = bodyDataStr;
        }
    }
}