问题描述： 在gateway中修改返回的响应体，在全局Filter中添加如下代码：

import org.springframework.core.Ordered;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

@Component
public class RequestGlobalFilter implements GlobalFilter, Ordered {
	//...
	
	@Override
	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
		//...
		ResponseDecorator decorator = new ResponseDecorator(exchange.getResponse());
		return chain.filter(exchange.mutate().response(decorator).build());
	}

	@Override
	public int getOrder() {
		return -1000;
	}
}

通过.response(decorator)设置一个响应装饰器（自定义），以下是装饰器具体实现：

import cn.hutool.json.JSONObject;
import org.reactivestreams.Publisher;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.io.buffer.DataBufferFactory;
import org.springframework.core.io.buffer.DataBufferUtils;
import org.springframework.core.io.buffer.DefaultDataBufferFactory;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.http.server.reactive.ServerHttpResponseDecorator;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import java.nio.charset.Charset;

/**
 * @author visy.wang
 * @desc 响应装饰器（重构响应体）
 */
public class ResponseDecorator extends ServerHttpResponseDecorator{
	public ResponseDecorator(ServerHttpResponse delegate){
		super(delegate);
	}

	@Override
	@SuppressWarnings(value = "unchecked")
	public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {

		if(body instanceof Flux) {
			Flux<DataBuffer> fluxBody = (Flux<DataBuffer>) body;

			return super.writeWith(fluxBody.buffer().map(dataBuffers -> {
				DataBufferFactory dataBufferFactory = new DefaultDataBufferFactory();
				DataBuffer join = dataBufferFactory.join(dataBuffers);

				byte[] content = new byte[join.readableByteCount()];
				join.read(content);
				DataBufferUtils.release(join);// 释放掉内存
				
				String bodyStr = new String(content, Charset.forName("UTF-8"));

                //修改响应体
				bodyStr = modifyBody(bodyStr);

				getDelegate().getHeaders().setContentLength(bodyStr.getBytes().length);
				return bufferFactory().wrap(bodyStr.getBytes());
			}));
		}
		return super.writeWith(body);
	}

    //重写这个函数即可
	private String modifyBody(String jsonStr){
		JSONObject json = new JSONObject(jsonStr);
        //TODO...修改响应体
		return json.toString();
	}
}