最近在使用SpringCloud Gateway进行网关的开发，我使用的版本是：SpringBoot的2.3.4.RELEASE+SpringCloud的Hoxton.SR8，在自定义过滤器时需要获取ServerHttpRequest中body的数据，发现一直无法获取到数据，经过各种百度、谷歌，再加上自己的实践，终于找到解决方案：
1、首先创建一个全局过滤器把body中的数据缓存起来

package com.cloudpath.gateway.portal.filter;

import lombok.extern.slf4j.Slf4j;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.core.Ordered;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.io.buffer.DataBufferUtils;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.http.server.reactive.ServerHttpRequestDecorator;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

/**
 * @author mazhen
 * @className CacheBodyGlobalFilter
 * @Description 把body中的数据缓存起来
 * @date 2020/10/28 18:02
 */
@Slf4j
@Component
public class CacheBodyGlobalFilter implements Ordered, GlobalFilter {

    //  public static final String CACHE_REQUEST_BODY_OBJECT_KEY = "cachedRequestBodyObject";

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        if (exchange.getRequest().getHeaders().getContentType() == null) {
            return chain.filter(exchange);
        } else {
            return DataBufferUtils.join(exchange.getRequest().getBody())
                    .flatMap(dataBuffer -> {
                        DataBufferUtils.retain(dataBuffer);
                        Flux<DataBuffer> cachedFlux = Flux
                                .defer(() -> Flux.just(dataBuffer.slice(0, dataBuffer.readableByteCount())));
                        ServerHttpRequest mutatedRequest = new ServerHttpRequestDecorator(
                                exchange.getRequest()) {
                            @Override
                            public Flux<DataBuffer> getBody() {
                                return cachedFlux;
                            }
                        };
                        //exchange.getAttributes().put(CACHE_REQUEST_BODY_OBJECT_KEY, cachedFlux);

                        return chain.filter(exchange.mutate().request(mutatedRequest).build());
                    });
        }
    }

    @Override
    public int getOrder() {
        return Ordered.HIGHEST_PRECEDENCE;
    }

}

CacheBodyGlobalFilter这个全局过滤器的目的就是把原有的request请求中的body内容读出来，并且使用ServerHttpRequestDecorator这个请求装饰器对request进行包装，重写getBody方法，并把包装后的请求放到过滤器链中传递下去。这样后面的过滤器中再使用exchange.getRequest().getBody()来获取body时，实际上就是调用的重载后的getBody方法，获取的最先已经缓存了的body数据。这样就能够实现body的多次读取了。
值得一提的是，这个过滤器的order设置的是Ordered.HIGHEST_PRECEDENCE，即最高优先级的过滤器。优先级设置这么高的原因是某些系统内置的过滤器可能也会去读body，这样就会导致我们自定义过滤器中获取body的时候报body只能读取一次这样的错误如下:

java.lang.IllegalStateException: Only one connection receive subscriber allowed.
	at reactor.ipc.netty.channel.FluxReceive.startReceiver(FluxReceive.java:279)
	at reactor.ipc.netty.channel.FluxReceive.lambda$subscribe$2(FluxReceive.java:129)
	at 

所以，必须把CacheBodyGlobalFilter的优先级设到最高。
2、在自定义的过滤器中尝试获取body中的数据

package com.cloudpath.iam.gateway.customerfilter;

import com.cloudpath.iam.gateway.utils.FilterRequestResponseUtil;
import lombok.extern.slf4j.Slf4j;
import org.springframework.cloud.gateway.filter.GatewayFilter;
import org.springframework.cloud.gateway.filter.factory.AbstractGatewayFilterFactory;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.stereotype.Component;
import reactor.core.publisher.Flux;

import java.util.Arrays;
import java.util.List;

/**
 * @author by mazhen
 * @Classname TestGatewayFilterFactory
 * @Description 自定义过滤器获取body中的数据
 * @Date 2020/10/27 14:38
 */
@Component
@Slf4j
public class TestGatewayFilterFactory extends AbstractGatewayFilterFactory<TestGatewayFilterFactory.Config> {


    @Override
    public List<String> shortcutFieldOrder() {
        return Arrays.asList("enabled");
    }

    public TestGatewayFilterFactory() {
        super(Config.class);
        log.info("Loaded TestGatewayFilterFactory");
    }

    @Override
    public GatewayFilter apply(Config config) {
        return (exchange, chain) -> {
            if (!config.isEnabled()) {
                return chain.filter(exchange);
            }

            if (null != exchange) {
                ServerHttpRequest httpRequest = exchange.getRequest();
                    try {

                        Flux<DataBuffer> dataBufferFlux = httpRequest.getBody();
                        //获取body中的数据
                        String body = FilterRequestResponseUtil.resolveBodyFromRequest(dataBufferFlux);
                        log.info("body:{}",body);

                    } catch (Exception e) {
                        log.error("异常:",e);
                        return chain.filter(exchange);
                    }
            }
            return chain.filter(exchange);
        };
    }


    public static class Config {
        /**
         * 控制是否开启统计
         */
        private boolean enabled;

        public Config() {
        }

        public boolean isEnabled() {
            return enabled;
        }

        public void setEnabled(boolean enabled) {
            this.enabled = enabled;
        }
    }
}

3、解析body的工具类

package com.cloudpath.iam.gateway.utils;


import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.io.buffer.DataBufferUtils;
import reactor.core.publisher.Flux;
import java.nio.CharBuffer;
import java.nio.charset.StandardCharsets;
import java.util.concurrent.atomic.AtomicReference;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * @author mazhen
 * @className FilterHeadersUtil
 * @Description 过滤器请求/响应工具类
 * @date 2020/10/29 9:31
 */
public final class FilterRequestResponseUtil {

    /**
     * spring cloud gateway 获取post请求的body体
     * @param body
     * @return
     */
    public static String resolveBodyFromRequest( Flux<DataBuffer> body){
        AtomicReference<String> bodyRef = new AtomicReference<>();
        // 缓存读取的request body信息
        body.subscribe(dataBuffer -> {
            CharBuffer charBuffer = StandardCharsets.UTF_8.decode(dataBuffer.asByteBuffer());
            DataBufferUtils.release(dataBuffer);
            bodyRef.set(charBuffer.toString());
        });
        //获取request body
        return bodyRef.get();

    }

    /**
     * 读取body内容
     * @param body
     * @return
     */
    public static String resolveBodyFromRequest2( Flux<DataBuffer> body){
        StringBuilder sb = new StringBuilder();

        body.subscribe(buffer -> {
            byte[] bytes = new byte[buffer.readableByteCount()];
            buffer.read(bytes);
            DataBufferUtils.release(buffer);
            String bodyString = new String(bytes, StandardCharsets.UTF_8);
            sb.append(bodyString);
        });
        return formatStr(sb.toString());
    }

    /**
     * 去掉空格,换行和制表符
     * @param str
     * @return
     */
    private static String formatStr(String str){
        if (str != null && str.length() > 0) {
            Pattern p = Pattern.compile("\\s*|\t|\r|\n");
            Matcher m = p.matcher(str);
            return m.replaceAll("");
        }
        return str;
    }
}

解析body的内容，网上普遍是上面的两种方式,亲测resolveBodyFromRequest方法解析body中的数据，没有1024字节的限制。
ps：我传的参数有1万多字节。。。。。。。
大家可以按需所选。