Spring cloud gateway response body modification

application.yml

spring:
  cloud:
    gateway:
      routes:
      - id: test
        uri: https://httpbin.org:443
        predicates:
        - Path=/uuid
        - Method=GET
server:
  port: 8080

GatewayApplication.java

package examples;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
@EnableAutoConfiguration
public class GatewayApplication {

	public static void main(String[] args) {
		SpringApplication.run(GatewayApplication.class, args);
	}	
}

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>examples</groupId>
  <artifactId>gateway</artifactId>
  <version>0.0.1-SNAPSHOT</version>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.0.0.M5</version>
		<relativePath /> <!-- lookup parent from repository -->
	</parent>

	<properties>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
		<java.version>1.8</java.version>
	</properties>
	
	<dependencies>
		<dependency>
			<groupId>org.springframework.cloud</groupId>
			<artifactId>spring-cloud-starter-gateway</artifactId>
			<version>2.0.0.M3</version>
		</dependency>
	</dependencies>

	<profiles>
		<profile>
			<id>spring</id>
			<activation>
				<activeByDefault>true</activeByDefault>
			</activation>
			<repositories>
				<repository>
					<id>spring-milestones</id>
					<name>Spring Milestones</name>
					<url>https://repo.spring.io/libs-milestone-local</url>
					<snapshots>
						<enabled>false</enabled>
					</snapshots>
				</repository>
			</repositories>
		</profile>
	</profiles>
</project>

ToUppercaseGlobalFilter.java

package examples;

import java.nio.charset.Charset;

import org.reactivestreams.Publisher;
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.DataBufferFactory;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.http.server.reactive.ServerHttpResponseDecorator;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;

import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

@Component
public class ToUppercaseGlobalFilter implements GlobalFilter, Ordered {

	@Override
	public int getOrder() {
		return -2; // -1 is response write filter, must be called before that
	}

	@Override
	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {

		ServerHttpResponse originalResponse = exchange.getResponse();
		DataBufferFactory bufferFactory = originalResponse.bufferFactory();
		ServerHttpResponseDecorator decoratedResponse = new ServerHttpResponseDecorator(originalResponse) {
            @Override
            public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
            	if (body instanceof Flux) {
                    Flux<? extends DataBuffer> fluxBody = (Flux<? extends DataBuffer>) body;
                    return super.writeWith(fluxBody.map(dataBuffer -> {
                    	// probably should reuse buffers 
                    	byte[] content = new byte[dataBuffer.readableByteCount()];
                    	dataBuffer.read(content);
                    	byte[] uppedContent = new String(content, Charset.forName("UTF-8")).toUpperCase().getBytes();
                    	return bufferFactory.wrap(uppedContent);
                    }));
            	}
            	return super.writeWith(body); // if body is not a flux. never got there.
            }			
		};
		return chain.filter(exchange.mutate().response(decoratedResponse).build()); // replace response with decorator
	}
}