How to get body of response in Spring WebClient - spring-webclient

Question

I am trying to write some code to integrate with an IRS e-service.
I can call the service manually with this command:
curl -k -POST https://api.alt.www4.irs.gov/auth/oauth/v2/token
I get this response:
{
"error_code":"ESRV103",
"error_msg": {
"error":"invalid_request",
"error_description":"Missing or duplicate parameters"
}
}
When I try to implement the call in Java using the Spring WebClient in a CommandLineRunner:
package springTest;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.reactive.function.client.WebClient;
import org.springframework.web.reactive.function.client.WebClient.RequestBodyUriSpec;
import org.springframework.web.reactive.function.client.WebClient.ResponseSpec;
import reactor.core.publisher.Mono;
#SpringBootApplication
public class WebClientTest implements CommandLineRunner {
public static void main(String[] args)
throws Exception {
SpringApplication.run(WebClientTest.class,args);
}
public void run(String... args)
throws Exception {
WebClient.Builder builder = WebClient.builder();
builder.baseUrl("https://api.alt.www4.irs.gov");
WebClient webClient = builder.build();
RequestBodyUriSpec requestSpec = webClient.post();
requestSpec.uri("/auth/oauth/v2/token");
ResponseSpec responseSpec = requestSpec.retrieve();
Mono<String> mono = responseSpec.bodyToMono(String.class);
System.out.println(mono.block());
}
}
I get the following exception from the block() method call:
java.lang.IllegalStateException: Failed to execute CommandLineRunner
at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:771) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:752) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:314) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1303) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1292) ~[spring-boot-2.7.6.jar:2.7.6]
at springTest.WebClientTest.main(WebClientTest.java:16) ~[test-classes/:na]
Caused by: org.springframework.web.reactive.function.client.WebClientResponseException$BadRequest: 400 Bad Request from POST https://api.alt.www4.irs.gov/auth/oauth/v2/token
at org.springframework.web.reactive.function.client.WebClientResponseException.create(WebClientResponseException.java:217) ~[spring-webflux-5.3.24.jar:5.3.24]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Error has been observed at the following site(s):
*__checkpoint ⇢ 400 from POST https://api.alt.www4.irs.gov/auth/oauth/v2/token [DefaultWebClient]
Original Stack Trace:
at org.springframework.web.reactive.function.client.WebClientResponseException.create(WebClientResponseException.java:217) ~[spring-webflux-5.3.24.jar:5.3.24]
at org.springframework.web.reactive.function.client.DefaultClientResponse.lambda$createException$1(DefaultClientResponse.java:207) ~[spring-webflux-5.3.24.jar:5.3.24]
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:106) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxOnErrorReturn$ReturnSubscriber.onNext(FluxOnErrorReturn.java:162) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:101) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:129) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:299) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onNext(FluxFilterFuseable.java:337) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1802) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.MonoCollect$CollectSubscriber.onComplete(MonoCollect.java:160) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.netty.channel.FluxReceive.onInboundComplete(FluxReceive.java:413) ~[reactor-netty-core-1.0.25.jar:1.0.25]
at reactor.netty.channel.ChannelOperations.onInboundComplete(ChannelOperations.java:419) ~[reactor-netty-core-1.0.25.jar:1.0.25]
at reactor.netty.channel.ChannelOperations.terminate(ChannelOperations.java:473) ~[reactor-netty-core-1.0.25.jar:1.0.25]
at reactor.netty.http.client.HttpClientOperations.onInboundNext(HttpClientOperations.java:702) ~[reactor-netty-http-1.0.25.jar:1.0.25]
at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:113) ~[reactor-netty-core-1.0.25.jar:1.0.25]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) ~[netty-codec-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:346) ~[netty-codec-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:318) ~[netty-codec-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1373) ~[netty-handler-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.ssl.SslHandler.decodeJdkCompatible(SslHandler.java:1236) ~[netty-handler-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.ssl.SslHandler.decode(SslHandler.java:1285) ~[netty-handler-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:529) ~[netty-codec-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:468) ~[netty-codec-4.1.85.Final.jar:4.1.85.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:290) ~[netty-codec-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562) ~[netty-transport-4.1.85.Final.jar:4.1.85.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.85.Final.jar:4.1.85.Final]
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.85.Final.jar:4.1.85.Final]
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.85.Final.jar:4.1.85.Final]
at java.base/java.lang.Thread.run(Thread.java:1589) ~[na:na]
Suppressed: java.lang.Exception: #block terminated with an error
at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:99) ~[reactor-core-3.4.25.jar:3.4.25]
at reactor.core.publisher.Mono.block(Mono.java:1742) ~[reactor-core-3.4.25.jar:3.4.25]
at springTest.WebClientTest.run(WebClientTest.java:30) ~[test-classes/:na]
at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:768) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:752) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:314) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1303) ~[spring-boot-2.7.6.jar:2.7.6]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1292) ~[spring-boot-2.7.6.jar:2.7.6]
at springTest.WebClientTest.main(WebClientTest.java:16) ~[test-classes/:na]
and the println is never called. I would like to see the error message in the body like I see above.
How do I do that?

Answer 1

First use a error handler to catch the errors and then return a response entity. use controllerAdvice to manage your handlers and throw them in your code. if you want to get precise on error messages create your own exception by extending another exception and create custom message for what went wrong.
https://www.baeldung.com/exception-handling-for-rest-with-spring
use Response Entity or just send back an object
// inside controller
GetMapping("/")
public ResponseEntity<> getObject(){
return new ResponseEntity(someObject,200)
}
or just return the someObject directly spring knows how to turn an object into json as long as it has getters on the class.