By Naresha in micronaut — Aug 17, 2020

Micronaut HTTP Client - Error Flow

We have seen Micronaut BlockingHttpClient in action and how to use exchange and retrieve methods to interact with an HTTP endpoint. What happens if your endpoint returns HTTP status code greater than or equal to 400 (they indicate that it's an error)?

To explore this, let's create an endpoint that returns an error as follows.

import io.micronaut.http.HttpResponse;
import io.micronaut.http.HttpResponseFactory;
import io.micronaut.http.HttpStatus;
import io.micronaut.http.annotation.*;

@Controller("/greeting")
public class GreetingController {

    @Get("/error")
    public HttpResponse<String> errorEndpoint() {
        return HttpResponse.status(HttpStatus.UNPROCESSABLE_ENTITY).body("ERROR");
    }

}

Here I decided to return the status code 422 (indicating something is wrong with the request) with a body of String type containing value "ERROR".

Let's attempt to invoke this endpoint from a test case. As usual, I am using Spock. We know how to invoke and endpoint that returns status 200. For now, let's assume the error response behave similarly except a different status code and response body. Since I am interested in the response status too, I prefer the exchange method to retrieve.

import io.micronaut.http.HttpResponse
import io.micronaut.http.HttpStatus
import io.micronaut.http.client.RxHttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.runtime.server.EmbeddedServer
import io.micronaut.test.annotation.MicronautTest
import spock.lang.AutoCleanup
import spock.lang.Shared
import spock.lang.Specification

import javax.inject.Inject

@MicronautTest
class ErrorResponseSpec extends Specification {

    @Shared
    @Inject
    EmbeddedServer embeddedServer

    @Shared
    @AutoCleanup
    @Inject
    @Client("/")
    RxHttpClient client

    def "when endpoint returns an error status, what happens?"() {
        when:
        HttpResponse response = client.toBlocking().exchange("/greeting/error", String)

        then:
        response.status == HttpStatus.UNPROCESSABLE_ENTITY
        and:
        response.getBody(String).get() == 'ERROR'
    }
}

Oops! we get an exception

io.micronaut.http.client.exceptions.HttpClientResponseException: Unprocessable Entity

	at io.micronaut.http.client.netty.DefaultHttpClient$11.channelRead0(DefaultHttpClient.java:2046)
	at io.micronaut.http.client.netty.DefaultHttpClient$11.channelRead0(DefaultHttpClient.java:1964)
	at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:99)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.micronaut.http.netty.stream.HttpStreamsHandler.channelRead(HttpStreamsHandler.java:190)
	at io.micronaut.http.netty.stream.HttpStreamsClientHandler.channelRead(HttpStreamsClientHandler.java:185)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:102)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:102)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436)
	at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:321)
	at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:295)
	at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
	at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163)
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:714)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:650)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:576)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.base/java.lang.Thread.run(Thread.java:834)

Micronaut HTTP client throws io.micronaut.http.client.exceptions.HttpClientResponseException when it receives a response with an error status. This helps you model your error handling flow using the typical error handling style of Java (I am talking about exceptions, not checked exceptions!). Now that we better understand how the API works let's modify our test accordingly.

def "error enpoint should result in throwing HttpClientResponseException"() {
    when:
    HttpResponse response = client.toBlocking().exchange("/greeting/error", String)

    then:
    HttpClientResponseException ex = thrown()
    and:
    ex.status == HttpStatus.UNPROCESSABLE_ENTITY
    and:
    ex.response.body == Optional.of('ERROR')
}

If you are not in a test class, you might use try-catch to handle error responses.

Micronaut version used: 2.0.0