My View and Experience: Error in console running simple rest client using RestTemplate : java.net.UnknownHostException: greet-service

Here Problem is whichever configured application cannot find the host=greet-service. If you have a application with hostname, the request will go to it, but in this scenario we are looking at service registered with Discovery service.

1. Service main Spring Boot class is missing @EnableDiscoveryClient and also

2. maven POM dependency for Discovery client

@SpringBootApplication

public class GreetServiceWithEurekaApplication {

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

}

Sample Controller

@RestController
public class GreetClientController {

    private RestTemplate restTemplate = new RestTemplate();

    @RequestMapping("/greet-call")
    Greet greet() {
        //Phase 1 - without Discovery/Eureka client
//        Greet greet = restTemplate.getForObject("http://localhost:8090/greet", Greet.class);
//With Discovery client - Now we can use service name - and with other option available via eureka server
        // you need to configure DiscoveryClient - else cannot find hte hostname "greet-service" which is registered inEureka server
        Greet greet = restTemplate.getForObject("http://greet-service/greet", Greet.class);
        return greet;
    }
}

Solution Steps

1. Add Discovery Client dependency to POM

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
</dependency>

you cannot simply add the dependency unless you don't have the dependency management. So add below dependency management block and property to identify the version.

<properties>
    
    <spring-cloud.version>2025.0.0</spring-cloud.version>
</properties>

<!--    added dependency mgt after adding discovery client dependency-->
    <dependencyManagement>
       <dependencies>
          <dependency>
             <groupId>org.springframework.cloud</groupId>
             <artifactId>spring-cloud-dependencies</artifactId>
             <version>${spring-cloud.version}</version>
             <type>pom</type>
             <scope>import</scope>
          </dependency>
       </dependencies>
    </dependencyManagement>

2. Add Discovery Client Enable annotations - @EnableDiscoveryClient

3. Make RestTemplate Autowired.

4. You need to define the RestTemplate bean

5. RestTamplate bean must be Load balanced

6. Make sure Discovery server and corresponding service greet-service up and running

So after this code blocks of main class and Sample controller would be like below

package com.example.rest_client;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;

@SpringBootApplication
@EnableDiscoveryClient
public class RestClientApplication {

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

    @LoadBalanced
    @Bean
    public RestTemplate restTemplate(){
       return new RestTemplate();
    }
}

package com.example.rest_client.controller;

import com.example.rest_client.model.Greet;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

@RestController
public class GreetClientController {

//    private RestTemplate restTemplate = new RestTemplate();
    @Autowired
    private RestTemplate restTemplate;

    @RequestMapping("/greet-call")
    Greet greet() {
        //Phase 1 - without Discovery/Eureka client
//        Greet greet = restTemplate.getForObject("http://localhost:8090/greet", Greet.class);
//With Discovery client - Now we can use service name - and with other option available via eureka server
        // you need to configure DiscoveryClient - else cannot find hte hostname "greet-service" which is registered inEureka server
        Greet greet = restTemplate.getForObject("http://greet-service/greet", Greet.class);
        return greet;
    }
}

So when you call with

http://localhost:8095/greet-call

you should get a response like below.

{"message":"Demo with RestTemplate"}

Below is the full stacktrace:

----------------------------

2025-06-03T16:54:02.056+05:30 ERROR 26616 --- [rest-client] [nio-8095-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: org.springframework.web.client.ResourceAccessException: I/O error on GET request for "http://greet-service/greet": greet-service] with root cause

java.net.UnknownHostException: greet-service

at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:567) ~[na:na]

at java.base/java.net.Socket.connect(Socket.java:751) ~[na:na]

at java.base/java.net.Socket.connect(Socket.java:686) ~[na:na]

at java.base/sun.net.NetworkClient.doConnect(NetworkClient.java:183) ~[na:na]

at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:531) ~[na:na]

at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:636) ~[na:na]

at java.base/sun.net.www.http.HttpClient.<init>(HttpClient.java:280) ~[na:na]

at java.base/sun.net.www.http.HttpClient.New(HttpClient.java:386) ~[na:na]

at java.base/sun.net.www.http.HttpClient.New(HttpClient.java:408) ~[na:na]

at java.base/sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1304) ~[na:na]

at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1237) ~[na:na]

at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1123) ~[na:na]

at java.base/sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:1052) ~[na:na]

at org.springframework.http.client.SimpleClientHttpRequest.executeInternal(SimpleClientHttpRequest.java:79) ~[spring-web-6.2.7.jar:6.2.7]

at org.springframework.http.client.AbstractStreamingClientHttpRequest.executeInternal(AbstractStreamingClientHttpRequest.java:71) ~[spring-web-6.2.7.jar:6.2.7]

at org.springframework.http.client.AbstractClientHttpRequest.execute(AbstractClientHttpRequest.java:81) ~[spring-web-6.2.7.jar:6.2.7]

at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:900) ~[spring-web-6.2.7.jar:6.2.7]

at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:801) ~[spring-web-6.2.7.jar:6.2.7]

at org.springframework.web.client.RestTemplate.getForObject(RestTemplate.java:415) ~[spring-web-6.2.7.jar:6.2.7]

at com.example.rest_client.controller.GreetClientController.greet(GreetClientController.java:19) ~[classes/:na]

at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[na:na]

at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[na:na]

at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:258) ~[spring-web-6.2.7.jar:6.2.7]

at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:191) ~[spring-web-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:118) ~[spring-webmvc-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:986) ~[spring-webmvc-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:891) ~[spring-webmvc-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1089) ~[spring-webmvc-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:979) ~[spring-webmvc-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014) ~[spring-webmvc-6.2.7.jar:6.2.7]

at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903) ~[spring-webmvc-6.2.7.jar:6.2.7]

at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564) ~[tomcat-embed-core-10.1.41.jar:6.0]

at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.2.7.jar:6.2.7]

at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658) ~[tomcat-embed-core-10.1.41.jar:6.0]

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:195) ~[tomcat-embed-core-10.1.41.jar:10.1.41]