SpringBoot MockRestServiceServerAutoConfiguration with RestTemplate and RestClient together throws incorrect exception

It should be possible to have together RestTemplate and RestClient in application and test them independently.

Unfortunately, currently if RestTemplate is used, RestClient cannot be tested:

org.springframework.boot.test.autoconfigure.web.client.MockRestServiceServerAutoConfiguration.DeferredRequestExpectationManager#getDelegate

if (!restTemplateExpectationManagers.isEmpty()) {
    Assert.state(restTemplateExpectationManagers.size() == 1,
            "Unable to use auto-configured MockRestServiceServer since "
                    + "MockServerRestTemplateCustomizer has been bound to more than one RestTemplate");
    return restTemplateExpectationManagers.values().iterator().next();
}
Assert.state(restClientExpectationManagers.size() == 1,
        "Unable to use auto-configured MockRestServiceServer since "
                + "MockServerRestClientCustomizer has been bound to more than one RestClient");
return restClientExpectationManagers.values().iterator().next();

**Comment From: philwebb**

I'm not sure I understand your setup. Could you please provide a minimal sample project (either a GitHub project or zip file) that shows what you are trying to do.

**Comment From: michaldo**

Voila: https://github.com/michaldo/rest-client-and-template-together

**Comment From: mhalbritter**

The problem is in `org.springframework.boot.test.autoconfigure.web.client.MockRestServiceServerAutoConfiguration.DeferredRequestExpectationManager#getDelegate`:

When a `RestTemplate` is in the context, `restTemplateExpectationManagers` contains 1 entry, which is then always used. The expectation manager, which may be in `restClientExpectationManagers` is then ignored.

I suspect that the RestClient case then uses the wrong expectation manager when the mocked response is returned. 

That's the error I'm getting:

java.lang.AssertionError: No further requests expected: HTTP GET / 0 request(s) executed.

at org.springframework.test.web.client.AbstractRequestExpectationManager.createUnexpectedRequestError(AbstractRequestExpectationManager.java:193)
at org.springframework.test.web.client.SimpleRequestExpectationManager.matchRequest(SimpleRequestExpectationManager.java:59)
at org.springframework.test.web.client.AbstractRequestExpectationManager.validateRequest(AbstractRequestExpectationManager.java:91)
at org.springframework.test.web.client.MockRestServiceServer$MockClientHttpRequestFactory$1.executeInternal(MockRestServiceServer.java:324)
at org.springframework.mock.http.client.MockClientHttpRequest.execute(MockClientHttpRequest.java:126)
at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.exchangeInternal(DefaultRestClient.java:453)
at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.retrieve(DefaultRestClient.java:424)
at com.example.restclientjson.RestClientAndTemplateTogetherTest.test(RestClientAndTemplateTogetherTest.java:42)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

**Comment From: scottfrederick**

If you have two `RestTemplate` beans or two `RestClient` beans in your application, you can't use an auto-wired `MockRestServiceServer` in your test. If you do, you will get an exception with a message like `Unable to use auto-configured MockRestServiceServer since MockServerRestTemplateCustomizer has been bound to more than one RestTemplate` when you try to set an expectation. This is mentioned in the [JavaDoc](https://github.com/spring-projects/spring-boot/blob/3.2.x/spring-boot-project/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/web/client/AutoConfigureMockRestServiceServer.java#L34-L44) for `AutoConfigureMockRestServiceServer.java` but the language is not completely clear:

> If multiple [RestTemplates](https://docs.spring.io/spring-framework/docs/6.1.2/javadoc-api/org/springframework/web/client/RestTemplate.html) or [RestClients](https://docs.spring.io/spring-framework/docs/6.1.2/javadoc-api/org/springframework/web/client/RestClient.html) are in use, inject a [MockServerRestTemplateCustomizer](https://docs.spring.io/spring-boot/docs/3.2.x/api/org/springframework/boot/test/web/client/MockServerRestTemplateCustomizer.html) and use [getServer(RestTemplate)](https://docs.spring.io/spring-boot/docs/3.2.x/api/org/springframework/boot/test/web/client/MockServerRestTemplateCustomizer.html#getServer(org.springframework.web.client.RestTemplate)), or inject a [MockServerRestClientCustomizer](https://docs.spring.io/spring-boot/docs/3.2.x/api/org/springframework/boot/test/web/client/MockServerRestClientCustomizer.html) and use [ getServer(RestClient.Builder)](https://docs.spring.io/spring-boot/docs/3.2.x/api/org/springframework/boot/test/web/client/MockServerRestClientCustomizer.html#getServer(org.springframework.web.client.RestClient.Builder)), or bind a [MockRestServiceServer](https://docs.spring.io/spring-framework/docs/6.1.2/javadoc-api/org/springframework/test/web/client/MockRestServiceServer.html) directly.

The same is true when you have one `RestTemplate` and one `RestClient`, but this condition is not currently detected in the code like it should be so that an exception with a descriptive message can be thrown. We'll fix that. 

Applying the suggestion from the JavaDoc to the provided sample would look like this: 

```java
    @Test
    void test(@Autowired RestClient restClient, @Autowired MockServerRestClientCustomizer customizer) {

        // given
        customizer.getServer().expect(requestTo("/")).andRespond(withSuccess());

        // when then
        restClient.get()
                .uri("/")
                .retrieve();
    }