RESTEASY003145: Unable to find a MessageBodyReader of content-type application/json and type class org.keycloak.representations.AccessTokenResponse

I used the dependency to fix this issue

   <dependency>
        <groupId>org.jboss.resteasy</groupId>
        <artifactId>resteasy-jackson2-provider</artifactId>
        <version>3.1.0.Final</version>
    </dependency>

I solved !! You must add org.jboss.resteasy.plugins.providers.jackson.ResteasyJacksonProvider into META-INF/services/javax.ws.rs.ext.Providers file then run your executable jar.

Or if you create client with proxy then better to initialize provider factory before:

    ResteasyProviderFactory instance=ResteasyProviderFactory.getInstance();
    RegisterBuiltin.register(instance);
    instance.registerProvider(ResteasyJackson2Provider.class);