Unable to find a MessageBodyReader of content-type application/json and type class java.lang.String

You could try to add the following dependency to your maven pom.

   <dependency>
    <groupId>org.jboss.resteasy</groupId>
    <artifactId>resteasy-jackson-provider</artifactId>
    <version>2.3.4.Final</version>
   </dependency>

The problem actually is that RestEasy is unable to find the Jackson provider. I had to manually register it by the following code:

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

Everything is working fine with this. But I am still unhappy with the solution as Resteasy is supposed to scan for the providers and register them automatically.

Client client = ClientBuilder.newBuilder().register(ResteasyJacksonProvider.class).build();

Just adding the line org.jboss.resteasy.plugins.providers.jackson.ResteasyJacksonProvider into META-INF/services/javax.ws.rs.ext.Providers file, solves the problem.

This file is included into resteasy-jackson-providers.jar but same file is also included into another jar, restasy-jaxrs.jar and for an executable jar file, that use both these jars, they are not merged !!

I had a similar problem and I realized that the problem was related with the version of resteasy-jackson-provider. I just moved from 3.0.4.Final to 3.0.5.Final and the problem disappeared.

Additionally I also realized that if I change the third line to the following the result was the expected with no need to change the dependencies.

Response response = request.get(Object.class).toString();