Going against the grain on best practices, and presenting an annotation to shunt requests for a given path to a component that forces a return of a different object. Avoids boilerplate in each of the controller methods to search for paths.

RestTestable.java

/*
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 * and open the template in the editor.
 */
package com.tresback.testing;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 *
 * @author rbailey
 */
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface RestTestable {
    Class target();
    String[] path() default {};
}

RestTestableAspect.java

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package com.tresback.testing;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.reflect.MethodSignature;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;

/**
 *
 * @author rbailey
 */
@Component
@Aspect
public class RestTestableAspect {
    protected final Logger log = LoggerFactory.getLogger(getClass());
    
    @Around(value = "@annotation(com.tresback.testing.RestTestable) && @annotation(RestTestable)", argNames="RestTestable")
    public Object processAnnotations(ProceedingJoinPoint pjp, RestTestable restTestable) throws Throwable {
        log.debug("Processing annotations");
        if ( restTestable != null ) {
            log.debug("Testing enabled on path: "+ restTestable.path());
        }
        int index = 0;
        
        MethodSignature sig = (MethodSignature) pjp.getSignature();
        Class[] parameterTypes = sig.getParameterTypes();
        log.debug("Got signature: "+ parameterTypes);
        log.debug("Argument 0: "+ pjp.getArgs()[0]);
        
        for (Object arg : pjp.getArgs() ) {
            log.debug("Arg to check: "+ arg.toString() +" vs: "+ restTestable.path());
            if ( (index < restTestable.path().length) &&
                    (String.class == parameterTypes[index] ) &&
                    !(arg.toString().equals(restTestable.path()[index])) ) {
                log.debug("proceeding");
                return pjp.proceed();
            }
            index++;
        }
        Class klass = restTestable.target();
        Object obj = klass.newInstance();
        log.debug("Returning the default object: "+ obj.toString());
        return obj;
        
    }
}