出乎意料的＆＃39;＆lt;＆＃39;在做REST请求时？

Question

我最近潜入Java Rest。但是，每当我尝试通过Postman做一些事情并发出GET请求时，结果都是一样的。

我的REST是这样写的：

/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package rest;

import DBMapper.DBMapper;
import com.google.gson.Gson;
import entity.Country;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.ws.rs.core.Context;
import javax.ws.rs.core.UriInfo;
import javax.ws.rs.Consumes;
import javax.ws.rs.Produces;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PUT;
import javax.ws.rs.PathParam;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;

/**
 * REST Web Service
 *
 * @author William Pfaffe
 */
@Path("world")
public class WorldResource {
    private EntityManager em;
    private EntityManagerFactory emf;
    private DBMapper db = new DBMapper(em, emf);
    private List<Country> list;
    private final  Gson gson = new com.google.gson.GsonBuilder().setPrettyPrinting().create();

    @Context
    private UriInfo context;

    /**
     * Creates a new instance of WorldResource
     */
    public WorldResource() {

    }

    /**
     * Retrieves representation of an instance of rest.WorldResource
     * @return an instance of java.lang.String
     */
    @GET
    @Produces(MediaType.APPLICATION_XML)
    public String getXml() {
        //TODO return proper representation object
        throw new UnsupportedOperationException();
    }

    @GET
    @Path("/countries")
    @Produces(MediaType.APPLICATION_JSON)
    public Response getCountries() {
        list = db.getAllCountries();
        String jsonEx = gson.toJson(list, List.class);
        return Response.status(200).entity(jsonEx).build();
    }

    /**
     * PUT method for updating or creating an instance of WorldResource
     * @param content representation for the resource
     */
    @PUT
    @Consumes(MediaType.APPLICATION_XML)
    public void putXml(String content) {
    }
}

我的ApplicationConfig是这样的：

/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package rest;

import java.util.Set;
import javax.ws.rs.core.Application;

/**
 *
 * @author William Pfaffe
 */
@javax.ws.rs.ApplicationPath("api")
public class ApplicationConfig extends Application {

    @Override
    public Set<Class<?>> getClasses() {
        Set<Class<?>> resources = new java.util.HashSet<>();
        addRestResourceClasses(resources);
        return resources;
    }

    /**
     * Do not modify addRestResourceClasses() method.
     * It is automatically populated with
     * all resources defined in the project.
     * If required, comment out calling this method in getClasses().
     */
    private void addRestResourceClasses(Set<Class<?>> resources) {
        resources.add(rest.WorldResource.class);
    }

}

我尝试了几乎所有的东西，尝试不同的返回类型，而不是返回响应对象。每次结果都是一样的。

**编辑：问题来自我的ENTITYMANAGER！ 我的DBMAPPER的代码显示在下面！

MultiException有2个例外。他们是： 1. java.lang.NoClassDefFoundError：javax / persistence / Persistence 2. java.lang.IllegalStateException：无法执行操作：在rest.TestResource上创建

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package DBMapper;

import entity.Country;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

/**
 *
 * @author William Pfaffe
 */
public class DBMapper {

    private EntityManagerFactory emf = Persistence.createEntityManagerFactory("pu");
    private EntityManager em = emf.createEntityManager();

    public List<Country> getAllCountries() {
        Query query = em.createQuery("SELECT c from Country c");
        List<Country> list = query.getResultList();
        return list;
    }

    public List<Country> getCountryGreaterThanPopulation(int population) {
        Query query = em.createQuery("SELECT c from Country c where c.population > :popu");
        query.setParameter("popu", population);
        List<Country> list = query.getResultList();
        return list;
    }

    public List<Country> getCountryByCountryCode(String cc) {
        Query query = em.createQuery("SELECT c from Country c where c.code = :cc");
        query.setParameter("cc", cc);
        List<Country> list = query.getResultList();
        return list;
    }

    public Country getDenmark() {
        Query query = em.createQuery("SELECT c from Country c where c.code = :cc");
        query.setParameter("cc", "DNK");
        Country list = (Country) query.getSingleResult();
        return list;
    }

}