Retrieve results JAX-RS + Jersey + Jackson - jackson

Question

I have this entity class
#Entity
#XmlRootElement
#Table(name="user")
#NamedQuery(name="User.findAll", query="SELECT u FROM User u")
public class User implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
#Column(name="id_user", unique=true, nullable=false)
private String idUser;
#Column(nullable=false, length=50)
private String docnum;
#Column(nullable=false, length=50)
private String email;
#Column(nullable=false, length=50)
private String firstname;
#Column(nullable=false, length=50)
private String lastname;
#Column(nullable=false, length=45)
private String pwd;
//bi-directional many-to-many association to Transaction
#ManyToMany
#JoinTable(
name="transaction_users"
, joinColumns={
#JoinColumn(name="user", nullable=false)
}
, inverseJoinColumns={
#JoinColumn(name="transaction", nullable=false)
}
)
private List<Transaction> transactions;
public User() {
}
public String getIdUser() {
return this.idUser;
}
public void setIdUser(String idUser) {
this.idUser = idUser;
}
public String getDocnum() {
return this.docnum;
}
public void setDocnum(String docnum) {
this.docnum = docnum;
}
public String getEmail() {
return this.email;
}
public void setEmail(String email) {
this.email = email;
}
public String getFirstname() {
return this.firstname;
}
public void setFirstname(String firstname) {
this.firstname = firstname;
}
public String getLastname() {
return this.lastname;
}
public void setLastname(String lastname) {
this.lastname = lastname;
}
public List<Transaction> getTransactions() {
return this.transactions;
}
public void setTransactions(List<Transaction> transactions) {
this.transactions = transactions;
}
}
generated from a database table. Then i have this rest service
#Path("service/2.0")
public class ServiceTest {
#GET
#Path("/users")
#Produces(MediaType.APPLICATION_JSON)
public Response getUser() {
EntityManager entityManager = EntityManagerUtil.getEntityManager();
entityManager.getTransaction().begin();
Query q = entityManager.createQuery("SELECT u FROM User u");
#SuppressWarnings("unchecked")
List<User> listOfUser = q.getResultList();
System.out.print(listOfUser);
return Response.ok(listOfUser).build();
}
I'm (supposed) to be using the jackson API to handle json but i'm not using maven. For this reason, i've added in my buildpath the following .jars:
jackson-annotations-2.9.3.jar
jackson-core-2.9.3.jar
jackson-databind-2.9.3.jar
jackson-jaxrs-base-2.9.3.jar
jackson-module-jaxb-annotations-2.9.3.jar
jersey-media-json-jackson-2.26.jar
jackson-jaxrs-json-provider-2.9.3.jar
Then i have an ApplicationConfig.java class
package prova;
import com.fasterxml.jackson.jaxrs.json.*;
import org.glassfish.jersey.server.ResourceConfig;
import javax.ws.rs.ApplicationPath;
#ApplicationPath("rest")
public class ApplicationConfig extends ResourceConfig {
public ApplicationConfig() {
packages("com.fasterxml.jackson.jaxrs.json");
packages("prova");
}
}
When i try to submit a GET request with postman, i obtain an "HTTP 500 internal server error" with the description:
"The server encountered an unexpected condition that prevented it from fulfilling the request."
While from the eclipse console i can see
[EL Fine]: sql: 2017-12-16 17:44:54.251--ServerSession(1869059368)--
Connection(771012214)--Thread(Thread[http-nio-8080-exec-80,5,main])--
SELECT id_user, DOCNUM, EMAIL, FIRSTNAME, LASTNAME, PWD FROM user
[prova.User#3c713cb0, prova.User#49e51730, prova.User#d9ecdd7,
prova.User#383fe468]dic 16, 2017 5:44:54
PM org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWr
iterInterceptor aroundWriteTo
GRAVE: MessageBodyWriter not found for media type=application/json,
type=class java.util.Vector, genericType=class java.util.Vector.
So what i can deduce is that the query is correctly executed and it returns an array of 4 object (prova is the name of my entity manager) but then i have the GRAVE:MessageBodyWriter Error
What the hell i'm not doing correctly?why i can not retrieve my JSON data?
Thanks
UPDATE
Following the advice, i've modified the GET resource into
#GET
#Path("/users")
#Produces(MediaType.APPLICATION_JSON)
#Consumes(MediaType.APPLICATION_JSON)
public String getUser() {
return String.valueOf(10+4);
}
and submitting the GET request gives me the expected JSON answer from Postman "14"....
Can the problem be the conversion of a List into Json?if yes, what to do?
Thanks
UPDATE 2
I've edited the code of the REST resource in this way:
#GET
#Path("/users")
#Produces(MediaType.APPLICATION_JSON)
#Consumes(MediaType.APPLICATION_JSON)
public String getUser() {
EntityManager entityManager = EntityManagerUtil.getEntityManager();
entityManager.getTransaction().begin();
Query q = entityManager.createQuery("SELECT u FROM User u");
List<User> listOfUser = q.getResultList();
System.out.print(listOfUser);
if (listOfUser.isEmpty()) {
System.out.println("VOID LIST");
entityManager.close();
return String.valueOf(2);
}
for (User user : listOfUser) {
System.out.println(user.getFirstname());
System.out.println("---");
}
return String.valueOf(3);
}
The postman Output is "3" so, everything fine while the consoloe output is:
[EL Fine]: sql: 2017-12-17 13:48:33.214--ServerSession(286337061)--
Connection(2132504260)--Thread(Thread[http-nio-8080-exec-2,5,main])--
SELECT id_user, DOCNUM, EMAIL, FIRSTNAME, LASTNAME, PWD FROM USER
[prova.User#2d3017ff, prova.User#6361d00, prova.User#7ab0944a,
prova.User#5945162f]
matteo
---
tony
---
bruce
---
peter
---
which is perfectly consistent with what i have in the table of the DB... :(

Answer 1

Try to convert your response entity in an array of users. There is proper equivalent for array in json.
Other way ist to have a wrapper class for your list of users.
#XmlType
#XmlRootElement
class Wrapper {
#XmlElement
List<User> users;
}
Return this in your response.