尝试将List从数据库转换为Java中的json时的Stackoverflow

Question

I have a problem. Here my entities user and userRole

User

package com.springapp.model;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Entity
@Table(name = "users", catalog = "users")
public class User {

private String username;
private String password;
private boolean enabled;
private String email;


private String token;
private Set<UserRole> userRole = new HashSet<UserRole>(0);

public User() {
    this.enabled=false;
}

@Column(name = "token",
        nullable = false, length = 80)
public String getToken() {
    return this.token;
}

public void setToken(String token) {
    this.token = token;
}

public String getEmail() {
    return this.email;
}
@Column(name = "email",
        nullable = false, length = 80)
public void setEmail(String email) {
    this.email = email;
}

@Id
@Column(name = "username", unique = true,
        nullable = false, length = 45)
public String getUsername() {
    return this.username;
}

public void setUsername(String username) {
    this.username = username;
}

@Column(name = "password",
        nullable = false, length = 60)
public String getPassword() {
    return this.password;
}

public void setPassword(String password) {
    this.password = password;
}

@Column(name = "enabled", nullable = false)
public boolean isEnabled() {
    return this.enabled;
}

public void setEnabled(boolean enabled) {
    this.enabled = enabled;
}

@OneToMany(fetch = FetchType.EAGER, mappedBy = "user")
public Set<UserRole> getUserRole() {
    return this.userRole;
}

public void setUserRole(Set<UserRole> userRole) {
    this.userRole = userRole;
}

}

UserRole

package com.springapp.model;

import javax.persistence.*;

import static javax.persistence.GenerationType.IDENTITY;

@Entity
@Table(name = "user_roles", uniqueConstraints = @UniqueConstraint(
            columnNames = { "role", "username" }))
public class UserRole{

private Integer userRoleId;
private User user;

private String role;

public UserRole() {
}

public UserRole(User user, String role) {
    this.user = user;
    this.role = role;
}

@Id
@GeneratedValue(strategy = IDENTITY)
@Column(name = "user_role_id",
        unique = true, nullable = false)
public Integer getUserRoleId() {
    return this.userRoleId;
}

public void setUserRoleId(Integer userRoleId) {
    this.userRoleId = userRoleId;
}

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "username", nullable = false)
public User getUser() {
    return this.user;
}

public void setUser(User user) {
    this.user = user;
}

@Column(name = "role", nullable = false, length = 45)
public String getRole() {
    return this.role;
}

public void setRole(String role) {
    this.role = role;
}

}

Now I want to get list of users with lists of their roles to json using gson lib

 List<User> users = userService.showAllUsers();
 Gson gson = new Gson();
 return  gson.toJson(users);

But it fails with stackoverflow error. I think it's because every user refers to roles and roles refers to users (because we have User user in Userroles and Set of roles in users). And it turns "recursion". How can I avoid this error?

Answer 1

That happens when you bidirectional associations and JSon doesn't ignore one side.

You need to mark the many-to-one side with @JsonIgnore :

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "username", nullable = false)
@JsonIgnore
public User getUser() {
    return this.user;
}