如何修复休眠 JPA 中的 LazyInitializationException

Question

There is a @ManyToMany table: User , user_role , roleuser .

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The roleuser table contains roles: ROLE_USER , ROLE_ADMIN and others.

This is how I make the @ManyToMany tables relate to each other:

User.class

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, fetch = FetchType.LAZY)
    @JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
    public List<Role> getRoleList() {
        return roleList;
    }

Role.class

    @ManyToMany(mappedBy = "roleList", cascade = {CascadeType.PERSIST, CascadeType.MERGE}, fetch = FetchType.EAGER)
    public List<User> getUserList() {
        return userList;
    }    



    public void addPerson(User user) {
        userList.add( user );
        user.getRoleList().add( this );
    }

And this is how I add the user:

AddUser.class

    User user = new User("Michael Joseph Jackson");

    Role role = serviceJpa.findRoleByRole("ROLE_USER");
    role.addPerson(user);

    serviceJpa.saveRole(role);

The bottom line is that I don't want to use fetch = FetchType.EAGER . One role can have thousands of users, and I don't want to get all thousand users when I access the role. But to do this, I have to change FetchType.EAGER to FetchType.LAZY , but then in this case, a well-known error occurs when I add a user:

Message Request processing failed; nested exception is org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.test.shop.model.user.Role.userList, could not initialize proxy - no Session

I don't know what to do about it. Please tell me what am I doing wrong?

Answer 1

I thinking your code is a bad pattern, when you call serviceJpa.saveRole(role); jpa first loading roll.userList and adding user into it, then saving it's relation. therefore , jpa first load roll.userList , but you set roll.userList load as lazy so jpa can not load userList beacuse your session closed!!! please read this tutorial