jpa 中@ManyToMany 标签使用后，只对关联表进行删除的写法

1. 表单结构 ：

• 一个角色表TRole 
• 一个菜单表TMenu
• 一个角色-菜单表TRoleMenu

2.通过jpa生成pojo，用到了many2many 注释来表示关联关系后。只会生成2个实体pojo 类。中间关联表不会生成pojo类

 package com.alcor.acl.domain;
 import java.io.Serializable;
 import javax.persistence.*;
 import java.util.Set;
 /**
  * The persistent class for the t_menu database table.
  *
  */
 @Entity
 @Table(name="t_menu")
 public class TMenu implements Serializable {
     private static final long serialVersionUID = 1L;
     @Id
     @Column(name="menu_id")
     private String menuId;
     @Column(name="menu_name")
     private String menuName;
     private int menuorder;
     private String module;
     @Column(name="parent_id")
     private String parentId;
     private String url;
     //bi-directional many-to-many association to TRole
     @ManyToMany(mappedBy="TMenus", fetch=FetchType.EAGER)
     private Set TRoles;
     public TMenu() {
     }
     public String getMenuId() {
         return this.menuId;
     }
     public void setMenuId(String menuId) {
         this.menuId = menuId;
     }
     public String getMenuName() {
         return this.menuName;
     }
     public void setMenuName(String menuName) {
         this.menuName = menuName;
     }
     public int getMenuorder() {
         return this.menuorder;
     }
     public void setMenuorder(int menuorder) {
         this.menuorder = menuorder;
     }
     public String getModule() {
         return this.module;
     }
     public void setModule(String module) {
         this.module = module;
     }
     public String getParentId() {
         return this.parentId;
     }
     public void setParentId(String parentId) {
         this.parentId = parentId;
     }
     public String getUrl() {
         return this.url;
     }
     public void setUrl(String url) {
         this.url = url;
     }
     public Set getTRoles() {
         return this.TRoles;
     }
     public void setTRoles(Set TRoles) {
         this.TRoles = TRoles;
     }

 }

 package com.alcor.acl.domain;
 import java.io.Serializable;
 import javax.persistence.*;
 import java.util.Set;
 /**
  * The persistent class for the t_role database table.
  *
  */
 @Entity
 @Table(name="t_role")
 public class TRole implements Serializable {
     private static final long serialVersionUID = 1L;
     @Id
     @Column(name="role_id")
     private String roleId;
     private String description;
     @Column(name="role_name")
     private String roleName;
     //bi-directional many-to-many association to TMenu
     @ManyToMany(fetch=FetchType.EAGER)
     @JoinTable(
         name="t_role_menu"
         , joinColumns={
             @JoinColumn(name="role_id")
             }
         , inverseJoinColumns={
             @JoinColumn(name="menu_id")
             }
         )
     private Set TMenus;
     //bi-directional many-to-many association to TUser
     @ManyToMany(mappedBy="TRoles", fetch=FetchType.EAGER)
     private Set TUsers;
     public TRole() {
     }
     public String getRoleId() {
         return this.roleId;
     }
     public void setRoleId(String roleId) {
         this.roleId = roleId;
     }
     public String getDescription() {
         return this.description;
     }
     public void setDescription(String description) {
         this.description = description;
     }
     public String getRoleName() {
         return this.roleName;
     }
     public void setRoleName(String roleName) {
         this.roleName = roleName;
     }
     public Set getTMenus() {
         return this.TMenus;
     }
     public void setTMenus(Set TMenus) {
         this.TMenus = TMenus;
     }

     public Set getTUsers() {
         return this.TUsers;
     }
     public void setTUsers(Set TUsers) {
         this.TUsers = TUsers;
     }

 }

3.如果需求是只要求解除菜单和角色的关联关系，而不是去删除菜单或者角色。就需要用这样的语法

 tRole.getTMenus().remove(tMenu);
             em.merge(tRole);

注意：只能通过tRole来发起

 

不能写成这样

 tMenu.getTRole().remove(tRole);         em.merge(tMenu);

这是因为：在pojo的@Many2Many标签只有在TRole这个里面设置了joinColumns属性，指定关系维护端(owner side)，即TRole类本身.

使用JoinColumn,name指向数据库字段,referencedColumnName指向实体类里面的属 性,当数据库字段与实体类的名字相同的时候可以省略.

设置inverseJoinColumns,指定关系被维护端(inverse side),即映射到TMenu类,设置方法与joinColumns一样.