java - 我如何在 jsp/jSTL 中使用选定值进行多选？

Question

您好，我有一个具有某些角色的用户 用户类

public class User {

 private Long id;
 private String firstName;
 private String lastName;

 private Set<Role> roles = new HashSet<Role>(0);

public Long getId() { 返回 ID； } public void setId(Long id) { this.id = id; }

 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 Set<Role> getRoles() { return this.roles; }
 public void setRoles(Set<Role> roles) { this.roles = roles; }
}

角色.类

public class Role {

 private Long id;
 private String name;

 public Long getId() { return id; }
 public void setId(Long id) { this.id = id; }

 public String getName() { return name; }
 public void setName(String name) { this.name = name; }

}

在 jsp 文件中，我想进行多项选择，所有角色都具有所选值(用户拥有的角色)。

我试过了:

<select id="roles" name="roles" multiple="true" size="4">
 <c:forEach items="${allRoles}" var="role">
  <option value="${role.id}" <c:if test="${role.id == roleSelected.id}">selected</c:if> >${role.name}</option>
 </c:forEach>
</select>

其中allRoles代表所有角色，roleSelected代表user.roles。 但它不起作用，是否有任何方式可以在 jSTL 中说出类似“if role in user.roles then selected”的内容？ 感谢您的任何建议。

---

更新:

不知何故它不起作用，我在那个类中放了一个记录器我有这个:

 public static boolean contains(Collection<?> collection, Object object) {
  System.out.println("coll = " + collection.toString());
  System.out.println("obj="+ object.toString());
  System.out.println("res="+ collection.contains(object));
     return collection.contains(object);
   }

在日志中我有这个，它应该在第二次测试中结果为真:

coll = [Id :2;Code: TESTName: Temp Manager;Enabled: true;Comment: ;]
obj=Id :1;Code: ADMName: ADMIN;Enabled: true;Comment: For Adminstrators;
res=false
coll = [Id :2;Code: TESTName: Temp Manager;Enabled: true;Comment: ;]
obj=Id :2;Code: TESTName: Temp Manager;Enabled: true;Comment: ;
res=false
coll = [Id :2;Code: TESTName: Temp Manager;Enabled: true;Comment: ;]
obj=Id :3;Code: RHHName: TECHNOMEDIA;Enabled: true;Comment: Dfdf;
res=false
coll = [Id :2;Code: TESTName: Temp Manager;Enabled: true;Comment: ;]
obj=Id :4;Code: RESPONSName: Refd;Enabled: true;Comment: Sdsds;
res=false

Answer 1

我会为此创建一个 EL 函数。

package com.example;

import java.util.Collection;

public final class Functions {

    private Functions() {
        //
    }

    public static boolean contains(Collection<Object> collection, Object item) {
        return collection.contains(item);
    }

}

在 /WEB-INF/functions.tld 中定义它

<?xml version="1.0" encoding="UTF-8" ?>
<taglib 
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
    version="2.1">

    <display-name>Custom Functions</display-name>    
    <tlib-version>1.0</tlib-version>
    <uri>http://example.com/functions</uri>

    <function>
        <name>contains</name>
        <function-class>com.example.Functions</function-class>
        <function-signature>boolean contains(java.util.Collection, java.lang.Object)</function-signature>
    </function>
</taglib>

然后就可以如下使用了

<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@taglib uri="http://example.com/functions" prefix="f" %>
...
<select name="roles" multiple>
    <c:forEach items="${allRoles}" var="role">
        <option value="${role.id}" ${f:contains(user.roles, role) ? 'selected' : ''}>${role.name}</option>
    </c:forEach>
</select>

---

更新:为了正确比较集合中的对象，您必须相应地实现equals() 和hashCode()。你好像没有那样做。这是一个按技术 ID 进行比较的基本示例:

public boolean equals(Object other) {
    return other instanceof Role && id != null ? id.equals(((Role) other).id) : other == this;
}

public int hashCode() {
    return id != null ? getClass().hashCode() + id.hashCode() : super.hashCode();
}

另见:

• HashSet does not recognize same objects
• Understanding equals and hashcode
• Overriding equals and hashcode
• Right way to implement equals and hashcode