使用EL和JSTL访问枚举值

Question

I have an Enum called Status defined as such:

public enum Status { 

    VALID("valid"), OLD("old");

    private final String val;

    Status(String val) {
        this.val = val;
    }

    public String getStatus() {
        return val;
    }

}

I would like to access the value of VALID from a JSTL tag. Specifically the test attribute of the <c:when> tag. Eg

<c:when test="${dp.status eq Status.VALID">

I'm not sure if this is possible.

Answer 1

对字符串的简单比较：

<c:when test="${someModel.status == 'OLD'}">

Answer 2

If using Spring MVC, the Spring Expression Language (SpEL) can be helpful:

<spring:eval expression="dp.status == T(com.example.Status).VALID" var="isValid" />
<c:if test="${isValid}">
   isValid
</c:if>

Answer 3

You have 3 choices here, none of which is perfect:

1. You can use a scriptlet in the test attribute:

   <c:when test="<%= dp.getStatus() == Status.VALID %>">

   This uses the enum, but it also uses a scriptlet, which is not the "right way" in JSP 2.0. But most importantly, this doesn't work when you want to add another condition to the same when using ${} . And this means all the variables you want to test have to be declared in a scriptlet, or kept in request, or session ( pageContext variable is not available in .tag files).

2. You can compare against string:

   <c:when test="${dp.status == 'VALID'}">

   This looks clean, but you're introducing a string that duplicates the enum value and cannot be validated by the compiler. So if you remove that value from the enum or rename it, you will not see that this part of code is not accessible anymore. You basically have to do a search/replace through the code each time.

3. You can add each of the enum values you use into the page context:

   <c:set var="VALID" value="<%=Status.VALID%>"/>

   and then you can do this:

   <c:when test="${dp.status == VALID}">

I prefer the last option (3), even though it also uses a scriptlet. This is because it only uses it when you set the value. Later on you can use it in more complex EL expressions, together with other EL conditions. While in option (1) you cannot use a scriptlet and an EL expression in the test attribute of a single when tag.

Answer 4

So to get my problem fully resolved I needed to do the following:

<% pageContext.setAttribute("old", Status.OLD); %>

Then I was able to do:

<c:when test="${someModel.status == old}"/>...</c:when>

which worked as expected.

Answer 5

Here are two more possibilities:

JSP EL 3.0 Constants

As long as you are using at least version 3.0 of EL, then you can import constants into your page as follows:

<%@ page import="org.example.Status" %>
<c:when test="${dp.status eq Status.VALID}">

However, some IDEs don't understand this yet (eg IntelliJ ) so you won't get any warnings if you make a typo, until runtime.

This would be my preferred method once it gets proper IDE support.

Helper Methods

You could just add getters to your enum.

public enum Status { 
  VALID("valid"), OLD("old");

  private final String val;

  Status(String val) {
    this.val = val;
  }

  public String getStatus() {
    return val;
  }

  public boolean isValid() {
    return this == VALID;
  }

  public boolean isOld() {
    return this == OLD;
  }
}

Then in your JSP:

<c:when test="${dp.status.valid}">

This is supported in all IDEs and will also work if you can't use EL 3.0 yet. This is what I do at the moment because it keeps all the logic wrapped up into my enum.

Also be careful if it is possible for the variable storing the enum to be null. You would need to check for that first if your code doesn't guarantee that it is not null:

<c:when test="${not empty db.status and dp.status.valid}">

I think this method is superior to those where you set an intermediary value in the JSP because you have to do that on each page where you need to use the enum. However, with this solution you only need to declare the getter once.

Answer 6

For this purposes I do the following:

<c:set var="abc">
    <%=Status.OLD.getStatus()%>
</c:set>

<c:if test="${someVariable == abc}">
    ....
</c:if>

It's looks ugly, but works!

Answer 7

I do not have an answer to the question of Kornel, but I've a remark about the other script examples. Most of the expression trust implicitly on the toString() , but the Enum.valueOf() expects a value that comes from/matches the Enum.name() property. So one should use eg:

<% pageContext.setAttribute("Status_OLD", Status.OLD.name()); %>
...
<c:when test="${someModel.status == Status_OLD}"/>...</c:when>

Answer 8

Add a method to the enum like:

public String getString() {
    return this.name();
}

For example

public enum MyEnum {
    VALUE_1,
    VALUE_2;
    public String getString() {
        return this.name();
    }
}

Then you can use:

<c:if test="${myObject.myEnumProperty.string eq 'VALUE_2'}">...</c:if>

Answer 9

When using a MVC framework I put the following in my controller.

request.setAttribute(RequestParameterNamesEnum.INBOX_ACTION.name(), RequestParameterNamesEnum.INBOX_ACTION.name());

This allows me to use the following in my JSP Page.

<script> var url = 'http://www.nowhere.com/?${INBOX_ACTION}=' + someValue;</script>

It can also be used in your comparison

<c:when test="${someModel.action == INBOX_ACTION}">

Which I prefer over putting in a string literal.

Answer 10

<%@ page import="com.example.Status" %>

1. ${dp.status eq Title.VALID.getStatus()}
2. ${dp.status eq Title.VALID}
3. ${dp.status eq Title.VALID.toString()}

• Put the import at the top , in JSP page header
• If you want to work with getStatus method , use #1
• If you want to work with the enum element itself, use either #2 or #3
• You can use == instead of eq

Answer 11

I generally consider it bad practice to mix java code into jsps/tag files. Using 'eq' should do the trick :

<c:if test="${dp.Status eq 'OLD'}">
  ...
</c:if>

Answer 12

I do it this way when there are many points to use...

public enum Status { 

    VALID("valid"), OLD("old");

    private final String val;

    Status(String val) {
        this.val = val;
    }

    public String getStatus() {
        return val;
    }

    public static void setRequestAttributes(HttpServletRequest request) {
        Map<String,String> vals = new HashMap<String,String>();
        for (Status val : Status.values()) {
            vals.put(val.name(), val.value);
        }
        request.setAttribute("Status", vals);
    }

}

JSP

<%@ page import="...Status" %>
<% Status.setRequestAttributes(request) %>

<c:when test="${dp.status eq Status.VALID}">
...

Answer 13

In Java Class:

    public class EnumTest{
    //Other property link
    private String name;
    ....

        public enum Status {
                ACTIVE,NEWLINK, BROADCASTED, PENDING, CLICKED, VERIFIED, AWARDED, INACTIVE, EXPIRED, DELETED_BY_ADMIN;
            }

        private Status statusobj ;

    //Getter and Setters
}

So now POJO and enum obj is created. Now EnumTest you will set in session object using in the servlet or controller class session.setAttribute("enumTest", EnumTest );

In JSP Page

<c:if test="${enumTest.statusobj == 'ACTIVE'}">

//TRUE??? THEN PROCESS SOME LOGIC