Super easy way to use enum with JSP

1.First of all

Unlike constants, Java enums are type-safe and are a convenient data type that can have multiple data. Therefore, you can define the code such as master and its meaning together. This time, I would like to explain how to easily display list boxes and checkboxes by using the enum in JSP. The web app is premised on spring-boot (spring mvc, TERASOLUNA 5.x).

2. Definition of enum

Define the enum used in this sample. I made an enum with the data of the code field of int and the label field to be displayed. If you do not use lombok, explicitly define the constructor and setter as well.

Status.java

package com.example.demo;

import lombok.AllArgsConstructor;
import lombok.Getter;

@AllArgsConstructor
@Getter
public enum Status {

    APPROVAL(1, "Approval"),
    DENIAL(2, "Denial"),
    WITHDRAWAL(3, "Withdrawal");

    int code;
    String label;

    //★ Point 1
    public String getName() {
        return name();
    }
}

** ★ Point 1 ** The enum has a method called name (), but we want to access it as a getter, so we define a wrapped getName () method. This is a measure to use enum in JSP.

3. Form class definition

Defines a Form class that stores the input data.

DemoForm.java

public class DemoForm implements Serializable {

    //★ Point 2
    @NotNull
    Status status;

    // ommited
}

** ★ Point 2 ** Define the defined enum as a field as it is. You don't have to receive it as an int code or a String label. Since enum is a data type, this field will always be Status. Since I want to make it a required item, I added the @NotNull annotation of Bean Validation.

4. JSP that outputs a list box with enum

Create a JSP that outputs a list box with enum, which is the point of this article. Use the standard JSP function and spring function (<form: xxx /> is a spring tag).

JSP that outputs a list box with enum

<%--★ Point 3--%>
<%@ page import="com.example.demo.Status" %>

...abridgement...

<%--★ Point 4--%>
<form:errors path="status"/>
<form:select path="status" items="${Status.values()}" itemLabel="label" itemValue="name" />

** ★ Point 3 ** Since I want to refer to Status in JSP, import the class with<% @ page import = "fqcn"%>. I often saw it in the days when JSP was written in scriptlets, but I don't see it much these days, but it is a standard function of JSP.

** ★ Point 4 ** Output the list box using the <form: select /> tag of spring. Since there are many attributes, each is explained below.

5. HTML of the output list box

The HTML of the output list box is shown below. The value attribute of<option>is set to name of enum, and the display item is set to label. If value matches the name defined in the `Statusʻenum, it can be bound to the Form class without any special processing.

Output HTML (add line breaks for readability)

<select id="status" name="status">
  <option value="APPROVAL">Approval</option>
  <option value="DENIAL">Denial</option>
  <option value="WITHDRAWAL">Withdrawal</option>
</select>

If you set value to a value not defined in enum (here HOGE), a Type Mismatch error will occur as shown below. It occurs when the HTML form is tampered with and an invalid value is set for value, or when the code field is mistakenly set at ʻitemValue` at point 4.

When HTTP request is made with a value that cannot be converted

Failed to convert property value of type java.lang.String to required type com.example.demo.Status for property status; 
nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] 
to type [com.example.demo.Status] for value HOGE; nested exception is java.lang.IllegalArgumentException: No enum constant com.example.demo.Status.HOGE

** As you can see from the explanation so far, by using enum in JSP, type safe of enum is applied when it is set in Form without preparing special input check. ** **

6. JSP that outputs a checkbox with enum

Change the Form class to an array so that you can select multiple forms.

DemoForm.java

public class DemoForm implements Serializable {

    @NotNull
    @Size(min = 1, max = 3)
    Status[] status;
    // ommited
}

I used the <form: select /> tag when outputting the list box, but I used the <form: checkboxes /> tag for the checkbox. Please note that there is also a <form: checkbox /> tag that outputs one checkbox.

JSP that outputs a checkbox with enum

<%--★ Point 5--%>
<form:errors path="status"/>
<ul>
  <form:checkboxes path="status" items="${Status.values()}" 
      itemLabel="label" itemValue="name" element="li" />
</ul>

** ★ Point 5 ** Describes the ʻelementattribute, which is different from the list box. Set this if you want to specify the HTML element that wraps the output checkbox. This time, I setli to make it a list format using <ul> <li>.

7. HTML of the output checkbox

The HTML of the output checkbox is shown below. The checkbox input form is wrapped in a <li> tag.

Output HTML (add line breaks for readability)

<ul>
  <li><input id="status1" name="status" type="checkbox" value="APPROVAL"/><label for="status1">Approval</label></li>
  <li><input id="status2" name="status" type="checkbox" value="DENIAL"/><label for="status2">Denial</label></li>
  <li><input id="status3" name="status" type="checkbox" value="WITHDRAWAL"/><label for="status3">Withdrawal</label></li>
  <input type="hidden" name="_status" value="on"/>
</ul>

8. Finally

This time, I explained how to easily display list boxes and checkboxes by using the enum in JSP. With this method, even if you change the definition of enum, neither JSP nor Form need to be modified. It is necessary to take measures to define the getName () method when defining the enum, but I was able to easily bind the input value of the form to the enum without implementing any special processing. Please try using enum in JSP.