Question

How do I disable the submit button until the user has chosen a file?

I have a form on my page:

<table>
    <form method="post" enctype="multipart/form-data" id="messageupload">
    <@spring.bind "messageUploadCommand.*"/>
        <tr>
            <td><@spring.formInput 'messageUploadCommand.multipartFile' '' 'file' /></td>
            <td><button type="submit" id="searchButton">Upload</button></td>
            <td><@spring.formSingleSelect 
                    'messageUploadCommand.messageFormat', 
                    messageFormats, '' /> 
                <@spring.showErrors  '<br>', 'error' /> </td>
        </tr>
    </form>
</table>

If the user submits the form before choosing a file I get

freemarker.template.TemplateException: Error executing macro: formSingleSelect required parameter: options is not specified. 
 at freemarker.core.Macro$Context.sanityCheck(Macro.java:211) 
 at freemarker.core.Macro$Context.runMacro(Macro.java:169) 
 at freemarker.core.Environment.visit(Environment.java:603) 
 at freemarker.core.UnifiedCall.accept(UnifiedCall.java:106) 
 at freemarker.core.Environment.visit(Environment.java:210) 
 at freemarker.core.MixedContent.accept(MixedContent.java:92) 
 at freemarker.core.Environment.visit(Environment.java:210)
 at freemarker.core.Environment.process(Environment.java:190)
 at freemarker.template.Template.process(Template.java:237)
 at ...

Answer 1

This is how I did it:

<table>
    <form method="post" enctype="multipart/form-data" id="messageupload">
    <@spring.bind "messageUploadCommand.*"/>
        <tr>
            <#assign onChangeScript = "document.getElementById('uploadButton').disabled = (value.length=0); ">
            <td><@spring.formInput 'messageUploadCommand.multipartFile' 'onchange=\"${onChangeScript}\"' 'file' /></td>
            <td><button type="submit" disabled="true"
                          id="searchButton">Upload</button></td>

            <td><@spring.formSingleSelect 
                    'messageUploadCommand.messageFormat', 
                    messageFormats, '' /> 
                <@spring.showErrors  '<br>', 'error' /> </td>
        </tr>
    </form>
</table>

I added the file-chooser onchange script via a Freemarker variable, and disabled the button by default. The onchange script tests for the length of the value, i.e. the filename, being zero.