How to iterate Apache velocity template variable attributes

Question

As title, is there any way to iterate or display Apache velocity template attributes?

for example, I have following code :

<code>
${ctx.messages.headerMessage}
</code>

And I want to know how many other attributes the variable ${ctx} has

Did you check [this](https://stackoverflow.com/questions/5683690/how-to-use-for-loop-in-velocity-template) — soorapadman, Apr 12 '19 at 10:55
@soorapadman thank you for your comment, this is for iterate the collection or array, not iterate the variable attributes — vincent zhang, Apr 12 '19 at 13:01
@vincentzhang It's hard to answer your question without knowing the class of `$ctx`. What does `$ctx.class.name` display? — Claude Brisson, Apr 13 '19 at 18:24
hi @ClaudeBrisson , thank you for your comment. the $ctx.class.name you could consider it is Person.java, if, there are only two attributes, for example, String firstName, String lastName. then how to iterate these two attributes on Velocity template? — vincent zhang, Apr 14 '19 at 23:06

score 1 · Accepted Answer · answered Apr 15 '19 at 17:08

It's only possible to discover and to loop on an object properties (that is, the ones with getters and/or setters) if you can add a new tool to your Velocity context. If you can't, you're rather stuck.

There are several ways to do this, I illustrate below how to do this with commons-beanutils.

First, add Apache commons-beanutils in your class path, and add it to your Velocity context from Java:

import org.apache.commons.beanutils.PropertyUtils;
...
    context.put("beans", new PropertyUtils());
...

One remark: if you do not have access to the Java part, but if by chance commons-beanutils is already in the classpath, there is one hakish way of having access to it: #set($beans = $foo.class.forName('org.apache.commons.beanutils.PropertyUtils').newInstance()).

Then, let's say that I have the following object:

class Foo
{
    public boolean isSomething() { return true; }
    public String getName() { return "Nestor"; }
}

which is present in my context under $foo. Using your newly $beans properties introspector, you can do:

#set ($properties = $beans.getPropertyDescriptors($foo.class))
#foreach ($property in $properties)
  $property.name ($property.propertyType) = $property.readMethod.invoke($foo)
#end

This will produce:

  bar (boolean) = true
  class (class java.lang.Class) = class Foo
  name (class java.lang.String) = Robert

(you'll need to filter out the class property, of course)

One last remark, though. Templates are for coding the View layer of an MVC application, and doing such a generic introspection of objects in them is rather inadequate in the view layer. You're far better of moving all this introspection code on the Java side.

How to iterate Apache velocity template variable attributes

1 Answers1