How to add or change HTML attributes from Java

To modify tag attributes in a component’s HTML markup we can use class org.apache.wicket.AttributeModifier. This class extends org.apache.wicket.behavior.Behavior and can be added to any component via the Component’s add method. Class Behavior is used to expand component functionalities and it can also modify component markup. We will see this class in detail later in chapter 19.1.

As first example of attribute manipulation let’s consider a Label component bound to the following markup:

<span wicket:id="simpleLabel"></span>

Suppose we want to add some style to label content making it red and bolded. We can add to the label an AttributeModifier which creates the tag attribute style with value color:red;font-weight:bold:

label.add(new AttributeModifier("style", "color:red;font-weight:bold"));

If attribute style already exists in the original markup, it will be replaced with the value specified by AttributeModifier. If we don’t want to overwrite the existing value of an attribute we can use subclass AttributeAppender which will append its value to the existing one:

label.add(new AttributeAppender("style", "color:red;font-weight:bold"));

We can also create attribute modifiers using factory methods provided by class AttributeModifier and it’s also possible to prepend a given value to an existing attribute:

//replaces existing value with the given one
label.add(AttributeModifier.replace("style", "color:red;font-weight:bold"));

//appends the given value to the existing one
label.add(AttributeModifier.append("style", "color:red;font-weight:bold"));

//prepends the given value to the existing one
label.add(AttributeModifier.prepend("style", "color:red;font-weight:bold"));