The Java EE 5 Tutorial

Setting Dynamic Attributes

Simple tag handlers that support dynamic attributes must declare that they do so in the tag element of the TLD (see Declaring Tag Handlers). In addition, your tag handler must implement the setDynamicAttribute method of the DynamicAttributes interface. For each attribute specified in the tag invocation that does not have a corresponding attribute element in the TLD, the web container calls setDynamicAttribute, passing in the namespace of the attribute (or null if in the default namespace), the name of the attribute, and the value of the attribute. You must implement the setDynamicAttribute method to remember the names and values of the dynamic attributes so that they can be used later when doTag is executed. If the setDynamicAttribute method throws an exception, the doTag method is not invoked for the tag, and the exception must be treated in the same manner as if it came from an attribute setter method.

The following implementation of setDynamicAttribute saves the attribute names and values in lists. Then, in the doTag method, the names and values are echoed to the response in an HTML list.

private ArrayList keys = new ArrayList();
private ArrayList values = new ArrayList();

public void setDynamicAttribute(String uri,
     String localName, Object value ) throws JspException {
    keys.add( localName );
    values.add( value );

public void doTag() throws JspException, IOException {
    JspWriter out = getJspContext().getOut();
    for( int i = 0; i < keys.size(); i++ ) {
        String key = (String)keys.get( i );
        Object value = values.get( i );
        out.println( "<li>" + key + " = " + value + "</li>" );