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package javax.servlet.jsp.tagext;

/**
 * Optional class provided by the tag library author to describe additional
 * translation-time information not described in the TLD.
 * The TagExtraInfo class is mentioned in the Tag Library Descriptor file (TLD).
 *
 * <p>
 * This class can be used:
 * <ul>
 * <li> to indicate that the tag defines scripting variables
 * <li> to perform translation-time validation of the tag attributes.
 * </ul>
 *
 * <p>
 * It is the responsibility of the JSP translator that the initial value
 * to be returned by calls to getTagInfo() corresponds to a TagInfo
 * object for the tag being translated. If an explicit call to
 * setTagInfo() is done, then the object passed will be returned in
 * subsequent calls to getTagInfo().
 * 
 * <p>
 * The only way to affect the value returned by getTagInfo()
 * is through a setTagInfo() call, and thus, TagExtraInfo.setTagInfo() is
 * to be called by the JSP translator, with a TagInfo object that
 * corresponds to the tag being translated. The call should happen before
 * any invocation on validate() and before any invocation on
 * getVariableInfo().
 *
 * <p>
 * <tt>NOTE:</tt> It is a (translation time) error for a tag definition
 * in a TLD with one or more variable subelements to have an associated
 * TagExtraInfo implementation that returns a VariableInfo array with
 * one or more elements from a call to getVariableInfo().
 */

public abstract class TagExtraInfo {

    /**
     * Sole constructor. (For invocation by subclass constructors, 
     * typically implicit.)
     */
    public TagExtraInfo() {
    }
    
    /**
     * information on scripting variables defined by the tag associated with
     * this TagExtraInfo instance.
     * Request-time attributes are indicated as such in the TagData parameter.
     *
     * @param data The TagData instance.
     * @return An array of VariableInfo data, or null or a zero length array
     *         if no scripting variables are to be defined.
     */
    public VariableInfo[] getVariableInfo(TagData data) {
	return ZERO_VARIABLE_INFO;
    }

    /**
     * Translation-time validation of the attributes. 
     * Request-time attributes are indicated as such in the TagData parameter.
     * Note that the preferred way to do validation is with the validate()
     * method, since it can return more detailed information.
     *
     * @param data The TagData instance.
     * @return Whether this tag instance is valid.
     * @see TagExtraInfo#validate
     */

    public boolean isValid(TagData data) {
	return true;
    }

    /**
     * Translation-time validation of the attributes.
     * Request-time attributes are indicated as such in the TagData parameter.
     * Because of the higher quality validation messages possible, 
     * this is the preferred way to do validation (although isValid() 
     * still works).  
     * 
     * <p>JSP 2.0 and higher containers call validate() instead of isValid().
     * The default implementation of this method is to call isValid().  If 
     * isValid() returns false, a generic ValidationMessage[] is returned
     * indicating isValid() returned false.</p>
     *
     * @param data The TagData instance.
     * @return A null object, or zero length array if no errors, an 
     *     array of ValidationMessages otherwise.
     * @since 2.0
     */
    public ValidationMessage[] validate( TagData data ) {
	ValidationMessage[] result = null;

	if( !isValid( data ) ) {
	    result = new ValidationMessage[] {
		new ValidationMessage( data.getId(), "isValid() == false" ) };
	}

	return result;
    }

    /**
     * Set the TagInfo for this class.
     *
     * @param tagInfo The TagInfo this instance is extending
     */
    public final void setTagInfo(TagInfo tagInfo) {
	this.tagInfo = tagInfo;
    }

    /**
     * Get the TagInfo for this class.
     *
     * @return the taginfo instance this instance is extending
     */
    public final TagInfo getTagInfo() {
	return tagInfo;
    }
    
    // private data
    private TagInfo tagInfo;

    // zero length VariableInfo array
    private static final VariableInfo[] ZERO_VARIABLE_INFO = { };
}