HstComponent Java Class

A HstComponent can be invoked by a HstComponent container during three different request lifecycle phases: ACTION, RESOURCE and RENDER. A HstComponent is not thread-safe because the HST container creates one instance to serve concurrent requests. This means that in general you do not want to store instance variables in a HstComponent class, and if you do so, make sure that it support concurrent access. Storing request specific objects in an instance variable of a HstComponent does in general indicate a concurrency issue.

More specific:

The HST instantiates a HstComponent only once per hst component configuration. Thus assume you have 2 hst component configuration (below hst:hst/hst:configurations/{myproject}/hst:pages or hst:hst/hst:configurations/{myproject}/hst:components) that both have a hst:componentclassname that is com.myproject.components.Detail, then Detail.java will be instantiated twice.

An HstComponent implementation must implement the following interface:

public interface HstComponent {
  /**
   * Allows the component to initialize itself
   *
   * @param servletConfig the servletConfig of the HST container servlet
   * @param componentConfig the componentConfigBean configuration
   * @throws HstComponentException
   */
  void init(ServletContext servletContext,
             ComponentConfiguration componentConfig)
                        throws HstComponentException;

  /**
   * Allows the component to do some business logic processing before
   * rendering
   *
   * @param request
   * @param response
   * @throws HstComponentException
   */
  void doBeforeRender(HstRequest request, HstResponse response)
                                          throws HstComponentException;

  /**
   * Allows the component to process actions
   *
   * @param request
   * @param response
   * @throws HstComponentException
   */
  void doAction(HstRequest request, HstResponse response)
                                           throws HstComponentException;

  /**
   * Allows the component to do some business logic processing before
   * serving resource
   *
   * @param request
   * @param response
   * @throws HstComponentException
   */
  void doBeforeServeResource(HstRequest request, HstResponse response)
                                               throws HstComponentException;

  /**
   * Allows the component to destroy itself
   *
   * @throws HstComponentException
   */
  void destroy() throws HstComponentException;

  /**
   * Returns the ComponentConfiguration for this component or <code>null</code> 
   * if not implemented by a subclass
   */
  default ComponentConfiguration getComponentConfiguration() {
     return null;
  }

}⁞

The most important methods are #doBeforeRender and #doAction as these are typically the only methods a custom Component implements/overrides.

In practice, a custom Component always extends the org.hippoecm.hst.component.support.bean.BaseHstComponent which in turns extends org.hippoecm.hst.core.component.GenericHstComponent. The GenericHstComponent only implements the interface methods without adding specific behavior. If there is no hst:componentclassname property defined, the GenericHstComponent will be used. The BaseHstComponent does not add behavior for doBeforeRender, doAction or doBeforeServeResource, but does add lots of common utilities. Note that #getComponentConfiguration is already implemented in the GenericHstComponent and in general never needs to be overridden.