src/java/de/uapcore/lightpit/ModuleManager.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 * 
 * Copyright 2017 Mike Becker. All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   1. Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *
 *   2. Redistributions in binary form must reproduce the above copyright
 *      notice, this list of conditions and the following disclaimer in the
 *      documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * 
 */
package de.uapcore.lightpit;

import javax.servlet.Registration;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Scans registered servlets for LightPIT modules.
 */
@WebListener
public class ModuleManager implements ServletContextListener {
    
    private static final Logger LOG = LoggerFactory.getLogger(ModuleManager.class);
    
    /**
     * The attribute name in the servlet context under which an instance of this class can be found.
     */
    public static final String SC_ATTR_NAME = ModuleManager.class.getName();
    
    private ServletContext sc;
    
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        sc = sce.getServletContext();
        reloadAll();
        sc.setAttribute(SC_ATTR_NAME, this);
        LOG.info("Module manager injected into ServletContext.");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        assert sc != null && sc.equals(sce.getServletContext());
        sc.removeAttribute(SC_ATTR_NAME);
        unloadAll();
    }
    
    private boolean isRegisteredAsModule(Registration reg) {
        try {
            final Class scclass = Class.forName(reg.getClassName());
            
            final boolean lpservlet = LightPITServlet.class.isAssignableFrom(scclass);
            final boolean lpmodule = scclass.isAnnotationPresent(LightPITModule.class);
            
            if (lpservlet && !lpmodule) {
                LOG.warn(
                    "Servlet is a LightPITServlet but is missing the module annotation: {}",
                    reg.getClassName()
                );
            } else if (!lpservlet && lpmodule) {
                LOG.warn(
                    "Servlet is annotated as a LightPITModule but does not extend LightPITServlet: {}",
                    reg.getClassName()
                );
            }
            
            return lpservlet && lpmodule;
        } catch (ClassNotFoundException ex) {
            LOG.error(
                    "Servlet registration refers to a class which cannot be found by the class loader: {}",
                    reg.getClassName()
            );
            return false;
        }        
    }
    
    private void handleServletRegistration(String name, Registration reg) {
        if (isRegisteredAsModule(reg)) {
            LOG.info("Module detected: {}", name);
        } else {
            LOG.debug("Servlet {} is no module, skipping.", name);
        }
    }
    
    /**
     * Scans for modules and reloads them all.
     */
    public void reloadAll() {
        sc.getServletRegistrations().forEach(this::handleServletRegistration);
        LOG.info("Modules loaded.");
    }
    
    /**
     * Unloads all found modules.
     */
    public void unloadAll() {
        LOG.info("All modules unloaded.");
    }
}