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

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 * 
 * Copyright 2018 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.
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 */
package de.uapcore.lightpit;

import de.uapcore.lightpit.dao.CoreDAOFactory;
import de.uapcore.lightpit.dao.ModuleDao;
import de.uapcore.lightpit.entities.Module;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.servlet.Registration;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.*;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.stream.Collectors;

/**
 * Scans registered servlets for LightPIT modules.
 */
@WebListener
public final 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;
    
    /**
     * Maps class names to module information.
     */
    private final Map<String, LightPITModule> registeredModules = new HashMap<>();
    
    /**
     * This flag is true, when synchronization is needed.
     */
    private final AtomicBoolean dirty = new AtomicBoolean(true);
    
    @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) {
        unloadAll();
    }
    
    private Optional<LightPITModule> getModuleInfo(Registration reg) {
        try {
            final Class<?> scclass = Class.forName(reg.getClassName());
            
            final boolean lpservlet = AbstractLightPITServlet.class.isAssignableFrom(scclass);
            final boolean lpmodule = scclass.isAnnotationPresent(LightPITModule.class);
            
            if (lpservlet && !lpmodule) {
                LOG.warn(
                    "{} is a LightPIT Servlet but is missing the module annotation.",
                    reg.getClassName()
                );
            } else if (!lpservlet && lpmodule) {
                LOG.warn(
                    "{} is annotated as a LightPIT Module but does not extend {}.",
                    reg.getClassName(),
                    AbstractLightPITServlet.class.getSimpleName()
                );
            }
            
            if (lpservlet && lpmodule) {
                final LightPITModule moduleInfo = scclass.getAnnotation(LightPITModule.class);
                return Optional.of(moduleInfo);
            } else {
                return Optional.empty();
            }
        } catch (ClassNotFoundException ex) {
            LOG.error(
                    "Servlet registration refers to class {} which cannot be found by the class loader (Reason: {})",
                    reg.getClassName(),
                    ex.getMessage()
            );
            return Optional.empty();
        }        
    }
    
    private void handleServletRegistration(String name, Registration reg) {
        final Optional<LightPITModule> moduleInfo = getModuleInfo(reg);
        if (moduleInfo.isPresent()) {
            registeredModules.put(reg.getClassName(), moduleInfo.get());            
            LOG.info("Module detected: {}", name);
        } else {
            LOG.debug("Servlet {} is no module, skipping.", name);
        }
    }
    
    /**
     * Scans for modules and reloads them all.
     */
    public void reloadAll() {
        registeredModules.clear();
        sc.getServletRegistrations().forEach(this::handleServletRegistration);
        
        // TODO: implement dependency resolver
        
        dirty.set(true);
        LOG.info("Modules loaded.");
    }

    /**
     * Synchronizes module information with the database.
     * <p>
     * This must be called from the {@link AbstractLightPITServlet}.
     * Admittedly the call will perform the synchronization once after reload
     * and be a no-op, afterwards.
     * However, since the DatabaseFacade might be loaded after the module
     * manager, we must defer the synchronization to the first request
     * handled by the Servlet.
     *
     * @param db interface to the database
     */
    public void syncWithDatabase(DatabaseFacade db) {
        if (dirty.compareAndSet(true, false)) {
            if (db.getDataSource().isPresent()) {
                try (Connection conn = db.getDataSource().get().getConnection()) {
                    final ModuleDao moduleDao = CoreDAOFactory.getModuleDao(db.getSQLDialect());
                    moduleDao.syncRegisteredModuleClasses(conn, registeredModules.entrySet());
                } catch (SQLException ex) {
                    LOG.error("Unexpected SQL Exception", ex);
                }
            } else {
                LOG.error("No datasource present. Cannot sync module information with database.");
            }
        } else {
            LOG.trace("Module information clean - no synchronization required.");
        }
    }
    
    /**
     * Unloads all found modules.
     */
    public void unloadAll() {
        registeredModules.clear();
        LOG.info("All modules unloaded.");
    }

    /**
     * Returns the main menu.
     * 
     * @param db the interface to the database
     * @return a list of menus belonging to the main menu
     */
    public List<Menu> getMainMenu(DatabaseFacade db) {
        // TODO: user specific menu
        
        if (db.getDataSource().isPresent()) {
            try (Connection conn = db.getDataSource().get().getConnection()) {
                final ModuleDao dao = CoreDAOFactory.getModuleDao(db.getSQLDialect());
                final List<Module> modules = dao.listAll(conn);

                return modules
                        .stream()
                        .filter(Module::isVisible)
                        .sorted(new Module.PriorityComparator())
                        .map(mod -> new Menu(
                                mod.getClassname(),
                                new ResourceKey(
                                        registeredModules.get(mod.getClassname()).bundleBaseName(),
                                        registeredModules.get(mod.getClassname()).menuKey()),
                                registeredModules.get(mod.getClassname()).modulePath()))
                        .collect(Collectors.toList());
            } catch (SQLException ex) {
                LOG.error("Unexpected SQLException when loading the main menu", ex);
                return Collections.emptyList();
            }
        } else {
            return Collections.emptyList();
        }
    }
    
    /**
     * Returns an unmodifiable map of all registered modules.
     * 
     * The key is the classname of the module.
     * 
     * @return the map of registered modules
     */
    public Map<String, LightPITModule> getRegisteredModules() {
        return Collections.unmodifiableMap(registeredModules);
    }
}