src/main/java/de/uapcore/lightpit/AbstractLightPITServlet.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.DataAccessObjects;
import de.uapcore.lightpit.dao.postgres.PGDataAccessObjects;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.*;
import java.util.function.Function;

/**
 * A special implementation of a HTTPServlet which is focused on implementing
 * the necessary functionality for {@link LightPITModule}s.
 */
public abstract class AbstractLightPITServlet extends HttpServlet {

    private static final Logger LOG = LoggerFactory.getLogger(AbstractLightPITServlet.class);

    private static final String SITE_JSP = Functions.jspPath("site");

    /**
     * The EL proxy is necessary, because the EL resolver cannot handle annotation properties.
     */
    private LightPITModule.ELProxy moduleInfo = null;

    @FunctionalInterface
    protected interface SQLFindFunction<K, T> {
        T apply(K key) throws SQLException;

        default <V> SQLFindFunction<V, T> compose(Function<? super V, ? extends K> before) throws SQLException {
            Objects.requireNonNull(before);
            return (v) -> this.apply(before.apply(v));
        }

        default <V> SQLFindFunction<K, V> andThen(Function<? super T, ? extends V> after) throws SQLException {
            Objects.requireNonNull(after);
            return (t) -> after.apply(this.apply(t));
        }

        static <K> Function<K, K> identity() {
            return (t) -> t;
        }
    }

    /**
     * Invocation mapping gathered from the {@link RequestMapping} annotations.
     * <p>
     * Paths in this map must always start with a leading slash, although
     * the specification in the annotation must not start with a leading slash.
     * <p>
     * The reason for this is the different handling of empty paths in
     * {@link HttpServletRequest#getPathInfo()}.
     */
    private final Map<HttpMethod, Map<String, Method>> mappings = new HashMap<>();

    /**
     * Gives implementing modules access to the {@link ModuleManager}.
     *
     * @return the module manager
     */
    protected final ModuleManager getModuleManager() {
        return (ModuleManager) getServletContext().getAttribute(ModuleManager.SC_ATTR_NAME);
    }


    /**
     * Creates a set of data access objects for the specified connection.
     *
     * @param connection the SQL connection
     * @return a set of data access objects
     */
    private DataAccessObjects createDataAccessObjects(Connection connection) throws SQLException {
        final var df = (DatabaseFacade) getServletContext().getAttribute(DatabaseFacade.SC_ATTR_NAME);
        if (df.getSQLDialect() == DatabaseFacade.Dialect.Postgres) {
            return new PGDataAccessObjects(connection);
        }
        throw new AssertionError("Non-exhaustive if-else - this is a bug.");
    }

    private ResponseType invokeMapping(Method method, HttpServletRequest req, HttpServletResponse resp, DataAccessObjects dao) throws IOException {
        try {
            LOG.trace("invoke {}#{}", method.getDeclaringClass().getName(), method.getName());
            final var paramTypes = method.getParameterTypes();
            final var paramValues = new Object[paramTypes.length];
            for (int i = 0; i < paramTypes.length; i++) {
                if (paramTypes[i].isAssignableFrom(HttpServletRequest.class)) {
                    paramValues[i] = req;
                } else if (paramTypes[i].isAssignableFrom(HttpServletResponse.class)) {
                    paramValues[i] = resp;
                }
                if (paramTypes[i].isAssignableFrom(DataAccessObjects.class)) {
                    paramValues[i] = dao;
                }
            }
            return (ResponseType) method.invoke(this, paramValues);
        } catch (ReflectiveOperationException | ClassCastException ex) {
            LOG.error("invocation of method {} failed: {}", method.getName(), ex.getMessage());
            LOG.debug("Details: ", ex);
            resp.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
            return ResponseType.NONE;
        }
    }

    @Override
    public void init() throws ServletException {
        moduleInfo = Optional.ofNullable(this.getClass().getAnnotation(LightPITModule.class))
                .map(LightPITModule.ELProxy::new).orElse(null);

        if (moduleInfo != null) {
            scanForRequestMappings();
        }

        LOG.trace("{} initialized", getServletName());
    }

    private void scanForRequestMappings() {
        try {
            Method[] methods = getClass().getDeclaredMethods();
            for (Method method : methods) {
                Optional<RequestMapping> mapping = Optional.ofNullable(method.getAnnotation(RequestMapping.class));
                if (mapping.isPresent()) {
                    if (!Modifier.isPublic(method.getModifiers())) {
                        LOG.warn("{} is annotated with {} but is not public",
                                method.getName(), RequestMapping.class.getSimpleName()
                        );
                        continue;
                    }
                    if (Modifier.isAbstract(method.getModifiers())) {
                        LOG.warn("{} is annotated with {} but is abstract",
                                method.getName(), RequestMapping.class.getSimpleName()
                        );
                        continue;
                    }
                    if (!ResponseType.class.isAssignableFrom(method.getReturnType())) {
                        LOG.warn("{} is annotated with {} but has the wrong return type - 'ResponseType' required",
                                method.getName(), RequestMapping.class.getSimpleName()
                        );
                        continue;
                    }

                    boolean paramsInjectible = true;
                    for (var param : method.getParameterTypes()) {
                        paramsInjectible &= HttpServletRequest.class.isAssignableFrom(param)
                                || HttpServletResponse.class.isAssignableFrom(param)
                                || DataAccessObjects.class.isAssignableFrom(param);
                    }
                    if (paramsInjectible) {
                        String requestPath = "/" + mapping.get().requestPath();

                        if (mappings
                                .computeIfAbsent(mapping.get().method(), k -> new HashMap<>())
                                .putIfAbsent(requestPath, method) != null) {
                            LOG.warn("{} {} has multiple mappings",
                                    mapping.get().method(),
                                    mapping.get().requestPath()
                            );
                        }

                        LOG.debug("{} {} maps to {}::{}",
                                mapping.get().method(),
                                requestPath,
                                getClass().getSimpleName(),
                                method.getName()
                        );
                    } else {
                        LOG.warn("{} is annotated with {} but has the wrong parameters - only HttpServletRequest. HttpServletResponse, and DataAccessObjects are allowed",
                                method.getName(), RequestMapping.class.getSimpleName()
                        );
                    }
                }
            }
        } catch (SecurityException ex) {
            LOG.error("Scan for request mappings on declared methods failed.", ex);
        }
    }

    @Override
    public void destroy() {
        mappings.clear();
        LOG.trace("{} destroyed", getServletName());
    }

    /**
     * Sets the name of the dynamic fragment.
     * <p>
     * It is sufficient to specify the name without any extension. The extension
     * is added automatically if not specified.
     * <p>
     * The fragment must be located in the dynamic fragments folder.
     *
     * @param req          the servlet request object
     * @param fragmentName the name of the fragment
     * @see Constants#DYN_FRAGMENT_PATH_PREFIX
     * @see Constants#REQ_ATTR_FRAGMENT
     */
    protected void setDynamicFragment(HttpServletRequest req, String fragmentName) {
        req.setAttribute(Constants.REQ_ATTR_FRAGMENT, Functions.dynFragmentPath(fragmentName));
    }

    /**
     * Sets the breadcrumbs menu.
     *
     * @param req         the servlet request object
     * @param breadcrumbs the menu entries for the breadcrumbs menu
     * @see Constants#REQ_ATTR_BREADCRUMBS
     */
    protected void setBreadcrumbs(HttpServletRequest req, List<MenuEntry> breadcrumbs) {
        req.setAttribute(Constants.REQ_ATTR_BREADCRUMBS, breadcrumbs);
    }

    /**
     * @param req      the servlet request object
     * @param location the location where to redirect
     * @see Constants#REQ_ATTR_REDIRECT_LOCATION
     */
    protected void setRedirectLocation(HttpServletRequest req, String location) {
        if (location.startsWith("./")) {
            location = location.replaceFirst("\\./", Functions.baseHref(req));
        }
        req.setAttribute(Constants.REQ_ATTR_REDIRECT_LOCATION, location);
    }

    /**
     * Specifies the name of an additional stylesheet used by the module.
     * <p>
     * Setting an additional stylesheet is optional, but quite common for HTML
     * output.
     * <p>
     * It is sufficient to specify the name without any extension. The extension
     * is added automatically if not specified.
     *
     * @param req        the servlet request object
     * @param stylesheet the name of the stylesheet
     */
    public void setStylesheet(HttpServletRequest req, String stylesheet) {
        req.setAttribute(Constants.REQ_ATTR_STYLESHEET, Functions.enforceExt(stylesheet, ".css"));
    }

    /**
     * Obtains a request parameter of the specified type.
     * The specified type must have a single-argument constructor accepting a string to perform conversion.
     * The constructor of the specified type may throw an exception on conversion failures.
     *
     * @param req   the servlet request object
     * @param clazz the class object of the expected type
     * @param name  the name of the parameter
     * @param <T>   the expected type
     * @return the parameter value or an empty optional, if no parameter with the specified name was found
     */
    protected <T> Optional<T> getParameter(HttpServletRequest req, Class<T> clazz, String name) {
        final String paramValue = req.getParameter(name);
        if (paramValue == null) return Optional.empty();
        if (clazz.equals(String.class)) return Optional.of((T) paramValue);
        try {
            final Constructor<T> ctor = clazz.getConstructor(String.class);
            return Optional.of(ctor.newInstance(paramValue));
        } catch (ReflectiveOperationException e) {
            throw new RuntimeException(e);
        }

    }

    /**
     * Tries to look up an entity with a key obtained from a request parameter.
     *
     * @param req   the servlet request object
     * @param clazz the class representing the type of the request parameter
     * @param name  the name of the request parameter
     * @param find  the find function (typically a DAO function)
     * @param <T>   the type of the request parameter
     * @param <R>   the type of the looked up entity
     * @return the retrieved entity or an empty optional if there is no such entity or the request parameter was missing
     * @throws SQLException if the find function throws an exception
     */
    protected <T, R> Optional<R> findByParameter(HttpServletRequest req, Class<T> clazz, String name, SQLFindFunction<? super T, ? extends R> find) throws SQLException {
        final var param = getParameter(req, clazz, name);
        if (param.isPresent()) {
            return Optional.ofNullable(find.apply(param.get()));
        } else {
            return Optional.empty();
        }
    }

    private void forwardToFullView(HttpServletRequest req, HttpServletResponse resp)
            throws IOException, ServletException {

        final var mainMenu = new ArrayList<MenuEntry>(getModuleManager().getMainMenu());
        for (var entry : mainMenu) {
            if (Functions.fullPath(req).startsWith("/" + entry.getPathName())) {
                entry.setActive(true);
            }
        }
        req.setAttribute(Constants.REQ_ATTR_MENU, mainMenu);
        req.getRequestDispatcher(SITE_JSP).forward(req, resp);
    }

    private String sanitizeRequestPath(HttpServletRequest req) {
        return Optional.ofNullable(req.getPathInfo()).orElse("/");
    }

    private Optional<Method> findMapping(HttpMethod method, HttpServletRequest req) {
        return Optional.ofNullable(mappings.get(method)).map(rm -> rm.get(sanitizeRequestPath(req)));
    }

    private void forwardAsSpecified(ResponseType type, HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        switch (type) {
            case NONE:
                return;
            case HTML:
                forwardToFullView(req, resp);
                return;
            // TODO: implement remaining response types
            default:
                throw new AssertionError("ResponseType switch is not exhaustive - this is a bug!");
        }
    }

    private void doProcess(HttpMethod method, HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        // choose the requested language as session language (if available) or fall back to english, otherwise
        HttpSession session = req.getSession();
        if (session.getAttribute(Constants.SESSION_ATTR_LANGUAGE) == null) {
            Optional<List<String>> availableLanguages = Functions.availableLanguages(getServletContext()).map(Arrays::asList);
            Optional<Locale> reqLocale = Optional.of(req.getLocale());
            Locale sessionLocale = reqLocale.filter((rl) -> availableLanguages.map((al) -> al.contains(rl.getLanguage())).orElse(false)).orElse(Locale.ENGLISH);
            session.setAttribute(Constants.SESSION_ATTR_LANGUAGE, sessionLocale);
            LOG.debug("Setting language for new session {}: {}", session.getId(), sessionLocale.getDisplayLanguage());
        } else {
            Locale sessionLocale = (Locale) session.getAttribute(Constants.SESSION_ATTR_LANGUAGE);
            resp.setLocale(sessionLocale);
            LOG.trace("Continuing session {} with language {}", session.getId(), sessionLocale);
        }

        // set some internal request attributes
        final String fullPath = Functions.fullPath(req);
        req.setAttribute(Constants.REQ_ATTR_BASE_HREF, Functions.baseHref(req));
        req.setAttribute(Constants.REQ_ATTR_PATH, fullPath);
        Optional.ofNullable(moduleInfo).ifPresent((proxy) -> req.setAttribute(Constants.REQ_ATTR_MODULE_INFO, proxy));

        // if this is an error path, bypass the normal flow
        if (fullPath.startsWith("/error/")) {
            final var mapping = findMapping(method, req);
            if (mapping.isPresent()) {
                forwardAsSpecified(invokeMapping(mapping.get(), req, resp, null), req, resp);
            }
            return;
        }

        // obtain a connection and create the data access objects
        final var db = (DatabaseFacade) req.getServletContext().getAttribute(DatabaseFacade.SC_ATTR_NAME);
        final var ds = db.getDataSource();
        if (ds == null) {
            resp.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE, "JNDI DataSource lookup failed. See log for details.");
            return;
        }
        try (final var connection = ds.getConnection()) {
            final var dao = createDataAccessObjects(connection);
            try {
                connection.setAutoCommit(false);
                // call the handler, if available, or send an HTTP 404 error
                final var mapping = findMapping(method, req);
                if (mapping.isPresent()) {
                    forwardAsSpecified(invokeMapping(mapping.get(), req, resp, dao), req, resp);
                } else {
                    resp.sendError(HttpServletResponse.SC_NOT_FOUND);
                }
                connection.commit();
            } catch (SQLException ex) {
                LOG.warn("Database transaction failed (Code {}): {}", ex.getErrorCode(), ex.getMessage());
                LOG.debug("Details: ", ex);
                resp.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, "Unhandled Transaction Error - Code: " + ex.getErrorCode());
                connection.rollback();
            }
        } catch (SQLException ex) {
            LOG.error("Severe Database Exception (Code {}): {}", ex.getErrorCode(), ex.getMessage());
            LOG.debug("Details: ", ex);
            resp.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, "Database Error - Code: " + ex.getErrorCode());
        }
    }

    @Override
    protected final void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doProcess(HttpMethod.GET, req, resp);
    }

    @Override
    protected final void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doProcess(HttpMethod.POST, req, resp);
    }
}