src/main/java/de/uapcore/lightpit/DatabaseFacade.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
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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.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.SQLException;
import java.util.Optional;

/**
 * Provides access to different privilege layers within the database.
 */
@WebListener
public final class DatabaseFacade implements ServletContextListener {

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

    /**
     * Timeout in seconds for the validation test.
     */
    private static final int DB_TEST_TIMEOUT = 10;

    /**
     * Specifies the database dialect.
     */
    public enum Dialect {
        Postgres
    }

    /**
     * The database dialect to use.
     * <p>
     * May be overridden by context parameter.
     *
     * @see Constants#CTX_ATTR_DB_DIALECT
     */
    private Dialect dialect = Dialect.Postgres;

    /**
     * The default schema to test against when validating the connection.
     * <p>
     * May be overridden by context parameter.
     *
     * @see Constants#CTX_ATTR_DB_SCHEMA
     */
    private static final String DB_DEFAULT_SCHEMA = "lightpit";

    /**
     * The attribute name in the Servlet context under which an instance of this class can be found.
     */
    public static final String SC_ATTR_NAME = DatabaseFacade.class.getName();

    private static final String DS_JNDI_NAME = "jdbc/lightpit/app";
    private DataSource dataSource;
    private DataAccessObjects dataAccessObjects;

    /**
     * Returns the data source.
     * <p>
     * The Optional returned should never be empty. However, if something goes
     * wrong during initialization, the data source might be absent.
     * Hence, users of this data source are forced to check the existence.
     *
     * @return a data source
     */
    public Optional<DataSource> getDataSource() {
        // TODO: this should not be an optional, if an empty optional is actually an exception
        return Optional.ofNullable(dataSource);
    }

    /**
     * Returns the data access objects.
     *
     * @return an interface to obtain the data access objects
     */
    public DataAccessObjects getDataAccessObjects() {
        return dataAccessObjects;
    }

    public Dialect getSQLDialect() {
        return dialect;
    }

    private static void checkConnection(DataSource ds, String testSchema) {
        try (Connection conn = ds.getConnection()) {
            if (!conn.isValid(DB_TEST_TIMEOUT)) {
                throw new SQLException("Validation check failed.");
            }
            if (conn.isReadOnly()) {
                throw new SQLException("Connection is read-only and thus unusable.");
            }
            if (!conn.getSchema().equals(testSchema)) {
                throw new SQLException(String.format("Connection is not configured to use the schema %s.", testSchema));
            }
            DatabaseMetaData metaData = conn.getMetaData();
            LOG.info("Connections as {} to {}/{} ready to go.", metaData.getUserName(), metaData.getURL(), conn.getSchema());
        } catch (SQLException ex) {
            LOG.error("Checking database connection failed", ex);
        }
    }

    private static DataSource retrieveDataSource(Context ctx) {
        DataSource ret = null;
        try {
            ret = (DataSource) ctx.lookup(DS_JNDI_NAME);
            LOG.info("Data source retrieved.");
        } catch (NamingException ex) {
            LOG.error("Data source {} not available.", DS_JNDI_NAME);
            LOG.error("Reason for the missing data source: ", ex);
        }
        return ret;
    }

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        ServletContext sc = sce.getServletContext();

        dataSource = null;

        final String contextName = Optional
                .ofNullable(sc.getInitParameter(Constants.CTX_ATTR_JNDI_CONTEXT))
                .orElse("java:comp/env");
        final String dbSchema = Optional
                .ofNullable(sc.getInitParameter(Constants.CTX_ATTR_DB_SCHEMA))
                .orElse(DB_DEFAULT_SCHEMA);
        final String dbDialect = sc.getInitParameter(Constants.CTX_ATTR_DB_DIALECT);
        if (dbDialect != null) {
            try {
                dialect = Dialect.valueOf(dbDialect);
            } catch (IllegalArgumentException ex) {
                LOG.error("Unknown or unsupported database dialect {}. Defaulting to {}.", dbDialect, dialect);
            }
        }

        dataAccessObjects = createDataAccessObjects(dialect);

        try {
            LOG.debug("Trying to access JNDI context {}...", contextName);
            Context initialCtx = new InitialContext();
            Context ctx = (Context) initialCtx.lookup(contextName);

            dataSource = retrieveDataSource(ctx);

            if (dataSource != null) {
                checkConnection(dataSource, dbSchema);
            }
        } catch (NamingException | ClassCastException ex) {
            LOG.error("Cannot access JNDI resources.", ex);
        }

        sc.setAttribute(SC_ATTR_NAME, this);
        LOG.info("Database facade injected into ServletContext.");
    }

    private static DataAccessObjects createDataAccessObjects(Dialect dialect) {
        switch (dialect) {
            case Postgres:
                return new PGDataAccessObjects();
            default:
                throw new AssertionError("Non-exhaustive switch - this is a bug.");
        }
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        dataSource = null;
    }
}