/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.catalina.loader;

import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

/**
 * This class is loaded by {@link WebappClassLoaderBase} to enable it to deregister JDBC drivers forgotten by the web
 * application. There are some classloading hacks involved - see {@link WebappClassLoaderBase#clearReferences()} for
 * details - but the short version is do not just create a new instance of this class with the new keyword.
 */
public class JdbcLeakPrevention {

    public List<String> clearJdbcDriverRegistrations() throws SQLException {
        List<String> driverNames = new ArrayList<>();

        /*
         * DriverManager.getDrivers() has a nasty side-effect of registering drivers that are visible to this class
         * loader but haven't yet been loaded. Therefore, the first call to this method a) gets the list of originally
         * loaded drivers and b) triggers the unwanted side effect. The second call gets the complete list of drivers
         * ensuring that both original drivers and any loaded as a result of the side effects are all de-registered.
         */
        Set<Driver> originalDrivers = new HashSet<>();
        Enumeration<Driver> drivers = DriverManager.getDrivers();
        while (drivers.hasMoreElements()) {
            originalDrivers.add(drivers.nextElement());
        }
        drivers = DriverManager.getDrivers();
        while (drivers.hasMoreElements()) {
            Driver driver = drivers.nextElement();
            // Only unload the drivers this web app loaded
            if (driver.getClass().getClassLoader() != this.getClass().getClassLoader()) {
                continue;
            }
            // Only report drivers that were originally registered. Skip any
            // that were registered as a side effect of this code.
            if (originalDrivers.contains(driver)) {
                driverNames.add(driver.getClass().getCanonicalName());
            }
            DriverManager.deregisterDriver(driver);
        }
        return driverNames;
    }
}
