/* 
    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.wiki;

import org.apache.log4j.Logger;
import org.apache.wiki.api.core.Engine;
import org.apache.wiki.event.WikiEngineEvent;
import org.apache.wiki.event.WikiEvent;
import org.apache.wiki.event.WikiEventListener;


/**
 * Abstract Thread subclass that operates in the background; when it detects the {@link WikiEngineEvent#SHUTDOWN} event,
 * it terminates itself. Subclasses of this method need only implement the method {@link #backgroundTask()}, instead of
 * the normal {@link Thread#run()}, and provide a constructor that passes the Engine and sleep interval. This class is thread-safe.
 */
public abstract class WikiBackgroundThread extends Thread implements WikiEventListener {
	
    private static final Logger LOG = Logger.getLogger( WikiBackgroundThread.class );
    private volatile boolean m_killMe = false;
    private final Engine m_engine;
    private final int m_interval;
    private static final long POLLING_INTERVAL = 1_000L;
    
    /**
     * Constructs a new instance of this background thread with a specified sleep interval, and adds the new instance 
     * to the wiki engine's event listeners.
     * 
     * @param engine the wiki engine
     * @param sleepInterval the interval between invocations of
     * the thread's {@link Thread#run()} method, in seconds
     */
    public WikiBackgroundThread( final Engine engine, final int sleepInterval ) {
        super();
        m_engine = engine;
        m_interval = sleepInterval;
        engine.addWikiEventListener( this );
        setDaemon( false );
    }
    
    /**
     * Listens for {@link org.apache.wiki.event.WikiEngineEvent#SHUTDOWN} and, if detected, marks the thread for death.
     * 
     * @param event {@inheritDoc}
     * @see org.apache.wiki.event.WikiEventListener#actionPerformed(org.apache.wiki.event.WikiEvent)
     */
    @Override
    public final void actionPerformed( final WikiEvent event ) {
        if ( event instanceof WikiEngineEvent ) {
            if ( event.getType() == WikiEngineEvent.SHUTDOWN ) {
                LOG.warn( "Detected wiki engine shutdown: killing " + getName() + "." );
                m_killMe = true;
            }
        }
    }
    
    /**
     * Abstract method that performs the actual work for this background thread; subclasses must implement this method.
     * 
     * @throws Exception Any exception can be thrown
     */
    public abstract void backgroundTask() throws Exception;
    
    /**
     * Returns the Engine that created this background thread.
     * 
     * @return the wiki engine
     */
    public Engine getEngine() {
        return m_engine;
    }
    
    /**
     *  Requests the shutdown of this background thread.  Note that the shutdown is not immediate.
     *  
     *  @since 2.4.92
     */
    public void shutdown() {
        m_killMe = true;
    }
    
    /**
     * Runs the background thread's {@link #backgroundTask()} method at the interval specified at construction.
     * The thread will initially pause for a full sleep interval before starting, after which it will execute 
     * {@link #startupTask()}. This method will cleanly terminate the thread if it has previously been marked as 
     * dead, before which it will execute {@link #shutdownTask()}. If any of the three methods return an exception, 
     * it will be re-thrown as a {@link org.apache.wiki.InternalWikiException}.
     * 
     * @see java.lang.Thread#run()
     */
    @Override
    public final void run() {
        try {
            // Perform the initial startup task
            final String name = getName();
            LOG.warn( "Starting up background thread: " + name + ".");
            startupTask();
            
            // Perform the background task; check every second for thread death
            while( !m_killMe ) {
                // Perform the background task
                // log.debug( "Running background task: " + name + "." );
                backgroundTask();
                
                // Sleep for the interval we're supposed to, but wake up every POLLING_INTERVAL to see if thread should die
                boolean interrupted = false;
                try {
                    for( int i = 0; i < m_interval; i++ ) {
                        Thread.sleep( POLLING_INTERVAL );
                        if( m_killMe ) {
                            interrupted = true;
                            LOG.warn( "Interrupted background thread: " + name + "." );
                            break;
                        }
                    }
                    if( interrupted ) {
                        break;
                    }
                } catch( final Throwable t ) {
                    LOG.error( "Background thread error: (stack trace follows)", t );
                }
            }
            
            // Perform the shutdown task
            shutdownTask();
        } catch( final Throwable t ) {
            LOG.error( "Background thread error: (stack trace follows)", t );
            throw new InternalWikiException( t.getMessage() ,t );
        }
    }
    
    /**
     * Executes a task after shutdown signal was detected. By default, this method does nothing; override it 
     * to implement custom functionality.
     * 
     * @throws Exception Any exception can be thrown.
     */
    public void shutdownTask() throws Exception {
    }
    
    /**
     * Executes a task just after the thread's {@link Thread#run()} method starts, but before the {@link #backgroundTask()} task executes.
     * By default, this method does nothing; override it to implement custom functionality.
     * 
     * @throws Exception Any exception can be thrown.
     */
    public void startupTask() throws Exception {
    }
    
}