org.gss_project.gss.server.ejb.TransactionHelper.java Source code

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Here is the source code for org.gss_project.gss.server.ejb.TransactionHelper.java

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/*
 * Copyright 2009 Electronic Business Systems Ltd.
 *
 * This file is part of GSS.
 *
 * GSS is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * GSS is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GSS.  If not, see <http://www.gnu.org/licenses/>.
 */
package org.gss_project.gss.server.ejb;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

import javax.ejb.EJBTransactionRolledbackException;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * A helper class that provides a method for repeatedly trying a transaction
 * that is rolled back due to optimistic locking exceptions.
 *
 * @author  past
 */
public class TransactionHelper<T> {
    /**
     * The logger.
     */
    private static Log logger = LogFactory.getLog(TransactionHelper.class);

    /**
     * Number of times to retry a transaction that was rolled back due to
     * optimistic locking.
     */
    private static final int TRANSACTION_RETRIES = 20;

    /**
     * The minimum retry timeout in milliseconds.
     */
    private static final int MIN_TIMEOUT = 200;

    /**
     * Execute the supplied command until it completes, ignoring transaction
     * rollbacks. Try at least TRANSACTION_RETRIES times before giving up,
     * each time waiting a random amount of time, using an exponential
     * backoff scheme. See http://en.wikipedia.org/wiki/Exponential_backoff
     * for the basic idea.
     *
     * @param command the command to execute
     * @return the value returned by the command
     * @throws Exception any other exception thrown by the command
     */
    public T tryExecute(final Callable<T> command) throws Exception {
        T returnValue = null;
        // Schedule a Future task to call the command after delay milliseconds.
        int delay = 0;
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
        for (int i = 0; i < TRANSACTION_RETRIES; i++) {
            final int retry = i;
            ScheduledFuture<T> future = executor.schedule(new Callable<T>() {

                @Override
                public T call() throws Exception {
                    return command.call();
                }
            }, delay, TimeUnit.MILLISECONDS);

            try {
                returnValue = future.get();
                break;
            } catch (ExecutionException e) {
                Throwable cause = e.getCause();
                if (!(cause instanceof EJBTransactionRolledbackException) || retry == TRANSACTION_RETRIES - 1) {
                    logger.info("Transaction retry #" + (i + 1) + " failed due to " + cause);
                    executor.shutdownNow();
                    if (cause instanceof Exception)
                        throw (Exception) cause;
                    if (cause instanceof Error)
                        throw (Error) cause;
                }
                delay = MIN_TIMEOUT + (int) (MIN_TIMEOUT * Math.random() * (i + 1));
                String origCause = cause.getCause() == null ? cause.getClass().getName()
                        : cause.getCause().getClass().getName();
                logger.info(
                        "Transaction retry #" + (i + 1) + " scheduled in " + delay + " msec due to " + origCause);
            }

        }
        executor.shutdownNow();
        return returnValue;
    }
}