Steven Willems

Today I had to fix a bug in a web application deployed on Tomcat saying that a user had to wait too long before he received the message on his screen ‘The mail has been sent’.

Wild ideas like queues, saving the mail to the database and process it with a separate polling service or posting it to a separate http request were the first solutions I could think of. But that would have been some overkill as the mail sending functionality isn’t used that much.

So I started googling and found out Spring 3.0 has some nice task management support. You can read all about it here Spring 3.0 Reference documentation

I was already using the MailSender interface of Spring, so I implemented a different implementation that sends the mail in a different thread that it gets from a thread pool. There is even a waiting queue.

And this is how it looks like:

package com.idevelop.altiplano.services.tasks;

import javax.annotation.Resource;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.task.TaskExecutor;
import org.springframework.mail.MailException;
import org.springframework.mail.MailSender;
import org.springframework.mail.SimpleMailMessage;
import org.springframework.stereotype.Service;

/**
 * Task executor for sending mails asynchronously.
 *
 * @author Steven Willems
 */
@Service(value = "asyncMailSender")
public class AsyncMailSender implements MailSender {

    @Resource(name = "mailSender")
    private MailSender mailSender;

    private TaskExecutor taskExecutor;

    @Autowired
    public AsyncMailSender(TaskExecutor taskExecutor) {
        this.taskExecutor = taskExecutor;
    }

    public void send(SimpleMailMessage simpleMessage) throws MailException {
        taskExecutor.execute(new AsyncMailTask(simpleMessage));
    }

    public void send(SimpleMailMessage[] simpleMessages) throws MailException {
        for (SimpleMailMessage message : simpleMessages) {
            send(message);
        }
    }

    private class AsyncMailTask implements Runnable {

        private SimpleMailMessage message;

        private AsyncMailTask(SimpleMailMessage message) {
            this.message = message;
        }

        public void run() {
            mailSender.send(message);
        }
    }
}

Unit test and corresponding application context:

package com.idevelop.altiplano.services.tasks;

import javax.annotation.Resource;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.mail.MailSender;
import org.springframework.mail.SimpleMailMessage;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

/**
 * Test class for {@link com.idevelop.altiplano.services.tasks.AsyncMailSender}.
 *
 * @author Steven Willems
 */
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
public class TestAsyncMailSender {

    @Resource(name = "asyncMailSender")
    private MailSender mailSender;

    @Test
    public void testSend() throws Exception {

        SimpleMailMessage[] mailMessages = new SimpleMailMessage[50];

        for (int i = 0; i < mailMessages.length; i++) {
            SimpleMailMessage message = new SimpleMailMessage();
            message.setSubject(String.valueOf(i));

            mailMessages[i] = message;
        }

        mailSender.send(mailMessages);
    }
}

In our application context, we define ThreadPoolTaskExecutor who will manage the thread and the waiting queue.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context" xmlns:p="http://www.springframework.org/schema/p"

       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">

    <context:component-scan base-package="com.idevelop.altiplano.services.tasks"/>

    <!--
        Thread pool definition for tasks to be executed asynchronously.
    -->
    <bean id="taskExecutor" class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor" p:corePoolSize="5"
          p:maxPoolSize="10" p:queueCapacity="100" p:waitForTasksToCompleteOnShutdown="true"/>

    <!--
        Mail sender
    -->
    <bean id="mailSender" class="com.idevelop.altiplano.util.DummyMailSender"/>

</beans>

The DummyMailSender doesn’t really send a mail, it just prints out the subject of the mail message.

package com.idevelop.altiplano.util;

import org.springframework.mail.MailException;
import org.springframework.mail.MailSender;
import org.springframework.mail.SimpleMailMessage;

/**
 * Dummy mail sender
 *
 * @author Steven Willems
 */
public class DummyMailSender implements MailSender {

    public void send(SimpleMailMessage message) throws MailException {
        System.out.println("sending mail message '" + message.getSubject() + "'");
    }

    public void send(SimpleMailMessage[] simpleMessages) throws MailException {
    }
}

And to prove it did it asynchronously, here is the output:

sending mail message '0'
sending mail message '1'
sending mail message '2'
sending mail message '3'
sending mail message '4'
sending mail message '5'
sending mail message '9'
sending mail message '8'
sending mail message '7'
sending mail message '6'
sending mail message '10'
sending mail message '11'
sending mail message '14'
sending mail message '15'
sending mail message '13'
sending mail message '19'
sending mail message '20'
sending mail message '21'
sending mail message '22'
sending mail message '23'
sending mail message '24'
sending mail message '25'
sending mail message '26'
sending mail message '27'
sending mail message '28'
sending mail message '29'
sending mail message '30'
sending mail message '31'
sending mail message '32'
sending mail message '33'
sending mail message '34'
sending mail message '35'
sending mail message '37'
sending mail message '38'
sending mail message '39'
sending mail message '40'
sending mail message '41'
sending mail message '42'
sending mail message '43'
sending mail message '44'
sending mail message '12'
sending mail message '36'
sending mail message '48'
sending mail message '49'
sending mail message '18'
sending mail message '17'
sending mail message '16'
sending mail message '47'
sending mail message '45'
sending mail message '46'

Again, very simple and it just works. Thank you Spring