Retrieving static constants with Spring

In a Spring configuration it is possible to inject constant values defined in some class (or interface). For this you don't need to know the actual value of the constant, but can instruct Spring to retrieve it. For instance for an IBM MQ TransportType:

<property name="transportType">
    <bean id="com.ibm.mq.jms.JMSC.MQJMS_TP_CLIENT_MQ_TCPIP" 
          class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean" />
</property>

Unit testing with JavaMail

It can be inconvenient to unit test a class which uses Javamail (needs SMTP server, hard to verify email contents). It is possible however to replace the standard mail Transport with a mock implementation. First you need two files in META-INF which make sure the mock gets used; in a Maven project, src/test/resources would be a good place for these. The file javamail.providers gives the transport class to use for the smtp protocol:

# set the javamail default provider to a mock implementation for SMTP only.
#
protocol=smtp; type=transport; class=com.myCompany.MockTransport; vendor=Acme Corporation;

In addition, the file javamail.address.map maps address types to the transport to use, its contents are:

rfc822=smtp

Finally, you need a mock Transport subclass which overrides the connect() methods and adds an implementation for sendMessage(). This example stores the last message sent in a static member; this makes it possible to retrieve the message afterwards in a unit test by using MockTransport.getLastMessage().

package com.myCompany;
import javax.mail.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Mock implementation of {@link Transport} for unit testing. This needs property file overrides
 * in a META-INF on the classpath in order to work; this is in src/test/resources .
 * See http://sujitpal.blogspot.com/2006/12/mock-objects-for-javamail-unit-tests.html for more information.
 * This implementation stores the last message sent and adds a getter for retrieval and verification.
 * @author bartas
 */
public class MockTransport extends Transport {
	 
	private static final Logger logger = LoggerFactory.getLogger(MockTransport.class);
	
	private static Message lastMessage;
	
	public MockTransport(Session session, URLName urlName) {
		super(session, urlName);
		logger.warn("constructed MockTransport instance - javamail is mocked");
	}

	/**
	 * Stores the message to send in this instance.
	 * @see javax.mail.Transport#sendMessage(javax.mail.Message, javax.mail.Address[])
	 */
	@Override
	public void sendMessage(Message arg0, Address[] arg1)
			throws MessagingException {
		
		String subject = arg0.getSubject();
		StringBuffer addresses = new StringBuffer("[");
		for (int i=0; i<arg1.length; i++) {
			addresses.append(arg1[i]);
			if (i<arg1.length-1) addresses.append(",");
		}
		addresses.append("]");
		logger.debug("sendMessage(\"{}\",{})", subject, addresses.toString());
		lastMessage = arg0;
	}
	
	
	@Override
	public void connect() throws MessagingException {
		logger.debug("connect()");
	}

	@Override
	public void connect(String arg0, int arg1, String arg2, String arg3)
			throws MessagingException {
		logger.debug("connect({},{},{},{})", new Object[] {arg0,arg1,arg2,arg3});
	}

	@Override
	public void connect(String arg0, String arg1, String arg2)
			throws MessagingException {
		logger.debug("connect({},{},{})", new Object[] {arg0,arg1,arg2});
	}

	@Override
	public void connect(String arg0, String arg1) throws MessagingException {
		logger.debug("connect({},{})", arg0,arg1);
	}

	@Override
	public synchronized void close() throws MessagingException {
		logger.debug("close()");
	}

	public static Message getLastMessage() {
		return lastMessage;
	}

}