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Conclusions

Generalizing

This article describes creating a class that encapsulates SMTP, but many other protocols are simple enough to encapsulate in a single class, the key thing to get from this article and the associated source code is an understanding of simple sockets programming, the design considerations, and some boilerplate code.  If you want to get some experience writing your own clients, two simple protocols to start with are HTTP and POP3.

An HTTP client would be different from the SMTP client in several respects. HTTP is a simpler protocol, and you would probably block while the information was retrieved.  HTTP 1.0 is described in RFC 1945.

HTTP is extremely simple, after connecting to a server, the client requests a file, for example:
GET /index.html HTTP/1.0
Then reads from the input stream until the the end of the file.  When the file has been sent, the server drops the connection.

POP3 is similar to SMTP as there is a conversation between the client and server, but you would probably block until the conversation was done, rather than spawn a thread as shown in the code for emc.java.  There are three state for the POP3 conversation, authentication, transaction and update, each of which has a limited number of commands that can be executed.  The relevant RFC is 1939.
 

Sources for More Information

There are two key things that you need to be able to write clients:  An understanding of sockets, and a description of the protocol you are interested in. To get more information about Internet protocols, you will need to read RFCs.  There are a lot of places on the net to get RFCs.  The place I usually go to is:
http://rfc.fh-koeln.de/
 

Closing remarks

This should give you a start on creating client programs with Java, which is pretty straightforward.  Writing servers is more complicated due to threading issues, but the basic ideas of using sockets and adhering to protocols are similar.  The really nice thing is when you write a client and it works properly.
 

Author Biography

John Keogh is president of Lithic Software Corporation, a provider of Applets, applications, and custom programming.  He was written many programs for Java, Windows, and UNIX, as well as articles for software magazines.  The Lithic website is at http://www.lithic.com/.  John can be contacted at keo@lithic.com.
 

An Email Component, Copyright 2000, John Keogh, Lithic Software Corporation
This article may be redistributed, provided that no modifications are made.
First distributed on the Javaboutique
 

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