An Email Component
Introduction
Adding email capabilities can extend the usefulness of many applets
and application. Email provides a convenient mechanism for transporting
information, and integrating this functionality with your software increases
the ease of use for your end users.
The challenge is to create a class that is flexible and easy to use,
yet that handles email properly and returns immediately so that the user
can get back to work.
This article describes a class that extends the Java Thread class (so
that it returns immediately) and which encapsulates Simple Mail Transfer
Protocol (SMTP). The class that instantiates it can determine its
state using public variables in the email component (emc) class.
The class as described uses Java 1.0 event handling, so that it can be
used with Applets with most browsers (most browsers still have problems
with the Java 1.1 event model). At the end of the article, migrating
to Java 1.1 and 1.2, which involves only a few changes, is described.
During the course of the article, several source files are mentioned.
The source files (emc.java, emctest.java, and emc.html) can be downloaded
together in emc.zip.
Beginning sockets programming
There are three basic approaches to writing clients: Using RMI,
using the openStream() method of the URL class to get a stream, and creating
a socket to interact directory with a server. The last approach,
which will be examined in this article, is the least abstract and permits
the programmer to have the most control over the interaction with the server.
Creating a socket is easy:
Socket s=new Socket(strHost, 25);
In this case, the connection is made to strHost on port 25 (which is
the port for SMTP). This assumes that you initialized strHost to
the host you want to connect to (e.g. www.yourhostname.com). The
port number must match the port number that the server is listening to,
for example, 80 for HTTP, 25 for SMTP, 21 for FTP, 23 for Telnet (there
are many more that are common, generally called "well known ports").
Socket creation needs to occur in a try/catch block.
Important:
If you are writing an applet, you can only connect back to the server the
applet was downloaded from (unless it is a signed applet, but that is another
topic). In order to make this easy, you can pass the Applet in to
the client class, then create a socket as follows (this assumes the Applet
was passed in as the variable parent).
Socket s=new Socket(parent.getCodeBase().getHost(), 25);
After creating a socket, the next step is to get streams from the socket,
which is how you will actually communicate with the server.
For example:
try{out = new PrintStream(MailSocket.getOutputStream());}
catch (Exception e) { otherError(); return;}
try{in=new DataInputStream(MailSocket.getInputStream());}
catch (Exception e) { otherError(); return;}
With the streams, you can converse with the server, using out.println()
to send something to the server and in.readLine() to read the server's
response. What will be read from and written to the server depends
on the protocol. In this case we will use SMTP as described in RFC
821 (RFCs, Request For Comments, are how Internet
standards for clients and server are generally promulgated, more later
on this topic).
NEXT ->
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New Review:
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set, and it's free!
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Reverse Complement is a simple applet that converts DNA or RNA
sequences into three useful formats.
Elsewhere on internet.com:
WebDeveloper Java
Lots of Java information on webdeveloper.com
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Thorough Java resource at the Web Developer's Virtual Library.
ScriptSearch Java
Hundreds of free Java code files to download.
jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.
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