Simply put, j8583 is a Java library to generate and read ISO8583 messages. It does not handle sending or reading them over a network connection, but it does parse the data you have read and can generate the data you need to write, or write it directly to an OutputStream.
ISO8583 is a message format used for credit card transactions, banking and other commercial interaction between different systems. It has an ASCII variant and a binary one, and it is somewhat convoluted and difficult to implement.
The main format of the ISO8583 is something like this:
ISO header (optional)|Message Type|primary bitmap|secondary bitmap (optional)|data fields
The ISO header is a string containing some code that can vary according to the message type.
The message type is a number expressed as 4 hex digits (or 2 bytes when using binary format).
The bitmap is 64 bits long and it is encoded as 16 hex characters or as 8 bytes when using binary format. Every bit that is set in the bitmap indicates that the corresponding field is present. If the first bit is set, then field 1 is present, and so on.
The fields in the message are numbered from 1 to 64. Field 1 is the secondary bitmap, if present. The secondary bitmap allows for the message to have fields from 65 to 128.
You can check the ISO8583 guide for more information; Wikipedia has a very good article on the whole specification.
j8583 offers a MessageFactory, which once properly configured, can create different message types with some values predefined, and can also parse a byte array to create an ISO message. Messages are represented by IsoMessage objects, which store IsoValue instances for their data fields. You can work with the IsoValues or use the convenience methods of IsoMessage to work directly with the stored values.
j8583 can directly handle the basic data types: numeric values, strings, amounts, dates and byte arrays. If you want to encode a custom object in a field, you can implement a custom field encoder. More information is available in the user's guide.