We have already seen a processing instruction. The XML declaration is a processing instruction. And if you recall, when we introduced the XML declaration we promised to return to the concept of processing instructions to explain them as a category.

So here we are.

A processing instruction is a bit of information meant for the application using the XML document. That is, they are not really of interest to the XML parser. Instead, the instructions are passed intact straight to the application using the parser.

The application can then pass this on to another application or interpret it itself.

All processing instructions follow the generic format of

<?NAME_OF_APPLICATION_INSTRUCTION_IS_FOR INSTRUCTIONS?>

As you might imagine, you cannot use any combination of "xml" as the NAME_OF_APPLICATION_INSTRUCTION_IS_FOR since "xml" is reserved. However, you might have something like:

<?JAVA_OBJECT JAR_FILE = "/java/myjar.jar"?>