StAX (Streaming API for XML) is a Java API for reading and writing XML documents in a streaming fashion. It allows you to process XML documents as a series of events, rather than loading the entire document into memory at once. This can be useful for working with very large XML documents, or for situations where you only need to process a subset of the document.

The StAX API includes two main interfaces:

1. XMLStreamReader: This interface provides a way to read XML documents as a series of events. The application can advance through the document by calling methods such as next(), which advances to the next event in the document. The XMLStreamReader interface can be used to read both XML elements and attributes, as well as other types of events such as processing instructions and comments.

2. XMLStreamWriter: This interface provides a way to write XML documents as a series of events. The application can write out events such as start elements, end elements, and attributes, as well as other types of events such as processing instructions and comments. The XMLStreamWriter interface can be used to write both well-formed and valid XML documents.

Here's an example of using the XMLStreamReader to read an XML document from a file:

import javax.xml.stream.*;
import java.io.*;

public class StAXExample {

    public static void main(String[] args) throws Exception {
        InputStream inputStream = new FileInputStream("example.xml");
        XMLInputFactory factory = XMLInputFactory.newInstance();
        XMLStreamReader reader = factory.createXMLStreamReader(inputStream);
        while (reader.hasNext()) {
            int eventType = reader.next();
            if (eventType == XMLStreamConstants.START_ELEMENT) {
                System.out.println("Start element: " + reader.getLocalName());
            } else if (eventType == XMLStreamConstants.END_ELEMENT) {
                System.out.println("End element: " + reader.getLocalName());
            } else if (eventType == XMLStreamConstants.CHARACTERS) {
                System.out.println("Text: " + reader.getText());
            }
        }
        reader.close();
    }
}

In this example, we create an InputStream object from an input file using a FileInputStream. We then create an XMLInputFactory object using the newInstance() method, which we use to create an XMLStreamReader object by passing in the InputStream object to the createXMLStreamReader() method.

We then use the XMLStreamReader object to read the XML document using a loop that advances through the document by calling the next() method. We use the getLocalName() and getText() methods to get the name and text content of elements and attributes, and we print out the event type and relevant information using System.out.println().

Finally, we close the reader using the close() method.

Note that the StAX API provides a low-level, event-based API for reading and writing XML documents, which may be more difficult to use than the higher-level, object-oriented DOM (Document Object Model) API or the SAX (Simple API for XML) API. However, StAX is generally more memory-efficient than the DOM API and faster than the SAX API for large documents, making it a good choice for certain types of applications.