Convert an InputStream to a DataHandler in Java

When retrieving large files from a database in Java, loading the entire content as a byte array causes OutOfMemoryError. The solution is to implement a custom DataSource that wraps an InputStream, then pass it to a DataHandler constructor. This avoids loading all data into memory at once. The tutorial walks through creating an InputStreamDataSource class implementing the DataSource interface, using it to build a DataHandler, and optionally supporting dynamic MIME type specification. Note that since Java 11, the Java Activation Framework (javax.activation) must be added as an explicit dependency.