Loading classes at runtime from JAR file bytes

Question

I have byte array of a JAR file and I need to load all the JAR classes at runtime from these bytes.
How can I do this without converting JAR bytes to a file?

Answer 1

You have to wrap that array of bytes into a java.util.ByteArrayInputStream and that in turn into a java.util.jar.JarInputStream.

The latter has methods to get one entry (most probably a class file, but also possible are some kinds of resources) after the other as an instance of JarEntry, and to read the bytes for each of that.

final var classes = new ArrayList<byte[]>();
try( final var inputStream = new JarInputStream( new ByteArrayInputStream( bytes ) ) )
{
  var entry = inputStream.getNextJarEntry();
  while( nonNull( entry ) )
  {
    var buffer = new bytes [entry.getSize()];
    inputStream.read( buffer, 0, entry.getSize() );
    classes.add( buffer );
    entry = inputStream.getNextJarEntry();
  }
}

Ok, this is not tested, and of course there is none of the required error handling, and perhaps you would prefer to store the class data in a map, with the class name as the key …

Have fun!

• ByteArrayInputStream

• JarInputStream