Question

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Loading from JAR as an InputStream?

Is it possible for me to load a jar file from an input stream (from a url connection, for example), load it into a classloader, and execute it?
Thanks for your time.

Answer 1

Yes; URLClassLoader is intended for this purpose. It can load classes from an array of URLs.

URL externalJar = new URL("http://example.com/app.jar");
URL localJar = new URL("C:/Documents/app.jar");
URLClassLoader cl = new URLClassLoader(URL[]{ externalJar, localJar });
Class<?> clazz = cl.loadClass("SomeClass"); // you now can load classes

If your InputStream is not based on a URL, you could instead write the stream’s contents to a temporary jar file which you then load using the above approach. (You can load a jar entirely in memory, meaning no temp file is created, but this method takes considerably more effort to do right, because you will need define a custom ClassLoader).