Load Java-Byte-Code at Runtime

Question

I got some java-byte-code (so compiled java-source) which is generated in my program. Now I want to load this byte-code into the currently running Java-VM and run a specific function. I'm not sure how to accomplish this, I digged a little bit into the Java Classloaders but found no straight way.

I found a solution which takes a class-file on the harddisk, but the bytecode I got is in a Byte-Array and I dont want to write it down to the disk but use it directly instead.

Thanks!

I think under this link you should find what you are looking for: tutorials.jenkov.com/java-reflection/… Look at the last section "ClassLoader Load / Reload Example". — Janick Bernet
– Janick Bernet, Commented Jul 4, 2010 at 11:23
My question was somehow unclear: I haven't got a class-file but a byte-array and I want to load it directly. Thanks anyway! — theomega
– theomega, Commented Jul 4, 2010 at 13:04
And I'm pretty sure my link provided exactly that. At least I found this through it: java.sun.com/j2se/1.4.2/docs/api/java/lang/…, int, int) Also you can obviously always save your byte-array to a temporary directory. — Janick Bernet
– Janick Bernet, Commented Jul 4, 2010 at 15:06

Henrik Aasted Sørensen · Accepted Answer · 2016-12-22 08:47:23Z

11

you need to write a custom class loader that overloads the findClass method

public Class findClass(String name) {
    byte[] b = ... // get the bytes from wherever they are generated
    return defineClass(name, b, 0, b.length);
}

edited Dec 22, 2016 at 8:47

Henrik Aasted Sørensen

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answered Jul 4, 2010 at 11:32

Nikolaus Gradwohl

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2 Comments

theomega Over a year ago

Thanks, sounds like a way to got, but there is no direct way without writing a custon ClassLoader?

Nikolaus Gradwohl Over a year ago

at least none i have found so far

Thorbjørn Ravn Andersen · Accepted Answer · 2010-07-04 11:24:26Z

2

If the byte code is not on the classpath of the running program, you can use URLClassLoader. From http://www.exampledepot.com/egs/java.lang/LoadClass.html

// Create a File object on the root of the directory containing the class file
File file = new File("c:\\myclasses\\");

try {
    // Convert File to a URL
    URL url = file.toURL();          // file:/c:/myclasses/
    URL[] urls = new URL[]{url};

    // Create a new class loader with the directory
    ClassLoader cl = new URLClassLoader(urls);

    // Load in the class; MyClass.class should be located in
    // the directory file:/c:/myclasses/com/mycompany
    Class cls = cl.loadClass("com.mycompany.MyClass");
} catch (MalformedURLException e) {
} catch (ClassNotFoundException e) {
}

answered Jul 4, 2010 at 11:24

Thorbjørn Ravn Andersen

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2 Comments

theomega Over a year ago

My question was somehow unclear: I haven't got a class-file but a byte-array and I want to load it directly. Thanks anyway!

Thorbjørn Ravn Andersen Over a year ago

Feel free to edit your question to be more precise. The code quoted works with class files ón the harddisk.

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