To generate Java Class on Fly and add to class path

Question

I am generating a java class on fly and trying to invoke a method on it. For this, seems like I have to do the following

Compile the class (javac filename will not work as it depends on may other dependencies)
Add the class to the class path at runtime

How can I achieve this?

maybe the [JavaCompiler](https://docs.oracle.com/javase/8/docs/api/javax/tools/JavaCompiler.html) interface is what you require? — Abra, May 24 '19 at 04:22
Possibly something already answered https://stackoverflow.com/questions/1011443/extending-or-adding-new-classes-at-runtime-in-java ? — Naman, May 24 '19 at 05:40
Possible duplicate of [Creating classes dynamically with Java](https://stackoverflow.com/questions/2320404/creating-classes-dynamically-with-java) — AshwinK, May 24 '19 at 18:38
The existing question doesn't have compilation and class loader idea combined, I guess. Anyways, @Abra answer helped me to come up with the solution. Thanks — Minisha, May 24 '19 at 23:53

score 2 · Answer 1 · answered May 24 '19 at 23:49

I made it work with JavaCompiler and Custom class loader like below.

 private Path compileSource(Path javaFile, String contractFileNameWithoutExtension) {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        compiler.run(null, null, null, javaFile.toFile().getAbsolutePath());
        return javaFile.getParent().resolve(contractFileNameWithoutExtension+".class");
    }


public Class findClass(String name) {
        String filePath = sourceCodeLocation +"/"+ name.replace(".", "/")+".class";
        byte[] b = loadClassFromFile(filePath);
        return defineClass(name, b, 0, b.length);
    }

    private byte[] loadClassFromFile(String fileName)  {
        try {
            InputStream inputStream = FileUtils.getFileInputStream.apply(fileName);
            byte[] buffer;
            ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
            int nextValue = 0;
            try {
                while ((nextValue = inputStream.read()) != -1) {
                    byteStream.write(nextValue);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
            buffer = byteStream.toByteArray();
            return buffer;
        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
    }

To generate Java Class on Fly and add to class path

1 Answers1