Generating Classes From Scratch - Hello World!

A simple example of the classic “Hello World!” program. This example will generate a ClassFile equivalent to this Java:

class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

"""
An example showing how to create a "Hello World" class from scratch.
"""
from jawa.cf import ClassFile
from jawa.assemble import assemble

cf = ClassFile.create('HelloWorld')

main = cf.methods.create('main', '([Ljava/lang/String;)V', code=True)
main.access_flags.acc_static = True
main.code.max_locals = 1
main.code.max_stack = 2

main.code.assemble(assemble([
    ('getstatic', cf.constants.create_field_ref(
        'java/lang/System',
        'out',
        'Ljava/io/PrintStream;'
    )),
    ('ldc', cf.constants.create_string('Hello World!')),
    ('invokevirtual', cf.constants.create_method_ref(
        'java/io/PrintStream',
        'println',
        '(Ljava/lang/String;)V'
    )),
    ('return',)
]))

with open('HelloWorld.class', 'wb') as fout:
    cf.save(fout)

Our example can then be run in the standard JVM:

$ python hello_world.py
$ java HelloWorld                                                                                                                                                                                                                                   develop
Hello World!