Python - Execute Java - Could not find or load main class [duplicate]

Question

I followed the SO Answer to execute a java program from python

import os.path, subprocess
from subprocess import STDOUT, PIPE


def compile_java(java_file):
    subprocess.check_call(['javac', java_file])


def execute_java(java_file, stdin):
    java_class, ext = os.path.splitext(java_file)
    cmd = ['java', java_class]
    process = subprocess.Popen(cmd, stdin=PIPE, stdout=PIPE, stderr=STDOUT, universal_newlines=True)
    stdout, stderr = process.communicate(stdin)
    print(stdout)


compile_java(os.path.join('model', 'classification', 'Model.java'))
execute_java('Model', '5 6 7 8')

The python code compiles the /model/classification/Model.java without problems. But when the python code executes the java program, Java can not find or load the main class Model. Executing java Model in the command line in the same directory (with the compiled version, which was triggered by the python snippet above) works.

Have you tried execute_java('model.classification.Model', '5 6 7 8') (you may need to remove the use of splitext!)? — Mark Rotteveel
– Mark Rotteveel, Commented May 23, 2018 at 18:58

Constantin Müller · Accepted Answer · 2018-05-23 18:47:52Z

0

The problem seems to be, that you have to add the directory of your *.class file to the classpath! You compiled the class in the directory model/classification so the path of your *.class file is model/classification/Model.class. For executing this bytecode you need to add the directory to classpath by writing java -cp model/classification Model...

This python code works for me:

import os.path,subprocess
from subprocess import STDOUT,PIPE

def compile_java(java_file):
    subprocess.check_call(['javac', java_file])

def execute_java(java_file, stdin):
    java_class,ext = os.path.splitext(java_file)

    // I changed the following line by adding the directory to the classpath
    cmd = ['java', '-cp', 'model/classification', java_class]

    proc = subprocess.Popen(cmd, stdin=PIPE, stdout=PIPE, stderr=STDOUT)
    stdout,stderr = proc.communicate(stdin)
    print ('This was "' + stdout + '"')

compile_java(os.path.join('model', 'classification', 'Model.java'))
execute_java('Model.java', '5 6 7 8')

In this case, the directory path is relative to the execution directory of your python script!

answered May 23, 2018 at 18:47

Constantin Müller

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4 Comments

Mark Rotteveel Over a year ago

I'd sooner guess the problem is that the class is model.classification.Model (so Model in the package model.classification), and not Model (or Model.java as your code tries) in the model/classification folder.

Constantin Müller Over a year ago

@MarkRotteveel Okay yea that would be also one possible source for this problem...

Constantin Müller Over a year ago

@MarkRotteveel But he compiled the class in a sub-directory without adding the directory to classpath so I assumed that the problem is the missing directory and not any package problem...

Mark Rotteveel Over a year ago

No, javac can compile (assuming it doesn't reference other files in that package), but java can't execute that way.

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