Java-to-Python

Did you ever want to call Python code from Java?

Until Java 5, I wasn't much of a fan of the Java language. The addition of enums, generics, and some syntactic sugar (e.g. foreach) makes it more bearable to develop in. But all of this doesn't change the fact that Java purposely hides non-portable operating system functions, such as signal handling and Windows registry access. Sure, you can do these things with JNI, but that makes application deployment significantly more complex.

Python, on the other hand, embraces the differences between operating systems. Certainly where it makes sense, platform-independent interfaces are provided - just like Java, and every other modern language. But just because one platform doesn't deal in signals shouldn't mean that we can't use signals on Linux/UNIX.

I realise it's probably not the most common thing to do, but calling Python code from Java needn't be difficult. There's Jython, which makes available a large portion of the Python standard library, but still has some way to go. Some modules are missing (e.g. _winreg, ctypes), and others are incomplete or buggy. That's not to say that the project isn't useful, it's just not 100% there yet.

Enter Pushy (version 0.3). In version 0.3, a Java API was added to Pushy, making it possible to connect to a Python interpreter from a Java program, and access objects and invoke functions therein. So, for example, you now have the ctypes module available to your Java program, meaning you can load shared objects / DLLs and call arbitrary functions from them. See below for a code sample.

import pushy.Client;
import pushy.Module;
import pushy.PushyObject;

public class TestCtypes {
    public static void main(String[] args) throws Exception {
        Client client = new Client("local:");
        try {
            // Import the "ctypes" Python module, and get a reference to the
            // GetCurrentProcessId function in kernel32.
            Module ctypes = client.getModule("ctypes");
            PushyObject windll = (PushyObject)ctypes.__getattr__("windll");
            PushyObject kernel32 = (PushyObject)windll.__getattr__("kernel32");
            PushyObject GetCurrentProcessId =
                (PushyObject)kernel32.__getattr__("GetCurrentProcessId");

            // Call GetCurrentProcessId. Note that the Python interpreter is
            // running in a separate process, so this is NOT the process ID
            // running the Java program.
            Number pid = (Number)GetCurrentProcessId.__call__();
            System.out.println(pid);
        } finally {
            client.close();
        }
    }
}

Neat, eh? What I have demonstrated here is the following:

• Connecting a Java program to a freshly created Python interpreter on the local host.
• Importing the ctypes module therein, and then getting a reference to kernel32.dll (this assumes Windows, obviously. It would be much the same on Linux/UNIX platforms.)
• Executing the GetCurrentProcessId function to obtain the process ID of the Python interpreter, and returning the result as a java.lang.Number object.

The final point is an important one. Pushy automatically casts types from Python types to their Java equivalent. For example, a Python dictionary object will be returned as a java.util.Map. If that map is modified, then the changes will be made in the remote dictionary object also. Tuples, which are immutable, are returned as arrays, whilst Python lists are returned as java.util.List objects.

The Pushy Java API aims to provide Java standard library equivalent interfaces to Python standard library modules where possible. For example, there is a pushy.io package for dealing with files in the Python interpreter using  java.io.-like classes. Similarly, a pushy.net package exists for performing socket operations using a java.net-like interface.

One can also connect to SSH hosts, as is possible through the Pushy Python API. This is achieved by creating a connection to a local Python interpreter, and thence to the remote system using the Pushy Python API in the subprocess. This is all done transparently if a target other than "local:" is specified in the Java program.

Enjoy!