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Welcome to Py4J

Py4J enables Python programs running in a Python interpreter to dynamically access Java objects in a Java Virtual Machine. Methods are called as if the Java objects resided in the Python interpreter and Java collections can be accessed through standard Python collection methods. Py4J also enables Java programs to call back Python objects. Py4J is distributed under the BSD license.

Here is a brief example of what you can do with Py4J. The following Python program creates a java.util.Random instance from a JVM and calls some of its methods. It also accesses a custom Java class, AdditionApplication to add the generated numbers.

>>> from py4j.java_gateway import JavaGateway
>>> gateway = JavaGateway()                   # connect to the JVM
>>> random = gateway.jvm.java.util.Random()   # create a java.util.Random instance
>>> number1 = random.nextInt(10)              # call the Random.nextInt method
>>> number2 = random.nextInt(10)
>>> print(number1,number2)
(2, 7)
>>> addition_app = gateway.entry_point        # get the AdditionApplication instance
>>> addition_app.addition(number1,number2)    # call the addition method
9

This is the Java program that was executing at the same time (no code was generated and no tool was required to run these programs). The AdditionApplication app instance is the gateway.entry_point in the previous code snippet. Note that the Java program must be started before executing the Python code above. In other words, the Py4J does not start a JVM.

public class AdditionApplication {

  public int addition(int first, int second) {
    return first + second;
  }

  public static void main(String[] args) {
    AdditionApplication app = new AdditionApplication();
    // app is now the gateway.entry_point
    GatewayServer server = new GatewayServer(app);
    server.start();
  }
}

Resources

News

  • December 26th 2013 - Py4J 0.8.1 has been released. See the Changelog for more details about the new features.
  • June 14th 2013 - Py4J 0.8 has been released. See the Changelog for more details about the new features.
  • June 2nd 2011 - Py4J 0.7 has been released. See the Changelog for more details about the new features.
  • February 17th 2011 - Py4J 0.6 has been released. See the Changelog for more details about the new features.
  • October 30th 2010 - Py4J 0.5 has been released. See the Changelog for more details about the new features.
  • September 19th 2010 - Py4J 0.4 has been released. See the Changelog for more details about the new features.
  • April 27th 2010 - Py4J 0.3 has been released. See the Changelog for more details about the new features!
  • February 11th 2010 - Py4J 0.2 has been released. See the Changelog for more details about all the new features that found their way in the latest release!
  • December 23rd 2009 - Py4J 0.1 has been released. Rejoice!
  • December 11th 2009 - Py4J is still in the planning phase, but the code currently works for basic scenarios. A release and a tutorial should be available in the following weeks.