I want to call java from python with Py4J library,
from py4j.java_gateway import JavaGateway
gateway = JavaGateway() # connect to the JVM
gateway.jvm.java.lang.System.out.println('Hello World!')
I've got the following error: "Py4JNetworkError: An error occurred while trying to connect to the Java server". It's seems that no JVM is running, how to fix that?
Minimal working example:
//AdditionApplication.java
import py4j.GatewayServer;
public class AdditionApplication {
public static void main(String[] args) {
AdditionApplication app = new AdditionApplication();
// app is now the gateway.entry_point
GatewayServer server = new GatewayServer(app);
server.start();
}
}
Compile (make sure that the -cp path to the py4j is valid, otherwise adjust it such that it points to the right place):
javac -cp /usr/local/share/py4j/py4j0.9.jar AdditionApplication.java
Run it:
java -cp .:/usr/local/share/py4j/py4j0.9.jar AdditionApplication
Now, if you run your python script, in the terminal where the java AdditionApplication is running you should see something like:
>>> Hello World!
; not a : in your class path.
: on UNIX or ; on Windows. A little bit more information on the topic can be found here.package test.test;
import py4j.GatewayServer;
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();
}
}
create a new class and run it(import py4j0.8.jar at 'py4j-0.8\py4j-0.8\py4j-java' first),then run python program
You should first start the java program, and then invoke java method from python.
py4j doesn't start jvm, it just connects to the already started java process.
