来自Java Runnable

时间:2017-12-02 11:33:01

标签: java python multithreading py4j

我目前正在尝试使用Py4J执行以下操作:

  • 在Python中定义一个调用JVM方法的方法(“executor”)
  • 定义实现JVM接口的Python(“回调”)对象
  • 在给定此回调对象
  • 的情况下构造JVM对象
  • 在此对象上调用一个方法,该方法将在Java中生成一个新线程,在回调对象上调用回调,这将(在Python端)执行“executor”方法

这就是我在Java方面所拥有的东西:

package javabridge.test;
public interface PythonCallback {
    Object notify(Object source);
}


package javabridge.test;
public class ScheduledRunnable implements Runnable {
    private PythonCallback callback;
    public ScheduledRunnable(PythonCallback callback) {
        this.callback = callback;
    }
    @Override
    public void run() {
        System.out.println("[ScheduledRunnable] run -> notify");
        this.callback.notify(this);
    }
}


package javabridge.test;
import py4j.GatewayServer;
public class Test {
    private PythonCallback callback;
    public Test(PythonCallback callback) {
        this.callback = callback;
    }
    public void runSynchronous() {
        System.out.println("[runSynchronous] run -> notify");
        this.callback.notify(this);
    }
    public void runAsynchronous() {
        System.out.println("[runAsynchronous] run -> spawn thread");
        ScheduledRunnable runnable = new ScheduledRunnable(callback);
        Thread t = new Thread(runnable);
        t.start();
    }
    public static void main(String[] args) {
        GatewayServer server = new GatewayServer();
        server.start(true);
    }   
}

在Python方面,我有以下脚本:

from py4j.java_gateway import JavaGateway, java_import, get_field, CallbackServerParameters
from py4j.clientserver import ClientServer, JavaParameters, PythonParameters

gateway = JavaGateway(callback_server_parameters=CallbackServerParameters())
#gateway = ClientServer(java_parameters=JavaParameters(), python_parameters=PythonParameters())

java_import(gateway.jvm, 'javabridge.test.*')

class PythonCallbackImpl(object):
    def __init__(self, execfunc):
        self.execfunc = execfunc
    def notify(self, obj):
        print('[PythonCallbackImpl] notified from Java')
        self.execfunc()
        return 'dummy return value'
    class Java:
        implements = ["javabridge.test.PythonCallback"]

def simple_fun():
    print('[simple_fun] called')
    gateway.jvm.System.out.println("[simple_fun] Hello from python!")

# Test 1: Without threading
input('Ready to begin test 1')
python_callback = PythonCallbackImpl(simple_fun)
nothread_executor = gateway.jvm.Test(python_callback)
nothread_executor.runSynchronous()

# Test 2: With threading
input('Ready to begin test 2')
python_callback = PythonCallbackImpl(simple_fun)
nothread_executor = gateway.jvm.Test(python_callback)
nothread_executor.runAsynchronous()

gateway.shutdown()

以下是尝试执行此脚本时发生的情况。首先,使用gateway = ClientServer(java_parameters=JavaParameters(), python_parameters=PythonParameters()),两个测试都失败了:

Test 1:

py4j.protocol.Py4JJavaError: An error occurred while calling o0.runSynchronous.
: py4j.Py4JException: Command Part is Empty or is the End of Command Part
        at py4j.Protocol.getObject(Protocol.java:277)
        at py4j.Protocol.getReturnValue(Protocol.java:458)

Test 2:

Exception in thread "Thread-4" py4j.Py4JException: Error while obtaining a new communication channel
        at py4j.CallbackClient.getConnectionLock(CallbackClient.java:218)
        at py4j.CallbackClient.sendCommand(CallbackClient.java:337)
        at py4j.CallbackClient.sendCommand(CallbackClient.java:316)

但是,如果我注释掉self.execfunc()行,则测试1可以正常工作而不会出现错误。然而,测试2仍然失败:

Exception in thread "Thread-5" py4j.Py4JException: Error while sending a command.
        at py4j.CallbackClient.sendCommand(CallbackClient.java:357)
        at py4j.CallbackClient.sendCommand(CallbackClient.java:316)

现在切换到gateway = JavaGateway(callback_server_parameters=CallbackServerParameters())。当我保持self.execfunc()注释时,测试2仍然在这里失败:

Exception in thread "Thread-5" py4j.Py4JException: Error while sending a command.
        at py4j.CallbackClient.sendCommand(CallbackClient.java:357)
        at py4j.CallbackClient.sendCommand(CallbackClient.java:316)

但至少测试1适用于启用self.execfunc()

我的问题是:如何在self.execfunc()调用时使用线程方法?这可能与Py4J有关吗?

编辑:为了使事情变得更加棘手,self.execfunc()调用的Java命令应该在调用.notify()的同一Java线程中运行。

1 个答案:

答案 0 :(得分:2)

解决。结果很简单:

  1. 在Python端和Java端使用ClientServer!
  2. 不要调用gateway.shutdown(),因为这会在收到回调之前断开Python(呃!)
  3. 然后,Java将完全遵循预期的线程模型,即接收Python回调调用的Java命令在执行回调的同一Java线程中执行。

    通过一个简单的Python函数,可以添加shutdown_when_done方法,等待所有回调在退出之前回来。