使用Java RMI，为什么远程方法调用会产生客户端的时区而不是服务器的时区？

Question

I'm trying to grasp the basics of Java RMI.

The idea is to have two separate processes running, each primed by a separate main in one of two classes:

• Client running as a client
• Server running as a server

A third class Document acts as the shared object (it being a simple collector of Strings ), with its method addTimestamp() saving the current Timestamp for further printing in the console. This method is also one available as a remote method, as defined in DocumentInterface .

Executing the two mains on two different cmd windows yields a perfectly functioning system:

• Server : java -cp rmi.jar Server

• Client : java -cp rmi.jar Client

As expected the client's output is:

CLIENT - Viewed on: 2016.09.30 01.53.01

SERVER - Viewed on: 2016.09.30 01.53.01

When I start the server under a different timezone though:

• Server : java -Duser.timezone=PST -cp rmi.jar Server

• Client : java -cp rmi.jar Client

I still get the original client output:

CLIENT - Viewed on: 2016.09.30 01.53.01

SERVER - Viewed on: 2016.09.30 01.53.01

I would expect the second line to have the server's PST-based Timestamp . I checked the flag was set correctly by having it printed directly by the server's main, and it is indeed different:

2016.09.29 16.55.57

From what I've understood so far, when calling the addTimestamp() method remotely on the remote object:

• the current Document is passed by copy to the server
• the Timezone is appended by the server, using its instance of the Document class
• the returned Document is passed by copy back to the client
• the Document is displayed by the client

In this case I would thus expect that Timezone to be based on the server's settings, not the client's. Why is this not the case?

Here are some code snippets from the four classes:

Document.java :

public Document addTimestamp(Document document) throws RemoteException
    {
    String timestamp = new SimpleDateFormat("yyyy.MM.dd HH.mm.ss").format(new Date());

    document.strings.add("Viewed on: "+timestamp);

    return document;
    }

DocumentInterface.java :

public interface DocumentInterface extends Remote
    {
    public Document addTimestamp(Document document) throws RemoteException;
    }

Server.java - main :

Registry registry = LocateRegistry.createRegistry(1099);
Document document = new Document();
Naming.bind("rmi:///Document", document);

Client.java - main :

Document document = new Document();
DocumentInterface remoteDocument;
try
    {
    remoteDocument = (DocumentInterface) Naming.lookup("rmi:///Document");

    document.addString("USER - ");
    document.addTimestamp(document);
    document.addString("\n");
    document.addString("SERVER - ");
    document = remoteDocument.addTimestamp(document);

    System.out.println(document.toString());
    }
catch (Exception except)
    {
    }

Answer 1

Because Document is an exported remote object, it is running as a callback at the client. Not by copy at the server.

Remove the remote interface and extends UnicastRemoteObject , and make it serializable.