几天前我开始使用 Java RMI。我想知道以下示例是否正确同步。
考虑以下向客户端提供资源字符串的服务器类。它永远不会两次提供相同的资源,因此它将提供的字符串存储在列表中。这是 ServerEngine 类:
package dummy;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.UnicastRemoteObject;
import java.util.LinkedList;
public class ServerEngine implements Server {
private final String s1 = "Resource Object 1";
private final String s2 = "Resource Object 2";
private final LinkedList<String> list = new LinkedList<>();
private final int timer = 5000;
public static void main(String[] args) {
try {
String name = "server";
ServerEngine engine = new ServerEngine();
Server stub = (Server) UnicastRemoteObject.exportObject(engine, 0);
Registry registry = LocateRegistry.getRegistry();
registry.rebind(name, stub);
System.out.println("ServerEngine bound");
} catch (Exception e) {
System.err.println("ServerEngine exception:");
}
}
@Override
public String getResource() throws RemoteException {
Object lock = new Object();
if ( ! list.contains(s1)) {
synchronized (lock) {
// wait to ensure concurrency
try {
lock.wait(timer);
} catch (InterruptedException ex) {}
}
list.add(s1);
return s1;
}
if ( ! list.contains(s2)) {
list.add(s2);
return s2;
}
return null;
}
}
服务器接口(interface):
package dummy;
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface Server extends Remote {
public String getResource(boolean synced) throws RemoteException;
}
和客户:
package dummy;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
public class Client {
public static void main(String[] args) {
try {
String name = "server";
Registry registry = LocateRegistry.getRegistry();
Server server = (Server) registry.lookup(name);
boolean sync = args.length > 0;
String s = server.getResource(sync);
System.out.println("Resource: " + s);
} catch (Exception e) {
System.err.println("Client exception:");
}
}
}
ServerEngine 的实现方式会导致并发问题。如果在五秒内从两个不同的虚拟机启动两个客户端,那么它们都将获得相同的返回字符串。
根据我迄今为止的研究,这是我解决问题的方法:
package dummy;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.UnicastRemoteObject;
import java.util.LinkedList;
public class ServerEngine implements Server {
private final String s1 = "Resource Object 1";
private final String s2 = "Resource Object 2";
private final LinkedList<String> list = new LinkedList<>();
private final int timer = 5000;
public static void main(String[] args) {
try {
String name = "server";
ServerEngine engine = new ServerEngine();
Server stub = (Server) UnicastRemoteObject.exportObject(engine, 0);
Registry registry = LocateRegistry.getRegistry();
registry.rebind(name, stub);
System.out.println("ServerEngine bound");
} catch (Exception e) {
System.err.println("ServerEngine exception:");
}
}
private synchronized String localGetResource() {
Object lock = new Object();
if ( ! list.contains(s1)) {
synchronized (lock) {
// wait to ensure concurrency
try {
lock.wait(timer);
} catch (InterruptedException ex) {}
}
list.add(s1);
return s1;
}
if ( ! list.contains(s2)) {
list.add(s2);
return s2;
}
return null;
}
@Override
public String getResource() throws RemoteException {
return localGetResource();
}
}
我想知道这是否是一个可行的解决方案。有什么注意事项吗?我真的需要第二个函数还是可以直接同步 getResource() ?
最佳答案
您的同步在多个级别上被破坏:
- 除非您希望其他线程
notify()
您,否则您不应该wait()
某些事情。 - 您只实现了 double-checked locking 的一半,翻译为“无锁定”,因为相同的值可能会多次出现在列表中。
- 您应该查看
java.util.concurrent
下正确的线程安全集合实现,而不是手动执行此操作。
关于java - RMI:多个客户端访问服务器时正确同步,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/47231982/