Spring实现RMI调用

传统的实现RMI，需要
1.服务接口必须从Remote派生，每个方法抛出RemoteException
2.实现类必须从UnicastRemoteObject派生
3.所有方法的参数和返回值，必须是基本类型，或者实现了Serializable接口

public class User implements Serializable {
  private String username;
  private String password;
  public User(String username, String password) {
    this.username = username;
    this.password = password;
  }
  public String getUsername() {
    return username;
  }
  public String getPassword() {
    return password;
  }
}
public interface RmiUserService extends Remote {
  User login(String username, String password) throws RemoteException;
  void create(String username, String password) throws RemoteException;
}
public class RmiUserServiceImpl extends UnicastRemoteObject implements RmiUserService {
  protected RmiUserServiceImpl() throws RemoteException {
  }
  private Map<String, String> users = new HashMap<String, String>();
  public void create(String username, String password) {
    if (username == null || password == null)
      throw new IllegalArgumentException("Invalid args.");
    if (users.get(username) != null)
      throw new RuntimeException("User exist!");
    users.put(username, password);
  }
  public User login(String username, String password) {
    if (username == null || password == null)
      throw new IllegalArgumentException("Invalid args.");
    if (password.equals(users.get(username)))
      return new User(username, password);
    throw new RuntimeException("Login failed.");
  }
  public static void main(String[] args) throws RemoteException, MalformedURLException, AlreadyBoundException {
    LocateRegistry.createRegistry(1099);
    Naming.bind("rmi://localhost:1099/UserService", new RmiUserServiceImpl());
  }
}
public class Client {
  public static void main(String[] args) throws RemoteException, MalformedURLException, NotBoundException {
    RmiUserService service = (RmiUserService) Naming.lookup("rmi://localhost:1099/UserService");
    service.create("xace", "1");
    System.out.println(service.login("xace", "1"));
  }
}

调用：
>rmic RmiUserServiceImpl
>java Client

Spring对RMI提供的支持
不用写一行代码，直接在Spring的配置文件中声明，就可以将一个传统的java类作为RMI服务输出

  <bean id="userService" class="example.rmi.UserServiceImpl" />

  <bean id="rmiService" class="org.springframework.remoting.rmi.RmiServiceExporter">
    <property name="serviceName" value="UserService"/>
    <property name="service" ref="userService"/>
    <property name="serviceInterface" value="example.rmi.UserService"/>
    <property name="registryPort" value="1099"/>
  </bean>

唯一编写的代码是main()方法，启动Spring容器

    public static void main(String[] args) {
        new ClassPathXmlApplicationContext("config.xml");
    }

客户端代码

public class Client {
  public static void main(String[] args) throws Exception {
    RmiProxyFactoryBean factory = new RmiProxyFactoryBean();
    factory.setServiceInterface(UserService.class);
    factory.setServiceUrl("rmi://localhost:1099/UserService");
    factory.afterPropertiesSet();
    UserService service = (UserService) factory.getObject();
    service.create("test", "password");
    System.out.println(service.login("test", "password"));
    try {
      service.login("test", "bad-password");
    } catch (Exception e) {
      System.out.println(e.getMessage());
    }
  }
}

如果客户端也在Spring容器中启动，完全可以在XML配置文件中定义UserService并直接使用

  <bean id="userServiceRmi" class="org.springframework.remoting.rmi.RmiProxyFactoryBean">
    <property name="serviceUrl" value="rmi://localhost:1099/UserService" />
    <property name="serviceInterface" value="example.rmi.UserService" />
  </bean>

RMI虽然是Java标准的远程调用模式，但是它使用特定的Java Remote Method Protocol二进制协议，很难穿透防火墙，如果要跨防火墙，应该使用HTTP协议为基础的远程调用。
Hessian; Burlap; Spring HTTP Invoker。使用私有协议的Http远程调用没有成为标准，实际应用较少，只能用于Java应用程序之间的远程调用。如果希望和异构平台实现远程调用，就必须使用标准的Web服务（Web Services）