理解Java RMI 一篇就够

Java RMI是什么

Java RMI(Java Remote Method Invocation),即Java远程方法调用。是Java编程语言里,一种用于实现远程过程调用的应用程序编程接口

注:很多文章或博客把RMI说成是一种消息协议,官方定义是java 编程接口。

RMI 使用 JRMP(Java Remote Message Protocol,Java远程消息交换协议)实现,使得客户端运行的程序可以调用远程服务器上的对象。是实现RPC的一种方式。

RMI 的使用

1、server端:创建远程对象,并注册远程对象

//定义远程对象的接口
public interface HelloService extends Remote {
    String say() throws RemoteException;
}

//接口的实现
public class HelloServiceImpl extends UnicastRemoteObject implements HelloService {

    public HelloServiceImpl() throws RemoteException{
        super();
    }

    @Override
    public String say() throws RemoteException {
        return "Hello";
    }
}

//注册远程对象
public class Service {
    public static void main(String[] args) throws RemoteException, AlreadyBoundException, MalformedURLException {
        HelloServiceImpl helloService = new HelloServiceImpl();
        LocateRegistry.createRegistry(1099);
        Naming.bind("rmi://127.0.0.1/hello",helloService);

    }
}

2、client端:查找远程对象,调用远程方法

public class Client {
    public static void main(String[] args) throws RemoteException, NotBoundException, MalformedURLException {
        HelloService helloService = (HelloService) Naming.lookup("rmi://127.0.0.1/hello");
        System.out.println(helloService.say());

    }
}

RMI 的原理

RMI本质是TCP网络通信,内部封装了序列化和通信过程,使用代理实现接口调用。下一篇文章带大家手写一个RPC框架,会更加清晰的明白RMI原理。


调用过程

1、服务端

//调用UnicastRemoteObject构造函数,发布对象
  protected UnicastRemoteObject(int port) throws RemoteException
    {
        this.port = port;
        exportObject((Remote) this, port);
    }

//创建UnicastServerRef对象,对象内有引用LiveRef(tcp通信)
    public static Remote exportObject(Remote obj, int port)
        throws RemoteException
    {
        return exportObject(obj, new UnicastServerRef(port));
    }


   public Remote exportObject(Remote var1, Object var2, boolean var3) throws RemoteException {
        Class var4 = var1.getClass();

        Remote var5;
        try {
//创建远程代理类,getClientRef提供的InvocationHandler提供了TCP连接
            var5 = Util.createProxy(var4, this.getClientRef(), this.forceStubUse);
        } catch (IllegalArgumentException var7) {
            throw new ExportException("remote object implements illegal remote interface", var7);
        }

        if (var5 instanceof RemoteStub) {
            this.setSkeleton(var1);
        }
//包装实际对象,并将其暴露在TCP端口上,等待客户端调用
        Target var6 = new Target(var1, this, var5, this.ref.getObjID(), var3);
        this.ref.exportObject(var6);
        this.hashToMethod_Map = (Map)hashToMethod_Maps.get(var4);
        return var5;
    }
时序图

2、客户端

//客户端通过LocateRegistry的getRegistry方法创建RegistryImpl_Stub代理,调用RegistryImpl_Stub的newCall方法建立与服务端Skeleton的映射
public static Registry getRegistry(String host, int port,
                                       RMIClientSocketFactory csf)
        throws RemoteException
    {
        Registry registry = null;

        if (port <= 0)
            port = Registry.REGISTRY_PORT;

        if (host == null || host.length() == 0) {
            // If host is blank (as returned by "file:" URL in 1.0.2 used in
            // java.rmi.Naming), try to convert to real local host name so
            // that the RegistryImpl's checkAccess will not fail.
            try {
                host = java.net.InetAddress.getLocalHost().getHostAddress();
            } catch (Exception e) {
                // If that failed, at least try "" (localhost) anyway...
                host = "";
            }
        }

//调用RegistryImpl_Stub的lookup方法时,看似本地调用,实则通过tcp连接发送消息到服务端
 public Remote lookup(String var1) throws AccessException, NotBoundException, RemoteException {
        try {
            RemoteCall var2 = super.ref.newCall(this, operations, 2, 4905912898345647071L);

            try {
                ObjectOutput var3 = var2.getOutputStream();
                var3.writeObject(var1);
            } catch (IOException var18) {
                throw new MarshalException("error marshalling arguments", var18);
            }

            super.ref.invoke(var2);

            Remote var23;
            try {
                ObjectInput var6 = var2.getInputStream();
                var23 = (Remote)var6.readObject();
            } catch (IOException var15) {
                throw new UnmarshalException("error unmarshalling return", var15);
            } catch (ClassNotFoundException var16) {
                throw new UnmarshalException("error unmarshalling return", var16);
            } finally {
                super.ref.done(var2);
            }

            return var23;
        } catch (RuntimeException var19) {
            throw var19;
        } catch (RemoteException var20) {
            throw var20;
        } catch (NotBoundException var21) {
            throw var21;
        } catch (Exception var22) {
            throw new UnexpectedException("undeclared checked exception", var22);
        }
    }

RMI 的优劣

1、优势
给分布计算的系统设计、编程都带来了极大的方便。只要按照RMI规则设计程序,可以不必再过问在RMI之下的网络细节了,如:TCP和Socket等等。任意两台计算机之间的通讯完全由RMI负责。调用远程计算机上的对象就像本地对象一样方便。
2、劣势
RMI对服务器的IP地址和端口依赖很紧密,但是在开发的时候不知道将来的服务器IP和端口如何,而客户端程序又依赖这个IP和端口。即客户端如何维护服务端地址和动态感知服务端地址变化的问题。
另一局限性是,RMI是Java语言的远程调用,两端的程序语言必须是Java实现。