Java RMI refers to Remote Method Invocation. It is a mechanism that allows an object in one Java virtual machine to call methods on an object in another Java virtual machine. Any object that can be called with this method must implement the remote interface. (Recommended: java video tutorial)

Java RMI is not a new technology (it was available in the era of Java 1.1), but it is a very important underlying technology.

The famous EJBs are all based on rmi. There are also some open source remote calling components, and their underlying technology is also rmi.

In the era of vigorously advocating Web Service and SOA, should every application use clumsy Web Service components to implement it? After comparative testing, RMI is the simplest. In some small applications, it is most suitable.

The following uses a simple example to illustrate the principles and applications of RMI. The following example is a simple HelloWorld, but it covers the core applications and development models of RMI.

/**
* Created by IntelliJ IDEA.
* User: leizhimin
* Date: 2008-8-7 21:50:02
* 定義一個(gè)遠(yuǎn)程接口，必須繼承Remote接口，其中需要遠(yuǎn)程調(diào)用的方法必須拋出RemoteException異常
*/
public interface IHello extends Remote {

    /**
     * 簡(jiǎn)單的返回“Hello World！"字樣
     * @return 返回“Hello World！"字樣
     * @throws java.rmi.RemoteException
     */
    public String helloWorld() throws RemoteException;

    /**
     * 一個(gè)簡(jiǎn)單的業(yè)務(wù)方法，根據(jù)傳入的人名返回相應(yīng)的問候語
     * @param someBodyName  人名
     * @return 返回相應(yīng)的問候語
     * @throws java.rmi.RemoteException
     */
    public String sayHelloToSomeBody(String someBodyName) throws RemoteException;
}

/**
* Created by IntelliJ IDEA.
* User: leizhimin
* Date: 2008-8-7 21:56:47
* 遠(yuǎn)程的接口的實(shí)現(xiàn)
*/
public class HelloImpl extends UnicastRemoteObject implements IHello {
    /**
     * 因?yàn)閁nicastRemoteObject的構(gòu)造方法拋出了RemoteException異常，因此這里默認(rèn)的構(gòu)造方法必須寫，必須聲明拋出RemoteException異常
     *
     * @throws RemoteException
     */
    public HelloImpl() throws RemoteException {
    }

    /**
     * 簡(jiǎn)單的返回“Hello World！"字樣
     *
     * @return 返回“Hello World！"字樣
     * @throws java.rmi.RemoteException
     */
    public String helloWorld() throws RemoteException {
        return "Hello World!";
    }

    /**
     * 一個(gè)簡(jiǎn)單的業(yè)務(wù)方法，根據(jù)傳入的人名返回相應(yīng)的問候語
     *
     * @param someBodyName 人名
     * @return 返回相應(yīng)的問候語
     * @throws java.rmi.RemoteException
     */
    public String sayHelloToSomeBody(String someBodyName) throws RemoteException {
        return "你好，" + someBodyName + "!";
    }
}

/**
* Created by IntelliJ IDEA.
* User: leizhimin
* Date: 2008-8-7 22:03:35
* 創(chuàng)建RMI注冊(cè)表，啟動(dòng)RMI服務(wù)，并將遠(yuǎn)程對(duì)象注冊(cè)到RMI注冊(cè)表中。
*/
public class HelloServer {
    public static void main(String args[]) {

        try {
            //創(chuàng)建一個(gè)遠(yuǎn)程對(duì)象
            IHello rhello = new HelloImpl();
            //本地主機(jī)上的遠(yuǎn)程對(duì)象注冊(cè)表Registry的實(shí)例，并指定端口為8888，這一步必不可少（Java默認(rèn)端口是1099），必不可缺的一步，缺少注冊(cè)表創(chuàng)建，則無法綁定對(duì)象到遠(yuǎn)程注冊(cè)表上
            LocateRegistry.createRegistry(8888);

            //把遠(yuǎn)程對(duì)象注冊(cè)到RMI注冊(cè)服務(wù)器上，并命名為RHello
            //綁定的URL標(biāo)準(zhǔn)格式為：rmi://host:port/name(其中協(xié)議名可以省略，下面兩種寫法都是正確的）
            Naming.bind("rmi://localhost:8888/RHello",rhello);
//            Naming.bind("//localhost:8888/RHello",rhello);

            System.out.println(">>>>>INFO:遠(yuǎn)程IHello對(duì)象綁定成功！");
        } catch (RemoteException e) {
            System.out.println("創(chuàng)建遠(yuǎn)程對(duì)象發(fā)生異常！");
            e.printStackTrace();
        } catch (AlreadyBoundException e) {
            System.out.println("發(fā)生重復(fù)綁定對(duì)象異常！");
            e.printStackTrace();
        } catch (MalformedURLException e) {
            System.out.println("發(fā)生URL畸形異常！");
            e.printStackTrace();
        }
    }
}

/**
* Created by IntelliJ IDEA.
* User: leizhimin
* Date: 2008-8-7 22:21:07
* 客戶端測(cè)試，在客戶端調(diào)用遠(yuǎn)程對(duì)象上的遠(yuǎn)程方法，并返回結(jié)果。
*/
public class HelloClient {
    public static void main(String args[]){
        try {
            //在RMI服務(wù)注冊(cè)表中查找名稱為RHello的對(duì)象，并調(diào)用其上的方法
            IHello rhello =(IHello) Naming.lookup("rmi://localhost:8888/RHello");
            System.out.println(rhello.helloWorld());
            System.out.println(rhello.sayHelloToSomeBody("熔巖"));
        } catch (NotBoundException e) {
            e.printStackTrace();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (RemoteException e) {
            e.printStackTrace();  
        }
    }
}

Run the RMI server program:

Run the RMI client program:

Summary :

From the above process, RMI is very dependent on the IP address and port of the server. However, during development, we do not know what the future server IP and port will be, but the client program depends on this IP and port. .

This is also one of the limitations of RMI. There are two ways to solve this problem: one is to solve it through DNS, and the other is to expose the IP outside the program code through encapsulation.

The second limitation of RMI is that RMI is a remote call in Java language. The programming language at both ends must be implemented in Java. For communication between different languages, you can consider using Web Service or Common Object Request Agency (CORBA). to fulfill.

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