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This article mainly introduces "how to use RMI of Java". In daily operation, I believe many people have doubts about how to use RMI of Java. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "how to use RMI of Java". Next, please follow the editor to study!

RMI introduction

RMI (Remote Method Invocation) model is a kind of distributed object application. Using RMI technology, an object in one JVM can call an object method in another JVM and get the result of the call. The other JVM here can be on the same computer or on a remote computer. Therefore, RMI means that you need a Server side and a Client side.

The Server side usually creates an object and makes it remotely accessible.

This object is called a remote object. The Server side needs to register that this object can be accessed remotely by Client.

The Client side calls the methods on the objects that can be accessed remotely, and the Client side can communicate with the Server side and transmit information to each other.

At this point, it is found that it is very convenient to use RMI to build a distributed application. Like RPC, it can communicate with each other between distributed applications, and even is very similar to the current micro-service idea.

How RMI works

As the saying goes, before you start writing RMI code, it is necessary to understand how RMI works and how the Client side communicates with the Server side in RMI.

The following figure can help us understand the workflow of RMI.

As you can see from the figure, there is something called Stub on the Client side, which is sometimes called a stub. It is the proxy object of RMI Client. The main function of Stub is to construct an information block when requesting remote methods, and the RMI protocol will send this information block to the Server side.

This block of information consists of several parts:

Remote object identifier.

The method description of the call.

Parameter values after marshalling (object serialization is used in the RMI protocol).

Since there is a Stub on the Client side that can construct a block of information to send to the Server side, then the Server side must have an object that receives this information quickly, called Skeleton.

Its main work is as follows:

Parse the calling object identifier and method description in the information fast, and call the specific object method on the Server side.

Gets the return value or outlier of the call.

The return value is grouped and returned to the client Stub.

At this point, the result of a call from the Client side to the Server side can be obtained.

RMI development

Through the above introduction, we know the concept of RMI and how RMI works, and the following describes the development process of RMI.

This will be demonstrated through a scenario, assuming that the Client side needs to query user information, and the user information exists on the Server side, so the RMI protocol interface is opened on the Server side for the client to call the query.

RMI Server

The main purpose of the Server side is to build a class User that can be transferred and a class UserService that can be accessed remotely. At the same time, this object should be registered to RMI and open to the client.

1. Define the server interface (you need to inherit the Remote class, and the method needs to throw RemoteException).

Package com.wdbyte.rmi.server;import java.rmi.Remote;import java.rmi.RemoteException;/** * RMI Server * * @ author www.wdbyte.com * @ date 2021-05-08 * / public interface UserService extends Remote {/ * find users * * @ param userId * @ return * @ throws RemoteException * / User findUser (String userId) throws RemoteException;}

The User object is defined in step 3.

two。 Implement the server interface (you need to inherit the UnicastRemoteObject class and implement the defined interface).

Package com.wdbyte.rmi.server;import java.rmi.RemoteException;import java.rmi.server.UnicastRemoteObject;/** * @ author www.wdbyte.com * @ date 2021-05-08 * / public class UserServiceImpl extends UnicastRemoteObject implements UserService {protected UserServiceImpl () throws RemoteException {} @ Override public User findUser (String userId) throws RemoteException {/ / loaded in query if ("00001" .equals (userId)) {User user = new User () User.setName (Jin Yong); user.setAge; user.setSkill (Writing); return user;} throw new RemoteException (check No such person);}}

3. Define the object to be transferred, which needs to implement the Serializable interface.

Classes that need to be transferred must implement the serialization interface, otherwise an error will be reported during the transfer. A simple tutorial on how to generate serialVersionUID in IDEA is attached at the end of the article.

Package com.wdbyte.rmi.server;import java.io.Serializable;/** @ author www.wdbyte.com * @ date 2021-05-08 * / public class User implements Serializable {private static final long serialVersionUID = 6490921832856589236L; private String name; private Integer age; private String skill; public String getName () {return name;} public void setName (String name) {this.name = name } public Integer getAge () {return age;} public void setAge (Integer age) {this.age = age;} public String getSkill () {return skill;} public void setSkill (String skill) {this.skill = skill } @ Override public String toString () {return "User {" + "name='" + name +'+ ", age=" + age + ", skill='" + skill +''+'}';}}

4. Register (rmiregistry) the remote object and start the server program.

The server binds the UserService object as the object for remote access, and the port is set to 1900 at startup.

Package com.wdbyte.rmi.server;import java.rmi.Naming;import java.rmi.registry.LocateRegistry;/** * RMI Server side * * @ author https://www.wdbyte.com * @ date 2021-05-08 * / public class RmiServer {public static void main (String [] args) {try {UserService userService = new UserServiceImpl (); LocateRegistry.createRegistry (1900) Naming.rebind ("rmi://localhost:1900/user", userService); System.out.println ("start server,port is 1900");} catch (Exception e) {e.printStackTrace ();} RMI Client

Compared with the Server side, the Client side is much simpler. Classes that are remotely accessible and need to be transferred are introduced directly, and a call can be made through the port and the address bound to the Server side.

Package com.wdbyte.rmi.client;import java.rmi.Naming;import com.wdbyte.rmi.server.User;import com.wdbyte.rmi.server.UserService;/** * @ author https://www.wdbyte.com * @ date 2021-05-08 * / public class RmiClient {public static void main (String args []) {User answer; String userId = "00001" Try {/ / lookup method to find reference of remote object UserService access = (UserService) Naming.lookup ("rmi://localhost:1900/user"); answer = access.findUser (userId); System.out.println ("query:" + userId); System.out.println ("result:" + answer);} catch (Exception ae) {System.out.println (ae) }} RMI Test

Start the Server side.

Start server,port is 1900

Start the Client side.

Query:00001result:User {name=' Jin Yong', age=100, skill=' Writing'}

If the Client side passes in a userId that does not exist.

Java.rmi.ServerException: RemoteException occurred in server thread; nested exception is: java.rmi.RemoteException: check the generation of serialVersionUID for this person

Generate serialVersionUID in IDEA, open the settings, and check it as shown in the following figure.

Select the class for which you want to generate serialVersionUID and press the smart prompt shortcut.

At this point, the study on "how to use the RMI of Java" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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