Mediator design pattern is one of the behavioral design pattern, so it deals with the behaviors of objects. Mediator design pattern is used to provide a centralized communication medium between different objects in a system.
According to GoF, mediator pattern intent is:
Allows loose coupling by encapsulating the way disparate sets of objects interact and communicate with each other. Allows for the actions of each object set to vary independently of one another.
Mediator design pattern is very helpful in an enterprise application where multiple objects are interacting with each other. If the objects interact with each other directly, the system components are tightly-coupled with each other that makes higher maintainability cost and not hard to extend. Mediator pattern focuses on provide a mediator between objects for communication and help in implementing lose-coupling between objects. Air traffic controller is a great example of mediator pattern where the airport control room works as a mediator for communication between different flights. Mediator works as a router between objects and it can have it’s own logic to provide way of communication. The system objects that communicate each other are called Colleagues. Usually we have an interface or abstract class that provides the contract for communication and then we have concrete implementation of mediators. For our example, we will try to implement a chat application where users can do group chat. Every user will be identified by it’s name and they can send and receive messages. The message sent by any user should be received by all the other users in the group.
First of all we will create Mediator interface that will define the contract for concrete mediators. ChatMediator.java
package com.journaldev.design.mediator;
public interface ChatMediator {
public void sendMessage(String msg, User user);
void addUser(User user);
}
Users can send and receive messages, so we can have User interface or abstract class. I am creating User as abstract class like below. User.java
package com.journaldev.design.mediator;
public abstract class User {
protected ChatMediator mediator;
protected String name;
public User(ChatMediator med, String name){
this.mediator=med;
this.name=name;
}
public abstract void send(String msg);
public abstract void receive(String msg);
}
Notice that User has a reference to the mediator object, it’s required for the communication between different users.
Now we will create concrete mediator class, it will have a list of users in the group and provide logic for the communication between the users. ChatMediatorImpl.java
package com.journaldev.design.mediator;
import java.util.ArrayList;
import java.util.List;
public class ChatMediatorImpl implements ChatMediator {
private List<User> users;
public ChatMediatorImpl(){
this.users=new ArrayList<>();
}
@Override
public void addUser(User user){
this.users.add(user);
}
@Override
public void sendMessage(String msg, User user) {
for(User u : this.users){
//message should not be received by the user sending it
if(u != user){
u.receive(msg);
}
}
}
}
Now we can create concrete User classes to be used by client system. UserImpl.java
package com.journaldev.design.mediator;
public class UserImpl extends User {
public UserImpl(ChatMediator med, String name) {
super(med, name);
}
@Override
public void send(String msg){
System.out.println(this.name+": Sending Message="+msg);
mediator.sendMessage(msg, this);
}
@Override
public void receive(String msg) {
System.out.println(this.name+": Received Message:"+msg);
}
}
Notice that send() method is using mediator to send the message to the users and it has no idea how it will be handled by the mediator.
Let’s test this our chat application with a simple program where we will create mediator and add users to the group and one of the user will send a message. ChatClient.java
package com.journaldev.design.mediator;
public class ChatClient {
public static void main(String[] args) {
ChatMediator mediator = new ChatMediatorImpl();
User user1 = new UserImpl(mediator, "Pankaj");
User user2 = new UserImpl(mediator, "Lisa");
User user3 = new UserImpl(mediator, "Saurabh");
User user4 = new UserImpl(mediator, "David");
mediator.addUser(user1);
mediator.addUser(user2);
mediator.addUser(user3);
mediator.addUser(user4);
user1.send("Hi All");
}
}
Notice that client program is very simple and it has no idea how the message is getting handled and if mediator is getting user or not. Output of the mediator pattern example program is:
Pankaj: Sending Message=Hi All
Lisa: Received Message:Hi All
Saurabh: Received Message:Hi All
David: Received Message:Hi All
That’s all for mediator design pattern and it’s implementation in java.
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This is something mix of mediator and visitor
- Fatih
nice explanation, but i can’t quite see the difference between mediator pattern and facade pattern, what is the difference between these two ?
- mohamed
hello,please help me i want to know, how use jms from mediator pattern? thankful
- fatemeh
Nice example.
- robothy
I knew at this and I can´t see the differences between mediator pattern and observe pattern
- David
Nice explanation.
- Annappa
Really crisp and good writeups
- Ashwin
how mediator pattern is implemented in ExecutorService API.Kindly explain
- Aditya
hi. tanck you for your nice tutorial.
- mohammad
Awesome explaination thanks :)
- Mohit Gupta