Monitor

Intent

Monitor pattern is used to create thread-safe objects and prevent conflicts between threads in concurrent applications.

Explanation

In plain words

Monitor pattern is used to enforce single-threaded access to data. Only one thread at a time is allowed to execute code within the monitor object.

Wikipedia says

In concurrent programming (also known as parallel programming), a monitor is a synchronization construct that allows threads to have both mutual exclusion and the ability to wait (block) for a certain condition to become false. Monitors also have a mechanism for signaling other threads that their condition has been met.

Programmatic Examples

Consider there is a bank that transfers money from an account to another account with transfer method . it is synchronized mean just one thread can access to this method because if many threads access to it and transfer money from an account to another account in same time balance changed !

class Bank {

     private int[] accounts;
     Logger logger;
 
     public Bank(int accountNum, int baseAmount, Logger logger) {
         this.logger = logger;
         accounts = new int[accountNum];
         Arrays.fill(accounts, baseAmount);
     }
 
     public synchronized void transfer(int accountA, int accountB, int amount) {
         if (accounts[accountA] >= amount) {
             accounts[accountB] += amount;
             accounts[accountA] -= amount;
             logger.info("Transferred from account :" + accountA + " to account :" + accountB + " , amount :" + amount + " . balance :" + getBalance());
         }
     }

getBalance always return total amount and the total amount should be same after each transfers

     private synchronized int getBalance() {
         int balance = 0;
         for (int account : accounts) {
             balance += account;
         }
         return balance;
     }
 }

Class diagram

Applicability

Use the Monitor pattern when

• we have a shared resource and there is critical section .
• you want to create thread-safe objects .
• you want to achieve mutual exclusion in high level programming language .