JMM(Java Memory Model)

              Java is an object-oriented programming language. Any java program is compiled and executed to get the desired output. It follows an architecture called JVM(Java virtual Machine).

Java code is compiled by java compiler(javac). Compiled code is executed, and it gives the byte code. During this process JMM takes the control over to the memory. It involves the threads.

Thread is a lightweight process. Multithreading deals with multiple threads running at same time.

Why JMM?

              JMM deals with interactions between threads and memory. The memory can be classified into two types.

·       Main memory – It’s the place where execution takes place.

·       Working memory- It stores the details of variables of the thread.

Features of JMM:

Volatile variables:

• Read and write operations on thread is visible for threads when it is declared as volatile.

Eg: private volatile boolean a = false;

Atomic functions:

• Volatile read and write operations are synchronized in atomic manner.
• Built in java package: “java.util.concurrent.atomic”

Synchronization:

              When multiple threads executed and a specific thread want to run in the execution block,it needs to be synchronized.

Locks and monitor:

    When threads are running, one should use critical region. Another one should be wait for critical region. This can be done by Locks and monitor.

Built in java package for implementing this concept: java.util.concurrent.locks

Ordering:

              This can be done by two categories.

Happens-Before Relationship: It deals with visibility of operations on memory.

Sequential consistency: The operations(read and write) is consistent.

Let us create a simple thread sample in java.

• A class is created using Thread.
• Run() is a method in thread to execute the process.
• An object is created for the class.
• The thread is started by start() function.

Program:

import java.io.*;

import java.util.*;

 public class sampleTh extends Thread {

    // code for run method

    public void run()

    {

        System.out.println("It's the time to start the thread...");

    }

    public static void main(String[] args)

    {

        sampleTh s1 = new sampleTh();

        // let us start the thread

        s1.start();

    }

}

Here is the output.

C:\raji\blog>javac sampleTh.java

C:\raji\blog>java sampleTh

It's the time to start the thread...

This is a simple thread operation in java uses JMM.