The Volatile Keyword in Java Explained with Example | Interview Question | Multithreading |

Summary

This video script delves into the Java 'volatile' keyword, crucial for multi-threading scenarios and often misunderstood. It explains how 'volatile' ensures visibility of changes across threads by bypassing the cache and directly interacting with main memory. The concept is exemplified using the Singleton pattern, where 'volatile' prevents multiple instances due to thread caching issues. The script aims to clarify the importance and application of 'volatile' for those new to the concept, promising a valuable insight for bragging rights among peers.

Takeaways

• 💡 The 'volatile' keyword in Java is crucial for ensuring visibility of changes across multiple threads.
• 🔍 Volatile is often misunderstood, but it's essential for understanding synchronization in multi-threaded environments.
• 💬 It's used to prevent issues that arise from caching, where a thread might not see updates made by another thread.
• 🖥️ CPUs use cache for faster data access, which can lead to inconsistencies if not managed properly with shared variables.
• 🔑 The 'volatile' keyword forces threads to read variables directly from main memory, not from their local cache.
• 🔄 This keyword is particularly important in the Singleton design pattern to prevent multiple instances from being created.
• 📺 The example of a 'TV set' class illustrates how 'volatile' can be used to ensure only one object is instantiated.
• 🛠️ The Singleton pattern restricts instantiation of a class to a single object, often using a 'volatile' static variable to control this.
• 🔄 In multi-threading, 'volatile' helps in maintaining the integrity of the application state by ensuring all threads see the most recent value of a variable.
• 📝 Understanding and correctly applying 'volatile' requires careful consideration and is a sign of a deeper grasp of Java's concurrency mechanisms.

Q & A

What is the 'volatile' keyword in Java?

The 'volatile' keyword in Java is used to indicate that a variable's value may change at any time, and its changes should be immediately visible to other threads. It prevents the compiler and processor from applying certain optimizations that might cache the variable in registers or thread-local storage.

Why is the 'volatile' keyword important in multi-threading?

In multi-threading, the 'volatile' keyword is crucial because it ensures that when one thread modifies a variable, the changes are immediately reflected in the main memory, and other threads reading the variable see the updated value, not a cached or stale value.

How does cache memory affect the visibility of variable changes in threads?

Cache memory can cause visibility issues in threads because each thread may have its own local cache. If a thread updates a variable, the change might be stored in its local cache first and not immediately propagated to the main memory, causing other threads to see an outdated value.

What problem does the 'volatile' keyword solve in the context of the Singleton pattern?

In the Singleton pattern, the 'volatile' keyword solves the issue of multiple threads creating multiple instances of a class when using double-checked locking. By declaring the instance variable as 'volatile', it ensures that when one thread creates an instance, other threads will see the updated instance and not create a new one.

Can you provide an example of a shared variable in Java that might require the 'volatile' keyword?

An example of a shared variable that might require the 'volatile' keyword is a status flag that controls the flow of multiple threads. If the flag's state is crucial for thread synchronization, marking it as 'volatile' ensures that all threads see the most recent value of the flag.

How does the 'volatile' keyword interact with the main memory and CPU cache?

When a variable is declared 'volatile', the CPU is instructed to read and write the variable directly from and to the main memory, bypassing the CPU cache. This ensures that every thread accessing the variable gets the most current value from memory.

What is the role of the 'volatile' keyword in ensuring the correct implementation of the Singleton pattern?

In the Singleton pattern, the 'volatile' keyword ensures that when an instance of the class is created, it is immediately visible to all threads. This prevents multiple threads from creating separate instances due to cache inconsistencies, thus maintaining the Singleton property.

Why might declaring a variable as 'volatile' not be enough to ensure thread safety?

Declaring a variable as 'volatile' alone is not enough for thread safety because it only ensures visibility of changes across threads. It does not provide atomicity for compound actions (like check-then-act operations) or protection against reordering of instructions by the compiler or CPU.

Can the 'volatile' keyword be applied to any variable type in Java?

Yes, the 'volatile' keyword can be applied to any variable type in Java, including primitives, object references, and arrays. However, its effectiveness is most relevant for variables that are accessed by multiple threads.

What are some scenarios where using 'volatile' might not be the best solution for thread synchronization?

Using 'volatile' might not be the best solution for thread synchronization when dealing with complex data structures that require atomic updates or when a more comprehensive locking mechanism is needed to maintain the order of operations. In such cases, other synchronization tools like 'synchronized' blocks or higher-level concurrency utilities from the `java.util.concurrent` package might be more appropriate.

Outlines

00:00

💻 Understanding the Volatile Keyword in Java

This paragraph introduces the concept of the volatile keyword in Java, emphasizing its importance in multi-threading scenarios, particularly in Singleton design patterns. It explains that while the volatile keyword is not inherently complex, its application can be challenging to grasp. The paragraph delves into the role of cache in computer memory and how it can lead to issues with shared variables in a multi-threaded environment. It uses the example of a shared 'flag' variable that multiple threads access and modify, potentially leading to visibility issues due to caching. The volatile keyword is introduced as a solution to ensure that changes to a variable are immediately visible to all threads and are not cached locally, thus maintaining a consistent state across all threads.

05:02

🔒 Ensuring Singleton Integrity with Volatile

The second paragraph continues the discussion on the volatile keyword, focusing on its application in the Singleton design pattern. It explains how the Singleton pattern restricts the instantiation of a class to a single object, and how the volatile keyword is crucial in a multi-threaded context to prevent multiple instances from being created inadvertently. The paragraph illustrates a scenario where, without the volatile keyword, multiple threads might create separate instances of a Singleton class due to caching issues. It highlights the role of the 'instance' variable as a flag that determines whether a new object should be created or an existing one returned. The paragraph concludes by emphasizing the importance of the volatile keyword in maintaining the integrity of the Singleton pattern and ensuring that only one instance of a class is ever created, even in a multi-threaded environment.

Keywords

💡volatile

The keyword 'volatile' in Java is used to indicate that a variable's value may change at any time, and it should not be cached in memory by the CPU. This keyword is crucial for multi-threaded applications where multiple threads may access and modify a variable concurrently. In the video, 'volatile' is explained in the context of ensuring that a shared variable is always read from the main memory, preventing issues with thread visibility. The example of a 'flag' variable used in a multi-threaded environment illustrates how 'volatile' ensures that changes made by one thread are immediately visible to others.

💡multi-threading

Multi-threading refers to the ability of a system to execute multiple threads concurrently. Threads are the smallest unit of processing within an application that can be scheduled by the operating system. In the video, multi-threading is discussed in the context of how threads interact with CPU, memory, and cache, and the challenges it presents, such as ensuring that changes to shared variables are visible to all threads. The Singleton pattern example demonstrates a real-world scenario where multi-threading can lead to issues if not handled correctly.

💡cache

Cache is a component of a computer system that stores copies of frequently accessed data to reduce the time it takes to access that data. In the video, cache is mentioned as a layer that can cause issues with data consistency in multi-threaded applications. When a thread updates a shared variable, the change might be stored in the local cache before being written to the main memory, leading to other threads seeing outdated values. The concept is integral to understanding the need for 'volatile'.

💡main memory

Main memory, also known as RAM, is where a computer's operating system, applications, and data are held for immediate access by the processor. In the video, main memory is discussed in relation to how threads read and write data. The concept is central to understanding the 'volatile' keyword, as it explains why declaring a variable as 'volatile' forces threads to read and write directly to the main memory, ensuring visibility of changes across threads.

💡Singleton pattern

The Singleton pattern is a software design pattern that restricts the instantiation of a class to one single instance and provides a global point of access to that instance. In the video, the Singleton pattern is used to illustrate the practical application of the 'volatile' keyword. The pattern is implemented in such a way that only one object of a particular class can be created, and the 'volatile' keyword is critical in ensuring that the instance variable is correctly handled in a multi-threaded environment to maintain the Singleton property.

💡flag variable

A flag variable is a boolean variable used to indicate the state of a condition or to control the flow of a program. In the video, the flag variable is used to demonstrate the concept of 'volatile'. It is a shared variable that, when not declared as 'volatile', can lead to issues where one thread may not see the updated value set by another thread due to caching. The video explains how declaring the flag as 'volatile' ensures that all threads see the most recent value.

💡double-checked locking

Double-checked locking is a design pattern used to reduce the overhead of acquiring a lock by first testing the locking criterion without actually acquiring the lock. If the criterion is true, the lock is not needed. In the video, double-checked locking is mentioned in the context of the Singleton pattern, where it is used to ensure thread safety while minimizing the performance overhead. The pattern involves checking whether an instance exists before locking and then creating the instance if it doesn't exist.

💡instance variable

An instance variable is a variable that is defined in the body of a class but outside any method, and is associated with each object of the class. In the video, the instance variable is used in the context of the Singleton pattern to control the creation of class instances. The 'volatile' keyword is applied to the instance variable to prevent issues with thread visibility in a multi-threaded environment, ensuring that only one instance of the class is created.

💡thread safety

Thread safety refers to the ability of a system to correctly handle operations on shared data when performed by multiple threads simultaneously. In the video, thread safety is a central concern when discussing the use of the 'volatile' keyword. Ensuring that threads can operate correctly without interfering with each other is critical in multi-threaded applications, and 'volatile' is one of the mechanisms used to achieve this by preventing caching of shared variables.

💡visibility

Visibility in the context of multi-threading refers to the ability of one thread to see changes made by another thread. In the video, visibility is a key issue addressed by the 'volatile' keyword. When a variable is declared 'volatile', it ensures that all threads see the most recent value of the variable, which is particularly important for shared variables that are subject to change by multiple threads.

Highlights

Introduction to the volatile keyword in Java.

Volatile's importance in multi-threading and the Singleton pattern.

The concept of cache and its role in reducing access time for the CPU.

Explanation of shared variables and their interaction with main memory and cache.

The problem of cache not updating immediately with changes made by threads.

How declaring a variable as volatile ensures visibility of changes across threads.

The role of volatile in ensuring a consistent state of a flag variable in multi-threaded environments.

Example of a Singleton class and the use of volatile in maintaining a single instance.

Detailed explanation of the Singleton design pattern and its implementation.

The issue of multiple threads creating multiple instances without volatile.

How volatile prevents the creation of multiple instances in a Singleton pattern.

The necessity of volatile for the correct implementation of design patterns in multi-threading.

The challenges in understanding and correctly using the volatile keyword.

Recommendation to watch the Singleton design pattern video for further insights.

The significance of volatile in direct updates to main memory and reads by threads.

Extensive explanation on the usage and implications of the volatile keyword.

Transcripts

okay so now let's learn a very important

concept that is the volatile keyboard

and it's often asked in a lot of sd2

interviews it's also used in Singleton

pattern and it's not a very easy concept

to grasp and very few people know about

it right so if you are a fresher you're

watching this video uh I can assure you

that there's something you can learn and

brag about your classmates because very

few people will actually know about this

and this is not a very hard concept to

grasp however it is not very easy to

figure out that where to use this

particular keyword okay but I will try

my best to let you guys understand this

keyword so volatile is a keyboard in

Java and now let's see when does this

usage can actually come into play so

Suppose there are two threads right and

you know what happens is a thread

interacts with your CPU right and the

CPU in turn interacts with the main

memory or the ram now let us introduce

our friend cache which basically helps

us in reducing the access time so we

know that uh it is far more efficient

for a CPU to access data from the cache

than for CPU to access data from the

main memory right and that's why cache

comes into picture because it gives you

fast data access time

so what happens ideally is whenever

there is a shared variable that exists

in the memory when I say a shared

variable you can consider the top of the

stack right which we saw in the previous

example right and that is The Shield

variable because multiple because that

variable exists in the main memory and

multiple threads are actually trying to

access that variable and work on it and

maybe update it or you know whatever it

wants to do with that particular shade

variable so let's consider here we have

a shared variable whose

black variable and initially it is set

to True okay so now what happens is this

thread both these threads they don't

directly read from the memory they have

their own cache and they read all these

threads read from read the value of this

flag variable locally from their cache

now the problem that might happen is if

let's say thread 2 changes the value of

this flag to false it won't directly

update it into the ram it would first

update in its local cache right so you

can see here it updates the flag

variable to false but the other thread

still can see the value of the thread as

true because it is not updated in its

local cache as well as in the main

memory it is still true now next step it

will take some time for this value of

the cache to be propagated to the main

memory as false because there was an

updation that was being done by thread 2

but you can see the thread one still

doesn't have num any visibility or that

hey this flag is actually changed to

false in order to get rid of this

problem we introduce the volatile

keyword if we declare the same variable

as volatile that is volatile Boolean

flag equals to true now what happens is

this threads no longer read it from the

cache or from the local copy they

directly read it from the main memory as

a result if the thread 2 changes if the

thread 2 changes this flag to false the

thread one will have access to it right

just in case where let's say there is

there is the status right there's a

status flag uh that is constantly

getting updated by multiple threads and

based on the status Flags the other

threads are doing some work right I mean

the condition of that status flag will

actually direct that how the threads

will behave and is very important for

all the threads to to have a consistent

state of that particular flag variable I

will give you an example if you remember

my Singleton pattern video uh if you

haven't checked out my singular design

pattern video I would highly recommend

you to go and check out that video but I

will just give you a

okay so now let's take a look at the

Singleton class that is the TV set which

we created while I was recording my

signal and design pattern video so here

what happens is what does a Singleton

design pattern say I will just briefly

tell you so in a Singleton design

pattern you can create only one object

of that particular class right so we

have to design the class in such a way

okay so that only one object can be

created for that particular class so

what we do is we maintain a static

variable uh of the of the instance right

and initially we set this to null right

because let's say that when this when

there is no object created uh the

instance is null right and we declare a

private Constructor we create a private

Constructor because we don't want uh any

other client to instantiate this class

from outside okay

so if now what if uh put any client one

wants to instantiate this particular uh

particular class right so for that we

create a static method right now why do

we make this Constructor private why do

we make a static method of get instance

all this I've covered in depth in that

Singleton design pattern video uh and

I'm not going to cover that in such that

but for now just want to say that

basically we don't expose our

Constructor we expose this static method

which basically checks that okay

whatever this instance if this is null

that means no object has been created

for this particular class and that is we

go ahead and we create an instance of

this particular class and we before

returning that instance to our client we

store it we store it we update this

reference variable uh to the new

instance that we just created and so the

next time if some other client wants to

Again instantiate by calling this get TV

instance object what we do is we return

this particular this particular instance

and we don't end go up go and create

another instance right as you can see

only if it is null only if this instance

is null then only we create a new TV set

otherwise we return otherwise we don't

create any new TV set and we just return

the same old instance and that way we

ensure that only one instance of this

object is always created right like

again why this is synchronized why this

is uh like why we are having two checks

right all these things which are called

which you call double check blocking and

all these things have covered during the

Singleton design pattern video so it is

highly recommended to go and check out

the video after you've watched this

multi-threading videos but here you can

see that there is a flag right there is

a flag that is this TV set instance this

this TV set instance is acting as a flag

right why this is acting as a flag is it

says that okay if this if this reference

is null

then create an object otherwise don't

create an object return that same object

so it is kind of acting as a flag right

that that particular variable the state

of the particular variable defines like

the op the following operations right so

now let's say in a multi-thread

environment uh multiple threads are

trying to access this class and let's

say one of the threads one of the

threads get access to this class and it

Updates this TV set instance so it's the

first thread comes in uh and first it

comes in and it Stacks that okay this TV

set instance is null then let me create

a new TV set instance uh and I will and

let me return it now let's say this

first thread has created this new TV set

instance and it has updated its value

and it has returned it right now if you

go back here you see that what this

thread did it updated the TV set

instance value in its local cache but it

is not yet propagated to the main memory

and let's say next is propagated to the

main memory but it's still not

propagated to the other threads cache so

the other thread is now if you again one

this other thread now wants to create a

new instance it will find in its local

cache that hey this TV set instance

value is still none and has a result

what it will do is it will again go and

create a new object so now there are two

objects being created which kind of

violates the uh Singleton design pattern

principle which says that only one

instance of you know of this particular

class can be created and that is the

reason I want this this flag instance

where you built to be directly updated

in the main memory and directly read by

the thread from the main memory and that

is the reason we declare it volatile

I've also explained in that video as

well but this is more an extensive

explanation on on volatile keyboard so I

hope you're able to grasp this concept

because it's a very very important

concept uh it's not very easy to

understand and well or to use volatile

in all of the cases so that might

require a lot of design discussions and

profiling but yeah that's that's mostly

about