When to use volatile vs synchronized in Java

When to use volatile vs synchronized in Java

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When to use volatile and synchronized keyword in Java is very important to understand as without the understanding you may go wrong in communicating between threads.

synchronized keyword:

synchronized keyword is used at method level and block level, but the whole purpose of synchronized keyword is to apply access restrictions as only one thread holding the monitor would be able to execute the code inside synchronized method/block.

volatile keyword:

Volatile keyword is used when you don't want local CPU core to cache the variable from main memory, instead all read/write to volatile variable should happen directly from main memory.

The Java volatile keyword guarantees visibility of changes to variables across threads.

So lets understand with practical scenario when to use which,

Scenario 1:
If multiple threads are trying to read the value of a variable and based on the current value it takes the decision of incrementing or decrementing a variable value, is marking a variable volatile in this situation enough?

No, Volatile only guarantees visibility, check and act is like we want two operations to be atomic in that case we either need to use synchronized method/block or Atomic variables (AtomicInteger for example).

Scenario 2:
There are multiple threads reading the value of shared variable but only one thread is writing to a shared variable, is marking a variable volatile in this situation enough?

Yes. as is there no situation where we need more than one operation to be atomic here and also only one thread is writing to a variable others are simply reading the shared variable, so volatile variable should work.

So to summarize, there are 3 problems associated with Multithreading,

1. Race Conditions. (check and act, like i++, i--)
2. Local CPU Caching OR Stale memory. (Threads running in CPU, caches the value from main memory to local CPU cache and now main memory and local CPU cache is out of sync)
3. Complier and CPU optimization (VM/CPU are free from reordering the instructions in the program for optimization and performance as long as the meaning of the instructions remain same.)

Volatile helps in resolving the problem happens due to point 2 and 3 but not 1.

Synchronization helps in resolving all of the above points.

Example demonstrating need of volatile keyword in Java

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package javabypatel;

public class VolatileExample extends Thread {

    //Remove the volatile keyword and see
    volatile boolean keepRunningFlag = true;

    public void run() {
        System.out.println("Volatile thread running");
        long counter = 0;
        while (keepRunningFlag) {
            counter++;
        }
        System.out.println("Volatile thread terminated." + counter);
    }

    public static void main(String[] args) throws InterruptedException {
        //Code below is executing under Main Thread
        VolatileExample volatileThread = new VolatileExample();
        volatileThread.start();

        Thread.sleep(1000);

        //Main Thread is changing the value of keepRunningFlag to false. Ideally after this changes,
        //volatileThread created above executing the run method should come out of the while loop,
        //but if you don't declare the variable keepRunningFlag as volatile then there is no guarantee when while loop will break.
        //How this works internally without volatile keyword,
        //Main thread say running in CPU core 1 updates the keepRunningFlag to false which is updated in Main thread CPU cache
        //volatileThread say running in CPU core 2 also has local CPU cache which before starting the thread had cached the variable
        //keepRunningFlag to its local cache as false.
        //So any updates on the local variable not marked as volatile will first get updated to its local CPU cache.
        //No when the local cache value will sync with Main memory is internal to CPU and that is where problem happens.
        //When we mark the variable as volatile, we are instructing CPU as not to read/write the value of this variable to
        //your local cache, instead directly read/write from main memory.
        volatileThread.keepRunningFlag = false;

        System.out.println("Main thread ended, keepRunningFlag = " + volatileThread.keepRunningFlag);
    }
}

Check the program below, what do you think we should use volatile or synchronization in below situation

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package javabypatel;

public class DBConnection {
    private boolean connected;

    public synchronized void setConnectionFlag(boolean connected) {
        this.connected = connected;
    }
    public synchronized boolean isConnected() {
        return connected;
    }
}

Think of synchronized keyword when you want to make more than one instruction to be marked as single unit that is Atomic operation.

Example: i++ where the operation is break down as,
1. Reading the value of i first
2. Incrementing the value of i.

So that is two step operation and you want to make this two step as a single Atomic operation.

Think of volatile when you want to resolve the issues of visibility, like updates from one thread should be immediately available to other thread for taking some action but there is no operation that you want to execute as atomic, like the example above. In the above example we are simply setting the value of connected in setConnectionFlag method and we are simply returning the value of connected variable in isConnected method, so better to mark the connected variable as volatile in this situation instead of marking the method as synchronized.

Remember whatever you could do with volatile keyword can also be done using synchronized but with synchronized keyword you also get the feature of making the multi step instructions as Atomic operation.

Using synchronized and volatile keyword together.

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There may be a situation where you may need to use volatile and synchronized keyword together. In the example below we are using for the reason of performance.

package javabypatel;

public class SingletonTest {
    volatile static SingletonTest instance = null;

    public static SingletonTest getInstance() {
        if (instance == null) {
            synchronized (SingletonTest.class) {
                if (instance == null) {
                    instance = new SingletonTest();
                    System.out.println("instance created");
                }
            }
        }
        return instance;
    }
}

In the example above since the outer line "if (instance == null)" is not inside synchronized block, what will happen is, say if Thread A gets a chance and is executing the line "instance = new SingletonTest();" and after that it gets preempted. Now, what will Thread 2 see which reached the line "if (instance == null) {" outside the synchronized block, whether the instance would be null or non-null. possibly it may see null if the local cache of the thread 1 is not flushed. so to make sure Thread 2 see the update value of the instance variable instance is marked as volatile here.

You may also like to see

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How ConcurrentHashMap works and ConcurrentHashMap interview questions.

How Thread.join() in Java works internally

Exception Handling Interview Question-Answer

Method Overloading - Method Hiding Interview Question-Answer

Type Casting Interview Questions In Java

How is ambiguous overloaded method call resolved in java

Method Overriding rules in Java

Interface interview questions and answers in Java

Enjoy !!!! 

If you find any issue in post or face any error while implementing, Please comment.