Guide to the Synchronized Keyword in Java

1. Overview

In this article, we’ll learn using the synchronized block in Java.

Simply put, in a multi-threaded environment, a race condition occurs when two or more threads attempt to update mutable shared data at the same time. Java offers a mechanism to avoid race conditions by synchronizing thread access to shared data.

A piece of logic marked with synchronized becomes a synchronized block, allowing only one thread to execute at any given time.

2. Why Synchronization?

Let’s consider a typical race condition where we calculate the sum, and multiple threads execute the calculate() method:

public class SynchronizedMethods {

    private int sum = 0;

    public void calculate() {
        setSum(getSum() + 1);
    }

    // standard setters and getters
}

Then let’s write a simple test:

@Test
public void givenMultiThread_whenNonSyncMethod() {
    ExecutorService service = Executors.newFixedThreadPool(3);
    SynchronizedMethods summation = new SynchronizedMethods();

    IntStream.range(0, 1000)
      .forEach(count -> service.submit(summation::calculate));
    service.awaitTermination(1000, TimeUnit.MILLISECONDS);

    assertEquals(1000, summation.getSum());
}

We’re using an ExecutorService with a 3-threads pool to execute the calculate() 1000 times.

If we executed this serially, the expected output would be 1000, but our multi-threaded execution fails almost every time with an inconsistent actual output:

java.lang.AssertionError: expected:<1000> but was:<965>
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:834)
...

Of course, we don’t find this result unexpected.

A simple way to avoid the race condition is to make the operation thread-safe using the synchronized keyword.

3. The Synchronized Keyword

We can use the synchronized keyword on different levels:

• Instance methods
• Static methods
• Code blocks

When we use a synchronized block, Java internally uses a monitor, also known as a monitor lock or intrinsic lock, to provide synchronization. These monitors are bound to an object; therefore, all synchronized blocks of the same object can have only one thread executing them at the same time.

3.1. Synchronized Instance Methods

We can add the synchronized keyword in the method declaration to make the method synchronized:

public synchronized void synchronisedCalculate() {
    setSum(getSum() + 1);
}

Notice that once we synchronize the method, the test case passes with the actual output as 1000:

@Test
public void givenMultiThread_whenMethodSync() {
    ExecutorService service = Executors.newFixedThreadPool(3);
    SynchronizedMethods method = new SynchronizedMethods();

    IntStream.range(0, 1000)
      .forEach(count -> service.submit(method::synchronisedCalculate));
    service.awaitTermination(1000, TimeUnit.MILLISECONDS);

    assertEquals(1000, method.getSum());
}

Instance methods are synchronized over the instance of the class owning the method, which means only one thread per instance of the class can execute this method.

3.2. Synchronized Static Methods

Static methods are synchronized just like instance methods:

 public static synchronized void syncStaticCalculate() {
     staticSum = staticSum + 1;
 }

These methods are synchronized on the Class object associated with the class. Since only one Class object exists per JVM per class, only one thread can execute inside a static synchronized method per class, irrespective of the number of instances it has.

Let’s test it:

@Test
public void givenMultiThread_whenStaticSyncMethod() {
    ExecutorService service = Executors.newCachedThreadPool();

    IntStream.range(0, 1000)
      .forEach(count -> 
        service.submit(SynchronizedMethods::syncStaticCalculate));
    service.awaitTermination(100, TimeUnit.MILLISECONDS);

    assertEquals(1000, SynchronizedMethods.staticSum);
}

3.3. Synchronized Blocks Within Methods

Sometimes we don’t want to synchronize the entire method, only some instructions within it. We can achieve this by applying synchronized to a block:

public void performSynchronisedTask() {
    synchronized (this) {
        setCount(getCount()+1);
    }
}

Then we can test the change:

@Test
public void givenMultiThread_whenBlockSync() {
    ExecutorService service = Executors.newFixedThreadPool(3);
    SynchronizedBlocks synchronizedBlocks = new SynchronizedBlocks();

    IntStream.range(0, 1000)
      .forEach(count -> 
        service.submit(synchronizedBlocks::performSynchronisedTask));
    service.awaitTermination(100, TimeUnit.MILLISECONDS);

    assertEquals(1000, synchronizedBlocks.getCount());
}

Notice that we passed a parameter this to the synchronized block. This is the monitor object. The code inside the block gets synchronized on the monitor object. Simply put, only one thread per monitor object can execute inside that code block.

If the method was static, we would pass the class name in place of the object reference, and the class would be a monitor for synchronization of the block:

public static void performStaticSyncTask(){
    synchronized (SynchronisedBlocks.class) {
        setStaticCount(getStaticCount() + 1);
    }
}

Let’s test the block inside the static method:

@Test
public void givenMultiThread_whenStaticSyncBlock() {
    ExecutorService service = Executors.newCachedThreadPool();

    IntStream.range(0, 1000)
      .forEach(count -> 
        service.submit(SynchronizedBlocks::performStaticSyncTask));
    service.awaitTermination(100, TimeUnit.MILLISECONDS);

    assertEquals(1000, SynchronizedBlocks.getStaticCount());
}

3.4. Reentrancy

The lock behind the synchronized methods and blocks is a reentrant. This means the current thread can acquire the same synchronized lock over and over again while holding it:

Object lock = new Object();
synchronized (lock) {
    System.out.println("First time acquiring it");

    synchronized (lock) {
        System.out.println("Entering again");

         synchronized (lock) {
             System.out.println("And again");
         }
    }
}

As shown above, while in a synchronized block, we can repeatedly acquire the same monitor lock.

4. Conclusion

In this brief article, we explored different ways of using the synchronized keyword to achieve thread synchronization.

We also learned how a race condition can impact our application and how synchronization helps us avoid that. For more about thread safety using locks in Java, refer to our java.util.concurrent.Locks article.