How to make a method thread-safe in Java?

Interview Question:

Is the following method thread-safe? How to make it thread-safe?

class MyCounter {
	private static int counter = 0;
 
	public static int getCount() {
		return counter++;
	}
}

This post explains a general interview question that has been asked by Google and a lot of companies. It's low-level and not about how to design concurrent program.

First of all, the answer is NO. The method is not thread-safe, because the counter++ operation is not atomic, which means it consists more than one atomic operations. In this case, one is accessing value and the other is increasing the value by one.

thread-safe method

When Thread 1 accesses the method at t1, Thread 2 may not be done with the method. So the value returned to Thread 1 is the value that has not been increased.

Make a method thread-safe - Method 1

Adding synchronized to this method will makes it thread-safe. When synchronized is added to a static method, the Class object is the object which is locked.

Is marking it synchronized enough? The answer is YES.

class MyCounter {
	private static int counter = 0;
 
	public static synchronized int getCount() {
		return counter++;
	}
}

If the method is not static, then adding synchronized keyword willsynchronize the instance of the class, not the Class object.

Make a method thread-safe - Method 2

In this particular counter example, we actually can make count++ atomic by using AtomicInteger from the package "java.util.concurrent.atomic".

import java.util.concurrent.atomic.AtomicInteger;
 
public class MyCounter {
	private static AtomicInteger counter = new AtomicInteger(0);
 
	public static int getCount() {
		return counter.getAndIncrement();
	}
}

Some other useful facts about thread-safe

Local variables are thread safe in Java.

Each thread has its own stack. Two different threads never shares the same stack. All local variables defined in a method will be allocated memory in stack. As soon as method execution is completed by the current thread, stack frame will be removed.

Category >> Concurrency >> Interview  
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