20 Thread Synchronization Interview Questions and Answers

Thread synchronization is a process that ensures that two or more concurrent threads do not simultaneously execute some section of code where they might interfere with each other. When applying for a position that involves thread synchronization, be prepared to answer questions about your experience and understanding of the concept. This article discusses some common questions that you may encounter during your job interview.

Thread Synchronization Interview Questions and Answers

Here are 20 commonly asked Thread Synchronization interview questions and answers to prepare you for your interview:

1. What is thread synchronization?

Thread synchronization is a process of ensuring that two or more threads do not access shared resources at the same time. This is done by using a lock, which is a mechanism that allows only one thread to access a resource at a time.

2. How do you synchronize threads in Java with the synchronized keyword?

The synchronized keyword is used to indicate that a method can only be accessed by one thread at a time. When a thread tries to access a synchronized method, it will first have to acquire a lock on the object that the method is a part of. Once the thread has acquired the lock, it can proceed with executing the method. When the method is finished, the thread will release the lock, allowing another thread to acquire it and execute the synchronized method.

3. Can you explain what a critical section is?

A critical section is a section of code that can only be executed by one thread at a time. This is usually accomplished by using a mutex, which is a lock that can be used to control access to a resource. When a thread wants to enter a critical section, it first must acquire the mutex. Once it has done so, it can execute the code in the critical section. When it is finished, it must release the mutex so that another thread can acquire it and enter the critical section.

4. What’s the difference between process-level and thread-level concurrency?

Process-level concurrency refers to the ability of a system to run multiple processes simultaneously. Thread-level concurrency, on the other hand, refers to the ability of a system to run multiple threads within the same process simultaneously.

5. What are some common use cases for thread synchronization?

Thread synchronization is often used in cases where multiple threads need to access the same data, but should not do so at the same time. This can help avoid race conditions and other problems that can occur when multiple threads are accessing the same data.

6. Why is it important to know about race conditions when creating multithreaded applications?

Race conditions can cause a lot of problems in multithreaded applications. If two threads are trying to access the same data at the same time, they can end up corrupting that data. This can lead to all sorts of problems, like crashes or data loss. Race conditions can be very difficult to track down and fix, so it’s important to try to avoid them in the first place.

7. Can you give me an example of how you would prevent deadlocks in multithreaded code?

There are a few ways to prevent deadlocks in multithreaded code:

1. Use a lock order: Always acquire locks in the same order to avoid the potential for deadlocks.

2. Use a lock manager: A lock manager can help to ensure that locks are acquired and released in the correct order.

3. Use a lock-free data structure: A lock-free data structure can help to avoid the need for locks altogether, and thus the potential for deadlocks.

8. If two threads need access to the same resource, how will you make sure that only one of them can access it at a time?

The most common way to achieve thread synchronization is to use a mutex. A mutex is a lock that can be placed on a resource, which will then only allow one thread to access that resource at a time. This ensures that the resource is not being accessed by multiple threads simultaneously, which could lead to data corruption or other problems.

9. What is your understanding of the term “reentrant”? In what situations does reentrancy become important?

A reentrant function is one that can be safely called from within a signal handler. A signal handler is a function that is executed when a signal is received by a process. When a signal is received, the process may be interrupted in the middle of executing a function. If the function that is interrupted is not reentrant, then the process may be left in an inconsistent state. Reentrancy is important in signal handlers because it allows the process to safely handle signals without corrupting its own state.

10. What happens when multiple threads try to modify the same object without proper locking mechanisms?

If multiple threads try to modify the same object without proper locking mechanisms, then it is possible for the object to become corrupted. This is because the different threads can end up overwriting each other’s changes, or one thread can read the object while another thread is in the process of modifying it. This can lead to unpredictable and incorrect results.

11. When should you use volatile variables as opposed to synchronized code blocks or methods?

Volatile variables are used when you need to ensure that all threads see the most up-to-date value of a variable. This is most often the case when the variable is being used as a flag to indicate that some event has occurred. Synchronized code blocks or methods should be used when you need to ensure that only one thread can execute a particular section of code at a time.

12. Does Java support atomic instructions? If yes, then can you name some of them?

Yes, Java does support atomic instructions. These include:

– The java.util.concurrent.atomic package, which contains classes that support atomic variables.
– The java.util.concurrent.locks package, which contains classes that support lock-based concurrency.

13. Can you describe a situation where using AtomicInteger instead of plain integers might be beneficial?

AtomicInteger is a class that provides atomic operations on integer values. This can be beneficial in a situation where you need to synchronize access to an integer value between multiple threads. By using an AtomicInteger, you can be sure that the integer value will be updated atomically, and that no thread will be able to see a partially updated value.

14. What is the best way to achieve thread-safety in Java?

The best way to achieve thread-safety in Java is to use the synchronized keyword. This keyword can be applied to methods, blocks of code, or entire classes. When a thread tries to execute a synchronized method or block of code, it will first have to acquire a lock on the object that the method or code is synchronized on. Once the thread has acquired the lock, it can proceed with execution. Other threads that try to execute the same method or block of code will be blocked until the first thread has released the lock.

15. Is there any difference between thread safety and concurrent programming?

Thread safety is a concept that applies to individual methods or blocks of code, and ensures that the code can be safely executed by multiple threads simultaneously without causing any data corruption or race conditions. Concurrent programming, on the other hand, is the process of writing code that can be executed by multiple threads simultaneously. So while thread safety is a subset of concurrent programming, the two are not the same thing.

16. Can you give me an example of a well-known application that uses thread synchronization extensively?

One example of an application that uses thread synchronization extensively is a web server. A web server typically has to handle multiple requests at the same time, so it needs to use thread synchronization in order to ensure that each request is handled properly.

17. What is a monitor in context with thread synchronization?

A monitor is a mechanism for enforcing mutual exclusion in a concurrent system. In other words, it is a way of making sure that only one thread is executing a particular section of code at a given time. This is important in order to prevent race conditions, where two threads try to access the same data at the same time and end up corrupting it.

18. Do all languages have built-in support for thread synchronization?

No, not all languages have built-in support for thread synchronization. However, most languages do have some kind of library or module that can be used to add this functionality.

19. What is a mutex?

A mutex is a lock that is used to protect access to a shared resource. When a thread wants to access the resource, it first acquires the mutex. This prevents other threads from also acquiring the mutex and accessing the resource. When the thread is finished with the resource, it releases the mutex, allowing other threads to acquire it.

20. What are some ways to avoid deadlocks in threaded code?

There are a few ways to avoid deadlocks in threaded code:

1. Use lock ordering: Always acquire locks in the same order. This will prevent two threads from ever trying to acquire the same locks in the opposite order, which could lead to a deadlock.

2. Use lock timeouts: If a thread tries to acquire a lock and it is already held by another thread, the thread can wait for a certain amount of time before giving up and continuing without the lock. This prevents the thread from getting stuck waiting forever for a lock that it will never be able to acquire.

3. Use lock polling: This is similar to using lock timeouts, but instead of waiting for a certain amount of time, the thread will periodically check to see if the lock is available. If it is, the thread will acquire the lock and continue, if not, the thread will go back to waiting.