Top Ten Coffee Wait, Notify, Locking Together With Synchronization Interview Questions

There has been a lot of articles in addition to books written on how to job hold back in addition to notify inwards Java, how they work, when do y'all demand synchronization, in addition to how to write concurrent code inwards Java, but, unfortunately, I withal meet many Java programmer struggles to solve even the classic producer-consumer problem. I know, writing right concurrent code is challenging in addition to it takes a lot of practice, mean value through powerfulness in addition to sense to acquire it right, merely at the same time, every Java programmer should travel familiar alongside basics of inter-thread communication, synchronization, locking in addition to empathise how all the things operate together. They should travel able to argue the output of plan when the same code is executed past times to a greater extent than than 1 thread, at the same time. They should know that compiler tin re-order their code to optimize performance in addition to how they tin foreclose it.


Since many Java programmer already has a fair thought of the wait, notify, locking, in addition to synchronization concepts, I am non going to explicate that, merely I'll introduce around interview questions to banking concern lucifer in addition to challenge your understanding. If y'all tin reply these enquiry right in addition to confidently without confusing yourself, in addition to thence y'all are inwards expert safe, otherwise, it's fourth dimension to read the Effective Java in addition to Java Concurrency inwards Practice again.

10 Locking, Synchronization, in addition to Inter-Thread Communication Questions inwards Java

Here is my listing of around of the best questions on locking, synchronization, inter-thread communication in addition to well-nigh the wait, notify, in addition to notifyAll methods inwards Java. These questions are genuinely asked inwards several Java interviews in addition to if y'all conduct maintain a duo of years of Java sense in addition to thence y'all tin reply in addition to thence without whatsoever difficulty.


When do y'all demand synchronization inwards Java?
First in addition to foremost, every bit a Java Programmer, y'all should know when your code needs synchronization. The reply is inwards the "synchronization motto" coined past times Brian Goetz, which says: "If y'all write a variable which may side past times side travel read past times around other thread, or y'all are reading a variable which may conduct maintain been finally written past times around other thread, y'all must job synchronization".

Suppose y'all conduct maintain next code:

while(!closed){   printf("Yes, losing weigth is possible"); }

Now, this closed is a boolean field, which may travel laid past times 1 thread, when the user presses a push clit in addition to read past times around other thread which is running this loop. In this case, y'all demand around form of synchronization thence that changes made past times 1 thread is visible to other.



You tin attain that synchronization past times using either synchronized or volatile keyword inwards Java. In fact, it is the right instance of using the volatile field, which provides low-cost synchronization.

Now, in that place are cases, where fifty-fifty though multiple threads execute the code merely y'all don't demand whatsoever synchronization e.g. reading from HashMap 1 time it is safely initialized. Reading value is a prophylactic functioning in addition to tin travel done without synchronization. provided the value y'all are reading is non going to alter 1 time initialized e.g. final variables.

So, don't nation inwards an interview that if a code is executed past times multiple threads in addition to thence y'all demand synchronization, y'all also needs to consider what does the code is doing. If 1 thread is reading in addition to other is writing in addition to thence y'all definitely demand around form of synchronization to attain the output y'all want.


When to job hold back in addition to notify method inwards Java?
The wait(), notify(), and notifyAll() methods are tools for inter-thread communication inwards Java. Another laid of similar tools is the await(), signal() in addition to signalAll() from Java five which operate alongside Lock in addition to Condition objects. You demand this tool when your plan has multiple threads in addition to they demand to communicate alongside each other. The best illustration of this is 1 time again the producer-consumer problem, where 2 threads demand to communicate alongside each other well-nigh their actions because that may touching on others.

For example, if the shared queue is empty in addition to the consumer is waiting ( y'all tin job wait() method or condition.await() inwards Java five to brand a thread hold back on a condition) for an item inwards the queue, the producer tin notify consumer subsequently successful insertion. Once the consumer receives that notification he tin start his labor again. See here to larn to a greater extent than well-nigh how to solve producer consumer job using hold back in addition to notify inwards Java.

What is the deviation betwixt notify in addition to notifyAll inwards Java?
Before I reply this question, I conduct maintain a full general advice for you, ever pay attending to the name, mostly it give away the purpose. As the refer suggests, the notify()  method transportation notification solely to 1 of the many threads waiting on a condition, piece notifyAll() transportation the notification to all of them.

Now, inwards the instance of notify() which thread volition acquire the notification? Well, a random thread is chosen. This is also 1 of the tricky multithreading questions in addition to y'all may acquire lots of follow-ups, thence brand certain y'all reply it correctly, every bit y'all volition meet inwards side past times side question.


Why using notifyAll is a safer pick than the notify method?
If for around reason, the random thread who receives notification from the notify() method is non able to snuff it on in addition to starts waiting again, in addition to thence your plan volition non progress further. Your application may number inwards consummate deadlock if the thread which calls notify() goes into waiting for set down subsequently sending a notification because in addition to thence in that place is no active thread to notify all waiting threads.

That's why notifyAll() is a safer pick than notify() because it sends a notification to all the threads. If 1 is non able to proceed, in that place is withal around to a greater extent than threads to do the job. See here to larn to a greater extent than well-nigh the deviation betwixt notify in addition to notifyAll methods inwards Java.



What is incorrect alongside this code inwards multi-threading surround inwards Java?

if(account.getBallance() >= withdrawl ){   double ballance = account.getBallance() - withdrawl;   account.setBallance(ballance); }

This code is inspired past times banking concern illustration given inwards Core Java Volume 1 past times Cay S. Horstmann, 1 of the best books to larn Java.  It's a mutual instance of a non-atomic operation which should occur together. If multiple threads execute this code in addition to thence it's possible that 1 thread got deactivate subsequently testing the status i.e. sufficient residue inwards the account, merely when it got upwards in addition to start again, the province of affairs mightiness conduct maintain changed past times around other thread past times processing around other withdrawal functioning in addition to similar a shot in that place is non plenty fund to consummate this transaction.

In lodge to brand this code thread-safe in addition to correct, y'all must roll both attempt in addition to transaction within a synchronized block every bit shown below:

// fellow member variable inwards class Object accountLock = new Object();  sychronized(accountLock){  if(account.getBallance() > withdrawl ){   double ballance = account.getBallance() - withdrawl;   account.setBallance(ballance);  } }

If y'all are using Java five in addition to knows how to job the java.util.concurrent.lock.Lock interface in addition to thence y'all tin also job a ReentrantLock to protect this code. This is also known every bit the critical section, a code segment which should solely travel executed past times 1 thread at a time.


Can a thread travel inwards a synchronized block without acquiring a lock inwards Java?
No, it's non possible. Influenza A virus subtype H5N1 thread must acquire the lock required past times synchronized block earlier entering. The synchronized keyword acquires the lock when a Thread enters in addition to releases the lock Thread leaves the block. Since in that place tin travel solely 1 lock in addition to if it is used past times around other thread than this thread needs to hold back until the lock is available.

Now in that place tin travel many gotchas, sometimes Java programmer uses dissimilar locks to protect the same resource, which is apparently wrong. It's similar 2 doors inwards your bath alongside dissever locks in addition to keys. You don't desire individual to meet y'all when y'all are within the privy right?

So, y'all must job same lock object or mutex to protect same resources, if y'all conduct maintain to job multiple locks in addition to thence job multi-level locks i.e. y'all opened upwards occupation solid door in addition to thence y'all opened upwards privy door, thence y'all demand both occupation solid in addition to privy keys to job the toilet.

What happens to the thread subsequently calling wait() method? Does it render lock?
This is 1 of the most interesting questions on this topic. I conduct maintain seen many programmers confuse themselves when asked during interviews, how come upwards the other thread got lock when showtime thread stuck subsequently calling wait()?

Well, when a thread sees that status to snuff it on farther is non ok, it decides to hold back past times calling the wait() method, merely it does liberate the lock. Yes, this is genuinely important, a thread is non proceeding farther merely the lock is released thence that other threads waiting for the lock tin proceed. This is also the telephone substitution deviation betwixt the sleep() in addition to the wait() method, which is used to suspension a thread (see here)


What happens to the waiting thread (who conduct maintain called wait() method) 1 time around other thread calls notifyAll()?
When a thread calls the notifyAll() or signalAll() method, all thread waiting on that status (a status associated alongside lock in addition to notifyAll() is called on an object, which holds the lock) gets a notification. Now they are costless from waiting for the set down merely they withal demand CPU to travel allocated past times thread scheduler to snuff it on further. When they acquire their chances, they snuff it on further. See here to larn to a greater extent than well-nigh the wait, notify in addition to notifyAll methods inwards Java.


What is the deviation betwixt a thread waiting for the lock in addition to waiting subsequently calling wait() method?
Influenza A virus subtype H5N1 thread waiting for lock gets activated (become eligible to run again) 1 time a lock is costless in addition to acquired past times this lock in addition to CPU is allocated to it. Influenza A virus subtype H5N1 thread waiting on status may non travel runnable 1 time again fifty-fifty if the lock is costless merely he hasn't received whatsoever notification from around other thread e.g. until individual telephone yell upwards notify() or notifyAll() inwards the instance of wait() in addition to signal() or signalAll() inwards the instance of waiting on a Condition object.

Why wait() method should travel called from synchronized method or block? 
I conduct maintain already answered this enquiry inwards expert detail, delight read this article to empathise the jeopardy in addition to dangers of calling hold back in addition to notify without whatsoever synchronized context.


Why y'all should banking concern lucifer waiting status on piece loop instead of if block?
One of the interesting in addition to Practical question. You demand to banking concern lucifer the waiting status inwards the loop because it's possible that fifty-fifty subsequently a wake-up telephone yell upwards past times notify() the waiting status withal holds. If a thread doesn't banking concern lucifer the waiting status subsequently wake up, it mightiness do terms past times proceeding further.

This could travel due to many reasons e.g. to a greater extent than than 1 thread waiting on that status in addition to until the instant thread gets a chance, showtime has already proceeded in addition to nullified that wake-up call.

For example, if 3 consumers are waiting in addition to a producer puts 1 item inwards the queue in addition to notify all three, The showtime consumer volition acquire the item in addition to other 2 needs to wait. This is solely possible when y'all banking concern lucifer the status inwards while() loop.

Btw, that's also the measure idiom to job wait() in addition to notify() as suggested past times Joshua Bloch inwards Effective Java.

Why hold back in addition to notify method are defined inwards java.lang.Object shape instead of Thread?
The brusque reply is because Object is also a monitor inwards Java. It comes alongside an associated lock in addition to the wait(), notify() are associated alongside waiting for the status on lock thence it makes sense to define them inwards the java.lang.Object. For the long answer, meet this article.


That's all well-nigh around Java interview questions on wait() in addition to notify(), locking, synchronization in addition to inter-thread communication. I know, it's 1 of the tricky concepts to empathise in addition to main in addition to that's why I conduct maintain asked tricky questions. Once y'all are comfortable answering this question, your agreement of wait() in addition to notify() volition snuff it solid.

Keep writing code in addition to thinking what happens if the same code block is run past times multiple threads at the same time. Remember, Thread tin halt at whatsoever line, in addition to the solely line of code may travel composed of multiple instructions e.g. ++ or != are non atomic.

I also advise reading Java Concurrency inwards Practice from start to cease for at to the lowest degree 1 time. It volition aid y'all to fill upwards gaps inwards your existing cognition of multi-threading, synchronization, in addition to concurrency inwards Java.


Other Java Multi-threading Interview Questions y'all may similar to explore

• Top 50 Java Thread Interview Questions alongside Answers (list)
• Top 12 Java Concurrency Questions for Experienced Programmers (see here)
• Top xv Multithreading in addition to Concurrency Questions from Investment banks (see here)
• 133 Core Java Interview Questions from finally five years (see here)
• How volatile variable plant inwards Java? (answer)
• Top 10 Java Concurrency in addition to multi-threading best practices (article)
• Difference betwixt start() in addition to run() method inwards Java? (answer)

Thanks for reading this article. If y'all similar this post service in addition to thence delight part alongside your friends in addition to colleagues. If y'all conduct maintain whatsoever proffer or feedback in addition to thence delight drib a comment. Btw, if y'all mean value that Java Concurrency inwards Practice is non relevant inwards the era of Java 8, in addition to thence delight read this article earlier making your judgment. 

Further Learning
Multithreading in addition to Parallel Computing inwards Java
Applying Concurrency in addition to Multi-threading to Common Java Patterns
Java Concurrency inwards Practice Course past times Heinz Kabutz

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