单线程的Java同步方法

Question

I'm having trouble understanding the synchronized keyword. As far as I know, it is used to make sure that only one thread can access the synchronized method/block at the same time. Then, is there sometimes a reason to make some methods synchronized if only one thread calls them?

Answer 1

If your program is single threaded, there's no need to synchronize methods.

Another case would be that you write a library and indicate that it's not thread safe. The user would then be responsible for handling possible multi-threading use, but you could write it all without synchronization.

Answer 2

If you are sure your class will be always used under single thread there is no reason to use any synchronized methods. But, the reality is - Java is inherently multi threaded environment. At some point of time somebody will use multiple threads. Therefore whichever class needs thread safety should have adequately synchonized methods/synchronized blocks to avoid problems.

Answer 3

No , you don't need synchronization if there is single thread involved.

Always specify thread-safety policy

Actually you never know how a class written by you is going to be used by others in future . So it is always better to explicitly state your policy. So that if in future someone tries to use it in multi-threaded way then they can be aware of the implications.

And the best place to specify the thread-safety policy is in JavaDocs. Always specify in JavaDocs as to whether the class that you are creating is thread safe or not.

Answer 4

When two or more threads need access to a shared resource , they need some way to ensure that the resource will be used by only one thread at a time.

Synchronized method is used to block the Shared resource between the multiple Threads.

So, No need to apply Synchronization for the Single Thread

Answer 5

Consider that you are designing a movie ticket seller application. And lets drop all the technology capabilities that are provided these days, for the sake of visualizing the problem.

There is only one ticket left for the show with 5 different counters selling tickets. Consider there are 2 people trying to buy the last ticket of the show at the counters. Consider your application workflow to be such

1. You take in details of the buyer, his name, and his credit card
   number. (this is the read operation)
2. Then you find out how many tickets are left for the show (this is again a read operation)
3. Then you book the ticket with the credit card (this is the write operation)

If this logic isnt synchronised, what would happen?

The details of Customer 1 and Customer 2 would be read up until step 2. Both will try to book the ticket and both their tickets would be booked.

If it is modified to be

1. You take in details of the buyer, his name, and his credit card number. (this is the read operation)

Synchronize (

1. Then you find out how many tickets are left for the show (this is again a read operation)
2. Then you book the ticket with the credit card (this is the write operation) )

There is no chance of overbooking the show due to a thread race condition.

Now, consider this example where you absolutely know that there will be only and only 1 person booking tickets. There is no need for synchronization.

The ticket seller here would be your single thread in case of your application

I have tried to put this in a very very simplistic manner. There are frameworks, and constraints which you put on the DB to avoid such a simple scenario. But the intent of the answer is to prove the theory of why thread synchronization, and not the capabilities of the way to avoid it.