Threads and synchronization

[Nidhi Sar]

Hi,

I don't understand the following lines from Thread chapter (page 756) of K & B book.

" ...changeable data stored in a field that is accessed by two different threads. We need to synchronize in order to access that changeable data safely. Fortunately, the same synchronized blocks that allow us to wait() and notify() also provide the required thread safety for our other access to changeable data."

Could someone explain this? How does synchronized(someObject) protect someObject from changes??

[Deepika Joshi]

- Multiple threads can be created in a process.
- Same object can be used in those threads.
- Suppose object has information of available tickets, say 20
- threadA checks available tickets
- threadB checks available tickets
- threadA books 18 tickets
- threadB tries to book 5 tickets (as per threadB there are 20 tickets & it can book 5 tickets but threadB is not aware that data has changed)

when one object is used in multiple threads; and you want to do following operation
- Read (because reading data is not changed, so it is always safe)
- Update (write) - you need to make sure when one thread is using that object, no other thread should use that object.
this can be done by locking that object, and locking can be done by synchronizing that object.
(when object is locked, no other thread can use the object until lock is released).

- notify() releases lock on object & other thread becomes eligible to have lock on that object.
- wait(), suspends the processing, until other thread does not notify().

you can use wait(), notify() in synchronization process -- this provide safety for access to changeable data.


Thanks.

[Neha Daga]

heyy thats a great explaination

[Nidhi Sar]

Deepika Joshi wrote:-(when object is locked, no other thread can use the object until lock is released).

Deepika,

Thanks for taking the time to write the details of the process. It is a great explanation. However... it doesn't answer my main question
1. An object (say myTicket) is locked by ThreadA,
2. Another thread ThreadB cannot get a lock on myTicket till it is released by ThreadA
3. However, can another thread CHANGE myTicket???

i.e. getting a lock on an object and changing its state are two different things, aren't they?

[Henry Wong]

Nidhi Sar wrote:
Thanks for taking the time to write the details of the process. It is a great explanation. However... it doesn't answer my main question
1. An object (say myTicket) is locked by ThreadA,
2. Another thread ThreadB cannot get a lock on myTicket till it is released by ThreadA
3. However, can another thread CHANGE myTicket???

i.e. getting a lock on an object and changing its state are two different things, aren't they?

Yes. Synchronization and thread safety are two different things. The threads must used synchronization in a cooperative manner. If you don't, it won't work -- as discribed in your example.

Henry

[Deepika Joshi]

Nidhi Sar wrote:

Deepika Joshi wrote:-(when object is locked, no other thread can use the object until lock is released).

Deepika,

Thanks for taking the time to write the details of the process. It is a great explanation. However... it doesn't answer my main question
1. An object (say myTicket) is locked by ThreadA,
2. Another thread ThreadB cannot get a lock on myTicket till it is released by ThreadA
3. However, can another thread CHANGE myTicket???

i.e. getting a lock on an object and changing its state are two different things, aren't they?

- A process can have multiple threads, & those threads can share someObj; those threads share ownership of someObj; and are eligible to change someObj's state
- Locking is explicit ownership of an object. So if another thread does not have ownership, can not use/update someObj.

Thanks.

[Nidhi Sar]

Deepika Joshi wrote:
- Locking is explicit ownership of an object. So if another thread does not have ownership, can not use/update someObj.
Thanks.

Deepika, Thanks!!! I did try it out with the following simple logic and that is absolutely the way it works. The first thread CANNOT read or change a shared object, till the second thread releases the object.

	public static void main(String[] args) {
		ThreadB t = new ThreadB();
		t.start();
		System.out.println("main going to sleep");
		try {
			Thread.sleep(500);
		} catch (InterruptedException e) {}
		System.out.println(t.sb2);
		System.out.println(t.sb); //This statement doesn't execute till ThreadB is done
	}

}

class ThreadB extends Thread{
	StringBuffer sb = new StringBuffer("red");
	StringBuffer sb2 = new StringBuffer("blue");
	public void run() {
		System.out.println("starting ThreadB.run()");
		synchronized(sb){
			System.out.println("ThreadB got the lock on sb");
			try {
				Thread.sleep(5000);
			} catch (InterruptedException e) {}
			System.out.println("ThreadB giving up lock on sb");
		}
	}
}