java interview question

[pooja jain]

1. what if synchronized keyword is not there and we want to achieve synchoronization?

2. why its not recommended to catch run time exception? dont answer me that they occured due to programmer's mistake and so...

[Joe Ess]

So what were your answers?

[ankur rathi]

 

Originally posted by pooja jain:
[QB]1. what if synchronized keyword is not there and we want to achieve synchoronization?
[QB]

One way could be:

boolean aThreadEntered = false;
public void synMethod() {
if(aThreadEntered) {
} else {
aThreadEntered = true;
// below code is supposed to be synchronized
//code
aThreadEntered = false;
}
}

[ January 17, 2008: Message edited by: ankur rathi ]

[Gabriel Claramunt]

Hmm... and what happens if a second thread enters the "if(aThreadEntered)" before the first one executed "aThreadEntered = true;" ?
I don't think that you can implement synchronization without resorting to language locking primitives (probably if you use atomic variables, you can modify your example to make it work)

Originally posted by ankur rathi:


One way could be:

boolean aThreadEntered = false;
public void synMethod() {
if(aThreadEntered) {
} else {
aThreadEntered = true;
// below code is supposed to be synchronized
//code
aThreadEntered = false;
}
}

[ January 17, 2008: Message edited by: ankur rathi ]

 

[Ayub ali khan]

Hi,

I think if we define aThreadEntered as static is should work?

Regarding runtime exceptions..How many exceptions will you catch ? there might be number of possible runtime exceptions might be caught..

Regards
Ayub

[ankur rathi]

 

Hmm... and what happens if a second thread enters the "if(aThreadEntered)" before the first one executed "aThreadEntered = true;" ?

I had recognized this issue in my code but I didn't find fix for it. So I posted the closest one I could get.

I think if we define aThreadEntered as static is should work?

Seems a good idea. Tough the above issue won�t solve with this but in case of two-instances-executing-this-code will work if we make it static.

Regarding runtime exceptions..How many exceptions will you catch ? there might be number of possible runtime exceptions might be caught..

You have an option of catching 'Exception'. :roll:

[Alec Lee]

What about this strategy:

1. Create a dummy file in the filesystem named "unlocked"

2. every running thread try to RENAME the file to "locked.pid". (pid a unique id identifying the thread)

e.g. by issuing os shell command:
rename unlocked locked.pid

- this command is guaranteed to be atomic since a file cannot be of 2 names and once it is renamed another thread cannot rename it anymore since the 'unlocked' filename no longer exist and the 'rename' command simply fails

3. every thread check for the existence of a 'locked.pid' file where pid is its own unique id.

4. At the end, the thread having the 'lock' rename it back to 'unlocked'

5. Potential problem is if a thread exit without renaming the 'locked.pid' back to 'unlocked' all other threads cannot obtained the lock anymore.

This is the algorithm I designed (not tested) to achieve multi-tasking when I used a scripting language (Autoit3 - a window keystroke emulator) which doesn't have interprocess communication built-in.

[Alec Lee]

Another idea is to obtain some OS provided resource which cannot be shared by 2 threads e.g. open a socket for a common port, open a common file in random write mode (reading and append mode not work), etc. The OS guarantees only 1 thread can obtain the resource and each thread just check if the target resource has been obtained by it before executing the synchronized code

[Maris Orbidans]

1) use explicit locks

class X {
private final ReentrantLock lock = new ReentrantLock();
// ...
public void m() {
lock.lock(); // block until condition holds
try {
// ... method body
} finally {
lock.unlock()
}
}
}

2) Because usually you can't recover from runtime exceptions. However you may want to catch runtime exceptions to write a message in a log file.