Does ThreadLocal need to be used here?

Question

I m wondering when do we need to use the threadlocal variable?, I have a code that runs multiple threads, each one read some files on S3, I wish to keep track of how many lines read out of the files altogether, here is my code:

final AtomicInteger logLineCounter = new AtomicInteger(0); 
for(File f : files) {
   calls.add(_exec.submit(new Callable<Void>() {
    @Override
    public Void call() throws Exception {
       readNWrite(f, logLineCounter);
       return null;
    }
    }));
}
for (Future<Void> f : calls) {
   try {
      f.get();
   } catch (Exception e) {
      //
   }
}
LOGGER.info("Total number of lines: " + logLineCounter);
...

private void readNWrite(File f, AtomicInteger counter) {
    Iterator<Activity> it = _dataReader.read(file);

    int lineCnt = 0;
    if (it != null && it.hasNext()) {
       while(it.hasNext()) {
         lineCnt++;
         // write to temp file here

       }
       counter.getAndAdd(lineCnt);
    }
}

my question is do I need to make the lineCnt in the readNWrite() method to be threadlocal?

Answer 1

No you don't need to use ThreadLocal here - your code looks perfectly fine:

• lineCnt is a local variable which is therefore not shared across thread => it is thread safe
• counter.getAndAdd(lineCnt); is an atomic and thread safe operation

If you are interested, there are several posts on SO about the use of ThreadLocal, such as this one.

Answer 2

lineCnt is already "thread local" since it's on the stack. Use ThreadLocal only when you need thread-local copies of instance member variables.

Answer 3

You dont need to make lineCnt a ThreadLocal explicitly. lineCnt is a local variable to the thread. It is not accessible by any other thread.

You can get more information about ThreadLocal here

ThreadLocal from javadoc

These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread

Answer 4

- A Thread has a Stack, Register, and Program Counter.

- lineCnt is already into the ThreadLocal.

- lineCnt is a personal copy of the instance variable lineCnt for this thread, and its not visible to any other thread .