Reading shared variable from another thread (Effective Java #66)

Question

In Effective Java: item 66, Joshua Bloch gave an example about life failure:

// Broken! - How long would you expect this program to run
class StopTh         


        

Answer 1

The problem is related to the memory value of the variable stopRequest.

This variable is not defined as volatile.

If you have a two processors the inner thread check the value of stopRequest taken from its registry.

The main thread alter the value of stopRequest in the registry of the other processor.

So the main thread modify a value of stopRequest but the thread see only a copy of it that never changes.

Modified after take a look at the source code of PrintStream (thanks to the commend of ΔλЛ): Using a System.out.print command will use an explicit synchronized block to print the value passed to it this will grant that the value of stopRequest is taken from the main memory and not from the registry of the processor.

Adding a volatile keyword will inform the JVM to take the value from the main memory instead from the registries of the processors and it solve the problem.

Also using the keyword synchronized will solve this problem because any variable used in the synchronized block is taken and update the main memory.

Memory model without volatile (Main thread use Processor 1 and explicit thread use Processor 2)

Processor 1         Processor 2     Main memory
-----------         -----------     -----------
  false                false           false
  true                 false           true       // After setting 
                                                  //stopRequest to true

Defining stopRequest as volatile where all threads read stopRequest from main memory.

    Processor 1         Processor 2     Main memory
-----------         -----------     -----------
       NA                   NA           false
       NA                   NA           true       // After setting 
                                                  //stopRequest to true