Question

This is a section from Core Java 8th edition Page 757

CAUTION:

public void flipDone() {
   done = !done;
}

// not atomic

I don’t understand why it’s not atomic. Can any one tell me why? thanks

Answer 1

The flipDone method is executed by the computer in three distinct steps:

Read the value of memory location labeled done into ALU
Flip the value (i.e true -> false or false -> true)
Write the new value into the memory location done

In Java, a piece of code can potentially be invoked in multiple threads. These threads should be thought of as executing the code concurrently.

Say, memory location labeled done contains the value false initially. Consider two threads calling flipDone, resulting in the following sequence of steps:

Thread 1                             Thread 2
-----------------------------------------------------------------------
Read value of done, false
Invert that value to true
Write the new value, true, to done       
                                     Read value of done, now true
                                     Invert that value to false
                                     Write the new value, false, to done

The flipDone method was called twice. done went from false to true and then back again to false – as one would expect. But since the threads execute concurrently, this is not the only ordering of steps. Consider this ordering instead:

Thread 1                             Thread 2
-----------------------------------------------------------------------
Read value of done, false
Invert that value to true            Read value of done, false
Write the new value, true, to done   Invert that value to true    
                                     Write the new value, true, to done

While the first thread is inverting the value it read, the second thread, concurrently, is reading the value. Similarly, while the first thread is writing the value to memory, the second thread is inverting the value it read. When Thread 2 finishes, the value of done will be true. Here, although flipDone was called twice, done was flipped only once! One of the updates seem to have been lost. This is the problem that the book is trying to warn you about.