Re: boolean and thread safety

Owen Jacobson <>
Fri, 11 Feb 2011 22:25:54 -0500
On 2011-02-11 14:24:36 -0500, Daniele Futtorovic said:

On 11/02/2011 19:09, allegedly wrote:

Are booleans inherently thread safe?

I am thinking of something like

public class AsyncBoolean {

boolean value = false;

public AsyncBoolean(boolean initValue) {
value = initValue;

public boolean getValue() {
return value;

public void setValue(boolean value) {
value = newValue;


Assuming I define the variable using

final AsyncBoolean varName = new AsyncBoolean(true);

does this in effect auto-sync, since you can't partially read a byte
and even if you could it is a boolean anyway?

Since it is declared as final, the reference can't change, so I could
then call varName.setValue(someValue) and varName.getValue() from
threads without the need for syncing.

You're mixing different things here. *Assigning* a boolean value, as in
value = true
is atomic, and hence thread-safe.

But what thread safety is about is only marginally atomicity (because
all assignments save those of double and long variables are atomic). The
main point is what a thread "sees". That is, whether it "sees" the
changes operated by another thread.

Not only that, but also in *what order* other threads see changes.
Given shared non-volatile boolean fields 'startedWork' and
'finishedWork' both initially false and no memory barriers (volatile,
synchronization, or otherwise), if Thread #1 executes

    foo.startedWork = true;
    /* ... some work ... */
    foo.finishedWork = true;

then Thread #2 may legitimately print "Can't happen!" from

    if (foo.finishedWork && !foo.startedWork) {
        System.out.println("Can't happen!");

even though Thread #1 ensures that foo.finishedWork only becomes true
after foo.startedWork is true. While this is relatively easy to
diagnose when both writes (and reads) affect closely-related shared
fields, the same surprising behaviour can arise from unrelated objects
and writes (reads) separated by large stretches of code.

Adding memory barriers or using AtomicFoo types not only ensures that
changes will be seen but also ensures that the changes will be seen in
a predictable order.

The Java memory model (defined at the end of the JLS) lays out the
specific guarantees for various scenarios.

With respects to that, no primitive type, no matter how small, is
thread-safe. You'll have to ensure synchronisation of information across
threads. The easiest way to do that, assuming the propagation of the
reference to the "AsyncBoolean" instance is ensured, would be to declare
the "value" field /volatile/. Another way would be to synchronise in the
getter and setter.

Have a look at java.util.concurrent.atomic.AtomicBoolean.

All good advice. I'll throw another vote on "read Java Concurrency in
Practice", too.


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