Why does notifyAll() raise IllegalMonitorStateException when synchronized on Integer?
Solution 1
You have noted correctly that notifyAll
must be called from a synchronized block.
However, in your case, because of auto-boxing, the object you synchronized on is not the same instance that you invoked notifyAll
on. In fact, the new, incremented foo
instance is still confined to the stack, and no other threads could possibly be blocked on a wait
call.
You could implement your own, mutable counter on which synchronization is performed. Depending on your application, you might also find that AtomicInteger meets your needs.
Solution 2
You should also be leery of locking or notifying on objects like String and Integer that can be interned by the JVM (to prevent creating a lot of objects that represent the integer 1 or the string "").
Solution 3
Incrementing the Integer makes the old foo disappear and be replaced with a brand new object foo which is not synchronized with the previous foo variable.
Here is an implementation of AtomicInteger that erickson suggested above. In this example foo.notifyAll(); does not produce a java.lang.IllegalMonitorStateException beause the AtomicInteger Object is not refreshed when foo.incrementAndGet(); is run.
import java.util.concurrent.atomic.AtomicInteger;
public class SynchronizeOnAPrimitive {
static AtomicInteger foo = new AtomicInteger(1);
public static void main(String[] args) {
synchronized (foo) {
foo.incrementAndGet();
foo.notifyAll();
}
System.out.println("foo is: " + foo);
}
}
Output:
foo is: 2
Solution 4
As erickson has noted, the code without the postincrement operator works without error:
static Integer foo = new Integer(1);
public static void main(String[] args) {
synchronized (foo) {
foo.notifyAll();
}
System.out.println("Success");
}
output:
Success
jjvainio
Updated on May 31, 2020Comments
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jjvainio almost 4 years
Why does this test program result in a
java.lang.IllegalMonitorStateException
?public class test { static Integer foo = new Integer(1); public static void main(String[] args) { synchronized(foo) { foo++; foo.notifyAll(); } System.err.println("Success"); } }
Result:
Exception in thread "main" java.lang.IllegalMonitorStateException at java.lang.Object.notifyAll(Native Method) at test.main(test.java:6)
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jjvainio over 15 yearsI failed to realize that incrementing the Integer would allocate a new object instead of changing the value of the existing object.
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dmitrii almost 13 yearsTrying to wait/notify using an Enum can also lead to this condition:
synchronized (myEnum) { myEnum=MyEnum.NEW_VALUE; myEnum.notify(); }
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Oded Breiner over 9 yearsFor those looking for a solution to a boolean, use AtomicBoolean as the wrapper type
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coffee_machine over 9 years@dmitrii, yes indeed, but here you are explicitly modifying the reference. That applies to ANY object reference, not only enums.
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vehsakul over 8 yearsIt's very tempting to use a primitive type wrapper both as the synchronizer and the condition data. What a mistake!