FULL PRODUCT VERSION :
java version "1.4.2-03"
Java(TM) 2 Runtime Environment, Standard Edition (build Blackdown-1.4.2-03)
Java HotSpot(TM) Client VM (build Blackdown-1.4.2-03, mixed mode)
adam@gradall:~/code/webslinger-fix/trunk/bug$ /usr/lib/j2sdk1.5-sun/bin/java -version
java version "1.5.0_11"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_11-b03)
Java HotSpot(TM) Client VM (build 1.5.0_11-b03, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Linux gradall 2.6.20.1 #1 PREEMPT Sun Feb 25 11:44:44 CST 2007 i686 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
In ThreadLocalMap, when there is a miss, and it calls initialValue, if, during that call, additional ThreadLocals are allocated, it could cause the 'map' to resize. When the initial getAfterMiss resumes, the index that it stores the Entry into is no longer valid, as the table size has changed.

The bug does not occur on java 1.6, because this class was rewritten, to only have the top-level get() set the value if not found, instead of inside getAfterMiss().

The following patch fixes it:
==
--- /tmp/ThreadLocal-1.5.java 2005-06-03 02:17:28.000000000 -0500
+++ ThreadLocal.java 2007-03-02 14:59:59.000000000 -0600
@@ -372,11 +372,7 @@
             }
 
             Object value = key.initialValue();
- tab[i] = new Entry(key, value);
- int sz = ++size;
- if (!cleanSomeSlots(i, sz) && sz >= threshold)
- rehash();
-
+ set(key, value);
             return value;
         }
 
==


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1: javac ThreadLocalBug
2: java -classpath . ThreadLocalBug


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expect to see 'initialValue called' printed one, and first/second/third printed with the same list of values.
ACTUAL -
'initialValue called' printed twice, with second/third having the same value, while first is different.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.ArrayList;

public class ThreadLocalBug {
    static int COUNT = 8;

    static ThreadLocal tl = new ThreadLocal() {
        protected Object initialValue() {
            System.err.println("initialValue called");
            ArrayList list = new ArrayList(COUNT);
            for (int i = 0; i < COUNT; i++) {
                MyThreadLocal mtl = new MyThreadLocal();
                mtl.get();
                list.add(mtl);
            }
            return list;
        }
    };

    public static void main(String[] args) throws Throwable {
        Object first = tl.get();
        Object second = tl.get();
        Object third = tl.get();
        System.err.println("first=" + first);
        System.err.println("second=" + second);
        System.err.println("second=" + third);
    }

    static class MyThreadLocal extends ThreadLocal {
        protected Object initialValue() {
            return Boolean.TRUE;
        }
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use ThreadLocal.get() == null, then ThreadLocal.set(object).