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The use of {{ThreadLocal}} objects is insecure in classes whose objects are required to be executed by several threads, together in a thread pool. The technique of thread pooling allows threads to be reused when thread creation cost is too high or creating an unbounded number of threads is a potential threat to the reliability of the system. Every thread that enters the pool expects to see an an object in its default, initialized form. However, when {{ThreadLocal}} objects are set from a thread which is subsequently made available for reuse, the reusing thread which takes its place may see the most recent state that was set by the previous thread instead of the expected, default state. \[[JPL 06|AA. Java References#JPL 06]\] |
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The issue is that the DiaryPool class uses a thread pool to execute multiple threads. This allows threads to be reused when the pool is full. When this happens, the thread local state of a previous thread may be inherited by a new thread that has just begun execution. In this case, even though the threads that were started using doSomething2() are expected to see the current day as Monday, one of them inherits the day Friday from the first thread when the thread is reused. Changing Increasing the thread pool size to a larger size (more than 2) appears to fix the problem because it prints the expected state (Friday occurs only once):
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class Diary {
static Day day;
Diary() {
day = day.getInitialDay(Day.MONDAY); // Default
}
private Day currentDay() {
return day;
}
public void setDay(Day d) {
day = d;
}
// Performs some thread-specific task
public void threadSpecificTask() {
// Do task ...
System.out.println("The day is: " + currentDay());
}
}
class DiaryPool {
final int NoOfThreads = 2; // Maximum number of threads allowed in pool
final Executor exec;
DiaryPool() {
exec = (Executor) Executors.newFixedThreadPool(NoOfThreads);
}
public void doSomething1() {
final Diary diary = new Diary(); // First instance
exec.execute(new Runnable() {
public void run() {
diary.setDay(Day.FRIDAY);
diary.threadSpecificTask();
}
});
}
public void doSomething2() {
final Diary diary = new Diary(); // Second instance
exec.execute(new Runnable() {
public void run() {
diary.threadSpecificTask();
}
});
}
public static void main(String[] args) {
DiaryPool dp = new DiaryPool();
dp.doSomething1(); // Thread 1, requires current day as Friday
dp.doSomething2(); // Thread 2, requires current day as Monday
dp.doSomething2(); // Thread 2, requires current day as Monday
}
}
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As expected, this code correctly prints the following or some other order with Friday occurring just oncean order in which Friday occurrs just once, such as:
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The current day is: FRIDAY The current day is: MONDAY The current day is: MONDAY |
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