Objects which are pooled and potentially reused should not be used for synchronization. If they are, it can cause unrelated threads to deadlock
with unhelpful stacktraces. Specifically, String literals, and boxed primitives such as Integers should not be used as lock objects
because they are pooled and reused. The story is even worse for Boolean objects, because there could possibly be only two instances of
Boolean, Boolean.TRUE and Boolean.FALSE and every class that uses a Boolean will be referring to one of the
two.
Here is the list of types which shouldn’t be used for synchronization:
- The primitive wrapper classes in java.lang;
- The class java.lang.Runtime.Version;
- The "optional" classes in java.util: Optional, OptionalInt, OptionalLong, and OptionalDouble;
- Many classes in the java.time API: Instant, LocalDate, LocalTime, LocalDateTime, ZonedDateTime, ZoneId, OffsetTime, OffsetDateTime, ZoneOffset,
Duration, Period, Year, YearMonth, and MonthDay, and, in java.time.chrono: MinguoDate, HijrahDate, JapaneseDate, and ThaiBuddhistDate;
- The interface java.lang.ProcessHandle and its implementation classes;
- The implementation classes of the collection factories in java.util: List.of, List.copyOf, Set.of, Set.copyOf, Map.of, Map.copyOf,
Map.ofEntries, and Map.entry.
Noncompliant Code Example
private static final Boolean bLock = Boolean.FALSE;
private static final Integer iLock = Integer.valueOf(0);
private static final String sLock = "LOCK";
private static final List<String> listLock = List.of("a", "b", "c", "d");
public void doSomething() {
synchronized(bLock) { // Noncompliant
// ...
}
synchronized(iLock) { // Noncompliant
// ...
}
synchronized(sLock) { // Noncompliant
// ...
}
synchronized(listLock) { // Noncompliant
// ...
}
Compliant Solution
private static final Object lock1 = new Object();
private static final Object lock2 = new Object();
private static final Object lock3 = new Object();
private static final Object lock4 = new Object();
public void doSomething() {
synchronized(lock1) {
// ...
}
synchronized(lock2) {
// ...
}
synchronized(lock3) {
// ...
}
synchronized(lock4) {
// ...
}
See
- CERT, LCK01-J. - Do not synchronize on objects that may be reused
- JEP-390. - JEP 390: Warnings for Value-Based Classes
