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JAVA SYNCHRONIZATION PITFALLS

What NOT to Synchronize On​

Bad TargetWhy
String / String.intern()String pool is JVM-global — any code synchronizing on the same string competes for the lock
Integer, Long, BooleanSmall values are cached (Integer -128..127) — same problem as interned strings
this (when exposed)External callers can synchronized(yourObj) and interfere
Class literals in librariesOther code may lock on the same Class object

Safe Alternative: Private Lock Objects​

// Per-instance lock
private final Object lock = new Object();

// Per-key lock map
private final ConcurrentHashMap<String, Object> locks = new ConcurrentHashMap<>();

Object lock = locks.computeIfAbsent(appId, k -> new Object());
synchronized (lock) {
    // safe — lock is private to this class
}

References​