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Java synchronizes on an object’s monitor, not on the value returned by equals(). Two distinct objects that compare equal therefore do not automatically block one another. To synchronize by a key’s value, map equal keys to the same lock object and synchronize on that shared lock.
Why synchronized does not lock by value
A synchronized statement evaluates its expression and attempts to acquire the monitor belonging to the resulting non-null object. The Java Language Specification describes it this way: “The synchronized statement (§14.19) computes a reference to an object; it then attempts to perform a lock action on that object’s monitor and does not proceed further until the lock action has successfully completed.” The rule is based on object identity, not an equals() comparison. See the Java Language Specification, Chapter 17.
For example, if a and b are separate key objects but a.equals(b) is true, synchronized (a) and synchronized (b) use separate monitors. The code can still proceed concurrently. Synchronizing on key works for a group only if every participant uses the exact same object reference.
A monitor excludes only code that also attempts to acquire that same monitor. It does not automatically prevent other code from reading or writing the object’s fields without synchronization. A monitor release followed by a later acquisition of that monitor also establishes a happens-before relationship, providing a visibility guarantee for properly synchronized accesses. The Oracle tutorial on intrinsic locks explains mutual exclusion and visibility; it was written for JDK 8.
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Use a shared lock per key value
For arbitrary keys, keep a lock registry and use a map operation that atomically creates or retrieves the lock. Each equality-equivalent key then resolves to the same lock object:
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
private static final ConcurrentMap<Key, Object> locks = new ConcurrentHashMap<>();
void update(Key key) {
Object lock = locks.computeIfAbsent(key, ignored -> new Object());
synchronized (lock) {
// Critical section for this key value
}
}ConcurrentHashMap.computeIfAbsent performs the mapping invocation atomically and invokes the mapping function once for that invocation when the key is absent. Keep that function short and simple; the Java SE 26 API documentation describes the operation.
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The map groups keys according to their equals() and hashCode() behavior. Those methods must be mutually consistent and stable while keys are in the registry; changing a key in a way that affects its hash code or equality can make lookups unreliable. Every operation that must coordinate for a key must use this same registry and locking protocol. Keep only the state that needs coordination inside the synchronized block.
Choose a lock strategy that fits the key set
| Approach | Does it share a monitor for equal values? | Ownership and lifecycle | Best fit |
|---|---|---|---|
Private ConcurrentHashMap<Key, Object> registry |
Yes, when map key equality and hashing are stable. | Component-owned; entries remain while mapped, so an unbounded key stream can grow memory use. | Arbitrary value keys with a manageable key set and explicit lifecycle. |
| Explicit private lock objects | Only when all operations for a key are deliberately routed to the same lock. | Private and straightforward; the set of locks is explicit. | A fixed small set of keys or operations. |
synchronized (key) |
No, not for equal-but-distinct key objects. | Lock identity depends on callers sharing the exact reference. | Cases where the same object reference is intentionally shared by all participants. |
String.intern() |
Can canonicalize equal strings. | Uses the shared string pool rather than a component-owned lock registry. | String-specific situations only; it is not a general solution for arbitrary key types. |
The table’s value-key distinction follows the monitor identity rule in the Java Language Specification; atomic lock creation for the registry approach is specific to the documented ConcurrentHashMap operation, not a blanket claim about every concurrent map.
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Plan the lock registry’s lifecycle
A registry retains its lock objects as long as mappings remain. That is simple for a bounded set of known keys, but user-generated or otherwise unbounded keys can cause memory usage to grow over time.
Do not remove an entry merely because a lock appears idle. A thread may already hold the old lock reference or be waiting to acquire it. If the mapping is removed and another lookup creates a replacement, threads for the same logical key could synchronize on different monitors. Cleanup therefore needs a lifecycle protocol that ensures no holder or waiter can use the old lock and no replacement lock is created concurrently. The cited map API documents mapping operations; it does not provide a general-purpose safe lock-eviction protocol.
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Common mistakes
- Assuming
synchronizedcallsequals(): it locks the monitor of the evaluated object reference. - Assuming equal key objects exclude each other: they need a shared lock identity, such as the one supplied by the registry.
- Assuming a lock protects every access to an object’s fields: only code that acquires the same monitor participates in that exclusion and visibility protocol.
- Using a long-running mapping function: keep the
computeIfAbsentfunction short and simple. - Removing locks without coordination: careless removal can allow two monitor identities for the same key value.
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