What does the transient keyword do?

transient marks a field so the default Java serialization mechanism skips it — its value is simply not written to the serialized byte stream, and on deserialization it's restored to its type's default (null for objects, 0/false for primitives), regardless of what it held before serialization.

class Session implements Serializable {
    String userId;              // serialized normally
    transient Connection dbConn; // skipped — Connection isn't serializable anyway
    transient String cachedToken; // skipped — recomputed after deserialization, not persisted
}

Typical reasons to mark a field transient:

• The field's type isn't serializable and can't reasonably be made so (a Thread, a Socket, a database Connection) — including it would throw NotSerializableException at serialization time.
• The value is derived/cacheable and cheaper to recompute than to persist and restore.
• The value is sensitive and shouldn't be persisted to disk or sent over the wire as part of the serialized form (e.g., a decrypted secret held only in memory).

If a class needs custom logic to restore a transient field's value after deserialization (rather than leaving it at its default), it can implement a private readObject(ObjectInputStream) method that calls defaultReadObject() for the normal fields and then manually recomputes/reinitializes the transient ones.