What Is an Object Header? Every single Java object — every String, every Integer, every record, every array — carries a header that the JVM uses for bookkeeping. Your code never sees this header; it lives alongside the object’s fields in memory. Before Java 25, on a 64-bit JVM, the header occupied 96 to 128 bits (12–16 bytes): ┌─────────────────────────────────────────────────────────┐ │ Mark Word (64 bits) │ │ ─ identity hash code │ │ ─ lock state (biased lock / thin lock / fat lock) │ │ ─ GC age bits │ ├─────────────────────────────────────────────────────────┤ │ Class Pointer (32 bits compressed / 64 bits full) │ │ ─ pointer to the object's class (Klass* in HotSpot) │ └─────────────────────────────────────────────────────────┘ With UseCompressedOops (default), the class pointer is compressed to 32 bits, giving a 96-bit (12-byte) header.