Can someone explain to me how the actual determination of the type of an object is performed in Java, what is the algorithm of actions?

  • The title of the object at the well-known index contains a link to the class metadata, with which it gets it - etki

1 answer 1

If we talk about OpenJDK / HotSpot, then each object in the heap has a header with two fields: mark and klass .

mark contains hashcode, object age and locks.

klass is a pointer to the structure:

  • in JDK 7: klassOop , lying in PermGen,
  • in JDK 8+: Klass , lying in Metaspace.

In fact, it is an internal analogue of java.lang.Class lying in the native memory.

As far as I know, this point is clearly not described in the JVM specification, and in other implementations it can be implemented differently.

UPD: In IBM's JVM, the order and set of header fields are different, but the essence is the same - a pointer to the class description.