本文记录jvm 对象内部实现

oopDesc

oopDesc is the class representation of java object in jvm, it has 2 fields : 1. markword
2. class pointer

class oopDesc {
    volatile markWord _mark
    Klass*      _klass
}
class markword {
    uintptr_t _value
}

class Handle {
    oop* _handle
}


Handle --> oopDesc
oopDesc -right-> markword: header 

Markword describes the header of the object.

Bit-format of an object header in in 64 bit:

unused:25 hash:31 -->| unused_gap:1  age:4  unused_gap:1  lock:2 (normal object)

1. hash contains the identity hash value.
2. 2 lock bits:

• the two lock bits are used to describe three states: locked/unlocked and monitor.

    [ptr             | 00]  locked             ptr points to real header on stack (stack-locking in use)
    [header          | 00]  locked             locked regular object header (fast-locking in use)
    [header          | 01]  unlocked           regular object header
    [ptr             | 10]  monitor            inflated lock (header is swapped out)
    [ptr             | 11]  marked             used to mark an object
    [0 ............ 0| 00]  inflating          inflation in progress (stack-locking in use)


We assume that stack/thread pointers have the lowest 2 bits cleared.

inflating is a distinguished markword value of all zeros that is used when inflating an existing stack-lock into an ObjectMonitor