synchronized

bytecodeInterpreter.cpp

  /* monitorenter and monitorexit for locking/unlocking an object */

      CASE(_monitorenter): {
        oop lockee = STACK_OBJECT(-1);
        // derefing's lockee ought to provoke implicit null check
        CHECK_NULL(lockee);
        // find a free monitor or one already allocated for this object
        // if we find a matching object then we need a new monitor
        // since this is recursive enter
        BasicObjectLock* limit = istate->monitor_base();
        BasicObjectLock* most_recent = (BasicObjectLock*) istate->stack_base();
        BasicObjectLock* entry = NULL;
        while (most_recent != limit ) {
          if (most_recent->obj() == NULL) entry = most_recent;
          else if (most_recent->obj() == lockee) break;
          most_recent++;
        }
        if (entry != NULL) {
          entry->set_obj(lockee);
          markOop displaced = lockee->mark()->set_unlocked();
          entry->lock()->set_displaced_header(displaced);
          if (Atomic::cmpxchg_ptr(entry, lockee->mark_addr(), displaced) != displaced) {
            // Is it simple recursive case?
            if (THREAD->is_lock_owned((address) displaced->clear_lock_bits())) {
              entry->lock()->set_displaced_header(NULL);
            } else {
              CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
            }
          }
          UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
        } else {
          istate->set_msg(more_monitors);
          UPDATE_PC_AND_RETURN(0); // Re-execute
        }
      }

真正的代码执行的不是这里。而是templateTable::monitor，里面会调用到lock_object