The Object class in the JDK does not provide an API to determine whether the object lock of an object is locked. But the following ideas may be feasible.
The layout of an object in memory includes the following parts
Object Head
Object instance data (not concerned here)
Aligned padding (not concerned here)
The object header contains some metadata of this object, including two parts: a mark word field and a klass field mark word contains a lock flag. When the object is not locked, the flag bit is 0 , otherwise it is 1.
The size of mark word is also different on different platforms:
For 32 bit JVM:
_mark : 4 byte constant
_klass : 4 byte pointer to class
For 64 bit JVM:
_mark : 8 byte constant
_klass : 8 byte pointer to class
For 64 bit JVM with compressed-oops:
_mark : 8 byte constant
_klass : 4 byte pointer to class
Therefore, as long as you can get the markword, you can get the lock flag, and then you can know whether an object is locked. However, in the JDK, we can only operate the instance data (variables and methods) of the object, and there is no way to get the object header of the object. Therefore, you may need to use the "black technology" in Javasun.misc.Unsafe.
Take 32-bit jdk as an example:
Object obj = new Object();
int markword = unsafe.getInt(obj, 0L);
The 0L here is the offset. Then perform a bit operation on the markword. The author needs to search for the specific content in the markword.
Back to the poster’s question, maybe the poster just wants a way to know whether the lock is held, and does not necessarily have to contend with the question of whether the object t is synchronized. If so, then the poster can abandon the synchronized keyword and use ReentrantLock in juc instead. There is a method to determine whether the lock is heldisLocked()
The Object class in the JDK does not provide an API to determine whether the object lock of an object is locked. But the following ideas may be feasible.
The layout of an object in memory includes the following parts
Object Head
Object instance data (not concerned here)
Aligned padding (not concerned here)
The object header contains some metadata of this object, including two parts: a mark word field and a klass field
mark word contains a lock flag. When the object is not locked, the flag bit is 0 , otherwise it is 1.
The size of mark word is also different on different platforms:
Therefore, as long as you can get the markword, you can get the lock flag, and then you can know whether an object is locked. However, in the JDK, we can only operate the instance data (variables and methods) of the object, and there is no way to get the object header of the object. Therefore, you may need to use the "black technology" in Java
sun.misc.Unsafe
.Take 32-bit jdk as an example:
The 0L here is the offset.
Then perform a bit operation on the markword. The author needs to search for the specific content in the markword.
Back to the poster’s question, maybe the poster just wants a way to know whether the lock is held, and does not necessarily have to contend with the question of whether the object t is synchronized.
If so, then the poster can abandon the synchronized keyword and use ReentrantLock in juc instead. There is a method to determine whether the lock is held
isLocked()