(layout.toPrintable()) Įvery time a live object's address changes, that's probably because of minor GC and movement between survivor spaces. Long currentAddr = VM.current().addressOf(instance) Long lastAddr = VM.current().addressOf(instance) ĬlassLayout layout = ClassLayout.parseInstance(instance) Then It adds the remaining 8 bytes after the long variable. So, we should add 12 more bytes to make it a multiple of 16.Īs shown above, it adds 4 internal padding bytes to start the long variable at offset 16 (enabling more aligned access). The object header and the long variable still consume 20 bytes in total. Space losses: 4 bytes internal + 8 bytes external = 12 bytes total For instance, for the same class, the memory layout with -XX:ObjectAlignmentInBytes=16 would be: SimpleLong object internals:Ģ4 8 (loss due to the next object alignment) We can also change the default alignment size via the -XX:ObjectAlignmentInBytes tuning flag. To make this size a multiple of 8 bytes, the JVM adds 4 bytes of padding. Space losses: 4 bytes internal + 0 bytes external = 4 bytes totalĪs shown above, the object header and the long state consume 20 bytes in total. Then JOL will print the memory layout: SimpleLong object internals: If we parse the class layout: (ClassLayout.parseClass(SimpleLong.class).toPrintable())
The most significant byte is 65 since the JVM stores that value in little-endian format. The HotSpot JVM stores the identity hashcode as “25 b2 74 65” in the mark word.
#OPENJDK 7 JVM INTERNALS CODE#
However, this will change if we call the System.identityHashCode()or even Object.hashCode() on the object instance: ("The identity hash code is " + System.identityHashCode(instance)) Space losses: 0 bytes internal + 0 bytes external = 0 bytes totalĪs shown above, the mark word currently doesn't seem to store anything significant yet. (ClassLayout.parseInstance(instance).toPrintable()) Let's see the memory layout for an object instance: SimpleInt instance = new SimpleInt() So, that's at least 16 bytes in 64-bit architectures because of 8 bytes of the mark, 4 bytes of klass, and another 4 bytes for padding.įor arrays, represented as arrayOop, the object header contains a 4-byte array length in addition to mark, klass, and paddings. Again, that would be at least 16 bytes because of 8 bytes of the mark, 4 bytes of klass, and another 4 bytes for the array length. After the object header, there may be zero or more references to instance fields.
#OPENJDK 7 JVM INTERNALS PLUS#
However, we'll only consider normal objects as Java 15 is going to deprecate biased locking.Īdditionally, the klass word encapsulates the language-level class information such as class name, its modifiers, superclass info, and so on.įor normal objects in Java, represented as instanceOop, the object header consists of mark and klass words plus possible alignment paddings. Moreover, the mark word state only contains a uintptr_t, therefore, its size varies between 4 and 8 bytes in 32-bit and 64-bit architectures, respectively. Also, the mark word for biased and normal objects are different. The HotSpot JVM uses this word to store identity hashcode, biased locking pattern, locking information, and GC metadata. The mark word describes the object header.
0 kommentar(er)
