Java 如何解释实例';什么是标记词?
我试图理解64位HotSpot VM(v8)上的输出。我不明白是如何使用的,根据链接类文件中的注释,应该指出是在实例上设置了有偏锁还是无偏锁 当我使用JOL by分析Java 如何解释实例';什么是标记词?,java,jvm-hotspot,object-layout,jol,biased-locking,Java,Jvm Hotspot,Object Layout,Jol,Biased Locking,我试图理解64位HotSpot VM(v8)上的输出。我不明白是如何使用的,根据链接类文件中的注释,应该指出是在实例上设置了有偏锁还是无偏锁 当我使用JOL by分析对象的实例时 ClassLayout layout = ClassLayout.parseClass(Object.class); Object object = new Object(); System.out.println(layout.toPrintable(object)); 我得到以下输出: java.lang.Obj
对象的实例时
ClassLayout layout = ClassLayout.parseClass(Object.class);
Object object = new Object();
System.out.println(layout.toPrintable(object));
我得到以下输出:
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (0000 0001 0000 0000 0000 0000 0000 0000)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) e5 01 00 f8 (1110 0101 0000 0001 0000 0000 1111 1000)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes (estimated, add this JAR via -javaagent: to get accurate result)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
根据(前8个字节),我理解我应该解释输出的上述位范围,如下所示:
00:01
-锁定标志(示例中为00
)
02:02
-偏置锁的标志(示例中为0
)
03:06
-年龄与年轻的垃圾收集迭代次数相同。(0000
示例中。)
07:07
-未使用。(似乎总是1
)
08:39
-标识哈希代码。(仅在计算之后,在填充零之前。)
40:63
-未使用。(似乎用零填充。)
确认哈希代码范围的布局
至少对于散列码,这可以通过计算标识散列码和比较值来容易地确认:
ClassLayout layout = ClassLayout.parseClass(Object.class);
Object object = new Object();
// Check that the hash code is not set.
System.out.println(layout.toPrintable(object));
System.out.println("hash code: 0x" + Integer.toHexString(object.hashCode()));
// Confirm that the value is set:
System.out.println(layout.toPrintable(object));
// Compute the hash code manually:
Field field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
Unsafe unsafe = (Unsafe) field.get(null);
long hashCode = 0;
for (long index = 7; index > 0; index--) {
hashCode |= (unsafe.getByte(object, index) & 0xFF) << ((index - 1) * 8);
}
System.out.println("hash code: 0x" + Long.toHexString(hashCode));
初始锁定后,偏置锁定在标记字中可见,如下所示:
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 f0 3f 02 (0000 0101 1111 0000 0011 1111 0000 0010)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) e5 01 00 f8 (1110 0101 0000 0001 0000 0000 1111 1000)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes (estimated, add this JAR via -javaagent: to get accurate result)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
一旦调用object.hashCode()
,我就可以验证这不代表哈希代码,因为标记字会发生变化(偏置锁被撤销)
我不明白的是,偏置锁和非偏置锁的标志仍然设置为零。HotSpot如何知道这是一个有偏锁,而不是对象中表示的哈希代码。另外,我想知道:历元位表示什么?答案很简单:OpenJDK的注释已经过时,而mark word今天的组织方式也不同。您说您正在运行HotSpot 8,但您正在链接JDK 6的源代码。
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 f0 3f 02 (0000 0101 1111 0000 0011 1111 0000 0010)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) e5 01 00 f8 (1110 0101 0000 0001 0000 0000 1111 1000)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes (estimated, add this JAR via -javaagent: to get accurate result)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total