JVM对象头
public static void main(String[] args) throws InterruptedException {
// 通过对象头说明a和b是同一个对象
System.out.println("==================JVM INFO====================");
System.out.println(VM.current().details());
String a = new String("a");
String b = a;
String c = new String("c");
System.out.println("加锁前---------------");
System.out.println(ClassLayout.parseInstance(a).toPrintable());
System.out.println(ClassLayout.parseInstance(b).toPrintable());
System.out.println(ClassLayout.parseInstance(c).toPrintable());
synchronized (a) {
System.out.println("\n");
System.out.println("加锁a ---------------");
System.out.println(ClassLayout.parseInstance(a).toPrintable());
System.out.println(ClassLayout.parseInstance(b).toPrintable());
System.out.println(ClassLayout.parseInstance(c).toPrintable());
new Thread(() -> {
System.out.println("子线程启动");
synchronized (b) {
System.out.println("获取到锁？");
System.out.println("加锁b ---------------");
System.out.println(ClassLayout.parseInstance(a).toPrintable());
System.out.println(ClassLayout.parseInstance(b).toPrintable());
System.out.println(ClassLayout.parseInstance(c).toPrintable());
}
}).start();
Thread.sleep(5000);
System.out.println("End....");
}
}
输出日志
==================JVM INFO====================
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
加锁前---------------
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [a]
16 4 int String.hash 0
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [a]
16 4 int String.hash 0
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [c]
16 4 int String.hash 99
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
加锁a ---------------
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 28 f2 c0 02 (00101000 11110010 11000000 00000010) (46199336)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [a]
16 4 int String.hash 0
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 28 f2 c0 02 (00101000 11110010 11000000 00000010) (46199336)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [a]
16 4 int String.hash 0
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [c]
16 4 int String.hash 99
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
子线程启动
End....
获取到锁？
加锁b ---------------
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 9a 9a 74 1c (10011010 10011010 01110100 00011100) (477403802)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [a]
16 4 int String.hash 0
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 9a 9a 74 1c (10011010 10011010 01110100 00011100) (477403802)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [a]
16 4 int String.hash 0
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) da 02 00 f8 (11011010 00000010 00000000 11111000) (-134216998)
12 4 char[] String.value [c]
16 4 int String.hash 99
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
因为是小端存储，所以你看的值是倒过来的，锁标识是第一行object header的第一个字节的最后两位
参考链接：https://blog.csdn.net/qq_33762302/article/details/113748613