jol是一款查看java对象内存结构的工具,第一次使用时着实踩了不少坑。 jol的maven依赖如下：
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.9</version>
</dependency>
测试类代码：
public class ObjectHeadTest {
private int intValue = 0;
public static void main(String[] args) {
ObjectHeadTest object = new ObjectHeadTest();
//打印hashcode
System.out.println(object.hashCode());
//查看字节序
System.out.println(ByteOrder.nativeOrder());
//打印当前jvm信息
System.out.println(VM.current().details());
String classLayout = ClassLayout.parseInstance(object).toPrintable();
System.out.println(classLayout);
}
}
上面的代码直接执行得到如下结果：
2034688500
LITTLE_ENDIAN
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 0x0000000800000000 base address and 0-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]
jdk_test.ObjectHeadTest object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 f4 e1 46 (00000001 11110100 11100001 01000110) (1189213185)
4 4 (object header) 79 00 00 00 (01111001 00000000 00000000 00000000) (121)
8 4 (object header) 40 70 06 00 (01000000 01110000 00000110 00000000) (421952)
12 4 int ObjectHeadTest.intValue 0
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
可以看到ObjectHeadTest实例的header只有12个字节。而64位的jvm对象的header应该占16个字节128位。造成这种情况的原因是jvm默认开启了指针压缩,在64位vm上会把64位的对象指针压缩成32位(原因和细节这里暂不延伸 有兴趣可以自行搜索下) 。这就导致原本应该占8byte的klass对象指针只占了4byte。使用-XX:-UseCompressedOops参数可以关闭指针压缩。开启和关闭指针压缩的头部信息如下两张表所示：
开启指针压缩：
-------------------------------------------------------------------------------------------------------------|--------------------|
| Object Header (96 bits) | State |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| Mark Word (64 bits) | Klass Word (32 bits) | |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| unused:25 | identity_hashcode:31 | cms_free:1 | age:4 | biased_lock:1 | lock:2 | OOP to metadata object | Normal |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| thread:54 | epoch:2 | cms_free:1 | age:4 | biased_lock:1 | lock:2 | OOP to metadata object | Biased |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| ptr_to_lock_record | lock:2 | OOP to metadata object | Lightweight Locked |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| ptr_to_heavyweight_monitor | lock:2 | OOP to metadata object | Heavyweight Locked |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| | lock:2 | OOP to metadata object | Marked for GC |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
关闭指针压缩
|------------------------------------------------------------------------------------------------------------|--------------------|
| Object Header (128 bits) | State |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| Mark Word (64 bits) | Klass Word (64 bits) | |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| unused:25 | identity_hashcode:31 | unused:1 | age:4 | biased_lock:1 | lock:2 | OOP to metadata object | Normal |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| thread:54 | epoch:2 | unused:1 | age:4 | biased_lock:1 | lock:2 | OOP to metadata object | Biased |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| ptr_to_lock_record:62 | lock:2 | OOP to metadata object | Lightweight Locked |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| ptr_to_heavyweight_monitor:62 | lock:2 | OOP to metadata object | Heavyweight Locked |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| | lock:2 | OOP to metadata object | Marked for GC |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
Object Header form thanks for gist.github.com/arturmkrtch…
还有一个我现在都不太明白的踩坑点 在上面我调用了一遍object.hashCode()来查看hash码，如果不手动调用一遍instance的hashCode方法 classLayout打印出来的hashcode就是空的
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 40 70 06 00 (01000000 01110000 00000110 00000000) (421952)
12 4 int ObjectHeadTest.intValue 0
20 4 (loss due to the next object alignment)
System.out.println(ByteOrder.nativeOrder()); 可以查看当前cpu的字节序。输出是LITTLE_ENDIAN意味着是小端序。
- 小端序：数据的高位字节存放在地址的高端 低位字节存放在地址低端
- 大端序： 数据的高位字节存放在地址的低端 低位字节存放在地址高端
比如一个整形0x1234567 ，1是高位数据，7是低位数据。按照小端序01放在内存地址的高位，比如放在0x100 ，23就放在0x101以此类推。大端序反之。
jdk_test.ObjectHeadTest object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 09 f4 e1 46 (00001001 11110100 11100001 01000110) (1189213193)
4 4 (object header) 79 00 00 00 (01111001 00000000 00000000 00000000) (121)
8 4 (object header) 18 55 a0 ab (00011000 01010101 10100000 10101011) (-1415555816)
12 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
16 4 int ObjectHeadTest.intValue 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
前8位是markword,后8位是Klass指针。先不管Klass指针，我们把markword转成大端序便于查看
#markword with big endian
00000000 00000000 00000000 01111001 01000110 11100001 11110100 00001001
前25位是未使用的，之后的31位是hashcode 001111001010001101110000111110100（第一个0为符号位）转成十进制即为2034688500和hashCode()方法的值一致 最后的8位00001001第一个0没使用 后面的001对应age,biased_lock和lock。这三个字段的详细说明如下：
- age - number of garbage collections the object has survived. It is incremented every time an object is copied within the young generation. When the age field reaches the value of max-tenuring-threshold, the object is promoted to the old generation.
- biased_lock - contains 1 if the biased locking is enabled for the class. 0 if the biased locking is disabled for the class. To find out more on biased locking check out my previous post.
- lock - the lock state of the object. 00 - Lightweight Locked, 01 - Unlocked or Biased, 10 - Heavyweight Locked, 11 - Marked for Garbage Collection. To find out more on locking/synchronization check out synchronization post.
age是对象经历的gc次数，biased_lock代表是否使用偏向锁，lock是锁的状态这里001表示没有使用锁
header占用12个字节，之后的4个字节是对象的int属性，之后又4个字节的对齐填充。
由于HotSpot VM的自动内存管理系统要求对象起始地址必须是8字节的整数倍，也就是说就是对象的大小必须是8字节的整数倍。如上header加上fileld是20字节，所以需要4字节的对齐填充。
