什么是java对象头文件 [英] What is in java object header
问题描述
您能否告诉我一些关于对象标题中存储内容的信息？我知道，它可能依赖于JVM，但至少可能对HotSpot有用吗？我正在寻找第一行的具体描述。
Could you give me some information on what is exactly stored in object header? I know, that it's probably JVM dependent, but maybe for HotSpot at least? I'm looking for exact description specifically for a first row.
我已经阅读了一些我无法用我发现的信息验证的信息。也许你有一个指向OpenJDK维基的链接？
I've read several information that I can't verify positively with information I find. Maybe you have a link to OpenJDK wiki that says it all?
推荐答案
对于HotSpot：
对象标题包含一个标记词和一个klass指针。
标记字在32位体系结构上具有字长（ 4字节，在64位上具有 8字节架构）和
The mark word has word size (4 byte on 32 bit architectures, 8 byte on 64 bit architectures) and
klass指针在 32位架构上的字大小。在 64位架构中，klass指针有字大小，但如果堆地址可以是 4字节以 4个字节编码。
the klass pointer has word size on 32 bit architectures. On 64 bit architectures the klass pointer either has word size, but can also have 4 byte if the heap addresses can be encoded in these 4 bytes.
此优化称为压缩oops，您也可以使用选项 UseCompressedOops 。
This optimization is called "compressed oops" and you can also control it with the option UseCompressedOops.
您还可以找到关于此的wiki条目 1 。
You can also find a wiki entry about this 1.
标记词实际上用于很多事情。
The mark word is actually used for many things.
- 一个是
有偏见的锁定2 ，HotSpot可以通过它实现高效锁定。 - 它也在
GC期间用于设置转发指针，而用于存储对象的年龄。对象的标识哈希码可以存储在标记内（System.identityHashCode/Object.hashCode一个） 。
- One is
Biased Locking2 through which HotSpot can implement efficient locking. - It is also used during
GC to set forward pointers, andto store the age of the objects. The identity hash code of an object can be stored inside the mark (theSystem.identityHashCode/Object.hashCodeone).
markOop.hpp ，根据体系结构描述布局：
There is a comment in the source code of markOop.hpp that describes the layout depending on the architecture:
// 32 bits:
// --------
// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
// size:32 ------------------------------------------>| (CMS free block)
// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
// 64 bits:
// --------
// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
// size:64 ----------------------------------------------------->| (CMS free block)
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
您还可以找到oop头文件这里。
You can also find the oop header file here.
- 1 https ：//wiki.openjdk.java.net/display/HotSpot/CompressedOops
- 2 https://wiki.openjdk.java.net/display/HotSpot/Synchronization
- 1 https://wiki.openjdk.java.net/display/HotSpot/CompressedOops
- 2 https://wiki.openjdk.java.net/display/HotSpot/Synchronization
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