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《Java并发编程的艺术》读书笔记二

    博客分类:
  • JVM
 
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java的对象构成


对于JVM来说,构造JAVA对象时,是以oops-klass二分模型来构建的,其中oops表示对象的相关信息。

基本结构如下:

class oopDesc {
  friend class VMStructs;
private:
  volatile markOop  _mark;
  union _metadata {
    wideKlassOop    _klass;
    narrowOop       _compressed_klass;
  } _metadata;


对于数组来说,其结构如下:
class arrayOopDesc : public oopDesc

从源码上没找到length字段,我们只能看注释:
// The layout of array Oops is:
//
//  markOop
//  klassOop  // 32 bits if compressed but declared 64 in LP64.
//  length    // shares klass memory or allocated after declared fields.

还有一段:
  // The _length field is not declared in C++.  It is allocated after the
  // declared nonstatic fields in arrayOopDesc if not compressed, otherwise
  // it occupies the second half of the _klass field in oopDesc.


这里的mark字段,用来表示一些JVM内部需要的相关信息

从OpenJDK源码中可以看到:

看下注释:

//  32 bits:
//  --------
//             hash:25 ------------>| age:4    biased_lock:1 lock:2 (normal object)

//  64 bits:
//  --------
//  unused:25 hash:31 -->| unused:1   age:4    biased_lock:1 lock:2 (normal object)

//  unused:25 hash:31 -->| cms_free:1 age:4    biased_lock:1 lock:2 (COOPs && normal object)

我们运行的时候,默认是使用UseCompressedOops的,所以对应COOPs

我的机器是64bits的,测试下 normal object的mark:

源码如下:

public class HashCodeMemViewDemo {

                 public static void main(String[] args) throws IOException {
                                DataEntity data = new DataEntity();
                                 data.setDataId(0x01020304);
                                
                                System. out.println(data .hashCode());
                                
                                System. in.read();
                }
}

class DataEntity {
                 private int dataId;
                 public int getDataId() {
                                 return dataId ;
                }
                 public void setDataId(int dataId) {
                                 this.dataId = dataId;
                }
}


运行后输出的值为:1359740460
(对应的二进制值为:1010001000010111111111000101100)

查看对象DataEntity的内存布局:

hsdb> mem 0xf5980a58 2
0x00000000f5980a58: 0x000000510bfe2c01
0x00000000f5980a60: 0x01020304dbc99c30

0x000000510bfe2c01 就是DataEntity对象的mark字段值,对应的二进制值为(已拆分):

0000000000000000000000000(unused:25)
1010001000010111111111000101100(hash:31)
0(cms_free:1)
0000(age:4)
0(biased_lock:1)
01(lock:2)

这里的(hash:31),根据 程序中输出的值 对比, 就是DataEntity对应的hash code值了。

另外,这里的hash是 System.identityHashCode

其中一份PPT里提到:

● First word of every object is the mark word
● Used for synchronization and garbage collection
     ● Also holds identity hash code if computed



因为mark字段中会存储对象的age,做个测试:

同样的代码,增加GC。

GC前:

identity hash code: 1359740460
DataEntity addr: 0xf5980b48

hsdb> mem 0xf5980b48 2
0x00000000f5980b48: 0x000000510bfe2c01
0x00000000f5980b50: 0x01020304dbc99cc0

mark字段解析后:
0000000000000000000000000(unused:25)
1010001000010111111111000101100(hash:31)
0(cms_free:1)
0000(age:4)
0(biased_lock:1)
01(lock:2)

经过1次GC:

DataEntity addr: 0xe0c00838
地址已经变了

hsdb> mem 0xe0c00838 2
0x00000000e0c00838: 0x000000510bfe2c09
0x00000000e0c00840: 0x01020304dbc99930

0000000000000000000000000
1010001000010111111111000101100
0
0001(age:4)
0
01

因为mark字段还用来表示锁相关信息,目前不介绍锁的细节,只用一个例子来说明一点问题。

代码如下:

public class LockMemViewDemo {

                 public static void main(String[] args) throws IOException {
                                DataEntity data = new DataEntity();
                                 data.setDataId(0x01020304);

                                 synchronized (data ) {
                                                System. out.println(System.identityHashCode( data));
                                                System. in.read();
                                }
                }
}

运行后输出的值为:353537765

查看对象DataEntity的内存布局:

hsdb> mem 0xf5980b78 2
0x00000000f5980b78: 0x00007f69cc004c1a
0x00000000f5980b80: 0x01020304dbc99ca0

其中mark字段的值为:0x00007f69cc004c1a ,对应的二进制值为(已拆分):
00000000000000000111111101101001110011000000000001001100000110
10

按照源码中的注释:
//    [ptr             | 10]  monitor            inflated lock (header is wapped out)

因此第一段就是指向monitor的指针,是0x1FDA73001306,具体是什么,指向哪个对象,无法验证。

有一段供参考:
In the uncontended case, a Java mutex consists of just 2 bits in the flags word. The JVM only associates a heavy-weight OS lock object with a Java mutex when the mutex is contended, and then releases the OS lock when the mutex has been exited by all threads.


参考:
http://arturmkrtchyan.com/java-object-header
http://stackoverflow.com/questions/4068562/how-heavy-are-java-monitors
http://www.programmershare.com/2077632/
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