《Java并发编程的艺术》读书笔记四

数组的可见性，与 数组元素 的可见性，是否是一回事？

 

比如private volatile int[] data;

 

这里的volatile，是保证data字段的可见性，那对于data里的元素，比如data[18]，是否也有volatile的语义呢？

 

看如下代码

package learn.concurrency.jcip;

import java.util.concurrent.TimeUnit;

public class VolatileArrayTest {

    private volatile int[] data;

    public VolatileArrayTest() {
        this.data = new int[20];
    }

    public void check() {
        while (this.getArray()[18] == 0) {

        }

        System.out.println("no equal, over");
    }

    public void change() {
        this.getArray()[18] = 1;
        
        int[] sameData = this.getArray(); // comment this in scenario 2
        this.setArray(sameData);// comment this in scenario 2
        
    }
    
    private int[] getArray() {
        return this.data;
    }
    
    private void setArray(int[] data) {
        this.data = data;
    }


    public static void main(String[] args) throws InterruptedException {

        final  VolatileArrayTest  test = new VolatileArrayTest();
        new Thread(new Runnable() {

            @Override
            public void run() {
                test.check();
            }
        }).start();

        TimeUnit.SECONDS.sleep(5);

        new Thread(new Runnable() {

            @Override
            public void run() {
                test.change();
            }
        }).start();

    }

}

 

架上神器HSDIS，查看check和change的汇编。

 

这里的汇编，关键的行，我都增加了注释

 

Decoding compiled method 0x00007fec29129bd0:
Code:
[Disassembling for mach='i386:x86-64']
[Entry Point]
[Constants]
  # {method} 'check' '()V' in 'learn/concurrency/jcip/VolatileArrayTest'
  #           [sp+0x30]  (sp of caller)
  0x00007fec29129d20: mov    0x8(%rsi),%r10d
  0x00007fec29129d24: cmp    %r10,%rax
  0x00007fec29129d27: jne    0x00007fec29037620  ;   {runtime_call}
  0x00007fec29129d2d: xchg   %ax,%ax
[Verified Entry Point]
  0x00007fec29129d30: mov    %eax,-0x14000(%rsp)
  0x00007fec29129d37: push   %rbp
  0x00007fec29129d38: sub    $0x20,%rsp         ;*synchronization entry
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@-1 (line 14)
  0x00007fec29129d3c: rex mov    0xc(%rsi),%ebp  ;*getfield data // 获取VolatileArrayTest实例的data字段，其实地址为12=0xc
                                                ; - learn.concurrency.jcip.VolatileArrayTest::getArray@1 (line 30)
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@1 (line 14)
  0x00007fec29129d40: mov    0xc(%rbp),%r10d    ; implicit exception: dispatches to 0x00007fec29129dc9

//获取数组的长度
  0x00007fec29129d44: cmp    $0x12,%r10d // 比较 索引18 与 数组长度，这就是java能够发现index越界的地方
  0x00007fec29129d48: jbe    0x00007fec29129da5  ;*iaload // 非法，则跳转到0x00007fec29129da5  
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@6 (line 14)
  0x00007fec29129d4a: mov    0x58(%rbp),%r10d //获取数组的地址为0x58=88的值，数组的内存布局头为8+4+4，第18个元素就是4*18
  0x00007fec29129d4e: test   %r10d,%r10d //测试data[18]是否为0
  0x00007fec29129d51: jne    0x00007fec29129d7d //如果data[18] !=0，则跳转到0x00007fec29129d7d 
  0x00007fec29129d53: nopw   0x0(%rax,%rax,1) //后续的指令不管了，是循环代码的重复
  0x00007fec29129d5c: xchg   %ax,%ax            ;*getstatic out
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@10 (line 18)
  0x00007fec29129d60: mov    0xc(%rsi),%r10d    ;*getfield data
                                                ; - learn.concurrency.jcip.VolatileArrayTest::getArray@1 (line 30)
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@1 (line 14)
  0x00007fec29129d64: mov    0xc(%r10),%r8d     ; implicit exception: dispatches to 0x00007fec29129db9
  0x00007fec29129d68: cmp    $0x12,%r8d
  0x00007fec29129d6c: jbe    0x00007fec29129d8d
  0x00007fec29129d6e: mov    0x58(%r10),%r11d   ; OopMap{rsi=Oop off=82}
                                                ;*ifeq
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@7 (line 14)
  0x00007fec29129d72: test   %eax,0x99f3288(%rip)        # 0x00007fec32b1d000
                                                ;   {poll}
  0x00007fec29129d78: test   %r11d,%r11d
  0x00007fec29129d7b: je     0x00007fec29129d60  ;*getstatic out
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@10 (line 18)
  0x00007fec29129d7d: mov    $0x18,%esi
  0x00007fec29129d82: nop    

Decoding compiled method 0x00007fec290e5b10:

Code:
[Disassembling for mach='i386:x86-64']
[Entry Point]
[Constants]
  # {method} 'change' '()V' in 'learn/concurrency/jcip/VolatileArrayTest'
  #           [sp+0x30]  (sp of caller)
  0x00007fec290e5c60: mov    0x8(%rsi),%r10d
  0x00007fec290e5c64: cmp    %r10,%rax
  0x00007fec290e5c67: jne    0x00007fec29037620  ;   {runtime_call}
  0x00007fec290e5c6d: xchg   %ax,%ax
[Verified Entry Point]
  0x00007fec290e5c70: mov    %eax,-0x14000(%rsp)
  0x00007fec290e5c77: push   %rbp
  0x00007fec290e5c78: sub    $0x20,%rsp         ;*synchronization entry
                                                ; - learn.concurrency.jcip.VolatileArrayTest::change@-1 (line 22)
  0x00007fec290e5c7c: mov    0xc(%rsi),%r11d    ;*getfield data //获取data字段
                                                ; - learn.concurrency.jcip.VolatileArrayTest::getArray@1 (line 30)
                                                ; - learn.concurrency.jcip.VolatileArrayTest::change@1 (line 22)
  0x00007fec290e5c80: mov    0xc(%r11),%r10d    ; implicit exception: dispatches to 0x00007fec290e5cd9 //获取data的length字段
  0x00007fec290e5c84: cmp    $0x12,%r10d //比较下标18和length
  0x00007fec290e5c88: jbe    0x00007fec290e5cc0 //小标非法，则跳转到0x00007fec290e5cc0 
  0x00007fec290e5c8a: movl   $0x1,0x58(%r11)    ;*synchronization entry // data[18]设置为1
                                                ; - learn.concurrency.jcip.VolatileArrayTest::getArray@-1 (line 30)
                                                ; - learn.concurrency.jcip.VolatileArrayTest::change@9 (line 24)
  0x00007fec290e5c92: mov    0xc(%rsi),%r10d // 获取data字段，存入%r10d
  0x00007fec290e5c96: mov    %r10d,0xc(%rsi) // %10d写入data字段

  0x00007fec290e5c9a: lock addl $0x0,(%rsp) // 内存屏障，用来刷新
  0x00007fec290e5c9f: mov    %rsi,%r10
  0x00007fec290e5ca2: shr    $0x9,%r10
  0x00007fec290e5ca6: mov    $0x7fec2fb69000,%r11
  0x00007fec290e5cb0: mov    %r12b,(%r11,%r10,1)  ;*synchronization entry
                                                ; - learn.concurrency.jcip.VolatileArrayTest::change@-1 (line 22)
  0x00007fec290e5cb4: add    $0x20,%rsp
  0x00007fec290e5cb8: pop    %rbp

 

红色部分，发现了lock指令， 对应的代码是：this.setArray(sameData)

而我们在check方法中，会读取data值，这就完成了data字段的volatile读和写，也就完成了HB

 

我们注释掉2行代码，继续执行：

Decoding compiled method 0x00007fb74d12b890:
Code:
[Disassembling for mach='i386:x86-64']
[Entry Point]
[Constants]
  # {method} 'check' '()V' in 'learn/concurrency/jcip/VolatileArrayTest'
  #           [sp+0x30]  (sp of caller)
  0x00007fb74d12b9e0: mov    0x8(%rsi),%r10d
  0x00007fb74d12b9e4: cmp    %r10,%rax
  0x00007fb74d12b9e7: jne    0x00007fb74d037620  ;   {runtime_call}
  0x00007fb74d12b9ed: xchg   %ax,%ax
[Verified Entry Point]
  0x00007fb74d12b9f0: mov    %eax,-0x14000(%rsp)
  0x00007fb74d12b9f7: push   %rbp
  0x00007fb74d12b9f8: sub    $0x20,%rsp         ;*synchronization entry
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@-1 (line 14)
  0x00007fb74d12b9fc: rex mov    0xc(%rsi),%ebp  ;*getfield data
                                                ; - learn.concurrency.jcip.VolatileArrayTest::getArray@1 (line 30)
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@1 (line 14)
  0x00007fb74d12ba00: mov    0xc(%rbp),%r10d    ; implicit exception: dispatches to 0x00007fb74d12ba89
  0x00007fb74d12ba04: cmp    $0x12,%r10d
  0x00007fb74d12ba08: jbe    0x00007fb74d12ba65  ;*iaload
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@6 (line 14)
  0x00007fb74d12ba0a: mov    0x58(%rbp),%r10d
  0x00007fb74d12ba0e: test   %r10d,%r10d
  0x00007fb74d12ba11: jne    0x00007fb74d12ba3d
  0x00007fb74d12ba13: nopw   0x0(%rax,%rax,1)
  0x00007fb74d12ba1c: xchg   %ax,%ax            ;*getstatic out
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@10 (line 18)
  0x00007fb74d12ba20: mov    0xc(%rsi),%r10d    ;*getfield data
                                                ; - learn.concurrency.jcip.VolatileArrayTest::getArray@1 (line 30)
                                                ; - learn.concurrency.jcip.VolatileArrayTest::check@1 (line 14)
  0x00007fb74d12ba24: mov    0xc(%r10),%r8d     ; implicit exception: dispatches to 0x00007fb74d12ba79
  0x00007fb74d12ba28: cmp    $0x12,%r8d
  0x00007fb74d12ba2c: jbe    0x00007fb74d12ba4d

Decoding compiled method 0x00007fb74d0d41d0:

Code:
[Disassembling for mach='i386:x86-64']
[Entry Point]
[Constants]
  # {method} 'change' '()V' in 'learn/concurrency/jcip/VolatileArrayTest'
  #           [sp+0x20]  (sp of caller)
  0x00007fb74d0d4320: mov    0x8(%rsi),%r10d
  0x00007fb74d0d4324: cmp    %r10,%rax
  0x00007fb74d0d4327: jne    0x00007fb74d037620  ;   {runtime_call}
  0x00007fb74d0d432d: xchg   %ax,%ax
[Verified Entry Point]
  0x00007fb74d0d4330: mov    %eax,-0x14000(%rsp)
  0x00007fb74d0d4337: push   %rbp
  0x00007fb74d0d4338: sub    $0x10,%rsp         ;*synchronization entry
                                                ; - learn.concurrency.jcip.VolatileArrayTest::change@-1 (line 22)
  0x00007fb74d0d433c: mov    0xc(%rsi),%r11d    ;*getfield data
                                                ; - learn.concurrency.jcip.VolatileArrayTest::getArray@1 (line 30)
                                                ; - learn.concurrency.jcip.VolatileArrayTest::change@1 (line 22)
  0x00007fb74d0d4340: mov    0xc(%r11),%r10d    ; implicit exception: dispatches to 0x00007fb74d0d4371
  0x00007fb74d0d4344: cmp    $0x12,%r10d
  0x00007fb74d0d4348: jbe    0x00007fb74d0d435e
  0x00007fb74d0d434a: movl   $0x1,0x58(%r11)    ;*synchronization entry
                                                ; - learn.concurrency.jcip.VolatileArrayTest::change@-1 (line 22)
  0x00007fb74d0d4352: add    $0x10,%rsp
  0x00007fb74d0d4356: pop    %rbp

 

这2段汇编，与第1种类似，唯一的区别在于： change方法中，没有lock指令了，也就没有了内存屏障，所以这就不满足HB原则， 对于 可见性来说是无法保障的。

 

尽管2个场景的代码，在执行上，目前都是正确的，都能结束循环。