在javaeye有位朋友问了我一个非常好的问题。

问题

一个线程向volatile的数组中设置值，而另一个线程向volatile的数组中读取。
比如seg.setValue(2)，随后另一个线程调用seg.getValue(2)，前一个线程设置的值对读取的线程是可见的吗？

我看书上说volatile的数组只针对数组的引用具有volatile的语义，而不是它的元素。

ConcurrentHashMap中也有这样的代码，我很疑惑，希望得到你的解答，谢谢。

public class Seg {

private volatile Object[] tabs = new Object[10];

public void setValue(int index) {
tabs[index] = new Object();
}

public Object getValue(int index) {
return tabs[index];
}
}

我的回答

我做了实验证实这句话是正确的，“volatile的数组只针对数组的引用具有volatile的语义，而不是它的元素”。

测试代码如下：
private static volatile Object[] tabs = new Object[10];

public static void main(String[] args) {
tabs[0]=1;

tabs=new Object[10];
}

编译成汇编语句如下

Java HotSpot(TM) Client VM warning: PrintAssembly is enabled; turning on DebugNonSafepoints to gain additional output
CompilerOracle: compileonly *ArrayTest.main
Loaded disassembler from D:/java/jdk1.6.0_33/jre/bin/client/hsdis-i386.dll
Decoding compiled method 0x009ec7c8:
Code:
[Disassembling for mach='i386']
[Entry Point]
[Verified Entry Point]
[Constants]
# {method} 'main' '([Ljava/lang/String;)V' in 'volatileTest/ArrayTest'
# parm0: ecx = '[Ljava/lang/String;'
# [sp+0x30] (sp of caller)
0x009ec8e0: mov %eax,-0x3000(%esp)
0x009ec8e7: push %ebp
0x009ec8e8: sub $0x28,%esp
0x009ec8eb: mov $0x32aa1eb0,%esi ; {oop('volatileTest/ArrayTest')}
0x009ec8f0: mov 0x150(%esi),%edi ;*getstatic tabs
; - volatileTest.ArrayTest::main@0 (line 7)
0x009ec8f6: mov $0x1,%ecx ;*invokestatic valueOf
; - volatileTest.ArrayTest::main@5 (line 7)
0x009ec8fb: mov %esi,0x14(%esp)
0x009ec8ff: mov %edi,0x10(%esp)
0x009ec903: call 0x009ad340 ; OopMap{[20]=Oop [16]=Oop off=40}
;*invokestatic valueOf
; - volatileTest.ArrayTest::main@5 (line 7)
; {static_call}
0x009ec908: mov 0x10(%esp),%edi
0x009ec90c: lea 0xc(%edi),%ebx
0x009ec90f: cmpl $0x0,0x8(%edi) ; implicit exception: dispatches to 0x009eca07
0x009ec916: jbe 0x009eca11
0x009ec91c: cmp $0x0,%eax
0x009ec91f: je 0x009ec960
0x009ec925: mov 0x4(%edi),%edx ; implicit exception: dispatches to 0x009eca1d
0x009ec928: mov 0x4(%eax),%esi
0x009ec92b: mov 0x88(%edx),%edx
0x009ec931: cmp %edx,%esi
0x009ec933: je 0x009ec960
0x009ec939: mov 0x10(%edx),%edi
0x009ec93c: cmp (%esi,%edi,1),%edx
0x009ec93f: je 0x009ec960
0x009ec945: cmp $0x14,%edi
0x009ec948: jne 0x009eca22
0x009ec94e: push %esi
0x009ec94f: push %edx
0x009ec950: call 0x009ebb40 ; {runtime_call}
0x009ec955: pop %esi
0x009ec956: pop %edx
0x009ec957: cmp $0x0,%edx
0x009ec95a: je 0x009eca22
0x009ec960: mov %eax,(%ebx)
0x009ec962: shr $0x9,%ebx
0x009ec965: movb $0x0,0x28eb100(%ebx) ;*aastore
; - volatileTest.ArrayTest::main@8 (line 7)
0x009ec96c: mov $0xa,%ebx
0x009ec971: mov $0x37a0e380,%edx ; {oop('java/lang/Object'[])}
0x009ec976: mov %ebx,%edi
0x009ec978: cmp $0xffffff,%ebx
0x009ec97e: ja 0x009eca27
0x009ec984: mov $0x13,%esi
0x009ec989: lea (%esi,%ebx,4),%esi
0x009ec98c: and $0xfffffff8,%esi
0x009ec98f: mov %fs:0x0(,%eiz,1),%ecx
0x009ec997: mov -0xc(%ecx),%ecx
0x009ec99a: mov 0x34(%ecx),%eax
0x009ec99d: lea (%eax,%esi,1),%esi
0x009ec9a0: cmp 0x3c(%ecx),%esi
0x009ec9a3: ja 0x009eca27
0x009ec9a9: mov %esi,0x34(%ecx)
0x009ec9ac: sub %eax,%esi
0x009ec9ae: movl $0x1,(%eax)
0x009ec9b4: mov %edx,0x4(%eax)
0x009ec9b7: mov %ebx,0x8(%eax)
0x009ec9ba: sub $0xc,%esi
0x009ec9bd: je 0x009ec9e3
0x009ec9c3: xor %ebx,%ebx
0x009ec9c5: shr $0x3,%esi
0x009ec9c8: jae 0x009ec9d8
0x009ec9ce: mov %ebx,0xc(%eax,%esi,8)
0x009ec9d2: je 0x009ec9e3
0x009ec9d8: mov %ebx,0x8(%eax,%esi,8)
0x009ec9dc: mov %ebx,0x4(%eax,%esi,8)
0x009ec9e0: dec %esi
0x009ec9e1: jne 0x009ec9d8
0x009ec9e3: mov 0x14(%esp),%esi
0x009ec9e7: mov %eax,0x150(%esi)
0x009ec9ed: shr $0x9,%esi
0x009ec9f0: movb $0x0,0x28eb100(%esi)
0x009ec9f7: lock addl $0x0,(%esp) ;*putstatic tabs
; - volatileTest.ArrayTest::main@14 (line 9)
0x009ec9fc: add $0x28,%esp
0x009ec9ff: pop %ebp
0x009eca00: test %eax,0x950100 ; {poll_return}
0x009eca06: ret
0x009eca07: call 0x009ea4d0 ; OopMap{[20]=Oop eax=Oop edi=Oop off=300}
;*aastore
; - volatileTest.ArrayTest::main@8 (line 7)
; {runtime_call}
0x009eca0c: call 0x009ea4d0 ; OopMap{[20]=Oop eax=Oop edi=Oop off=305}
;*aastore
; - volatileTest.ArrayTest::main@8 (line 7)
; {runtime_call}
0x009eca11: movl $0x0,(%esp)
0x009eca18: call 0x009ea1d0 ; OopMap{[20]=Oop eax=Oop edi=Oop off=317}
;*aastore
; - volatileTest.ArrayTest::main@8 (line 7)
; {runtime_call}
0x009eca1d: call 0x009ea4d0 ; OopMap{[20]=Oop eax=Oop off=322}
;*aastore
; - volatileTest.ArrayTest::main@8 (line 7)
; {runtime_call}
0x009eca22: call 0x009eb850 ; OopMap{[20]=Oop eax=Oop off=327}
;*aastore
; - volatileTest.ArrayTest::main@8 (line 7)
; {runtime_call}
0x009eca27: call 0x009eb1c0 ; OopMap{[20]=Oop off=332}
;*anewarray
; - volatileTest.ArrayTest::main@11 (line 9)
; {runtime_call}
0x009eca2c: jmp 0x009ec9e3
0x009eca2e: nop
0x009eca2f: nop
0x009eca30: mov %fs:0x0(,%eiz,1),%esi
0x009eca38: mov -0xc(%esi),%esi
0x009eca3b: mov 0x184(%esi),%eax
0x009eca41: movl $0x0,0x184(%esi)
0x009eca4b: movl $0x0,0x188(%esi)
0x009eca55: add $0x28,%esp
0x009eca58: pop %ebp
0x009eca59: jmp 0x009bb5c0 ; {runtime_call}
0x009eca5e: hlt
0x009eca5f: hlt
[Stub Code]
0x009eca60: nop ; {no_reloc}
0x009eca61: nop
0x009eca62: mov $0x0,%ebx ; {static_stub}
0x009eca67: jmp 0x009eca67 ; {runtime_call}
[Exception Handler]
0x009eca6c: call 0x009eb600 ; {runtime_call}
0x009eca71: push $0x6db038d4 ; {external_word}
0x009eca76: call 0x009eca7b
0x009eca7b: pusha
0x009eca7c: call 0x6da09de0 ; {runtime_call}
0x009eca81: hlt
[Deopt Handler Code]
0x009eca82: push $0x9eca82 ; {section_word}
0x009eca87: jmp 0x009ad970 ; {runtime_call}

可以看到line 7给数组的某个元素赋值时没有lock前缀的指令。
而修改数组line 9才有lock前缀的指令。

0x009ec9f7: lock addl $0x0,(%esp) ;*putstatic tabs
; - volatileTest.ArrayTest::main@14 (line 9)