http://ifeve.com/volatile-array-visiblity/

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。

`01`	`public` `class` `Seg {`

02

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

04

`05`	`public` `void` `setValue(int` `index) {`

`06`	`tabs[index] =` `new` `Object();`

07 }

08

`09`	`public` `Object getValue(int` `index) {`

`10`	`return` `tabs[index];`

11 }

12 }

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

001 Java HotSpot(TM) Client VM warning: PrintAssembly is enabled; turning on DebugNonSafepoints to gain additional output

002 CompilerOracle: compileonly *ArrayTest.main

003 Loaded disassembler from D:\java\jdk1.6.0_33\jre\bin\client\hsdis-i386.dll

004 Decoding compiled method 0x009ec7c8:

005 Code:

006 [Disassembling for mach='i386']

007 [Entry Point]

008 [Verified Entry Point]

009 [Constants]

010 # {method} 'main' '([Ljava/lang/String;)V' in 'volatileTest/ArrayTest'

011 # parm0: ecx = '[Ljava/lang/String;'

012 # [sp+0x30] (sp of caller)

013 0x009ec8e0: mov %eax,-0x3000(%esp)

014 0x009ec8e7: push %ebp

015 0x009ec8e8: sub $0x28,%esp

016 0x009ec8eb: mov $0x32aa1eb0,%esi ; {oop('volatileTest/ArrayTest')}

017 0x009ec8f0: mov 0x150(%esi),%edi ;*getstatic tabs

018 ; - volatileTest.ArrayTest::main@0 (line 7)

019 0x009ec8f6: mov $0x1,%ecx ;*invokestatic valueOf

020 ; - volatileTest.ArrayTest::main@5 (line 7)

021 0x009ec8fb: mov %esi,0x14(%esp)

022 0x009ec8ff: mov %edi,0x10(%esp)

023 0x009ec903: call 0x009ad340 ; OopMap{[20]=Oop [16]=Oop off=40}

024 ;*invokestatic valueOf

025 ; - volatileTest.ArrayTest::main@5 (line 7)

026 ; {static_call}

027 0x009ec908: mov 0x10(%esp),%edi

028 0x009ec90c: lea 0xc(%edi),%ebx

029 0x009ec90f: cmpl $0x0,0x8(%edi) ; implicit exception: dispatches to 0x009eca07

030 0x009ec916: jbe 0x009eca11

031 0x009ec91c: cmp $0x0,%eax

032 0x009ec91f: je 0x009ec960

033 0x009ec925: mov 0x4(%edi),%edx ; implicit exception: dispatches to 0x009eca1d

034 0x009ec928: mov 0x4(%eax),%esi

035 0x009ec92b: mov 0x88(%edx),%edx

036 0x009ec931: cmp %edx,%esi

037 0x009ec933: je 0x009ec960

038 0x009ec939: mov 0x10(%edx),%edi

039 0x009ec93c: cmp (%esi,%edi,1),%edx

040 0x009ec93f: je 0x009ec960

041 0x009ec945: cmp $0x14,%edi

042 0x009ec948: jne 0x009eca22

043 0x009ec94e: push %esi

044 0x009ec94f: push %edx

045 0x009ec950: call 0x009ebb40 ; {runtime_call}

046 0x009ec955: pop %esi

047 0x009ec956: pop %edx

048 0x009ec957: cmp $0x0,%edx

049 0x009ec95a: je 0x009eca22

050 0x009ec960: mov %eax,(%ebx)

051 0x009ec962: shr $0x9,%ebx

052 0x009ec965: movb $0x0,0x28eb100(%ebx) ;*aastore

053 ; - volatileTest.ArrayTest::main@8 (line 7)

054 0x009ec96c: mov $0xa,%ebx

055 0x009ec971: mov $0x37a0e380,%edx ; {oop('java/lang/Object'[])}

056 0x009ec976: mov %ebx,%edi

057 0x009ec978: cmp $0xffffff,%ebx

058 0x009ec97e: ja 0x009eca27

059 0x009ec984: mov $0x13,%esi

060 0x009ec989: lea (%esi,%ebx,4),%esi

061 0x009ec98c: and $0xfffffff8,%esi

062 0x009ec98f: mov %fs:0x0(,%eiz,1),%ecx

063 0x009ec997: mov -0xc(%ecx),%ecx

064 0x009ec99a: mov 0x34(%ecx),%eax

065 0x009ec99d: lea (%eax,%esi,1),%esi

066 0x009ec9a0: cmp 0x3c(%ecx),%esi

067 0x009ec9a3: ja 0x009eca27

068 0x009ec9a9: mov %esi,0x34(%ecx)

069 0x009ec9ac: sub %eax,%esi

070 0x009ec9ae: movl $0x1,(%eax)

071 0x009ec9b4: mov %edx,0x4(%eax)

072 0x009ec9b7: mov %ebx,0x8(%eax)

073 0x009ec9ba: sub $0xc,%esi

074 0x009ec9bd: je 0x009ec9e3

075 0x009ec9c3: xor %ebx,%ebx

076 0x009ec9c5: shr $0x3,%esi

077 0x009ec9c8: jae 0x009ec9d8

078 0x009ec9ce: mov %ebx,0xc(%eax,%esi,8)

079 0x009ec9d2: je 0x009ec9e3

080 0x009ec9d8: mov %ebx,0x8(%eax,%esi,8)

081 0x009ec9dc: mov %ebx,0x4(%eax,%esi,8)

082 0x009ec9e0: dec %esi

083 0x009ec9e1: jne 0x009ec9d8

084 0x009ec9e3: mov 0x14(%esp),%esi

085 0x009ec9e7: mov %eax,0x150(%esi)

086 0x009ec9ed: shr $0x9,%esi

087 0x009ec9f0: movb $0x0,0x28eb100(%esi)

088 0x009ec9f7: lock addl $0x0,(%esp) ;*putstatic tabs

089 ; - volatileTest.ArrayTest::main@14 (line 9)

090 0x009ec9fc: add $0x28,%esp

091 0x009ec9ff: pop %ebp

092 0x009eca00: test %eax,0x950100 ; {poll_return}

093 0x009eca06: ret

094 0x009eca07: call 0x009ea4d0 ; OopMap{[20]=Oop eax=Oop edi=Oop off=300}

095 ;*aastore

096 ; - volatileTest.ArrayTest::main@8 (line 7)

097 ; {runtime_call}

098 0x009eca0c: call 0x009ea4d0 ; OopMap{[20]=Oop eax=Oop edi=Oop off=305}

099 ;*aastore

100 ; - volatileTest.ArrayTest::main@8 (line 7)

101 ; {runtime_call}

102 0x009eca11: movl $0x0,(%esp)

103 0x009eca18: call 0x009ea1d0 ; OopMap{[20]=Oop eax=Oop edi=Oop off=317}

104 ;*aastore

105 ; - volatileTest.ArrayTest::main@8 (line 7)

106 ; {runtime_call}

107 0x009eca1d: call 0x009ea4d0 ; OopMap{[20]=Oop eax=Oop off=322}

108 ;*aastore

109 ; - volatileTest.ArrayTest::main@8 (line 7)

110 ; {runtime_call}

111 0x009eca22: call 0x009eb850 ; OopMap{[20]=Oop eax=Oop off=327}

112 ;*aastore

113 ; - volatileTest.ArrayTest::main@8 (line 7)

114 ; {runtime_call}

115 0x009eca27: call 0x009eb1c0 ; OopMap{[20]=Oop off=332}

116 ;*anewarray

117 ; - volatileTest.ArrayTest::main@11 (line 9)

118 ; {runtime_call}

119 0x009eca2c: jmp 0x009ec9e3

120 0x009eca2e: nop

121 0x009eca2f: nop

122 0x009eca30: mov %fs:0x0(,%eiz,1),%esi

123 0x009eca38: mov -0xc(%esi),%esi

124 0x009eca3b: mov 0x184(%esi),%eax

125 0x009eca41: movl $0x0,0x184(%esi)

126 0x009eca4b: movl $0x0,0x188(%esi)

127 0x009eca55: add $0x28,%esp

128 0x009eca58: pop %ebp

129 0x009eca59: jmp 0x009bb5c0 ; {runtime_call}

130 0x009eca5e: hlt

131 0x009eca5f: hlt

132 [Stub Code]

133 0x009eca60: nop ; {no_reloc}

134 0x009eca61: nop

135 0x009eca62: mov $0x0,%ebx ; {static_stub}

136 0x009eca67: jmp 0x009eca67 ; {runtime_call}

137 [Exception Handler]

138 0x009eca6c: call 0x009eb600 ; {runtime_call}

139 0x009eca71: push $0x6db038d4 ; {external_word}

140 0x009eca76: call 0x009eca7b

141 0x009eca7b: pusha

142 0x009eca7c: call 0x6da09de0 ; {runtime_call}

143 0x009eca81: hlt

144 [Deopt Handler Code]

145 0x009eca82: push $0x9eca82 ; {section_word}

146 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。

`1`	`0x009ec9f7: lock addl $0x0,(%esp) ;*putstatic tabs`

`2`	`; - volatileTest.ArrayTest::main@14 (line 9)`

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volatile是否能保证数组中元素的可见性？

问题

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