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)