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            ?

            ?1?package?test;
            ?2?
            ?3?public?class?testClone?{
            ?4?????volatile?boolean?isInit;
            ?5?
            ?6?????volatile?Foo?foo;
            ?7?
            ?8?????volatile?int?time?=?1;
            ?9?
            10?????public?class?Foo?{
            11?????????volatile?int?flg;
            12?
            13?????????public?Foo()?{
            14?????????????flg?=?0;
            15?????????????try?{
            16?????????????????Thread.sleep(time);
            17?????????????}?catch?(InterruptedException?e)?{
            18?????????????}
            19?????????????++flg;
            20?????????????System.out.println("Foo?inited");
            21?????????}
            22?????}
            23?
            24?????public?static?void?main(String[]?args)?throws?InterruptedException?{
            25?????????testClone?t?=?new?testClone();
            26?????????t.test();
            27?????}
            28?
            29?????public?void?test()?{
            30?????????for?(int?i?=?0;?i?<?5;?++i)?{
            31?????????????WorkThread?t?=?new?WorkThread();
            32?????????????t.start();
            33?????????}
            34?
            35?????????for?(;;)?{
            36?????????????try?{
            37?????????????????Thread.sleep(1000);
            38?????????????}?catch?(InterruptedException?e)?{
            39?????????????}
            40?????????????time?=?1000;
            41?????????????synchronized?(this)?{
            42?????????????????foo?=?null;
            43?????????????}
            44?????????}
            45?????}
            46?
            47?????public?Foo?bar()?{
            48?????????Foo?f?=?foo;
            49?????????if?(f?==?null)?{
            50?????????????synchronized?(this)?{
            51?????????????????if?(foo?==?null)?{
            52?????????????????????foo?=?new?Foo();
            53?????????????????}
            54?????????????????return?foo;
            55?????????????}
            56?????????}
            57?????????return?f;
            58?????}
            59?
            60?????public?class?WorkThread?extends?Thread?{
            61?????????public?void?run()?{
            62?????????????for?(;;)?{
            63?????????????????try?{
            64?
            65?????????????????????Foo?f?=?bar();
            66?????????????????????if?(f.flg?==?0)?{
            67?????????????????????????System.out.println(f.flg);
            68?????????????????????}
            69?????????????????}?catch?(Throwable?e)?{
            70?????????????????????e.printStackTrace();
            71?????????????????}
            72?????????????}
            73?????????}
            74?????}
            75?}
            76?



            1.4.2jdk編譯執行。長時間執行沒有發現有網上所說的由于jit優化導致的當分配完Foo的內存,Foo構造函數未初始化完成就將其地址賦值給foo的錯誤。相信這時候jit已經對代碼進行了優化。

            國外網址關于Double-Checked Locking的文章http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

            A test case showing that it doesn't work

            Paul Jakubik found an example of a use of double-checked locking that did not work correctly. A slightly cleaned up version of that code is available here.

            When run on a system using the Symantec JIT, it doesn't work. In particular, the Symantec JIT compiles

            singletons[i].reference = new Singleton();

            to the following (note that the Symantec JIT using a handle-based object allocation system).

            0206106A   mov         eax,0F97E78h
            0206106F   call        01F6B210                  ; allocate space for
                                                             ; Singleton, return result in eax
            02061074   mov         dword ptr [ebp],eax       ; EBP is &singletons[i].reference 
                                                            ; store the unconstructed object here.
            02061077   mov         ecx,dword ptr [eax]       ; dereference the handle to
                                                             ; get the raw pointer
            02061079   mov         dword ptr [ecx],100h      ; Next 4 lines are
            0206107F   mov         dword ptr [ecx+4],200h    ; Singleton's inlined constructor
            02061086   mov         dword ptr [ecx+8],400h
            0206108D   mov         dword ptr [ecx+0Ch],0F84030h
            

            As you can see, the assignment to singletons[i].reference is performed before the constructor for Singleton is called. This is completely legal under the existing Java memory model, and also legal in C and C++ (since neither of them have a memory model).
            上面是國外網站給出的jit代碼和說明。


            posted on 2007-01-19 14:59 含笑半步癲 閱讀(1375) 評論(0)  編輯 收藏 引用 所屬分類: java
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