通过实例解析JMM和Volatile底层原理

public class VolatileTest {
  private static volatile boolean flag = false;
  public static void main(String[] args) {
    update();
  }

  public static void update(){
    flag = true;
    System.out.println(flag);
  }
}
Volatile JIT编译器编译java代码为汇编指令查看
1.在jdk\jre\bin\ 目录下添加 hsdis-amd64.lib
2.在jdk1.8\jre\bin\server\目录下添加hsdis-amd64.dll文件
3.在IDEA中设置 JVM参数
-server -Xcomp -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -XX:CompileCommand=compileonly,VolatileTest.update
4.运行Java程序即可打印出
CompilerOracle: compileonly *VolatileTest.update
Loaded disassembler from E:\EclipseDev\jdk\jdk1.8\jre\bin\server\hsdis-amd64.dll
Decoding compiled method 0x000000000f11aad0:
Code:
Argument 0 is unknown.RIP: 0xf11ac40 Code size: 0x000002a8
[Disassembling for mach='amd64']
[Entry Point]
[Verified Entry Point]
[Constants]
 # {method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest'
 #      [sp+0x40] (sp of caller)
 0x000000000f11ac40: mov   dword ptr [rsp+0ffffffffffffa000h],eax
 0x000000000f11ac47: push  rbp
 0x000000000f11ac48: sub   rsp,30h
 0x000000000f11ac4c: mov   r8,8792d70h    ;  {metadata(method data for {method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest')}
 0x000000000f11ac56: mov   edx,dword ptr [r8+0dch]
 0x000000000f11ac5d: add   edx,8h
 0x000000000f11ac60: mov   dword ptr [r8+0dch],edx
 0x000000000f11ac67: mov   r8,8792b70h    ;  {metadata({method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest')}
 0x000000000f11ac71: and   edx,0h
 0x000000000f11ac74: cmp   edx,0h
 0x000000000f11ac77: je   0f11ad68h     ;*iconst_1
                        ; - com.yew.test.VolatileTest::update@0 (line 17)
 0x000000000f11ac7d: mov   r8,0d7b08a30h   ;  {oop(a 'java/lang/Class' = 'com/yew/test/VolatileTest')}
 0x000000000f11ac87: mov   edx,1h
 0x000000000f11ac8c: mov   byte ptr [r8+68h],dl
volatile修饰
 0x000000000f11ac90: lock add dword ptr [rsp],0h ;*putstatic flag
                        ; - com.yew.test.VolatileTest::update@1 (line 17)
无Volatile修饰
 0x000000000f113707: mov byte ptr [r8+68h],1h ;*putstatic flag
                        ; - com.yew.test.VolatileTest::update@1 (line 17)
通过比较可知：改变共享变量flag的值为true,该变量由Volatile修饰，进行汇编打印时，会有lock前缀修饰，根据IA-32架构软件开发者手册可知，lock前缀指令在多核CPU处理器下会引发两件事情：
【1】将当前处理器缓存行的数据立即写回系统内存
【2】wirte操作会使其他处理器中缓存该内存地址的数据无效
LOCK#声言期间，处理器独占任何共享内存。IA-32处理器和Intel 64处理器使用MESI（修改、独占、共享、无效）控制协议去维护内部缓存和其他处理器缓存的一致性。通过嗅探技术保证处理器内部缓存、系统缓存和其他处理器缓存的数据再总线上保持一致。当其他处理器打算回写内存地址，该地址是共享内存区域，那么嗅探的处理器会将它的缓存行设置为无效，下次访问相同内存时，强制执行缓存行填充。
0x000000000f11ac95: nop
 0x000000000f11ac98: jmp   0f11add4h     ;  {no_reloc}
 0x000000000f11ac9d: add   byte ptr [rax],al
 0x000000000f11ac9f: add   byte ptr [rax],al
 0x000000000f11aca1: add   byte ptr [rsi+0fh],ah
 0x000000000f11aca4: Fatal error: Disassembling failed with error code: 15Decoding compiled method 0x000000000f11ef50:
Code:
Argument 0 is unknown.RIP: 0xf11f080 Code size: 0x00000058
[Entry Point]
[Verified Entry Point]
[Constants]
 # {method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest'
 #      [sp+0x20] (sp of caller)
 0x000000000f11f080: mov   dword ptr [rsp+0ffffffffffffa000h],eax
 0x000000000f11f087: push  rbp
 0x000000000f11f088: sub   rsp,10h
 0x000000000f11f08c: mov   r10,0d7b08a30h  ;  {oop(a 'java/lang/Class' = 'com/yew/test/VolatileTest')}
 0x000000000f11f096: mov   byte ptr [r10+68h],1h
 0x000000000f11f09b: lock add dword ptr [rsp],0h ;*putstatic flag
                        ; - com.yew.test.VolatileTest::update@1 (line 17)
 0x000000000f11f0a0: mov   edx,1ch
 0x000000000f11f0a5: nop
 0x000000000f11f0a7: call  0f0557a0h     ; OopMap{off=44}
                        ;*getstatic out
                        ; - com.yew.test.VolatileTest::update@4 (line 18)
                        ;  {runtime_call}
 0x000000000f11f0ac: int3           ;*getstatic out
                        ; - com.yew.test.VolatileTest::update@4 (line 18)
 0x000000000f11f0ad: hlt
 0x000000000f11f0ae: hlt
 0x000000000f11f0af: hlt
 0x000000000f11f0b0: hlt
 0x000000000f11f0b1: hlt
 0x000000000f11f0b2: hlt
 0x000000000f11f0b3: hlt
 0x000000000f11f0b4: hlt
 0x000000000f11f0b5: hlt
 0x000000000f11f0b6: hlt
 0x000000000f11f0b7: hlt
 0x000000000f11f0b8: hlt
 0x000000000f11f0b9: hlt
 0x000000000f11f0ba: hlt
 0x000000000f11f0bb: hlt
 0x000000000f11f0bc: hlt
 0x000000000f11f0bd: hlt
 0x000000000f11f0be: hlt
 0x000000000f11f0bf: hlt
[Exception Handler]
[Stub Code]
 0x000000000f11f0c0: jmp   0f0883a0h     ;  {no_reloc}
[Deopt Handler Code]
 0x000000000f11f0c5: call  0f11f0cah
 0x000000000f11f0ca: sub   qword ptr [rsp],5h
 0x000000000f11f0cf: jmp   0f057600h     ;  {runtime_call}
 0x000000000f11f0d4: hlt
 0x000000000f11f0d5: hlt
 0x000000000f11f0d6: hlt
 0x000000000f11f0d7: hlt
true