{"id":9829,"date":"2023-11-28T21:15:26","date_gmt":"2023-11-28T13:15:26","guid":{"rendered":"https:\/\/wx.kaifamiao.info\/?p=9829"},"modified":"2023-11-28T21:15:31","modified_gmt":"2023-11-28T13:15:31","slug":"java-zhong-shen-me-shicas-shi-xian-yuan-li-shi-she","status":"publish","type":"post","link":"http:\/\/wx.kaifamiao.info\/index.php\/2023\/11\/28\/java-zhong-shen-me-shicas-shi-xian-yuan-li-shi-she\/","title":{"rendered":"Java\u4e2d\u4ec0\u4e48\u662fCAS,\u5b9e\u73b0\u539f\u7406\u662f\u4ec0\u4e48\uff1f"},"content":{"rendered":"

CAS\uff1a\u5168\u79f0Compare and swap,\u5b57\u9762\u610f\u601d\uff1a"\u6bd4\u8f83\u5e76\u4ea4\u6362"\uff0c\u4e00\u4e2aCAS\u6d89\u53ca\u5230\u4ee5\u4e0b\u64cd\u4f5c
\n\u6211\u4eec\u5047\u8bbe\u5185\u5b58\u4e2d\u7684\u539f\u6570\u636eV,\u65e7\u7684\u9884\u671f\u503cA,\u9700\u8981\u4fee\u6539\u7684\u65b0\u503cB\u30021.\u6bd4\u8f83A\u4e0eV\u662f\u5426\u76f8\u7b49\u3002
\n(\u6bd4\u8f83)2.\u5982\u679c\u6bd4\u8f83\u76f8\u7b49\uff0c\u5c06B\u5199\u5165V\u3002(\u4ea4\u6362)3.\u8fd4\u56de\u64cd\u4f5c\u662f\u5426\u6210\u529f\u3002
\n\u5f53\u591a\u4e2a\u7ebf\u7a0b\u540c\u65f6\u5bf9\u67d0\u4e2a\u8d44\u6e90\u8fdb\u884cCAS\u64cd\u4f5c\uff0c\u53ea\u80fd\u6709\u4e00\u4e2a\u7ebf\u7a0b\u64cd\u4f5c\u6210\u529f\uff0c\u4f46\u662f\u5e76\u4e0d\u4f1a\u963b\u585e\u5176\u4ed6\u7ebf \u7a0b\uff0c\u5176\u4ed6\u7ebf\u7a0b\u53ea\u4f1a\u6536\u5230\u64cd\u4f5c\u5931\u8d25\u7684\u4fe1\u53f7\u3002\u53ef\u89c1CAS\u5176\u5b9e\u662f\u4e00\u4e2a\u4e50\u89c2\u9501\u3002
\nCAS\u662f\u600e\u4e48\u5b9e\u73b0\u7684
\n\u8ddf\u968fAtominteger\u7684\u4ee3\u7801\u6211\u4eec\u4e00\u8def\u5f80\u4e0b\uff0c\u5c31\u80fd\u53d1\u73b0\u6700\u7ec8\u8c03\u7528\u7684\u662fsum.misc.Unsafe\u8fd9\u4e2a\u7c7b\u3002\u770b \u540d\u79f0Unsafe\u5c31\u662f\u4e00\u4e2a\u4e0d\u5b89\u5168\u7684\u7c7b\uff0c\u8fd9\u4e2a\u7c7b\u662f\u5229\u7528\u4e86 Java\u7684\u7c7b\u548c\u5305\u5728\u53ef\u89c1\u6027\u7684\u7684\u89c4\u5219\u4e2d\u7684\u4e00 \u4e2a\u6070\u5230\u597d\u5904\u5904\u7684\u6f0f\u6d1e\u3002Unsafe\u8fd9\u4e2a\u7c7b\u4e3a\u4e86\u901f\u5ea6\uff0c\u5728Java\u7684\u5b89\u5168\u6807\u51c6\u4e0a\u505a\u5c80\u4e86 \u4e00\u5b9a\u7684\u59a5\u534f\u3002
\n\u518d\u5f80\u4e0b\u5bfb\u627e\u6211\u4eec\u53d1\u73b0Unsafe\u7684compareAndSwapInt\u662fNative\u7684\u65b9\u6cd5\uff1a
\npublic final native boolean compareAndSwapInt(Object varl, long var2, int var4, int var5);
\n\u4e5f\u5c31\u662f\u8bf4\uff0c\u8fd9\u51e0\u4e2aCAS\u7684\u65b9\u6cd5\u5e94\u8be5\u662f\u4f7f\u7528\u4e86\u672c\u5730\u7684\u65b9\u6cd5\u3002\u6240\u4ee5\u8fd9\u51e0\u4e2a\u65b9\u6cd5\u7684\u5177\u4f53\u5b9e\u73b0\u9700\u8981\u6211\u4eec\u81ea\u5df1\u53bbjdk\u7684\u6e90\u7801\u4e2d\u641c\u7d22\u3002
\n\u4e8e\u662f\u6211\u4e0b\u8f7d\u4e00\u4e2aOpenJdk\u7684\u6e90\u7801\u7ee7\u7eed\u5411\u4e0b\u63a2\u7d22\uff0c\u6211\u4eec\u53d1\u73b0
\n\u5728 \/jdk9u\/hotspot\/src\/share\/vm\/unsafe.cpp\u4e2d\u6709\u8fd9\u6837\u7684\u4ee3\u7801\uff1a
\n{CC "compareAndSetlnt", CC " (" OBJFN_PTR(Unsafe_CompareAndSetInt)},
\n\u8fd9\u4e2a\u6d89\u53ca\u5230\uff0cJNI\u7684\u8c03\u7528\uff0c\u611f\u5174\u8da3\u7684\u540c\u5b66\u53ef\u4ee5\u81ea\u884c\u5b66\u4e60\u3002\u6211\u4eec\u641c\u7d22Unsafe_CompareAndSetInt \u540e\u53d1\u73b0\uff1a
\nUNSAFE_ENTRY(jboolean, Unsafe_CompareAndSetInt (JNIEnv env, jobject unsafe, jobject obj, jlong offset, jint e, jint x)) {
\noop p = JNIHandles::resolve(obj);
\njint<\/em> addr = (jint ) index_oop_from_field_offset_long(p, offset);
\nreturn (jint)(Atomic::cmpxchg(x, addr, e)) == e;
\n} UNSAFE_END
\n\u6700\u7ec8\u6211\u4eec\u7ec8\u4e8e\u770b\u5230\u4e86\u6838\u5fc3\u4ee3\u7801Atomic:: cmpxchg\u3002
\n\u7ee7\u7eed\u5411\u5e95\u5c42\u63a2\u7d22\uff0c\u5728\u6587\u4ef6
\njava\/jdk9u\/hotspot\/src\/os_cpu\/linux_x86\/vm\/atomic_linux_x86.hpp \u6709\u8fd9\u6837\u7684\u4ee3\u7801\uff1a
\ninline jint Atomic::cmpxchg (jintexchange_value, volatile jint<\/em> dest, jint
\ncompare_value, cmpxchg_memory_order order) {
\nint mp = os:: is_MP();
\nasm<\/em> volati.le (LOCK_IF_MP(%4) "cmpxchgl %1,(%3)"
\n:"=a" (exchange_value)
\n:"r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
\n: cc , memory );
\nreturn exchange_value ;
\n}
\n\u6211\u4eec\u901a\u8fc7\u6587\u4ef6\u540d\u53ef\u4ee5\u77e5\u9053\uff0c\u9488\u5bf9\u4e0d\u540c\u7684\u64cd\u4f5c\u7cfb\u7edf,JVM\u5bf9\u4e8eAtomic::cmpxchg\u5e94\u8be5\u6709\u4e0d\u540c\u7684\u5b9e \u73b0\u3002\u7531\u4e8e\u6211\u4eec\u670d\u52a1\u57fa\u672c\u90fd\u662f\u4f7f\u7528\u7684\u662f64\u4f4dlinux,\u6240\u4ee5\u6211\u4eec\u5c31\u770b\u770blinux_x86\u7684\u5b9e\u73b0\u3002
\n\u6211\u4eec\u7ee7\u7eed\u770b\u4ee3\u7801\uff1a
\n1\u3001\t_asm_\u7684\u610f\u601d\u662f\u8fd9\u4e2a\u662f\u4e00\u6bb5\u5185\u5d4c\u6c47\u7f16\u4ee3\u7801\u3002\u4e5f\u5c31\u662f\u5728C\u8bed\u8a00\u4e2d\u4f7f\u7528\u6c47\u7f16\u4ee3\u7801\u3002
\n2\u3001\t\u8fd9\u91cc\u7684volatile\u548cJAVA\u6709\u4e00\u70b9\u7c7b\u4f3c\uff0c\u4f46\u4e0d\u662f\u4e3a\u4e86\u5185\u5b58\u7684\u53ef\u89c1\u6027\uff0c\u800c\u662f\u544a\u8bc9\u7f16\u8bd1\u5668\u5bf9\u8bbf \u95ee\u8be5\u53d8\u91cf\u7684\u4ee3\u7801\u5c31\u4e0d\u518d\u8fdb\u884c\u4f18\u5316\u3002
\n3\u3001\tLOCK_IF_MP(%4)\u7684\u610f\u601d\u5c31\u6bd4\u8f83\u7b80\u5355\uff0c\u5c31\u662f\u5982\u679c\u64cd\u4f5c\u7cfb\u7edf\u662f\u591a\u7ebf\u7a0b\u7684\uff0c\u90a3\u5c31\u589e\u52a0\u4e00\u4e2aLOCK\u3002
\n4\u3001\tcmpxchgl\u5c31\u662f\u6c47\u7f16\u7248\u7684\u201c\u6bd4\u8f83\u5e76\u4ea4\u6362\u201d\u3002\u4f46\u662f\u6211\u4eec\u77e5\u9053\u6bd4\u8f83\u5e76\u4ea4\u6362\uff0c\u6709\u4e09\u4e2a\u6b65\u9aa4\uff0c\u4e0d\u662f \u539f\u5b50\u7684\u3002\u6240\u4ee5\u5728\u591a\u6838\u60c5\u51b5\u4e0b\u52a0\u4e00\u4e2aLOCK,\u7531CPU\u786c\u4ef6\u4fdd\u8bc1\u4ed6\u7684\u539f\u5b50\u6027\u3002
\n5,\u6211\u4eec\u518d\u770b\u770bLOCK\u662f\u600e\u4e48\u5b9e\u73b0\u7684\u5462\uff1f\u6211\u4eec\u53bbIntel\u7684\u5b98\u7f51\u4e0a\u770b\u770b\uff0c\u53ef\u4ee5\u77e5\u9053LOCK\u5728\u7684\u65e9 \u671f\u5b9e\u73b0\u662f\u76f4\u63a5\u5c06cup\u7684\u603b\u7ebf\u963b\u585e\uff0c\u8fd9\u6837\u7684\u5b9e\u73b0\u53ef\u89c1\u6548\u7387\u662f\u5f88\u4f4e\u4e0b\u7684\u3002\u540e\u6765\u4f18\u5316\u4e3aX86 cpu\u6709 \u9501\u5b9a\u4e00\u4e2a\u7279\u5b9a\u5185\u5b58\u5730\u5740\u7684\u80fd\u529b\uff0c\u5f53\u8fd9\u4e2a\u7279\u5b9a\u5185\u5b58\u5730\u5740\u88ab\u9501\u5b9a\u540e\uff0c\u5b83\u5c31\u53ef\u4ee5\u963b\u6b62\u5176\u4ed6\u7684\u7cfb\u7edf\u603b\u7ebf \u8bfb\u53d6\u6216\u4fee\u6539\u8fd9\u4e2a\u5185\u5b58\u5730\u5740\u3002
\n\u5173\u4e8eCAS\u7684\u5e95\u5c42\u63a2\u7d22\u6211\u4eec\u5c31\u5230\u6b64\u4e3a\u6b62\u3002\u6211\u4eec\u603b\u7ed3\u4e00\u4e0bJAVA\u7684cas\u662f\u600e\u4e48\u5b9e\u73b0\u7684\uff1a
\n1\u3001\tjava\u7684cas\u5229\u7528\u7684\u7684\u662funsafe\u8fd9\u4e2a\u7c7b\u63d0\u4f9b\u7684cas\u64cd\u4f5c\u3002
\n2\u3001\tunsafe\u7684cas\u4f9d\u8d56\u4e86\u7684\u662fjvm\u9488\u5bf9\u4e0d\u540c\u7684\u64cd\u4f5c\u7cfb\u7edf\u5b9e\u73b0\u7684Atomic::cmpxchg
\n3\u3001\tAtomic:: cmpxchg\u7684\u5b9e\u73b0\u4f7f\u7528\u4e86\u6c47\u7f16\u7684cas\u64cd\u4f5c\uff0c\u5e76\u4f7f\u7528cpu\u786c\u4ef6\u63d0\u4f9b\u7684lock\u4fe1\u53f7\u4fdd \u8bc1\u5176\u539f\u5b50\u6027
\nCAS\u7684\u4f7f\u7528\u80fd\u591f\u907f\u514d\u7ebf\u7a0b\u7684\u963b\u585e\u3002
\n\u591a\u6570\u60c5\u51b5\u4e0b\u6211\u4eec\u4f7f\u7528\u7684\u662fwhile true\u76f4\u5230\u6210\u529f\u4e3a\u6b62\u3002
\nCAS\u7f3a\u70b9<\/p>\n

1\u3001\tABA\u7684\u95ee\u9898\uff0c\u5c31\u662f\u4e00\u4e2a\u503c\u4eceA\u53d8\u6210\u4e86 B\u53c8\u53d8\u6210\u4e86 A,\u4f7f\u7528CAS\u64cd\u4f5c\u4e0d\u80fd\u53d1\u73b0\u8fd9\u4e2a\u503c\u53d1\u751f\u53d8 \u5316\u4e86\uff0c\u5904\u7406\u65b9\u5f0f\u662f\u53ef\u4ee5\u4f7f\u7528\u643a\u5e26\u7c7b\u4f3c\u65f6\u95f4\u6233\u7684\u7248\u672cAtomicStampedReference
\n2,\u6027\u80fd\u95ee\u9898\uff0c\u6211\u4eec\u4f7f\u7528\u65f6\u5927\u90e8\u5206\u65f6\u95f4\u4f7f\u7528\u7684\u662fwhile true\u65b9\u5f0f\u5bf9\u6570\u636e\u7684\u4fee\u6539\uff0c\u76f4\u5230\u6210\u529f\u4e3a \u6b62\u3002\u4f18\u52bf\u5c31\u662f\u76f8\u5e94\u6781\u5feb\uff0c\u4f46\u5f53\u7ebf\u7a0b\u6570\u4e0d\u505c\u589e\u52a0\u65f6\uff0c\u6027\u80fd\u4e0b\u964d\u660e\u663e\uff0c\u56e0\u4e3a\u6bcf\u4e2a\u7ebf\u7a0b\u90fd\u9700\u8981\u6267\u884c\uff0c\u5360\u7528CPU\u65f6\u95f4\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"

CAS\uff1a\u5168\u79f0Compare and swap,\u5b57\u9762\u610f\u601d\uff1a"\u6bd4\u8f83\u5e76\u4ea4\u6362"\uff0c\u4e00\u4e2aCAS\u6d89\u53ca\u5230\u4ee5 […]<\/p>\n","protected":false},"author":7,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[118],"tags":[],"class_list":["post-9829","post","type-post","status-publish","format-standard","hentry","category-java"],"_links":{"self":[{"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/posts\/9829","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/users\/7"}],"replies":[{"embeddable":true,"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/comments?post=9829"}],"version-history":[{"count":1,"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/posts\/9829\/revisions"}],"predecessor-version":[{"id":9830,"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/posts\/9829\/revisions\/9830"}],"wp:attachment":[{"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/media?parent=9829"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/categories?post=9829"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/wx.kaifamiao.info\/index.php\/wp-json\/wp\/v2\/tags?post=9829"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}