{"id":3283,"date":"2019-05-17T16:17:22","date_gmt":"2019-05-17T08:17:22","guid":{"rendered":"https:\/\/damogame.cn\/wordpress\/?p=3283"},"modified":"2019-05-17T16:17:22","modified_gmt":"2019-05-17T08:17:22","slug":"%e6%b7%b1%e5%85%a5%e7%90%86%e8%a7%a3%e8%af%bb%e5%86%99%e9%94%81reentrantreadwritelock","status":"publish","type":"post","link":"https:\/\/i007.cc\/wordpress\/archives\/3283","title":{"rendered":"\u6df1\u5165\u7406\u89e3\u8bfb\u5199\u9501ReentrantReadWriteLock"},"content":{"rendered":"
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\u539f\u6587\u5730\u5740<\/a><\/div>\n<\/div>\n<\/div>\n

1.\u8bfb\u5199\u9501\u7684\u4ecb\u7ecd<\/h1>\n

\u5728\u5e76\u53d1\u573a\u666f\u4e2d\u7528\u4e8e\u89e3\u51b3\u7ebf\u7a0b\u5b89\u5168\u7684\u95ee\u9898\uff0c\u6211\u4eec\u51e0\u4e4e\u4f1a\u9ad8\u9891\u7387\u7684\u4f7f\u7528\u5230\u72ec\u5360\u5f0f\u9501\uff0c\u901a\u5e38\u4f7f\u7528java\u63d0\u4f9b\u7684\u5173\u952e\u5b57synchronized\uff08\u5173\u4e8esynchronized\u53ef\u4ee5\u770b\u8fd9\u7bc7\u6587\u7ae0<\/a>\uff09\u6216\u8005concurrents\u5305\u4e2d\u5b9e\u73b0\u4e86Lock\u63a5\u53e3\u7684ReentrantLock<\/a>\u3002\u5b83\u4eec\u90fd\u662f\u72ec\u5360\u5f0f\u83b7\u53d6\u9501\uff0c\u4e5f\u5c31\u662f\u5728\u540c\u4e00\u65f6\u523b\u53ea\u6709\u4e00\u4e2a\u7ebf\u7a0b\u80fd\u591f\u83b7\u53d6\u9501\u3002\u800c\u5728\u4e00\u4e9b\u4e1a\u52a1\u573a\u666f\u4e2d\uff0c\u5927\u90e8\u5206\u53ea\u662f\u8bfb\u6570\u636e\uff0c\u5199\u6570\u636e\u5f88\u5c11\uff0c\u5982\u679c\u4ec5\u4ec5\u662f\u8bfb\u6570\u636e\u7684\u8bdd\u5e76\u4e0d\u4f1a\u5f71\u54cd\u6570\u636e\u6b63\u786e\u6027\uff08\u51fa\u73b0\u810f\u8bfb\uff09\uff0c\u800c\u5982\u679c\u5728\u8fd9\u79cd\u4e1a\u52a1\u573a\u666f\u4e0b\uff0c\u4f9d\u7136\u4f7f\u7528\u72ec\u5360\u9501\u7684\u8bdd\uff0c\u5f88\u663e\u7136\u8fd9\u5c06\u662f\u51fa\u73b0\u6027\u80fd\u74f6\u9888\u7684\u5730\u65b9\u3002\u9488\u5bf9\u8fd9\u79cd\u8bfb\u591a\u5199\u5c11\u7684\u60c5\u51b5\uff0cjava\u8fd8\u63d0\u4f9b\u4e86\u53e6\u5916\u4e00\u4e2a\u5b9e\u73b0Lock\u63a5\u53e3\u7684ReentrantReadWriteLock(\u8bfb\u5199\u9501)\u3002\u8bfb\u5199\u6240\u5141\u8bb8\u540c\u4e00\u65f6\u523b\u88ab\u591a\u4e2a\u8bfb\u7ebf\u7a0b\u8bbf\u95ee\uff0c\u4f46\u662f\u5728\u5199\u7ebf\u7a0b\u8bbf\u95ee\u65f6\uff0c\u6240\u6709\u7684\u8bfb\u7ebf\u7a0b\u548c\u5176\u4ed6\u7684\u5199\u7ebf\u7a0b\u90fd\u4f1a\u88ab\u963b\u585e<\/strong>\u3002\u5728\u5206\u6790WirteLock\u548cReadLock\u7684\u4e92\u65a5\u6027\u65f6\u53ef\u4ee5\u6309\u7167WriteLock\u4e0eWriteLock\u4e4b\u95f4\uff0cWriteLock\u4e0eReadLock\u4e4b\u95f4\u4ee5\u53caReadLock\u4e0eReadLock\u4e4b\u95f4\u8fdb\u884c\u5206\u6790\u3002\u66f4\u591a\u5173\u4e8e\u8bfb\u5199\u9501\u7279\u6027\u4ecb\u7ecd\u5927\u5bb6\u53ef\u4ee5\u770b\u6e90\u7801\u4e0a\u7684\u4ecb\u7ecd\uff08\u9605\u8bfb\u6e90\u7801\u65f6\u6700\u597d\u7684\u4e00\u79cd\u5b66\u4e60\u65b9\u5f0f\uff0c\u6211\u4e5f\u6b63\u5728\u5b66\u4e60\u4e2d\uff0c\u4e0e\u5927\u5bb6\u5171\u52c9\uff09\uff0c\u8fd9\u91cc\u505a\u4e00\u4e2a\u5f52\u7eb3\u603b\u7ed3\uff1a<\/p>\n

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  1. \u516c\u5e73\u6027\u9009\u62e9<\/strong>\uff1a\u652f\u6301\u975e\u516c\u5e73\u6027\uff08\u9ed8\u8ba4\uff09\u548c\u516c\u5e73\u7684\u9501\u83b7\u53d6\u65b9\u5f0f\uff0c\u541e\u5410\u91cf\u8fd8\u662f\u975e\u516c\u5e73\u4f18\u4e8e\u516c\u5e73\uff1b<\/li>\n
  2. \u91cd\u5165\u6027<\/strong>\uff1a\u652f\u6301\u91cd\u5165\uff0c\u8bfb\u9501\u83b7\u53d6\u540e\u80fd\u518d\u6b21\u83b7\u53d6\uff0c\u5199\u9501\u83b7\u53d6\u4e4b\u540e\u80fd\u591f\u518d\u6b21\u83b7\u53d6\u5199\u9501\uff0c\u540c\u65f6\u4e5f\u80fd\u591f\u83b7\u53d6\u8bfb\u9501\uff1b<\/li>\n
  3. \u9501\u964d\u7ea7<\/strong>\uff1a\u9075\u5faa\u83b7\u53d6\u5199\u9501\uff0c\u83b7\u53d6\u8bfb\u9501\u518d\u91ca\u653e\u5199\u9501\u7684\u6b21\u5e8f\uff0c\u5199\u9501\u80fd\u591f\u964d\u7ea7\u6210\u4e3a\u8bfb\u9501<\/li>\n<\/ol>\n

    \u8981\u60f3\u80fd\u591f\u5f7b\u5e95\u7684\u7406\u89e3\u8bfb\u5199\u9501\u5fc5\u987b\u80fd\u591f\u7406\u89e3\u8fd9\u6837\u51e0\u4e2a\u95ee\u9898\uff1a1. \u8bfb\u5199\u9501\u662f\u600e\u6837\u5b9e\u73b0\u5206\u522b\u8bb0\u5f55\u8bfb\u5199\u72b6\u6001\u7684\uff1f2. \u5199\u9501\u662f\u600e\u6837\u83b7\u53d6\u548c\u91ca\u653e\u7684\uff1f3.\u8bfb\u9501\u662f\u600e\u6837\u83b7\u53d6\u548c\u91ca\u653e\u7684\uff1f\u6211\u4eec\u5e26\u7740\u8fd9\u6837\u7684\u4e09\u4e2a\u95ee\u9898\uff0c\u518d\u53bb\u4e86\u89e3\u4e0b\u8bfb\u5199\u9501\u3002<\/p>\n

    2.\u5199\u9501\u8be6\u89e3<\/h1>\n

    2.1.\u5199\u9501\u7684\u83b7\u53d6<\/h2>\n

    \u540c\u6b65\u7ec4\u4ef6\u7684\u5b9e\u73b0\u805a\u5408\u4e86\u540c\u6b65\u5668\uff08AQS\uff09\uff0c\u5e76\u901a\u8fc7\u91cd\u5199\u91cd\u5199\u540c\u6b65\u5668\uff08AQS\uff09\u4e2d\u7684\u65b9\u6cd5\u5b9e\u73b0\u540c\u6b65\u7ec4\u4ef6\u7684\u540c\u6b65\u8bed\u4e49\uff08\u5173\u4e8e\u540c\u6b65\u7ec4\u4ef6\u7684\u5b9e\u73b0\u5c42\u7ea7\u7ed3\u6784\u53ef\u4ee5\u770b\u8fd9\u7bc7\u6587\u7ae0<\/a>\uff0cAQS\u7684\u5e95\u5c42\u5b9e\u73b0\u5206\u6790\u53ef\u4ee5\u770b\u8fd9\u7bc7\u6587\u7ae0<\/a>\uff09\u3002\u56e0\u6b64\uff0c\u5199\u9501\u7684\u5b9e\u73b0\u4f9d\u7136\u4e5f\u662f\u91c7\u7528\u8fd9\u79cd\u65b9\u5f0f\u3002\u5728\u540c\u4e00\u65f6\u523b\u5199\u9501\u662f\u4e0d\u80fd\u88ab\u591a\u4e2a\u7ebf\u7a0b\u6240\u83b7\u53d6\uff0c\u5f88\u663e\u7136\u5199\u9501\u662f\u72ec\u5360\u5f0f\u9501\uff0c\u800c\u5b9e\u73b0\u5199\u9501\u7684\u540c\u6b65\u8bed\u4e49\u662f\u901a\u8fc7\u91cd\u5199AQS\u4e2d\u7684tryAcquire\u65b9\u6cd5\u5b9e\u73b0\u7684\u3002\u6e90\u7801\u4e3a:<\/p>\n

    protected<\/span> final<\/span> boolean<\/span> tryAcquire<\/span>(int<\/span> acquires)<\/span> <\/span>{\r\n    \/*\r\n     * Walkthrough:\r\n     * 1. If read count nonzero or write count nonzero\r\n     *    and owner is a different thread, fail.\r\n     * 2. If count would saturate, fail. (This can only\r\n     *    happen if count is already nonzero.)\r\n     * 3. Otherwise, this thread is eligible for lock if\r\n     *    it is either a reentrant acquire or\r\n     *    queue policy allows it. If so, update state\r\n     *    and set owner.\r\n     *\/<\/span>\r\n    Thread current = Thread.currentThread();\r\n    \/\/ 1. \u83b7\u53d6\u5199\u9501\u5f53\u524d\u7684\u540c\u6b65\u72b6\u6001<\/span>\r\n    int<\/span> c = getState();\r\n    \/\/ 2. \u83b7\u53d6\u5199\u9501\u83b7\u53d6\u7684\u6b21\u6570<\/span>\r\n    int<\/span> w = exclusiveCount(c);\r\n    if<\/span> (c != 0<\/span>) {\r\n        \/\/ (Note: if c != 0 and w == 0 then shared count != 0)<\/span>\r\n        \/\/ 3.1 \u5f53\u8bfb\u9501\u5df2\u88ab\u8bfb\u7ebf\u7a0b\u83b7\u53d6\u6216\u8005\u5f53\u524d\u7ebf\u7a0b\u4e0d\u662f\u5df2\u7ecf\u83b7\u53d6\u5199\u9501\u7684\u7ebf\u7a0b\u7684\u8bdd<\/span>\r\n        \/\/ \u5f53\u524d\u7ebf\u7a0b\u83b7\u53d6\u5199\u9501\u5931\u8d25<\/span>\r\n        if<\/span> (w == 0<\/span> || current != getExclusiveOwnerThread())\r\n            return<\/span> false<\/span>;\r\n        if<\/span> (w + exclusiveCount(acquires) > MAX_COUNT)\r\n            throw<\/span> new<\/span> Error(\"Maximum lock count exceeded\"<\/span>);\r\n        \/\/ Reentrant acquire<\/span>\r\n        \/\/ 3.2 \u5f53\u524d\u7ebf\u7a0b\u83b7\u53d6\u5199\u9501\uff0c\u652f\u6301\u53ef\u91cd\u590d\u52a0\u9501<\/span>\r\n        setState(c + acquires);\r\n        return<\/span> true<\/span>;\r\n    }\r\n    \/\/ 3.3 \u5199\u9501\u672a\u88ab\u4efb\u4f55\u7ebf\u7a0b\u83b7\u53d6\uff0c\u5f53\u524d\u7ebf\u7a0b\u53ef\u83b7\u53d6\u5199\u9501<\/span>\r\n    if<\/span> (writerShouldBlock() ||\r\n        !compareAndSetState(c, c + acquires))\r\n        return<\/span> false<\/span>;\r\n    setExclusiveOwnerThread(current);\r\n    return<\/span> true<\/span>;\r\n}\r\n<\/code><\/pre>\n
    \u8fd9\u6bb5\u4ee3\u7801\u7684\u903b\u8f91\u8bf7\u770b\u6ce8\u91ca\uff0c\u8fd9\u91cc\u6709\u4e00\u4e2a\u5730\u65b9\u9700\u8981\u91cd\u70b9\u5173\u6ce8\uff0cexclusiveCount(c)\u65b9\u6cd5\uff0c\u8be5\u65b9\u6cd5\u6e90\u7801\u4e3a\uff1a<\/p>\n
    static<\/span> int<\/span> exclusiveCount<\/span>(int<\/span> c)<\/span> <\/span>{ return<\/span> c & EXCLUSIVE_MASK; }\r\n<\/code><\/pre>\n
    \u5176\u4e2dEXCLUSIVE_MASK<\/strong>\u4e3a:\u00a0static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;<\/code>\u00a0EXCLUSIVE _MASK\u4e3a1\u5de6\u79fb16\u4f4d\u7136\u540e\u51cf1\uff0c\u5373\u4e3a0x0000FFFF\u3002\u800cexclusiveCount\u65b9\u6cd5\u662f\u5c06\u540c\u6b65\u72b6\u6001\uff08state\u4e3aint\u7c7b\u578b\uff09\u4e0e0x0000FFFF\u76f8\u4e0e\uff0c\u5373\u53d6\u540c\u6b65\u72b6\u6001\u7684\u4f4e16\u4f4d\u3002\u90a3\u4e48\u4f4e16\u4f4d\u4ee3\u8868\u4ec0\u4e48\u5462\uff1f\u6839\u636eexclusiveCount\u65b9\u6cd5\u7684\u6ce8\u91ca\u4e3a\u72ec\u5360\u5f0f\u83b7\u53d6\u7684\u6b21\u6570\u5373\u5199\u9501\u88ab\u83b7\u53d6\u7684\u6b21\u6570\uff0c\u73b0\u5728\u5c31\u53ef\u4ee5\u5f97\u51fa\u6765\u4e00\u4e2a\u7ed3\u8bba\u540c\u6b65\u72b6\u6001\u7684\u4f4e16\u4f4d\u7528\u6765\u8868\u793a\u5199\u9501\u7684\u83b7\u53d6\u6b21\u6570<\/strong>\u3002\u540c\u65f6\u8fd8\u6709\u4e00\u4e2a\u65b9\u6cd5\u503c\u5f97\u6211\u4eec\u6ce8\u610f\uff1a<\/p>\n
    static<\/span> int<\/span> sharedCount<\/span>(int<\/span> c)<\/span>    <\/span>{ return<\/span> c >>> SHARED_SHIFT; }\r\n<\/code><\/pre>\n
    \u8be5\u65b9\u6cd5\u662f\u83b7\u53d6\u8bfb\u9501\u88ab\u83b7\u53d6\u7684\u6b21\u6570\uff0c\u662f\u5c06\u540c\u6b65\u72b6\u6001\uff08int c\uff09\u53f3\u79fb16\u6b21\uff0c\u5373\u53d6\u540c\u6b65\u72b6\u6001\u7684\u9ad816\u4f4d\uff0c\u73b0\u5728\u6211\u4eec\u53ef\u4ee5\u5f97\u51fa\u53e6\u5916\u4e00\u4e2a\u7ed3\u8bba\u540c\u6b65\u72b6\u6001\u7684\u9ad816\u4f4d\u7528\u6765\u8868\u793a\u8bfb\u9501\u88ab\u83b7\u53d6\u7684\u6b21\u6570<\/strong>\u3002\u73b0\u5728\u8fd8\u8bb0\u5f97\u6211\u4eec\u5f00\u7bc7\u8bf4\u7684\u9700\u8981\u5f04\u61c2\u7684\u7b2c\u4e00\u4e2a\u95ee\u9898\u5417\uff1f\u8bfb\u5199\u9501\u662f\u600e\u6837\u5b9e\u73b0\u5206\u522b\u8bb0\u5f55\u8bfb\u9501\u548c\u5199\u9501\u7684\u72b6\u6001\u7684\uff0c\u73b0\u5728\u8fd9\u4e2a\u95ee\u9898\u7684\u7b54\u6848\u5c31\u5df2\u7ecf\u88ab\u6211\u4eec\u5f04\u6e05\u695a\u4e86\uff0c\u5176\u793a\u610f\u56fe\u5982\u4e0b\u56fe\u6240\u793a\uff1a<\/p>\n
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    <\/div>\n<\/div>\n
    \u8bfb\u5199\u9501\u7684\u8bfb\u5199\u72b6\u6001\u8bbe\u8ba1.png<\/div>\n<\/div>\n
    \u73b0\u5728\u6211\u4eec\u56de\u8fc7\u5934\u6765\u770b\u5199\u9501\u83b7\u53d6\u65b9\u6cd5tryAcquire\uff0c\u5176\u4e3b\u8981\u903b\u8f91\u4e3a\uff1a\u5f53\u8bfb\u9501\u5df2\u7ecf\u88ab\u8bfb\u7ebf\u7a0b\u83b7\u53d6\u6216\u8005\u5199\u9501\u5df2\u7ecf\u88ab\u5176\u4ed6\u5199\u7ebf\u7a0b\u83b7\u53d6\uff0c\u5219\u5199\u9501\u83b7\u53d6\u5931\u8d25\uff1b\u5426\u5219\uff0c\u83b7\u53d6\u6210\u529f\u5e76\u652f\u6301\u91cd\u5165\uff0c\u589e\u52a0\u5199\u72b6\u6001\u3002<\/strong><\/p>\n
    2.2.\u5199\u9501\u7684\u91ca\u653e<\/h2>\n
    \u5199\u9501\u91ca\u653e\u901a\u8fc7\u91cd\u5199AQS\u7684tryRelease\u65b9\u6cd5\uff0c\u6e90\u7801\u4e3a\uff1a<\/p>\n
    protected<\/span> final<\/span> boolean<\/span> tryRelease<\/span>(int<\/span> releases)<\/span> <\/span>{\r\n    if<\/span> (!isHeldExclusively())\r\n        throw<\/span> new<\/span> IllegalMonitorStateException();\r\n    \/\/1. \u540c\u6b65\u72b6\u6001\u51cf\u53bb\u5199\u72b6\u6001<\/span>\r\n    int<\/span> nextc = getState() - releases;\r\n    \/\/2. \u5f53\u524d\u5199\u72b6\u6001\u662f\u5426\u4e3a0\uff0c\u4e3a0\u5219\u91ca\u653e\u5199\u9501<\/span>\r\n    boolean<\/span> free = exclusiveCount(nextc) == 0<\/span>;\r\n    if<\/span> (free)\r\n        setExclusiveOwnerThread(null<\/span>);\r\n    \/\/3. \u4e0d\u4e3a0\u5219\u66f4\u65b0\u540c\u6b65\u72b6\u6001<\/span>\r\n    setState(nextc);\r\n    return<\/span> free;\r\n}\r\n<\/code><\/pre>\n
    \u6e90\u7801\u7684\u5b9e\u73b0\u903b\u8f91\u8bf7\u770b\u6ce8\u91ca\uff0c\u4e0d\u96be\u7406\u89e3\u4e0eReentrantLock\u57fa\u672c\u4e00\u81f4\uff0c\u8fd9\u91cc\u9700\u8981\u6ce8\u610f\u7684\u662f\uff0c\u51cf\u5c11\u5199\u72b6\u6001int nextc = getState() - releases;<\/code>\u53ea\u9700\u8981\u7528\u5f53\u524d\u540c\u6b65\u72b6\u6001\u76f4\u63a5\u51cf\u53bb\u5199\u72b6\u6001\u7684\u539f\u56e0\u6b63\u662f\u6211\u4eec\u521a\u624d\u6240\u8bf4\u7684\u5199\u72b6\u6001\u662f\u7531\u540c\u6b65\u72b6\u6001\u7684\u4f4e16\u4f4d\u8868\u793a\u7684<\/strong>\u3002<\/p>\n
    3.\u8bfb\u9501\u8be6\u89e3<\/h1>\n
    3.1.\u8bfb\u9501\u7684\u83b7\u53d6<\/h2>\n
    \u770b\u5b8c\u4e86\u5199\u9501\uff0c\u73b0\u5728\u6765\u770b\u770b\u8bfb\u9501\uff0c\u8bfb\u9501\u4e0d\u662f\u72ec\u5360\u5f0f\u9501\uff0c\u5373\u540c\u4e00\u65f6\u523b\u8be5\u9501\u53ef\u4ee5\u88ab\u591a\u4e2a\u8bfb\u7ebf\u7a0b\u83b7\u53d6\u4e5f\u5c31\u662f\u4e00\u79cd\u5171\u4eab\u5f0f\u9501\u3002\u6309\u7167\u4e4b\u524d\u5bf9AQS\u4ecb\u7ecd\uff0c\u5b9e\u73b0\u5171\u4eab\u5f0f\u540c\u6b65\u7ec4\u4ef6\u7684\u540c\u6b65\u8bed\u4e49\u9700\u8981\u901a\u8fc7\u91cd\u5199AQS\u7684tryAcquireShared\u65b9\u6cd5\u548ctryReleaseShared\u65b9\u6cd5\u3002\u8bfb\u9501\u7684\u83b7\u53d6\u5b9e\u73b0\u65b9\u6cd5\u4e3a\uff1a<\/p>\n
    protected<\/span> final<\/span> int<\/span> tryAcquireShared<\/span>(int<\/span> unused)<\/span> <\/span>{\r\n    \/*\r\n     * Walkthrough:\r\n     * 1. If write lock held by another thread, fail.\r\n     * 2. Otherwise, this thread is eligible for\r\n     *    lock wrt state, so ask if it should block\r\n     *    because of queue policy. If not, try\r\n     *    to grant by CASing state and updating count.\r\n     *    Note that step does not check for reentrant\r\n     *    acquires, which is postponed to full version\r\n     *    to avoid having to check hold count in\r\n     *    the more typical non-reentrant case.\r\n     * 3. If step 2 fails either because thread\r\n     *    apparently not eligible or CAS fails or count\r\n     *    saturated, chain to version with full retry loop.\r\n     *\/<\/span>\r\n    Thread current = Thread.currentThread();\r\n    int<\/span> c = getState();\r\n    \/\/1. \u5982\u679c\u5199\u9501\u5df2\u7ecf\u88ab\u83b7\u53d6\u5e76\u4e14\u83b7\u53d6\u5199\u9501\u7684\u7ebf\u7a0b\u4e0d\u662f\u5f53\u524d\u7ebf\u7a0b\u7684\u8bdd\uff0c\u5f53\u524d<\/span>\r\n    \/\/ \u7ebf\u7a0b\u83b7\u53d6\u8bfb\u9501\u5931\u8d25\u8fd4\u56de-1<\/span>\r\n    if<\/span> (exclusiveCount(c) != 0<\/span> &&\r\n        getExclusiveOwnerThread() != current)\r\n        return<\/span> -1<\/span>;\r\n    int<\/span> r = sharedCount(c);\r\n    if<\/span> (!readerShouldBlock() &&\r\n        r < MAX_COUNT &&\r\n        \/\/2. \u5f53\u524d\u7ebf\u7a0b\u83b7\u53d6\u8bfb\u9501<\/span>\r\n        compareAndSetState(c, c + SHARED_UNIT)) {\r\n        \/\/3. \u4e0b\u9762\u7684\u4ee3\u7801\u4e3b\u8981\u662f\u65b0\u589e\u7684\u4e00\u4e9b\u529f\u80fd\uff0c\u6bd4\u5982getReadHoldCount()\u65b9\u6cd5<\/span>\r\n        \/\/\u8fd4\u56de\u5f53\u524d\u83b7\u53d6\u8bfb\u9501\u7684\u6b21\u6570<\/span>\r\n        if<\/span> (r == 0<\/span>) {\r\n            firstReader = current;\r\n            firstReaderHoldCount = 1<\/span>;\r\n        } else<\/span> if<\/span> (firstReader == current) {\r\n            firstReaderHoldCount++;\r\n        } else<\/span> {\r\n            HoldCounter rh = cachedHoldCounter;\r\n            if<\/span> (rh == null<\/span> || rh.tid != getThreadId(current))\r\n                cachedHoldCounter = rh = readHolds.get();\r\n            else<\/span> if<\/span> (rh.count == 0<\/span>)\r\n                readHolds.set(rh);\r\n            rh.count++;\r\n        }\r\n        return<\/span> 1<\/span>;\r\n    }\r\n    \/\/4. \u5904\u7406\u5728\u7b2c\u4e8c\u6b65\u4e2dCAS\u64cd\u4f5c\u5931\u8d25\u7684\u81ea\u65cb\u5df2\u7ecf\u5b9e\u73b0\u91cd\u5165\u6027<\/span>\r\n    return<\/span> fullTryAcquireShared(current);\r\n}\r\n<\/code><\/pre>\n
    \u4ee3\u7801\u7684\u903b\u8f91\u8bf7\u770b\u6ce8\u91ca\uff0c\u9700\u8981\u6ce8\u610f\u7684\u662f\u00a0\u5f53\u5199\u9501\u88ab\u5176\u4ed6\u7ebf\u7a0b\u83b7\u53d6\u540e\uff0c\u8bfb\u9501\u83b7\u53d6\u5931\u8d25<\/strong>\uff0c\u5426\u5219\u83b7\u53d6\u6210\u529f\u5229\u7528CAS\u66f4\u65b0\u540c\u6b65\u72b6\u6001\u3002\u53e6\u5916\uff0c\u5f53\u524d\u540c\u6b65\u72b6\u6001\u9700\u8981\u52a0\u4e0aSHARED_UNIT\uff08(1 << SHARED_SHIFT)<\/code>\u53730x00010000\uff09\u7684\u539f\u56e0\u8fd9\u662f\u6211\u4eec\u5728\u4e0a\u9762\u6240\u8bf4\u7684\u540c\u6b65\u72b6\u6001\u7684\u9ad816\u4f4d\u7528\u6765\u8868\u793a\u8bfb\u9501\u88ab\u83b7\u53d6\u7684\u6b21\u6570\u3002\u5982\u679cCAS\u5931\u8d25\u6216\u8005\u5df2\u7ecf\u83b7\u53d6\u8bfb\u9501\u7684\u7ebf\u7a0b\u518d\u6b21\u83b7\u53d6\u8bfb\u9501\u65f6\uff0c\u662f\u9760fullTryAcquireShared\u65b9\u6cd5\u5b9e\u73b0\u7684\uff0c\u8fd9\u6bb5\u4ee3\u7801\u5c31\u4e0d\u5c55\u5f00\u8bf4\u4e86\uff0c\u6709\u5174\u8da3\u53ef\u4ee5\u770b\u770b\u3002<\/p>\n
    3.2.\u8bfb\u9501\u7684\u91ca\u653e<\/h2>\n
    \u8bfb\u9501\u91ca\u653e\u7684\u5b9e\u73b0\u4e3b\u8981\u901a\u8fc7\u65b9\u6cd5tryReleaseShared\uff0c\u6e90\u7801\u5982\u4e0b\uff0c\u4e3b\u8981\u903b\u8f91\u8bf7\u770b\u6ce8\u91ca\uff1a<\/p>\n
    protected<\/span> final<\/span> boolean<\/span> tryReleaseShared<\/span>(int<\/span> unused)<\/span> <\/span>{\r\n    Thread current = Thread.currentThread();\r\n    \/\/ \u524d\u9762\u8fd8\u662f\u4e3a\u4e86\u5b9e\u73b0getReadHoldCount\u7b49\u65b0\u529f\u80fd<\/span>\r\n    if<\/span> (firstReader == current) {\r\n        \/\/ assert firstReaderHoldCount > 0;<\/span>\r\n        if<\/span> (firstReaderHoldCount == 1<\/span>)\r\n            firstReader = null<\/span>;\r\n        else<\/span>\r\n            firstReaderHoldCount--;\r\n    } else<\/span> {\r\n        HoldCounter rh = cachedHoldCounter;\r\n        if<\/span> (rh == null<\/span> || rh.tid != getThreadId(current))\r\n            rh = readHolds.get();\r\n        int<\/span> count = rh.count;\r\n        if<\/span> (count <= 1<\/span>) {\r\n            readHolds.remove();\r\n            if<\/span> (count <= 0<\/span>)\r\n                throw<\/span> unmatchedUnlockException();\r\n        }\r\n        --rh.count;\r\n    }\r\n    for<\/span> (;;) {\r\n        int<\/span> c = getState();\r\n        \/\/ \u8bfb\u9501\u91ca\u653e \u5c06\u540c\u6b65\u72b6\u6001\u51cf\u53bb\u8bfb\u72b6\u6001\u5373\u53ef<\/span>\r\n        int<\/span> nextc = c - SHARED_UNIT;\r\n        if<\/span> (compareAndSetState(c, nextc))\r\n            \/\/ Releasing the read lock has no effect on readers,<\/span>\r\n            \/\/ but it may allow waiting writers to proceed if<\/span>\r\n            \/\/ both read and write locks are now free.<\/span>\r\n            return<\/span> nextc == 0<\/span>;\r\n    }\r\n}\r\n<\/code><\/pre>\n
    4.\u9501\u964d\u7ea7<\/h1>\n
    \u8bfb\u5199\u9501\u652f\u6301\u9501\u964d\u7ea7\uff0c\u9075\u5faa\u6309\u7167\u83b7\u53d6\u5199\u9501\uff0c\u83b7\u53d6\u8bfb\u9501\u518d\u91ca\u653e\u5199\u9501\u7684\u6b21\u5e8f\uff0c\u5199\u9501\u80fd\u591f\u964d\u7ea7\u6210\u4e3a\u8bfb\u9501<\/strong>\uff0c\u4e0d\u652f\u6301\u9501\u5347\u7ea7\uff0c\u5173\u4e8e\u9501\u964d\u7ea7\u4e0b\u9762\u7684\u793a\u4f8b\u4ee3\u7801\u6458\u81eaReentrantWriteReadLock\u6e90\u7801\u4e2d\uff1a<\/p>\n
    void<\/span> processCachedData<\/span>()<\/span> <\/span>{\r\n        rwl.readLock().lock();\r\n        if<\/span> (!cacheValid) {\r\n            \/\/ Must release read lock before acquiring write lock<\/span>\r\n            rwl.readLock().unlock();\r\n            rwl.writeLock().lock();\r\n            try<\/span> {\r\n                \/\/ Recheck state because another thread might have<\/span>\r\n                \/\/ acquired write lock and changed state before we did.<\/span>\r\n                if<\/span> (!cacheValid) {\r\n                    data = ...\r\n            cacheValid = true<\/span>;\r\n          }\r\n          \/\/ Downgrade by acquiring read lock before releasing write lock<\/span>\r\n          rwl.readLock().lock();\r\n        } finally<\/span> {\r\n          rwl.writeLock().unlock(); \/\/ Unlock write, still hold read<\/span>\r\n        }\r\n      }\r\n \r\n      try<\/span> {\r\n        use(data);\r\n      } finally<\/span> {\r\n        rwl.readLock().unlock();\r\n      }\r\n    }\r\n}<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n
    <\/div>\n","protected":false},"excerpt":{"rendered":"
    \u539f\u6587\u5730\u5740 1.\u8bfb\u5199\u9501\u7684\u4ecb\u7ecd \u5728\u5e76\u53d1\u573a\u666f\u4e2d<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"colormag_page_container_layout":"default_layout","colormag_page_sidebar_layout":"default_layout","footnotes":""},"categories":[],"tags":[119],"class_list":["post-3283","post","type-post","status-publish","format-standard","hentry","tag-03-"],"_links":{"self":[{"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/posts\/3283","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/comments?post=3283"}],"version-history":[{"count":0,"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/posts\/3283\/revisions"}],"wp:attachment":[{"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/media?parent=3283"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/categories?post=3283"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/i007.cc\/wordpress\/wp-json\/wp\/v2\/tags?post=3283"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}