{"id":4000,"date":"2025-07-28T21:31:01","date_gmt":"2025-07-28T13:31:01","guid":{"rendered":"https:\/\/www.laixuexila.com\/?p=4000"},"modified":"2025-07-28T21:31:01","modified_gmt":"2025-07-28T13:31:01","slug":"%e4%b8%ba%e4%bb%80%e4%b9%88concurrenthashmap%e4%b8%adkey%e4%b8%8d%e5%85%81%e8%ae%b8%e4%b8%banull","status":"publish","type":"post","link":"https:\/\/www.laixuexila.com\/index.php\/2025\/07\/28\/%e4%b8%ba%e4%bb%80%e4%b9%88concurrenthashmap%e4%b8%adkey%e4%b8%8d%e5%85%81%e8%ae%b8%e4%b8%banull\/","title":{"rendered":"\u4e3a\u4ec0\u4e48ConcurrentHashMap\u4e2dkey\u4e0d\u5141\u8bb8\u4e3anull"},"content":{"rendered":"\n<p>\u5728 Java \u7684 <code>ConcurrentHashMap<\/code> \u4e2d\uff0c<strong>key \u4e0d\u5141\u8bb8\u4e3a <code>null<\/code><\/strong>\uff0c\u8fd9\u662f\u51fa\u4e8e<strong>\u7ebf\u7a0b\u5b89\u5168\u6027\u3001\u8bbe\u8ba1\u4e00\u81f4\u6027\u548c\u907f\u514d\u6b67\u4e49<\/strong>\u7b49\u591a\u65b9\u9762\u7684\u8003\u8651\u3002\u4e0b\u9762\u6211\u4eec\u4ece<strong>\u539f\u7406\u5206\u6790\u3001\u5b9e\u9645\u5bf9\u6bd4\u3001\u6e90\u7801\u5c42\u9762\u3001\u5f02\u5e38\u6848\u4f8b<\/strong>\u7b49\u7ef4\u5ea6\uff0c\u5168\u9762\u89e3\u7b54\u8fd9\u4e2a\u95ee\u9898\u3002<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e00\u3001\u5b98\u65b9\u660e\u786e\u8bf4\u660e\uff1aKey \u4e0d\u5141\u8bb8\u4e3a null<\/h2>\n\n\n\n<p>\u6765\u81ea JDK \u6587\u6863\uff1a<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\u2757 <code>ConcurrentHashMap<\/code> does <strong>not allow null to be used as a key or value<\/strong>.<\/p>\n\n\n\n<p>\u5b98\u65b9\u94fe\u63a5\uff1a<a href=\"https:\/\/docs.oracle.com\/javase\/8\/docs\/api\/java\/util\/concurrent\/ConcurrentHashMap.html\">ConcurrentHashMap Javadoc &#8211; Java SE 8<\/a><\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e8c\u3001\u4e3a\u4ec0\u4e48\u4e0d\u5141\u8bb8 key \u4e3a null\uff1f<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">1. <strong>\u907f\u514d\u6b67\u4e49\uff08Ambiguity\uff09<\/strong><\/h3>\n\n\n\n<p>\u5728\u975e\u7ebf\u7a0b\u5b89\u5168\u7684 <code>HashMap<\/code> \u4e2d\uff0c\u5141\u8bb8 <code>key = null<\/code>\uff0c\u662f\u4f5c\u4e3a\u201c\u7f3a\u7701\u952e\u201d\u7684\u4e00\u79cd\u7279\u4f8b\u5904\u7406\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>HashMap&lt;String, String&gt; map = new HashMap&lt;&gt;();\nmap.put(null, \"abc\");        \/\/ \u2705 \u5408\u6cd5\nSystem.out.println(map.get(null)); \/\/ \u8f93\u51fa abc<\/code><\/pre>\n\n\n\n<p>\u4f46\u5728\u5e76\u53d1\u573a\u666f\u4e0b\uff0c\u4ee5\u4e0b\u95ee\u9898\u4f1a\u4ea7\u751f\u6b67\u4e49\uff1a<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>get(null)<\/code> \u662f\u627e\u4e0d\u5230 key\uff0c\u8fd8\u662f key \u5c31\u662f null\uff1f<\/li>\n\n\n\n<li><code>put(null, \"x\")<\/code> \u662f\u8bbe\u7f6e\u201c\u7a7a key\u201d\uff1f\u8fd8\u662f\u8bbe\u7f6e\u5931\u8d25\uff1f<\/li>\n\n\n\n<li>\u591a\u7ebf\u7a0b\u5e76\u53d1\u5904\u7406 null key \u65f6\uff0c\u7ebf\u7a0b\u4e4b\u95f4\u5982\u4f55\u540c\u6b65\u8be5\u7279\u6b8a\u503c\uff1f<\/li>\n<\/ul>\n\n\n\n<p><strong>\u8fd9\u4e9b\u903b\u8f91\u7684\u542b\u4e49\u4e0d\u660e\u786e\uff0c\u4f1a\u589e\u52a0\u5b9e\u73b0\u590d\u6742\u6027\u4e0e\u51fa\u9519\u6982\u7387\u3002<\/strong><\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">2. <strong>ConcurrentHashMap \u9700\u8981\u66f4\u4e25\u683c\u7684\u8bed\u4e49\u4fdd\u8bc1<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>ConcurrentHashMap<\/code> \u8bbe\u8ba1\u76ee\u6807\u662f<strong>\u5e76\u53d1\u5b89\u5168\u3001\u9ad8\u6027\u80fd\u3001\u9ad8\u53ef\u9884\u6d4b\u6027<\/strong>\u3002<\/li>\n\n\n\n<li>\u5728\u5e76\u53d1\u73af\u5883\u4e2d\uff0c\u5141\u8bb8 null \u4f1a\u5bfc\u81f4\u5404\u79cd\u8fb9\u754c\u884c\u4e3a\u5fc5\u987b\u989d\u5916\u5904\u7406\uff0c\u4f8b\u5982\uff1a<\/li>\n\n\n\n<li><code>get(null)<\/code> \u662f\u5426\u7b49\u4ef7\u4e8e <code>map.containsKey(null)<\/code>\uff1f<\/li>\n\n\n\n<li><code>computeIfAbsent(null, f)<\/code> \u5e94\u8be5\u629b\u5f02\u5e38\uff0c\u8fd8\u662f\u6267\u884c\uff1f<\/li>\n<\/ul>\n\n\n\n<p>\u56e0\u6b64\uff1a<strong>\u4e3a\u4e86\u4fdd\u6301\u8bed\u4e49\u6e05\u6670\u4e0e\u5b9e\u73b0\u7b80\u5355\uff0c\u5e72\u8106\u5f7b\u5e95\u7981\u6b62 null\u3002<\/strong><\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">3. <strong>\u907f\u514d\u7a7a\u6307\u9488\u5f02\u5e38\uff08NPE\uff09\u95ee\u9898\u96be\u4ee5\u8ffd\u8e2a<\/strong><\/h3>\n\n\n\n<p>\u5728\u591a\u7ebf\u7a0b\u6267\u884c\u4e2d\uff0c\u5982\u679c\u6709\u4ee5\u4e0b\u4ee3\u7801\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>String value = map.get(key);   \/\/ key == null<\/code><\/pre>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\u5728 <code>HashMap<\/code> \u4e2d\u53ef\u80fd\u8fd4\u56de null\uff08key \u4e0d\u5b58\u5728\u6216 key \u662f null\uff09<\/li>\n\n\n\n<li>\u5728 <code>ConcurrentHashMap<\/code> \u4e2d\u76f4\u63a5\u629b\u51fa <code>NullPointerException<\/code><\/li>\n<\/ul>\n\n\n\n<p>\u8fd9\u79cd\u65e9\u671f\u5931\u8d25\uff08fail-fast\uff09\u8bbe\u8ba1\u66f4\u5b89\u5168\uff0c\u6709\u5229\u4e8e\u5feb\u901f\u5b9a\u4f4d\u9519\u8bef\u3002<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">4. <strong>\u5e95\u5c42\u5b9e\u73b0\u673a\u5236\u4e0d\u652f\u6301 null key<\/strong><\/h3>\n\n\n\n<p>\u5728 <code>ConcurrentHashMap<\/code> \u7684\u5185\u90e8\u5b9e\u73b0\u4e2d\uff08Java 8+ \u91c7\u7528<strong>\u5206\u6bb5+\u94fe\u8868\/\u7ea2\u9ed1\u6811+CAS+volatile<\/strong>\u7684\u673a\u5236\uff09\uff0c\u51e0\u4e4e\u6240\u6709\u64cd\u4f5c\u90fd\u9700\u8981\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>int hash = spread(key.hashCode());<\/code><\/pre>\n\n\n\n<p>\u82e5 key \u4e3a null\uff0c<code>key.hashCode()<\/code> \u4f1a\u76f4\u63a5\u629b\u51fa <code>NullPointerException<\/code>\u3002<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e09\u3001\u5bf9\u6bd4\uff1aHashMap vs ConcurrentHashMap<\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>\u7279\u6027<\/th><th>HashMap<\/th><th>ConcurrentHashMap<\/th><\/tr><\/thead><tbody><tr><td>\u662f\u5426\u652f\u6301 null key<\/td><td>\u662f<\/td><td>\u5426<\/td><\/tr><tr><td>\u662f\u5426\u7ebf\u7a0b\u5b89\u5168<\/td><td>\u5426<\/td><td>\u662f<\/td><\/tr><tr><td>null key \u4f7f\u7528\u573a\u666f<\/td><td>\u7279\u4f8b\u5904\u7406\uff0ckey=null \u4f5c\u4e3a\u4e00\u7c7b\u952e\u503c<\/td><td>\u4f1a\u5f15\u8d77\u6b67\u4e49\u548c\u540c\u6b65\u95ee\u9898<\/td><\/tr><tr><td>\u9519\u8bef\u68c0\u6d4b<\/td><td>\u5ef6\u8fdf\u62a5\u9519<\/td><td>\u65e9\u671f\u62a5\u9519\uff08fail-fast\uff09<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u56db\u3001\u9a8c\u8bc1\u793a\u4f8b\u4ee3\u7801<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>import java.util.concurrent.ConcurrentHashMap;\n\npublic class TestMap {\n    public static void main(String&#91;] args) {\n        ConcurrentHashMap&lt;String, String&gt; cmap = new ConcurrentHashMap&lt;&gt;();\n\n        cmap.put(\"a\", \"1\"); \/\/ OK\n        \/\/ cmap.put(null, \"x\"); \/\/ \u274c \u629b\u51fa NullPointerException\n        \/\/ cmap.get(null);      \/\/ \u274c \u629b\u51fa NullPointerException\n    }\n}<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u5b98\u65b9\u6e90\u4ee3\u7801\u5f15\u7528<\/h2>\n\n\n\n<p>Java 8 \u4e2d <code>ConcurrentHashMap.put()<\/code> \u6e90\u7801\uff08\u7b80\u5316\u7248\uff09\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>public V put(K key, V value) {\n    if (key == null || value == null)\n        throw new NullPointerException();\n    ...\n}<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e94\u3001\u603b\u7ed3\u4e00\u53e5\u8bdd<\/h2>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p><strong>ConcurrentHashMap \u7981\u6b62 null key\uff0c\u662f\u4e3a\u4e86\u907f\u514d\u8bed\u4e49\u6b67\u4e49\u3001\u63d0\u5347\u7ebf\u7a0b\u5b89\u5168\u6027\uff0c\u5e76\u7b80\u5316\u5b9e\u73b0\u903b\u8f91\u3002<\/strong><\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u63a8\u8350\u9605\u8bfb<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><a href=\"https:\/\/github.com\/openjdk\/jdk\/blob\/master\/src\/java.base\/share\/classes\/java\/util\/concurrent\/ConcurrentHashMap.java\">JDK8 ConcurrentHashMap \u6e90\u7801\u89e3\u6790<\/a><\/li>\n\n\n\n<li>\u300aJava \u5e76\u53d1\u7f16\u7a0b\u5b9e\u6218\u300b\u2014\u2014 \u7b2c 5 \u7ae0\uff1a\u6784\u5efa\u7ebf\u7a0b\u5b89\u5168\u7684\u7c7b<\/li>\n\n\n\n<li>\u300aEffective Java\u300b\u6761\u76ee 50\uff1a\u660e\u667a\u5730\u4f7f\u7528\u7ebf\u7a0b\u5b89\u5168\u96c6\u5408<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p>\u66f4\u591a\u8be6\u7ec6\u5185\u5bb9\u8bf7\u5173\u6ce8\u5176\u4ed6\u76f8\u5173\u6587\u7ae0\uff01<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u5728 Java \u7684 ConcurrentHashMap \u4e2d\uff0ckey \u4e0d\u5141\u8bb8\u4e3a null\uff0c\u8fd9\u662f\u51fa\u4e8e\u7ebf\u7a0b\u5b89\u5168\u6027\u3001\u8bbe\u8ba1 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":4001,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[86,68],"tags":[],"class_list":["post-4000","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-java-","category-java"],"_links":{"self":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/4000","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/comments?post=4000"}],"version-history":[{"count":1,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/4000\/revisions"}],"predecessor-version":[{"id":4002,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/4000\/revisions\/4002"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/media\/4001"}],"wp:attachment":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/media?parent=4000"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/categories?post=4000"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/tags?post=4000"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}