Java中的String到底占用多大的内存空间？你所了解的可能都是错误的！！

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"写在前面"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最近小伙伴加群时，我总是问一个问题：Java中的String类占用多大的内存空间？很多小伙伴的回答着实让我哭笑不得，有说不占空间的，有说1个字节的，有说2个字节的，有说3个字节的，有说不知道的，更让人哭笑不得的是竟然还有人说是2的31次方。那如果真是这样的话，服务器的内存空间还放不下一个字符串呀！作为程序员的我们，可不能闹这种笑话呀。今天，我们就一起来聊聊Java中的String到底占用多大的内存空间！"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Java对象的结构"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先，我们来下Java对象在虚拟机中的结构，这里，以HotSpot虚拟机为例。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/8c/8c539363612ade7cf8830e0fb8228a2f.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注：图片来源http://r6d.cn/wp7q"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"从上面的这张图里面可以看出，对象在内存中的结构主要包含以下几个部分："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Mark Word(标记字段)：对象的Mark Word部分占4个字节，其内容是一系列的标记位，比如轻量级锁的标记位，偏向锁标记位等等。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Klass Pointer（Class对象指针）：Class对象指针的大小也是4个字节，其指向的位置是对象对应的Class对象（其对应的元数据对象）的内存地址"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对象实际数据：这里面包括了对象的所有成员变量，其大小由各个成员变量的大小决定，比如：byte和boolean是1个字节，short和char是2个字节，int和float是4个字节，long和double是8个字节，reference是4个字节"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对齐：最后一部分是对齐填充的字节，按8个字节填充。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"换种说法就是："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对象头（object header）：8 个字节（保存对象的 class 信息、ID、在虚拟机中的状态）"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Java 原始类型数据：如 int, float, char 等类型的数据"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"引用（reference）：4 个字节"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"填充符（padding）"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Java中的String类型"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"空String占用的空间"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这里，我们以Java8为例进行说明。首先，我们来看看String类中的成员变量。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"/** The value is used for character storage. */\nprivate final char value[];\n \n/** Cache the hash code for the string */\nprivate int hash; // Default to 0\n \n/** use serialVersionUID from JDK 1.0.2 for interoperability */\nprivate static final long serialVersionUID = -6849794470754667710L;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在 Java 里数组也是对象，因此数组也有对象头。所以，一个数组所占的空间为对象头所占的空间加上数组长度加上数组的引用，即 8 + 4 + 4= 16 字节 。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以，我们可以得出一个空String对象所占用的内存空间，如下所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"对象头（8 字节）+ 引用 (4 字节 ) + char 数组（16 字节）+ 1个 int（4字节）+ 1个long（8字节）= 40 字节"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"所以，小伙伴们，你们的回答正确吗？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"非空String占用的空间"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果String字符串的长度大于0的话，我们也可以得出String占用内存的计算公式，如下所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"40 + 2 * n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其中，n为字符串的长度。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这里，可能有小伙伴会问，为什么是 40 + 2 "},{"type":"text","marks":[{"type":"italic"}],"text":" n 呢？这是因为40是空字符串占用的内存空间，这个我们上面已经说过了，String类实际上是把数据存储到char[]这个成员变量数组中的，而char[]数组中的一个char类型的数据占用2个字节的空间，所以，只是String中的数据就会占用 2 "},{"type":"text","text":" n（n为字符串的长度）个字节的空间，再加上空字符串所占用的40个字节空间，最终得出一个字符串所占用的存储空间为： 40 + 2 * n （n为字符串长度）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注：40 + 2 * n 这个公式我们可以看成是计算String对象占用多大内存空间的通用公式。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因此在代码中大量使用String对象时，应考虑内存的实际占用情况。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"验证结论"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"接下来，我们就一起来验证下我们上面的结论。首先，创建一个UUIDUtils类用来生成32位的UUID，如下所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"package io.mykit.binghe.string.test;\n\nimport java.util.UUID;\n\n/**\n * @author binghe\n * @version 1.0.0\n * @description 生成没有-的UUID\n */\npublic class UUIDUtils {\n\tpublic static String getUUID(){\n\t\tString uuid = UUID.randomUUID().toString();\n\t\treturn uuid.replace(\"-\", \"\");\n\t}\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"接下来，创建一个TestString类，在main()方法中创建一个长度为4000000的数组，然后在数组中放满UUID字符串，如下所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"package io.mykit.binghe.string.test;\n\nimport java.util.UUID;\n\n/**\n * @author binghe\n * @version 1.0.0\n * @description 测试String占用的内存空间\n */\npublic class TestString{\n public static void main(String[] args){\n String[] strContainer = new String[4000000];\n for(int i = 0; i < 4000000; i++){\n strContainer[i] = UUIDUtils.getUUID();\n System.out.println(i);\n }\n //防止程序退出\n while(true){\n\n }\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这里，4000000个字符串，每个字符串的长度为32，所以保存字符串数据所占用的内存空间为：(40 + 32 "},{"type":"text","marks":[{"type":"italic"}],"text":" 2) "},{"type":"text","text":" 4000000 = 416000000字节，约等于416MB。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们使用Jprofiler内存分析工具进行分析："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b5/b5eaffd7be02486915204a9a1f9f2193.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到，使用Jprofiler内存分析工具的结果为：321MB + 96632KB，约等于417MB。之所以使用Jprofiler内存分析工具得出的结果比我们计算的大些，是因为在程序实际运行的过程中，程序内部也会生成一些字符串，这些字符串也会占用内存空间！！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以，使用Jprofiler内存分析工具得出的结果符合我们的预期。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"好了，今天就到这儿吧，希望小伙伴们能有所收获，我是冰河，我们下期见！！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"重磅福利"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"微信搜一搜【冰河技术】微信公众号，关注这个有深度的程序员，每天阅读超硬核技术干货，公众号内回复【PDF】有我准备的一线大厂面试资料和我原创的超硬核PDF技术文档，以及我为大家精心准备的多套简历模板（不断更新中），希望大家都能找到心仪的工作，学习是一条时而郁郁寡欢，时而开怀大笑的路，加油。如果你通过努力成功进入到了心仪的公司，一定不要懈怠放松，职场成长和新技术学习一样，不进则退。如果有幸我们江湖再见！ "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"另外，我开源的各个PDF，后续我都会持续更新和维护，感谢大家长期以来对冰河的支持！！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"写在最后"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果你觉得冰河写的还不错，请微信搜索并关注「 "},{"type":"text","marks":[{"type":"strong"}],"text":"冰河技术"},{"type":"text","text":" 」微信公众号，跟冰河学习高并发、分布式、微服务、大数据、互联网和云原生技术，「 "},{"type":"text","marks":[{"type":"strong"}],"text":"冰河技术"},{"type":"text","text":" 」微信公众号更新了大量技术专题，每一篇技术文章干货满满！不少读者已经通过阅读「 "},{"type":"text","marks":[{"type":"strong"}],"text":"冰河技术"},{"type":"text","text":" 」微信公众号文章，吊打面试官，成功跳槽到大厂；也有不少读者实现了技术上的飞跃，成为公司的技术骨干！如果你也想像他们一样提升自己的能力，实现技术能力的飞跃，进大厂，升职加薪，那就关注「 "},{"type":"text","marks":[{"type":"strong"}],"text":"冰河技术"},{"type":"text","text":" 」微信公众号吧，每天更新超硬核技术干货，让你对如何提升技术能力不再迷茫！"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/86/86642f445ccf193620f77650ac1ae2bc.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}