深入理解JVM中的类加载机制

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"0、引言","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"现如今,各种IDE越来越智能,我们程序员的日常开发基本上都是在IDE上完成的,它可以帮助我们将更多的注意力放在实际的业务处理中,随着这种安逸的编码生活的持续,我们慢慢也就忘记了代码运行的底层原理。如果不学习,好像也没啥问题,毕竟我们的关注重点是代码逻辑实现上,当出现问题了,百度,谷歌一下,或者问问公司的狠人,问题好像也能愉快的解决,自己好像也理解了似的。但事实上呢,依此周而复始,仍旧不理解,学习一门技术,只有我们真正懂得了其底层原理,才能更好的解决问题。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"1、类加载概述","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们在前面几篇文章中分别讲解了","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/of3CbfuAGzMevevwkno60w","title":null},"content":[{"type":"text","text":"类文件结构","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#036aca","name":"user"}}]},{"type":"text","text":",","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/kDnHV0D6aA-tUjK-fVco5w","title":null},"content":[{"type":"text","text":"JVM内存管理","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#036aca","name":"user"}}]},{"type":"text","text":"。这两篇文章详细描述了Class文件存储格式的具体细节及JVM运行时数据区。而今天这篇文章将会讲解Class文件中的信息进入到虚拟机中会发生什么变化。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"**先来个官方叙述:**类加载是Java虚拟机把描述类的数据从Class文件加载到内存,并对数据进行校验、转换解析和初始化、最终形成可以被虚拟机直接使用的Java类型。通俗来讲,就是我们在完成代码的编写后,编译器会将我们的java文件编译成对应的class文件(二进制字节码文件),通过类载器将这些class的时候将其加载到JVM中,生成对应的class对象。下面,让我们详细来分析下类加载过程。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"2、类加载过程","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对于任意一个类,类加载过程可以分为","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"加载","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"验证","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"准备","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"解析","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"使用","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"卸载","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"七个阶段,如下图所示:","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/1a/1afd8cde74443ab0ac42011ae27c10a7.png","alt":"image-20210128214026739","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"图中的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"加载","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"验证","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"准备","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"、","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"初始化","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"卸载","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"这五个阶段的顺序是确定的,而","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"解析","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"则不一定,为了支持java语言的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"运行时绑定特性","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"u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解析","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"解析阶段就是将Class中的常量池中的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"符号引用","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"解析为","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"直接引用","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"。符号引是使用一组符号描述被引用的目标,引用的目标不一定加载到内存中;直接引用可以使直接指向目标地址的指针,相对偏移量或者间接定位到目标的句柄,有了直接引用,引用的目标一定存在在虚拟机中。主要包括四种类型引用的解析,分别是类或接口解析、字段解析、方法解析和接口方法解析。下面以字段解析和方法解析为例:","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"2.5 初始化","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"初始化是类加载过程的最后一步,到了初始化阶段,才开始正真的执行字节码文件,根据字节码文件的内容对类的各个字段进行赋值;初始化是执行类构造器()方法的过程。实际上,在连接的准备阶段,类变量已赋过一次系统要求的初始值,而在初始化阶段,则是根据程序员自己写的逻辑去初始化类变量和其他资源,举例如下:","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public static int number1 = 5;\n public static int number2 = 6;\n static{\n number = 68;\n 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