平台特定的启动类加载器深入分析与自定义系统类加载器详解

平台特定的启动类加载器深入分析：

这里以上一次【https://www.cnblogs.com/webor2006/p/9157626.html】的这个程序作为学习的开端，如下：

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/sunrsasign.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/classes
/Users/xiongwei/Library/Java/Extensions:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java
/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/lib/tools.jar:/Users/xiongwei/Documents/workspace/IntelliJSpace/jvm_lectue/out/production/classes

可见不同的系统类加载器所加载的目录是不一样的，那如果换成用java命令来执行这个程序会有啥不同呢，先进入该字节码文件所在的目录：

那为啥通过IDE运行看到应用类加载器的目录有这么多呢？这是因为IntelliJ IDEA在运行时会在classPath中增加一些特定的目录，而通过java命令执行此时就脱离开了Intellij IDEA了，所以只有一个当前为点的路径了，当然IntelliJ IDEA运行时也包含了当前目录的路径，如下：

另外回顾一下这个理论：“在运行期，一个Java类是由该类的完全限定名（binary name, 二进制名）和用于加载该类的定义类加载器（defining loader）所共同决定的。如果同样名字（既相同的完全限定名）的类是由两个不同的加载器所加载，那么这些类就是不同的，既便.class文件的字节码完全一样，并且从相同的位置加载亦如此。”

接下来如果咱们改变启动类加载器的加载目录，那结果会如何呢，下面来试一下：

接下来思考一个问题：类加载器的作用是用来加载类的，但是对于类加载器（如扩展类加载器ExtClassLoader、应用类加载器AppClassLoader）也是一个java文件，那它们是由谁来加载到JVM当中的呢？其实扩展类加载器和应用类加载器都是由启动类加载器所加载的，而启动类加载器是由C++来实现的，并非是java文件，它是内建于JVM当中的，也就是归属于JVM的一部分，也就是JVM启动时启动类加载器就会被创建，也就是说：内建于JVM中的启动类加载器会加载java.lang.ClassLoader以及其它的Java平台类，当JVM启动时，一块特殊的机器码会运行，它会加载扩展类加载器与系统类加载器，这块特殊的机器码叫做启动类加载器（Bootstrap）,而启动类加载器并不是Java类，而其它的加载器则都是Java类，启动类加载器是特定于平台的机器指令，它负责开启整个加载过程。所有类加载器（除了启动类加载器）都被实现为Java类，不过总归要有一个组件来加载第一个Java类加载器，从而让整个加载过程能够顺利进行下去，加载第一个纯Java类加载器就是启动类加载器的职责。启动类加载器还会负责加载供JRE正常运行所需要的基本组件，这包括java.util与java.lang包中的类等等。

接下来修改一下代码：