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Java ClassLoader 学习反馈

Classloader是Java运行时核心机制之一，它解决了我们的.class文件如何被jvm加载的问题。也就是我们的写的源码.java文件被编译成.class字节码后，如何被jvm加载解释。
Java在广义上有三种classloader
  1. BootstrapClassLoader 是JVM级别的classloader，使用c/c++来写的。详细解释如何来加载一个bootstrap级别的class文件，哪些class文件是bootstrapclassloader来加载的。
  2. ExtClassLoader  是ExtensionClassLoader，他是AppClassLoader的 ‘父类’，这个父类不是指AppClassLoader extends ExtClassLoader，而是指ExtClassLoader来创建了AppClassloader，后面源码部分可以解释这个关系。
  3. AppClassLoader 这是一般我们自己写的class的classloader，他加载了我们自己写的class，而且同时如果我们有自定义的classloader，那么他也是custom class loader的parent class loader.
他们的加载顺序也是从上往下，即 1->2 ->3。
  到这里可能会产生疑问，他们加载的class有何不同？ 
AppClassLoader是上面说的，他会加载我们自己定义的class，即我们workspace的classpath下的class
ExtClassLoader则是加载我们Jdk lib/ext 下的class
BootstrapClassLoader是加载rt.jar、resources.jar、charsets.jar和class等。 

那么他们是如何找到lib下这些class文件呢？ 答案是通过环境变量中的一些变量，而这些变量的定义也是通过我们设置的JAVA HOME等来取得。
源码中有这么两个地方解释了这个地方。
==》 private static String bootClassPath = System.getProperty("sun.boot.class.path");
==》 String s = System.getProperty("java.ext.dirs");

蓝色部分的是定义了BootstrapClassLoader如何找到他需要的那些class所在的目录。
红色部分则是定义了ExtClassLoader如何找到他需要的class

挑选第一个，我们起一个main函数来做一个测试。
System.out.println(System.getProperty("sun.boot.class.path"));
==>>>
C:\Program Files\Java\jdk1.8.0\jre\lib\resources.jar;
C:\Program Files\Java\jdk1.8.0\jre\lib\rt.jar;
C:\Program Files\Java\jdk1.8.0\jre\lib\sunrsasign.jar;
C:\Program Files\Java\jdk1.8.0\jre\lib\jsse.jar;
C:\Program Files\Java\jdk1.8.0\jre\lib\jce.jar;
C:\Program Files\Java\jdk1.8.0\jre\lib\charsets.jar;
C:\Program Files\Java\jdk1.8.0\jre\lib\jfr.jar;
C:\Program Files\Java\jdk1.8.0\jre\classes
  这就是classloader如何加载class的方式。

  • 找到一个指定的路径
  • 加载路径下指定的class文件





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