### 最常见的类加载器分类 * Bootstrap ClassLoader * ExtClassLoader * AppClassLoader * User Defined Classloader\(自定义类加载器\) #### Bootstrap ClassLoader * 称为启动类加载器，主要是用于加载JDK核心类，包括JRE\_HOME\lib下的**rt.jar、resources.jar、charsets.jar**等，或者由"-Xbootclasspath"指定路径中的所有类型 * 启动类加载器由C++编写并嵌在JVM内部，加载器名称为Null 可通过如下程序获得该类加载器从哪些地方加载了相关的jar或class文件： ``` URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs(); for (int i = 0; i < urls.length; i++) { System.out.println(urls[i].toExternalForm()); } output==> file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/resources.jar file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/rt.jar file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/sunrsasign.jar file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jsse.jar file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jce.jar file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/charsets.jar file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/jfr.jar file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/classes ``` 其实上述结果也是通过查找**sun.boot.class.path**这个系统属性所得知的。 ``` System.out.println(System.getProperty("sun.boot.class.path")); ``` #### ExtClassLoader ExtClassLoader派生于ClassLoader\(Java语言编写\)，负责加载JAVA\_HOME/lib/ext目录下的类 #### AppClassLoader AppClassLoader系统类加载器派生于ClassLoader\(Java语言编写\)，用于加载当前应用classpath下的所有类，也被称作SystemClassLoader ``` package com.quancheng.jvm; public class Appliction { public static void main(String[] args) { ClassLoader bootstrap = System.class.getClassLoader(); System.err.println(null != bootstrap ? bootstrap.getClass().getName() : null); ClassLoader ext = sun.security.ec.SunEC.class.getClassLoader(); System.err.println(ext); ClassLoader app = Appliction.class.getClassLoader(); System.err.println(app.getClass().getName()); } } output==> null sun.misc.Launcher$ExtClassLoader@eed1f14 sun.misc.Launcher$AppClassLoader ```