### 最常見的類加載器分類 * Bootstrap ClassLoader * ExtClassLoader * AppClassLoader * User Defined Classloader\(自定義類加載器\) #### Bootstrap ClassLoader * 稱為啟動類加載器，主要是用于加載JDK核心類，包括JRE\_HOME\lib下的**rt.jar、resources.jar、charsets.jar**等，或者由"-Xbootclasspath"指定路徑中的所有類型 * 啟動類加載器由C++編寫并嵌在JVM內部，加載器名稱為Null 可通過如下程序獲得該類加載器從哪些地方加載了相關的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 ``` 其實上述結果也是通過查找**sun.boot.class.path**這個系統屬性所得知的。 ``` System.out.println(System.getProperty("sun.boot.class.path")); ``` #### ExtClassLoader ExtClassLoader派生于ClassLoader\(Java語言編寫\)，負責加載JAVA\_HOME/lib/ext目錄下的類 #### AppClassLoader AppClassLoader系統類加載器派生于ClassLoader\(Java語言編寫\)，用于加載當前應用classpath下的所有類，也被稱作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 ```