关于JAVA 类加载器的研究

JAVA类加载器加载过程

JAVA有个Bootstrap ClassLoader引导类加载器（C++实现的，固化在JVM里），是java.exe加载JVM.dll启动虚拟机时就加载的，用来加载JAVA核心类库文件的，然后Bootstrap ClassLoader加载了EstCLassLoader(extension Classloader扩展类加载器)类加载器，ExtCLassLoader用于加载lib,ext等下的类，然后加载了AppCLassLoader类加载器，这个主要用于加载classpath下的类，我们程序里自已定义的类，基本都是由这个加载器加载的，也叫系统类加载器，ClassLoader.getSystemClassLoader就可以获取AppCLassLoader类加载器

继承关系:

java.lang.Object

      --- java.lang.ClassLoader
             --- java.security.SecureClassLoader
                     --- java.net.URLClassLoader
                          --- sun.misc.Launcher$ExtClassLoader

java.lang.Object

       --- java.lang.ClassLoader
             --- java.security.SecureClassLoader
                     --- java.net.URLClassLoader
                          --- sun.misc.Launcher$AppClassLoader

双亲委托加载机制

当一个类加载器要加载一个类时，会先调findLoadedClass()查找，这个类是否已经加载，如果未加载，就调findClass()方法，这个方法会先调用父类的findClass()方法,直到BootstrapClassLoader引导类加载器还未找到，则由BootstrapClassLoader引导类加载器尝试加载这个类，如果不能加载，则返回由EXTClassLoader类加载器尝试加载这个类，如果还不能加载则返回由AppClassLoader类加载器尝试加载这个类，直到返回最初的类加载器，如果还是不能加载，则会抛出ClassNotFoundException异常。最初调用loadClass的类加载器叫初始类加载器，而最终加载类的调用了definedClass方法的才是真正加载类的加载器，这种机制保证了核心库类总是由BootstrapClassLoader引导类加载器来加载，且JVM维护了一张核心库类的表，保证了JAVA核心库的一致和稳定。

每个类加载器有着各自的加载路径，如果你自定义一个类，然后让BootstrapClassLoader去加载这个类，会抛出ClassNotFoundException.下面一段代码可以打印出三个类加载器各自加载的路径。

<pre name="code" class="java">package jvm;import java.net.URL;import java.net.URLClassLoader;public class PrintClassLoaderURLs{    public static void main(String[] args)    {        //获取BootstrapClassLoader的加载路径        URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs();        for (int i = 0; i < urls.length; i++)        {            System.out.println(urls[i].toExternalForm());        }                //获取AppClassLoader        URLClassLoader cl = (URLClassLoader)ClassLoader.getSystemClassLoader();        do        {            System.out.println();            urls = cl.getURLs();            for (int i = 0; i < urls.length; i++)            {                System.out.println(urls[i].toExternalForm());            }        }while((cl = (URLClassLoader)cl.getParent()) != null);      //获取ExpClassLoader            }}

运行结果如下：

file:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/resources.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/rt.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/sunrsasign.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/jsse.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/jce.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/charsets.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/jfr.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/classesfile:/H:/Study/Myprogramme/Java/MyProject/test/bin/file:/H:/Study/Myprogramme/Java/MyProject/test/src/lib/commons-beanutils.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/access-bridge-32.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/dnsns.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/jaccess.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/localedata.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/sunec.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/sunjce_provider.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/sunmscapi.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/sunpkcs11.jarfile:/D:/Program%20Files%20(x86)/java/jdk1.7.0_17/jre/lib/ext/zipfs.jar