分析BootstrapClassLoader/ExtClassLoader/AppClassLoader的加载路径及"父委托机制"

import java.net.URL;
import java.net.URLClassLoader;
/*
分析BootstrapClassLoader/ExtClassLoader/AppClassLoader的加载路径
*
*/
public class ClassPath_of_Bootstrap_Ext_AppClassLoader
{
public static void main(String[] args)
{
System.out.println("BootstrapClassLoader 的加载路径: ");
URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs();
for(URL url : urls)
System.out.println(url);
System.out.println("----------------------------");
//取得扩展类加载器
URLClassLoader extClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader().getParent();
System.out.println(extClassLoader);
System.out.println("扩展类加载器 的加载路径: ");
urls = extClassLoader.getURLs();
for(URL url : urls)
System.out.println(url);
System.out.println("----------------------------");
//取得应用(系统)类加载器
URLClassLoader appClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();
System.out.println(appClassLoader);
System.out.println("应用(系统)类加载器 的加载路径: ");
urls = appClassLoader.getURLs();
for(URL url : urls)
System.out.println(url);
System.out.println("----------------------------");
}
}

运行时输出如下:

BootstrapClassLoader 的加载路径: 
file:/C:/jdk1.7.0_01/jre/lib/resources.jar
file:/C:/jdk1.7.0_01/jre/lib/rt.jar
file:/C:/jdk1.7.0_01/jre/lib/sunrsasign.jar
file:/C:/jdk1.7.0_01/jre/lib/jsse.jar
file:/C:/jdk1.7.0_01/jre/lib/jce.jar
file:/C:/jdk1.7.0_01/jre/lib/charsets.jar
file:/C:/jdk1.7.0_01/jre/classes
----------------------------
sun.misc.Launcher$ExtClassLoader@3e389405
扩展类加载器 的加载路径: 
file:/C:/jdk1.7.0_01/jre/lib/ext/dnsns.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/localedata.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/sunec.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/sunjce_provider.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/sunmscapi.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/zipfs.jar
----------------------------
sun.misc.Launcher$AppClassLoader@a200d0c
应用(系统)类加载器 的加载路径: 
file:/E:/JAVA/JVM_Class_Reflect_Thread/ClassPath_of_Bootstrap_Ext_AppClassLoader/bin/
----------------------------

现在我们就知道了这三个类加载器的加载路径了:

JDK源代码中,protected Class<?> loadClass(String name, boolean resolve)方法 

已经很好地体现出"父亲委托机制":

    

1:先调用parent.loadClass(name, false)

    

2:如果父亲加载失败,则调用loadClass(name, false) 方法 查找"当前类加载器"对应的"目录"来加载

     

public Class<?>loadClass(String name, boolean resolve)

此方法的默认实现将按以下顺序搜索类：

    

1.调用findLoadedClass(name)来检查是否已经加载类。

   

2.在父类加载器上调用 loadClass方法。如果父类加载器为 null，则使用虚拟机的内置类加载器。

   

3.调用findClass(name) 方法查找类。

PS: 如果用户想从自定义的目录下加载类(.class文件),则可以自定义类加载器: