类加载

来源自网络，很喜欢就收藏了；

需要注意的是：如果想要对这个实例进行测试的话，首先需要在c盘建立一个c://myjava的目录。然后将相应的java文件放在这个目录中。并将产生的.clas文件放在c://myjava/com/lg.test目录下，否则是找不到的。这是要注意的。。

class FileClassLoader ：

 

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package com.lg.test; import java.io.ByteArrayOutputStream; import java.io.FileInputStream; import java.io.IOException; import java.io.InputStream; /**  * Created by 鏉庢灉 on 2016/8/6.  */ public class FileClassLoader extends ClassLoader {     String rootDir=null;     public FileClassLoader(String rootDir) {         this.rootDir = rootDir;     }     @Override       protected Class<?> findClass(String className) throws ClassNotFoundException {         //首先检查是否已经被加载了。         Class<?> c = findLoadedClass(className);         String path = rootDir + "/" + className.replace('.', '/') + ".class";         if (c != null) {             return c;         } else {             /*双亲委托模式*/             ClassLoader loaderParent = this.getParent();             c = loaderParent.loadClass(className);             if (c != null) {                 return c;             } else {                 /*如果再不行的话，就再进行加载。因为字节码的本质就是一个字节数组*/                 InputStream is = null;                 ByteArrayOutputStream outputStream = new ByteArrayOutputStream();                 try {                     is = new FileInputStream(path);                     byte[] buffer = new byte[1024];                     int len = 0;                     while ((len = is.read(buffer)) != -1) {                         outputStream.write(buffer, 0, len);                     }                     c = defineClass(className, buffer, 0, buffer.length);                 }                 catch (Exception e) {                     e.printStackTrace();                 }                 finally {                     if (is != null) {                         try {                             is.close();                         }                         catch (IOException e) {                             e.printStackTrace();                         }                     }                 }             }             return c;         }     } }

class Demo ：

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package com.lg.test; /**  * Created by 鏉庢灉 on 2016/8/6.  */ /*相同的类加载器对同一个类进行加载，得到的hascode是相同的  * 不同的类加载器对同一类进行加载，得到的hascode是不一样的*/ public class Demo {     public static void main(String[] args) {         FileClassLoader loader = new FileClassLoader("c://myjava");         FileClassLoader loader2=new FileClassLoader("c://myjava");         try {             Class<?> c = loader.findClass("com.lg.test.HelloWorld");             Class<?> c0=loader.findClass("com.lg.test.HelloWorld");             Class<?> c1=loader2.findClass("com.lg.test.HelloWorld");             Class<?> c2=loader.findClass("com.lg.test.Demo01");             Class<?> c3=loader.findClass("java.lang.String");             System.out.println(c.hashCode());             System.out.println(c.getClassLoader());             System.out.println(c0.hashCode());             System.out.println(c0.getClassLoader());             System.out.println(c1.hashCode());             System.out.println(c1.getClassLoader());             System.out.println(c2.hashCode());             System.out.println(c2.getClassLoader());             System.out.println(c3.hashCode());             System.out.println(c3.getClassLoader());         }         catch (ClassNotFoundException e) {             e.printStackTrace();         }     } }

最后运行的结果为：

366712642
sun.misc.Launcher$AppClassLoader@4e0e2f2a
366712642
sun.misc.Launcher$AppClassLoader@4e0e2f2a
366712642
sun.misc.Launcher$AppClassLoader@4e0e2f2a
1829164700
sun.misc.Launcher$AppClassLoader@4e0e2f2a
2018699554
null