ClassLoader和类加载机制

   /**     
 * @param name    类名字      
* @param resolve 　是否解析，如果只是想知道该class是否存在可以设置该参数为false     
 * @return 返回一个class泛型     
 * @throws ClassNotFoundException      
*/     
protected Class<?> loadClass(String name, boolean resolve)             
throws ClassNotFoundException {         
/**          
* getClassLoadingLock(name)          
* 为类的加载操作返回一个锁对象。为了向后兼容，这个方法这样实现:如果当前的classloader对象注册了并行能力，         
 * 方法返回一个与指定的名字className相关联的特定对象，否则，直接返回当前的ClassLoader对象。         
 */         
synchronized (getClassLoadingLock(name)) {             
// 首先查看class是否已经被加载过            
 Class<?> c = findLoadedClass(name);             
if (c == null) {                
long t0 = System.nanoTime();                
 try {                    
 //如果父加载器不为空，则委托给父加载器去加载                     
if (parent != null) {                         
c = parent.loadClass(name, false);                     
} else {                        
 /**                         
 *  如果父加载器为空，说明父加载器已经是Bootstrap ClassLoader了，则直接使用根加载器加载,也就是使用虚拟机加                          
*  载器加载                          
*/                        
 c = findBootstrapClassOrNull(name);                    
 }                 
} catch (ClassNotFoundException e) {                    
 // ClassNotFoundException thrown if class not found                     
// from the non-null parent class loader                
 }                
 //如果以上的加载器在自己的路径上面都没有加载到,则调用findClass(name)调用用户自定义的加载器                
 if (c == null) {                    
 // If still not found, then invoke findClass in order                     
// to find the class.                    
 long t1 = System.nanoTime();                     
c = findClass(name);                      
// this is the defining class loader; record the stats                     sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);                     sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);                     sun.misc.PerfCounter.getFindClasses().increment();                
 }            
 }            
 //根据resolve参数决定是否解析该类             
if (resolve) {                
 resolveClass(c);            
 }            
 return c;        
 }     
}

package com.siyu;

import java.io.*;
public class ClassLoaderTest extends ClassLoader {     
//自定义加载器加载该路径下面的文件     private String directory;      
public ClassLoaderTest(String directory) {         
this.directory = directory;    
 }     
/**     
 * 重写findClass,用户可以做以下的事情      
* 1.可以加载boot、ext、system加载器所加载不了的路径下的文件      
* 2.可以解密加密后的class文件      
*/     
@Override     
protected Class<?> findClass(String name) throws ClassNotFoundException {         
//解密密钥        
byte key = (byte) 1;         
//加密文件的路径         
String fileName = directory + name + ".class";         
File file = new File(fileName);         
byte[] decryptedByte = readFromFile(file);         
//解密为原始的class文件         
for (int i = 0; i < decryptedByte.length; i++) {             
decryptedByte[i] = (byte) (decryptedByte[i] ^ key);         
}         
//defineClass实现了链接阶段的验证等         
return defineClass(null, decryptedByte, 0, decryptedByte.length);     
}       
private byte[] readFromFile(File fileName) {         
try {             
byte[] bytes = null;             
FileInputStream fin = new FileInputStream(fileName);              
int i;             
if ((i = fin.read()) != -1) {                 
//初始化数组大小和文件大小一样                 
bytes = new byte[fin.available()];                 
fin.read(bytes);             
}            
return bytes;         
} catch (FileNotFoundException e) {             
e.printStackTrace();             
return null;        
 } catch (IOException e) {             
e.printStackTrace();            
 return null;        
 }    
 }      
private byte[] encrypt(byte[] bytes) {         
byte key = (byte) 1;         
//依次加密的代码        
 for (int i = 0; i < bytes.length; i++) {             
bytes[i] = (byte) (bytes[i] ^ key); 
//利用异或加密         
}         
return bytes;    
 }      
public void encryptFile(String fileName, String directory) {        
 try {            
 String name = fileName.substring(fileName.lastIndexOf("\") + 1, fileName.length() - 6);             
//加密文件的路径             
String destFileName = directory + "encryted" + name + ".class";             
//如果加密文件不存在则创建加密文件             
File f = new File(destFileName);            
 if (f == null) {                 
f.createNewFile();             
}             
//加密             
byte[] encryptedByte = encrypt(readFromFile(new File(fileName)));             
FileOutputStream fos = new FileOutputStream(destFileName);             
//把加密后的字节写入到加密文件中             
fos.write(encryptedByte);        
 } catch (FileNotFoundException e) {             
e.printStackTrace();        
 } catch (IOException e) {             
e.printStackTrace();        
 }     
}      
 public static void main(String[] args) {         
//设置加密路径         
ClassLoaderTest classLoaderTest=new ClassLoaderTest("C:\EncryptedClass\");        
 //将test.class加密后存储到EncryptedClass目录下         classLoaderTest.encryptFile("C:\Users\jasonchu.zsy\IdeaProjects\BoKeTest\out\production\BoKeTest\com\siyu\test.class"              
,"C:\EncryptedClass\");         
try {             
Class<?> t=classLoaderTest.loadClass("encrytedtest");        
 } catch (ClassNotFoundException e) {             
e.printStackTrace();        
 }    
 }
}