详解JVM类加载机制及类缓存问题的处理方法

　　　　JAVA_HOME/jre/lib/resources.jar:
　　　　JAVA_HOME/jre/lib/rt.jar:
　　　　JAVA_HOME/jre/lib/sunrsasign.jar:
　　　　JAVA_HOME/jre/lib/jsse.jar:
　　　　JAVA_HOME/jre/lib/jce.jar:
　　　　JAVA_HOME/jre/lib/charsets.jar:
　　　　JAVA_HOME/jre/lib/jfr.jar:
　　　　JAVA_HOME/jre/classes

　　　　../Java/Extensions:
　　　　../JAVA_HOME/jre/lib/ext:　　　　../Library/Java/Extensions:/Network/Library/Java/Extensions:
　　　　../System/Library/Java/Extensions:
　　　　../lib/java

package java.lang.ClassLoader;
import ....
 protected Class<&＃63;> loadClass(String name, boolean resolve)
  throws ClassNotFoundException
 {
  synchronized (getClassLoadingLock(name)) {
   // First，在虚拟机内存中查找是否已经加载过此类...类缓存的主要问题所在！！！ 
   Class<&＃63;> c = findLoadedClass(name);
   if (c == null) {
    long t0 = System.nanoTime();
    try {
     if (parent != null) {
　　　　　　　　　　　　　　 //先让上一层加载器进行加载
      c = parent.loadClass(name, false);
     } else {
      c = findBootstrapClassOrNull(name);
     }
    } catch (ClassNotFoundException e) {
     // ClassNotFoundException thrown if class not found
     // from the non-null parent class loader
    }
    if (c == null) {
　　　　　　　　　　　　//调用此类加载器所实现的findClass方法进行加载
     c = findClass(name);
    }
   }
   if (resolve) {
    resolveClass(c);
   }
   return c;
  }
 }

package com.linuxidc.utils;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
public class ServiceClassLoader extends ClassLoader{
 private String classPath;
 public ServiceClassLoader(String classPath) {
  this.classPath = classPath;
 }
 /**
  * 重写父类的findClass 方法。 父类的loadClass会调用此方法
  */
 @Override
 protected Class<&＃63;> findClass(String name) throws ClassNotFoundException {
  Class<&＃63;> c = null;
  byte[] classData = getClassData(name);
  if (classData!=null) {
   c = defineClass(name, classData, 0, classData.length);
  }else {
   throw new ClassNotFoundException();
  }
  return c;
 }　
　　 // 将class文件通过IO流读取，转化为字节数组
 private byte[] getClassData(String name) {
  
  String path = classPath + "/"+ name.replace('.', '/') + ".class";
  
  InputStream iStream = null;
  ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
  try {
   iStream = new FileInputStream(path);
   
   byte[] buffer = new byte[1024];
   int temp = 0;
   while ((temp = iStream.read(buffer))!=-1) {
    byteArrayOutputStream.write(buffer, 0, temp);
   }
   if (byteArrayOutputStream!=null) {
    return byteArrayOutputStream.toByteArray();
   }
  } catch (Exception e) {
   e.printStackTrace();
  }finally {
   try {
    if (iStream!=null) {
     iStream.close();
    }
   } catch (IOException e) {
    e.printStackTrace();
   }
   try {
    if (byteArrayOutputStream!=null) {
     byteArrayOutputStream.close();
    }
   } catch (IOException e) {
    e.printStackTrace();
   }
  }
  return null;
 }
}

ServiceClassLoader serviceClassLoader = new ServiceClassLoader("c:\myclass");
Czlass<&＃63;> c = ServiceClassLoader.loadClass("com.linuxidc.service.Myclass");