一、前言

关于jasper的jasper-compiler源码中org.apache.jasper.servlet.JasperLoader类，继承java.net.URLClassLoader自定网络类加载器,另外这边基于抽象类ClassLoader自定义MyClassLoader类加载器实现示例，具体参见如下源码说明部分。

二、源码说明 

1.源码包中的JasperLoader类

package org.apache.jasper.servlet;@b@@b@import java.io.IOException;@b@import java.io.InputStream;@b@import java.net.URL;@b@import java.net.URLClassLoader;@b@import java.security.CodeSource;@b@import java.security.PermissionCollection;@b@@b@public class JasperLoader extends URLClassLoader@b@{@b@  private PermissionCollection permissionCollection;@b@  private CodeSource codeSource;@b@  private String className;@b@  private ClassLoader parent;@b@  private SecurityManager securityManager;@b@@b@  public JasperLoader(URL[] urls, ClassLoader parent, PermissionCollection permissionCollection, CodeSource codeSource)@b@  {@b@    super(urls, parent);@b@    this.permissionCollection = permissionCollection;@b@    this.codeSource = codeSource;@b@    this.parent = parent;@b@    this.securityManager = System.getSecurityManager();@b@  }@b@@b@  public Class loadClass(String name)@b@    throws ClassNotFoundException@b@  {@b@    return loadClass(name, false);@b@  }@b@@b@  public Class loadClass(String name, boolean resolve)@b@    throws ClassNotFoundException@b@  {@b@    Class clazz = null;@b@@b@    clazz = findLoadedClass(name);@b@    if (clazz != null) {@b@      if (resolve)@b@        resolveClass(clazz);@b@      return clazz;@b@    }@b@@b@    if (this.securityManager != null) {@b@      int dot = name.lastIndexOf(46);@b@      if (dot >= 0)@b@        try@b@        {@b@          if (!("org.apache.jasper.runtime".equalsIgnoreCase(name.substring(0, dot))))@b@            this.securityManager.checkPackageAccess(name.substring(0, dot));@b@        }@b@        catch (SecurityException se) {@b@          String error = "Security Violation, attempt to use Restricted Class: " + name;@b@@b@          se.printStackTrace();@b@          throw new ClassNotFoundException(error);@b@        }@b@@b@    }@b@@b@    if (!(name.startsWith("org.apache.jsp")))@b@    {@b@      clazz = this.parent.loadClass(name);@b@      if (resolve)@b@        resolveClass(clazz);@b@      return clazz;@b@    }@b@@b@    return findClass(name);@b@  }@b@@b@  public InputStream getResourceAsStream(String name)@b@  {@b@    InputStream is = this.parent.getResourceAsStream(name);@b@    if (is == null) {@b@      URL url = findResource(name);@b@      if (url != null)@b@        try {@b@          is = url.openStream();@b@        } catch (IOException e) {@b@          is = null;@b@        }@b@    }@b@@b@    return is;@b@  }@b@@b@  public final PermissionCollection getPermissions(CodeSource codeSource)@b@  {@b@    return this.permissionCollection;@b@  }@b@}

2.自定义MyClassLoader类

package test;@b@@b@import java.io.ByteArrayOutputStream;@b@import java.io.File;@b@import java.io.FileInputStream;@b@import java.io.FileNotFoundException;@b@import java.io.IOException;@b@import java.io.InputStream;@b@@b@public class MyClassLoader extends ClassLoader{@b@@b@    private String classpath;@b@    @b@    public MyClassLoader(String classpath) {@b@        @b@        this.classpath = classpath;@b@    }@b@@b@    @Override@b@    protected Class<?> findClass(String name) throws ClassNotFoundException {@b@        try {@b@            byte [] classDate=getDate(name);@b@            if(classDate==null){@b@            } else{@b@                //defineClass方法将字节码转化为类@b@                return defineClass(name,classDate,0,classDate.length);@b@            }@b@            @b@        } catch (IOException e) {@b@            e.printStackTrace();@b@        }@b@        return super.findClass(name);@b@    }@b@    //返回类的字节码@b@    private byte[] getDate(String className) throws IOException{@b@        InputStream in = null;@b@        ByteArrayOutputStream out = null;@b@        String path=classpath + File.separatorChar +className.replace('.',File.separatorChar)+".class";@b@        try {@b@            in=new FileInputStream(path);@b@            out=new ByteArrayOutputStream();@b@            byte[] buffer=new byte[2048];@b@            int len=0;@b@            while((len=in.read(buffer))!=-1){@b@                out.write(buffer,0,len);@b@            }@b@            return out.toByteArray();@b@        } @b@        catch (FileNotFoundException e) {@b@            e.printStackTrace();@b@        }@b@        finally{@b@            in.close();@b@            out.close();@b@        }@b@        return null;@b@    }@b@}