如何为RunTime动态添加自定义classpath

这几天在做一个小项目的时候 有这样一个需求, 程序需要动态引入一个.class文件,然后用Class.forName(name)将其载入,从而得到.class文件的信息.如果是在系统当前指定的classpath下,可以直接载入,但是若是自定义的路径下的.class文件,就会报找不到类文件的错.后来我问了老师,老师提供的方法是用ClassLoader.defineClass(byte[] b, int off, int len)来载入.class文件,这种方法已经成功.但我在想,怎么能将这个文件的路径加入classpath中呢,开始我想到了System.setProperty(String key, String value)这个方法,但后来证明是错的,这种方式是对classpath不起作用的,因为系统的类加载器在启动期间很早的时候就读取classpath了,系统在后来是不再读取classpath的,所以修改对系统没有任何作用.这几天在google看到一篇文章,这位国外的仁兄用了一种方法,可以实现动态添加自定义classpath,很好的程序,转载来学习一下.

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;                   

import java.io.File;
import java.io.IOException;

import java.net.URL;
import java.net.URLClassLoader;

/**
 * Allows programs to modify the classpath during runtime.              
 */                                                                     
public class ClassPathUpdater {                                         
  /** Used to find the method signature. */                             
  private static final Class[] PARAMETERS = new Class[]{ URL.class };   

  /** Class containing the private addURL method. */
  private static final Class> CLASS_LOADER = URLClassLoader.class;

  /**
   * Adds a new path to the classloader. If the given string points to a file,
   * then that file's parent file (i.e., directory) is used as the
   * directory to add to the classpath. If the given string represents a
   * directory, then the directory is directly added to the classpath.
   *
   * @param s The directory to add to the classpath (or a file, which
   * will relegate to its directory).
   */
  public static void add( String s )
    throws IOException, NoSuchMethodException, IllegalAccessException,
           InvocationTargetException {
    add( new File( s ) ); :evil: 
  }

  /**
   * Adds a new path to the classloader. If the given file object is
   * a file, then its parent file (i.e., directory) is used as the directory
   * to add to the classpath. If the given string represents a directory,
   * then the directory it represents is added.
   *
   * @param f The directory (or enclosing directory if a file) to add to the
   * classpath.
   */
  public static void add( File f )
    throws IOException, NoSuchMethodException, IllegalAccessException,
           InvocationTargetException {
    f = f.isDirectory() ? f : f.getParentFile();
    add( f.toURI().toURL() );
  }

  /**
   * Adds a new path to the classloader. The class must point to a directory,
   * not a file.
   *
   * @param url The path to include when searching the classpath.
   */
  public static void add( URL url )
    throws IOException, NoSuchMethodException, IllegalAccessException,
           InvocationTargetException {
    Method method = CLASS_LOADER.getDeclaredMethod("addURL", PARAMETERS );
    method.setAccessible( true );
    method.invoke( getClassLoader(), new Object[]{ url } );
  }

  private static URLClassLoader getClassLoader() {
    return (URLClassLoader)ClassLoader.getSystemClassLoader();
  }
}
 

Example Usage

The following example will add /home/user/dev/java/app/build/com/package to the classpath at runtime:

try {
  ClassPathUpdater.add(/home/user/dev/java/app/build/com/package/Filename.class);
}
catch( Exception e ) {
  e.printStackTrace();
}