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.
package com.dawn.scm.util;
import java.io.File;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
/**
* Allows programs to modify the classpath during runtime.
*
* @author Yucheng
*
*/
public class ClassPathUpdater {
/** Used to find the method signature. */
@SuppressWarnings("rawtypes")
private static final Class[] PARAMETERS = new Class[] { URL.class };
/** Class containing the private addURL method. */
@SuppressWarnings("rawtypes")
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 ) );
}
/**
* 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.
*/
private 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.
*/
@SuppressWarnings("unchecked")
private 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)Thread.currentThread().getContextClassLoader();
}
}
出处:
http://code.google.com/p/dawnwork/source/browse/trunk/dawn_saas_scm/src/com/dawn/scm/util/ClassPathUpdater.java?spec=svn195&r=195
http://ted0928.iteye.com/blog/543599