Java动态加载类

我试图动态地将类加载到组件中。 我正在使用文件选择器来选择将要加载的.JAR文件，然后使用选项窗格来获取类的名称。

我已经在互联网上搜索如何将java文件转换为URL以便在URLClassLoader中加载它我已经提出：

File myFile = filechooser.getSelectedFile(); String className = JOptionPane.showInputDialog( this, "Class Name:", "Class Name", JOptionPane.QUESTION_MESSAGE); URL myUrl= null; try { myUrl = myFile.toURL(); } catch (MalformedURLException e) { } URLClassLoader loader = new URLClassLoader(myUrl); loader.loadClass(className); 

我现在收到一个’无法找到符号’错误，将URL加载到URLClassLoader中

Answer 1

我喜欢Zellus在答案中提到的ClassPathHacker类，但它充满了不赞成的调用和不良做法，所以这里是一个重写版本，它还缓存了Classloader和addUrl方法：

 import java.lang.reflect.Method; import java.net.URL; import java.net.URLClassLoader; import java.io.IOException; import java.io.File; public class ClassPathHacker{ private static final Class URLCLASSLOADER = URLClassLoader.class; private static final Class[] PARAMS = new Class[] { URL.class }; public static void addFile(final String s) throws IOException{ addFile(new File(s)); } public static void addFile(final File f) throws IOException{ addURL(f.toURI().toURL()); } public static void addURL(final URL u) throws IOException{ final URLClassLoader urlClassLoader = getUrlClassLoader(); try{ final Method method = getAddUrlMethod(); method.setAccessible(true); method.invoke(urlClassLoader, new Object[] { u }); } catch(final Exception e){ throw new IOException( "Error, could not add URL to system classloader"); } } private static Method getAddUrlMethod() throws NoSuchMethodException{ if(addUrlMethod == null){ addUrlMethod = URLCLASSLOADER.getDeclaredMethod("addURL", PARAMS); } return addUrlMethod; } private static URLClassLoader urlClassLoader; private static Method addUrlMethod; private static URLClassLoader getUrlClassLoader(){ if(urlClassLoader == null){ final ClassLoader sysloader = ClassLoader.getSystemClassLoader(); if(sysloader instanceof URLClassLoader){ urlClassLoader = (URLClassLoader) sysloader; } else{ throw new IllegalStateException( "Not an UrlClassLoader: " + sysloader); } } return urlClassLoader; } } 

Answer 2

在此论坛post中找到的ClassPathHacker.java是一个动态加载类的选项。

 import java.lang.reflect.*; import java.io.*; import java.net.*; public class ClassPathHacker { private static final Class[] parameters = new Class[]{URL.class}; public static void addFile(String s) throws IOException { File f = new File(s); addFile(f); }//end method public static void addFile(File f) throws IOException { addURL(f.toURL()); }//end method public static void addURL(URL u) throws IOException { URLClassLoader sysloader = (URLClassLoader)ClassLoader.getSystemClassLoader(); Class sysclass = URLClassLoader.class; try { Method method = sysclass.getDeclaredMethod("addURL",parameters); method.setAccessible(true); method.invoke(sysloader,new Object[]{ u }); } catch (Throwable t) { t.printStackTrace(); throw new IOException("Error, could not add URL to system classloader"); }//end try catch }//end method }//end class 

Answer 3

看看这个相关的问题： 我应该如何在运行时动态加载Jars？

Answer 4

URLClassLoader的构造函数采用一组URL ，而不是单个URL。

Answer 5

我在scala中重写了这个，以防任何人需要，因为它不是100％琐碎的:)

 /* * Class which allows URLS to be "dynamically" added to system class loader */ object class_path_updater { val URLCLASSLOADER = classOf[URLClassLoader] var urlClassLoader = getUrlClassLoader var addUrlMethod = getAddUrlMethod /* * addFile - have to use reflection to retrieve and call class loader addURL method as it is protected */ def addFile(s: String) = { val urlClassLoader = getUrlClassLoader try { val method = getAddUrlMethod method.setAccessible(true) val v = (new File(s)).toURI.toURL invoke(urlClassLoader, method, Array[AnyRef](v)) def invoke(proxy: AnyRef, m: Method, args: Array[AnyRef]) = m.invoke(proxy, args: _*) } } private def getAddUrlMethod: Method = { if (addUrlMethod == null) addUrlMethod = URLCLASSLOADER.getDeclaredMethod("addURL", classOf[URL]) addUrlMethod } private def getUrlClassLoader: URLClassLoader = { if (urlClassLoader == null) { val sysLoader = ClassLoader.getSystemClassLoader sysLoader match { case x: URLClassLoader => urlClassLoader = sysLoader.asInstanceOf[URLClassLoader] case _ => throw new IllegalStateException("Not a UrlClassLoader: " + sysLoader) } } urlClassLoader } }