动态加载Jar并实例化已加载类的Object

我尝试动态地将jar加载到我的Java项目中。

这是类加载器的代码：

public class ClassLoad { public static void main(String[] args) { String filePath = new String("C:/Users/Mehdi/Desktop/JavaClassLoader/jarred.jar"); URL myJarFile = null; try { myJarFile = new URL("file://"+filePath); } catch (MalformedURLException e1) { System.out.println("1"); e1.printStackTrace(); } URLClassLoader cl = URLClassLoader.newInstance(new URL[]{myJarFile}); Class Jarred = null; try { Jarred = cl.loadClass("com.jarred.exp.Jarred"); } catch (ClassNotFoundException e) { System.out.println("2"); e.printStackTrace(); } Method simpleWrite = null; try { simpleWrite = Jarred.getMethod("simpleWrite", new Class[] {String.class}); } catch (SecurityException e) { System.out.println("3"); e.printStackTrace(); } catch (NoSuchMethodException e) { System.out.println("4"); e.printStackTrace(); } Object JarredObj = null; try { JarredObj = Jarred.newInstance(); } catch (InstantiationException e) { System.out.println("5"); e.printStackTrace(); } catch (IllegalAccessException e) { System.out.println("6"); e.printStackTrace(); } try { Object response = simpleWrite.invoke(JarredObj, "\nHello Mehdi ! It works hamdoulillah :D"); } catch (IllegalArgumentException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } catch (InvocationTargetException e) { e.printStackTrace(); } } } 

以及包含在Jar中的类：

 package com.jarred.exp; public class Jarred { public void simpleWrite(String str) { System.out.println(str); } 

}

它给了我：

 2 java.lang.ClassNotFoundException: com.jarred.exp.Jarred at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.net.FactoryURLClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at ClassLoad.main(ClassLoad.java:25) Exception in thread "main" java.lang.NullPointerException at ClassLoad.main(ClassLoad.java:32) 

你对此有什么想法吗？ 谢谢。

Answer 1

您的文件url似乎无效。

“Windows中的文件URI”说

对于本地Windows文件路径

C:\Documents and Settings\davris\FileSchemeURIs.doc

Windows中相应的有效文件URI是：

file:///C:/Documents%20and%20Settings/davris/FileSchemeURIs.doc

这表示冒号后需要三个斜杠 ，但是你要计算的URL

 String filePath = new String("C:/Users/Mehdi/Desktop/JavaClassLoader/jarred.jar"); URL myJarFile = null; try { myJarFile = new URL("file://"+filePath); 

file:后只有两个斜杠 file: 。 也许

  myJarFile = new URL("file://"+filePath); 

应该

  myJarFile = new URL("file:///"+filePath); 

或者你也可以使用java.io.File.toURI

 File myJarFile = new File("C:\\Users\\Mehdi\\Desktop\\JavaClassLoader\\jarred.jar"); if (!myJarFile.isFile()) { throw new FileNotFoundException("Missing required JAR: " + myJarFile.toString()); } URL myJarUrl = myJarFile.toURI().toURL(); 

适当的exception处理。