Java动态加载类

Posted 2023-03-25

【中文标题】Java动态加载类

【英文标题】：Java Dynamically Loading a class

【发布时间】：2011-04-04 14:10:43

【问题描述】：

我正在尝试将类动态加载到组件中。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。

我在网上搜索了如何将 java 文件转换为 URL 以便在 URLClassLoader 中加载它，我想出了：

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);

我现在在将 URL 加载到 URLClassLoader 时遇到“找不到符号”错误

【问题讨论】：

是一个词吗？谷歌唯一建议的是“被控制”:-) urbandictionary.com/define.php?term=trolled

@seanizer: “trawled”——作为搜索的一部分进行筛选。

@trashgod 听起来好多了...

相关：Adding files to java classpath at runtime

【参考方案1】：

URLClassLoadertakes an array of URLs的构造函数，不是单个URL。

【参考方案2】：

看看这个相关的问题：How should I load Jars dynamically at runtime?

【参考方案3】：

ClassPathHacker.java 在this forum thread 中找到，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

【参考方案4】：

我喜欢the answer by Zellus 中提到的 ClassPathHacker 类，但它充满了不推荐使用的调用和不良做法，所以这里有一个重写版本，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 =
        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;
    

【参考方案5】：

我在 scala 中重写了这个，以防万一有人需要，因为它不是 100% 微不足道的 :)

/*
 * 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