为什么用Java这么难？如果我们想要任何类型的模块系统，则需要能够动态加载jar。有人告诉我有一种方法可以通过编写自己的ClassLoader来完成，但是对于(至少在我看来)应该像调用带有jar文件作为其参数的方法一样容易的事情来说，这是很多工作。

对执行此操作的简单代码有何建议？

解决方案

回答

以下解决方案有些骇人，因为它使用反射来绕过封装，但是它可以完美地工作：

File file = ...
URL url = file.toURI().toURL();

URLClassLoader classLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);
method.invoke(classLoader, url);

回答

我们应该看一下OSGi，例如在Eclipse平台中实现。它确实做到了。我们可以安装，卸载，启动和停止所谓的捆绑软件，这些捆绑软件实际上是JAR文件。但是它提供了更多功能，例如可以在运行时在JAR文件中动态发现的服务。

或者参见Java模块系统的规范。

回答

很难的原因是安全性。类加载器是不可变的。我们不应在运行时随意向其添加类。实际上，我很惊讶能与系统类加载器一起使用。这是制作自己的子类加载器的方法：

URLClassLoader child = new URLClassLoader(myJar.toURL(), this.getClass().getClassLoader());
Class classToLoad = Class.forName("com.MyClass", true, child);
Method method = classToLoad.getDeclaredMethod("myMethod");
Object instance = classToLoad.newInstance();
Object result = method.invoke(instance);

很痛苦，但确实如此。

回答

我发现的最好的是org.apache.xbean.classloader.JarFileClassLoader，它是XBean项目的一部分。

这是我过去使用的一种简短方法，可以从特定目录中的所有lib文件创建类加载器

public void initialize(String libDir) throws Exception {
    File dependencyDirectory = new File(libDir);
    File[] files = dependencyDirectory.listFiles();
    ArrayList<URL> urls = new ArrayList<URL>();
    for (int i = 0; i < files.length; i++) {
        if (files[i].getName().endsWith(".jar")) {
        urls.add(files[i].toURL());
        //urls.add(files[i].toURI().toURL());
        }
    }
    classLoader = new JarFileClassLoader("Scheduler CL" + System.currentTimeMillis(), 
        urls.toArray(new URL[urls.size()]), 
        GFClassLoader.class.getClassLoader());
}

然后使用类加载器，只需执行以下操作：

classLoader.loadClass(name);

回答

JCL类加载器框架如何？我不得不承认，我还没有使用它，但是它看起来很有希望。

用法示例：

JarClassLoader jcl = new JarClassLoader();
jcl.add("myjar.jar"); // Load jar file  
jcl.add(new URL("http://myserver.com/myjar.jar")); // Load jar from a URL
jcl.add(new FileInputStream("myotherjar.jar")); // Load jar file from stream
jcl.add("myclassfolder/"); // Load class folder  
jcl.add("myjarlib/"); // Recursively load all jar files in the folder/sub-folder(s)

JclObjectFactory factory = JclObjectFactory.getInstance();
// Create object of loaded class  
Object obj = factory.create(jcl, "mypackage.MyClass");

回答

这是不推荐使用的版本。我修改了原始文件以删除不推荐使用的功能。

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/*
 * Created on Oct 6, 2004
 */
package tools;

import java.io.File;
import java.io.IOException;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

/**
 * Useful class for dynamically changing the classpath, adding classes during runtime. 
 */
public class ClasspathHacker {
    /**
     * Parameters of the method to add an URL to the System classes. 
     */
    private static final Class<?>[] parameters = new Class[]{URL.class};

    /**
     * Adds a file to the classpath.
     * @param s a String pointing to the file
     * @throws IOException
     */
    public static void addFile(String s) throws IOException {
        File f = new File(s);
        addFile(f);
    }

    /**
     * Adds a file to the classpath
     * @param f the file to be added
     * @throws IOException
     */
    public static void addFile(File f) throws IOException {
        addURL(f.toURI().toURL());
    }

    /**
     * Adds the content pointed by the URL to the classpath.
     * @param u the URL pointing to the content to be added
     * @throws IOException
     */
    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");
        }        
    }

    public static void main(String args[]) throws IOException, SecurityException, ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException, NoSuchMethodException{
        addFile("C:\dynamicloading.jar");
        Constructor<?> cs = ClassLoader.getSystemClassLoader().loadClass("test.DymamicLoadingTest").getConstructor(String.class);
        DymamicLoadingTest instance = (DymamicLoadingTest)cs.newInstance();
        instance.test();
    }
}