// 生成JAR包D:\TestClass.jar

package hand.java.loadjar;

public class TestClass {

    private String sayHello = "Hello World!!";

    public String sayHello() {

        return this.sayHello;

    }

}

// 创建一个测试类

package hand.java.loadjar;

import java.net.URL;

import java.net.URLClassLoader;

import java.lang.reflect.Method;

public class LoadJarTest {    

    public LoadJarTest(){

        try {

            URL url = new URL("file:D:\\TestClass.jar");  

            URLClassLoader  loader = new URLClassLoader(new URL[]{ url });   

            // Load class

            Class<?> myclass = loader.loadClass("hand.java.loadjar.TestClass");

            // Gene new object

            Object myobject = myclass.newInstance();   

            // Get function

            Method method = myclass.getDeclaredMethod("sayHello");

            // Make the function  valid

            method.setAccessible(true);

            // Invoke method

            System.out.println(method.invoke(myobject));   

        } catch (Exception ex) {

            ex.printStackTrace();

        }

    }

    @SuppressWarnings("unused")

    public static void main(String[] args){

        LoadJarTest loadjar = new LoadJarTest();

    }

}

运行，输出内容:

Hello World!!

 

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package hand.dynamic.jar;

import java.io.File;

import java.net.MalformedURLException;

import java.net.URL;

import java.net.URLClassLoader;

public class JarLoadUtil {

    private static String jar_path = "D:\\JAR";

    private static URLClassLoader loader = null;

    // private static String curDir = System.getProperty("user.dir");

    private static String separator = System.getProperty("file.separator");

    public static URLClassLoader getURLClassLoader() {

        if (loader == null) {

            String fileNames[] = new File(jar_path).list();

            if (fileNames != null && fileNames.length > 0) {

                URL urls[] = new URL[fileNames.length];

                for (int i = 0; i < fileNames.length; i++) {

                    try {

                        // if (fileNames[i].equals("dom4j-1.6.1.jar"))

                        urls[i] = new URL(jar_path + separator + fileNames[i]);

                    } catch (MalformedURLException e) {

                        System.out.println("加载指定目录下JAR文件出错！");

                        throw new RuntimeException("加载指定目录下JAR文件出错！", e);

                    }

                }

                loader = new URLClassLoader(urls);

            }

        }

        return loader;

    }

}