【0】README

0.1）本文主要对类加载器进行分析，且 URLClassLoader是 ClassLoader的子类；

0.2）关于如何设置类加载器的加载路径，参见 对servlet容器的补充

【1】URLClassLoader类加载器

1.1）URLClassLoader的一种加载方式

package com.tomcat.classloader;import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;public class URLClassLoaderTest {public final static String dir = System.getProperty("user.dir") + File.separator + "src" + File.separator;public static void main(String[] args) throws Exception {// Getting the jar URL which contains target classURL[] classLoaderUrls = new URL[]{new URL("file:" + dir)};// Create a new URLClassLoader URLClassLoader urlClassLoader = new URLClassLoader(classLoaderUrls);// Load the target classClass<?> beanClass = urlClassLoader.loadClass("com.tomcat.test.Bean");// Create a new instance from the loaded classConstructor<?> constructor = beanClass.getConstructor();Object beanObj = constructor.newInstance();// Getting a method from the loaded class and invoke itMethod method = beanClass.getMethod("sayHello");method.invoke(beanObj);}
}

package com.tomcat.test;public class Bean {public void sayHello() {System.out.println("Hello from loaded Bean class for test!!!");}
}

1.2）URLClassLoader的另一种加载方式

package com.tomcat.classloader;import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;public class URLClassLoaderTest {public final static String dir = System.getProperty("user.dir") + File.separator + "webroot" + File.separator;public static void main(String[] args) {		try {File file = new File(dir);URL url = file.toURI().toURL(); // attend this lineURL[] urls = new URL[]{url};URLClassLoader loader = new URLClassLoader(urls, null, null);  System.out.println("test = " + loader.getURLs()[0].toString());Class<?> beanClass = loader.loadClass("servlet.Bean");Object obj = beanClass.newInstance();Method method = beanClass.getMethod("sayHello");method.invoke(obj);} catch (Exception e) {e.printStackTrace();}  }
}

【2】ClassLoader类加载器的加载方式

package com.tomcat.test;import java.lang.reflect.Constructor;
import java.lang.reflect.Method;public class ClassLoaderTest {    // 第一种加载类的方式public static void main1(String[] args) throws Exception {    Class<?> newClass = Class.forName("com.tomcat.classloader.Bean");Constructor constructor = newClass.getConstructor();Object obj = constructor.newInstance();Method method = newClass.getMethod("sayHello");method.invoke(obj);}// 第二种加载类的方式public static void main(String[] args) throws Exception {    ClassLoader loader = Thread.currentThread().getContextClassLoader();Class<?> newClass = loader.loadClass("com.tomcat.classloader.Bean");Constructor constructor = newClass.getConstructor();Object obj = constructor.newInstance();Method method = newClass.getMethod("sayHello");method.invoke(obj);}
}