我在开发系统的一个插件功能，就是提供一个接口，动态加载插件jar，然后执行。所有的插件继承于抽象类BasicPlugin，例如我实现了一个PluginA，在主程序中动态加载：
- Java code
//BasicPlugin.javapackage byv;abstract class BasicPlugin { abstract public int test(int a);}//PluginA.javapackage byv;import byv.BasicPlugin;public class PluginA extends BasicPlugin { @Override public int test(int a) { return a + a; }}//主程序 private static void loadTest() throws Exception { URL url = new File("PluginA.jar").toURI().toURL(); URLClassLoader ClassLoader = URLClassLoader.newInstance(new URL[] {url}); Class<?> clazz = Class.forName("byv.PluginA", true, ClassLoader); BasicPlugin obj = (BasicPlugin) clazz.newInstance(); obj.test(2); }其中PluginA.java被打包成了PluginA.jar，里面仅仅包含PluginA.class，不包含BasicPlugin.class。这时运行会出现这样的错误：
- Java code
Exception in thread "main" java.lang.IllegalAccessError: class byv.PluginA cannot access its superclass byv.BasicPlugin at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:634) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) at java.net.URLClassLoader.defineClass(URLClassLoader.java:277) at java.net.URLClassLoader.access$000(URLClassLoader.java:73) at java.net.URLClassLoader$1.run(URLClassLoader.java:212) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:205) at java.lang.ClassLoader.loadClass(ClassLoader.java:321) at java.net.FactoryURLClassLoader.loadClass(URLClassLoader.java:615) at java.lang.ClassLoader.loadClass(ClassLoader.java:266) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:264) at byv.Main.loadTest1(Main.java:18) at byv.Main.main(Main.java:11)
但如果我把BasicPlugin.class也放入其中，类型转换会失败，两个BasicPlugin会被识别为不同的东西。怎么办？？
------解决方案--------------------
很简单：
public class Test {
public static void main(String... args) {
Class<?> clazz = Class.forName("byv.PluginA", true, ClassLoader);
BasicPlugin obj = (BasicPlugin) clazz.newInstance();
obj.test(2);
}
}
Test打包成Test.jar
public class Boot {
public static void main(String... args) {
URL url1 = new File("PluginA.jar").toURI().toURL();
URL url2 = new File("BasePluginA.jar").toURI().toURL();
URL url3 = new File("Test.jar").toURI().toURL();
URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] {url1, url2});
classLoader.loadClass("Test").getMethod("main",new Class[]{args.getClass()}).invoke(null,new Object[]{args});
}
}