java - 了解 urlclassloader，如何访问已加载的 jar 的类

Question

我试图了解如何使用 URLClassLoader 访问/提供 jar 文件。

首先我加载 jar 文件

    package myA;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

import org.jgroups.JChannel;



public class loader {

    JChannel channel;

        String user_name=System.getProperty("user.name", "n/a");


        private void start() throws Exception {

            channel=new JChannel(); // use the default config, udp.xml

            channel.connect("ChatCluster");

        }

    public void loadMe()throws ClassNotFoundException, MalformedURLException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
        URL classUrl;
        classUrl = new URL("file:///home/myJars/jgroups-3.4.2.Final.jar");
        URL[] classUrls = { classUrl };
        URLClassLoader ucl = new URLClassLoader(classUrls);
        Class<?> c = ucl.loadClass("org.jgroups.JChannel");
        for(Field f: c.getDeclaredFields()) {
            System.out.println("Field name=" + f.getName());
            }
        Object instance = c.newInstance();  
        //Method theMethod = c.getMethod("main");
        //theMethod.invoke(instance);
        }

     public static void main(String[] args) throws Exception {
         new loader().loadMe();
         new  loader().start();

     }
}

打印输出显示 jgroups-3.4.2.Final.jar 中声明的字段，但随后会抛出 classnotfound 错误。

java -cp myA.jar myA.loader
Field name=DEFAULT_PROTOCOL_STACK
Field name=local_addr
Field name=address_generator
Field name=name
Field name=cluster_name
Field name=my_view
Field name=prot_stack
Field name=state_promise
Field name=state_transfer_supported
Field name=flush_supported
Field name=config
Field name=stats
Field name=sent_msgs
Field name=received_msgs
Field name=sent_bytes
Field name=received_bytes
Field name=probe_handler
Exception in thread "main" java.lang.NoClassDefFoundError: org/jgroups/JChannel
        at myA.loader.start(loader.java:23)
        at myA.loader.main(loader.java:45)
Caused by: java.lang.ClassNotFoundException: org.jgroups.JChannel
        at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
        ... 2 more

我不明白为什么打印输出显示类已加载但后来找不到？

谢谢 艺术

Answer 1

您的代码可能有几个问题。首先，您在 main 中实例化加载器两次，因此第二个实例独立于第一个实例，并且可能不知道第一个实例加载了 JChannel 的类文件定义。

此外，您之前已将 JChannel 定义为 loader 的成员，因此 JRE 应该在启动时要求它的类定义 - 否则它不应该知道该字段应该是什么。我已将其替换为您通过 URLClassLoader 加载的类，您应该在 start() 中实例化该类。

package myA;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import org.jgroups.JChannel;

public class Loader 
{
    Class<?> clazz;
    String user_name=System.getProperty("user.name", "n/a");

    private void start() throws Exception 
    {
        if (this.clazz == null)
            throw new Exception("Channel class was not loaded properly");

        Object channel  = this.clazz.newInstance(); // use the default config, udp.xml
        Method chatCluster = this.clazz.getDeclaredMethod("connect", new Class[] { String.class });
        chatCluster.invoke(channel, "ChatCluster");
    }

    public void loadMe() throws Exception
    {
        URL classUrl;
        classUrl = new URL("file:///home/myJars/jgroups-3.4.2.Final.jar");
        URL[] classUrls = { classUrl };
        URLClassLoader ucl = new URLClassLoader(classUrls);
        Class<?> c = ucl.loadClass("org.jgroups.JChannel");
        for(Field f: c.getDeclaredFields()) 
        {
            System.out.println("Field name=" + f.getName());
        }
        this.clazz = c;
        Object instance = c.newInstance();  
        //Method theMethod = c.getMethod("main");
        //theMethod.invoke(instance);
     }

     public static void main(String[] args) throws Exception 
     {
         Loader loader = new Loader();
         loader.loadMe();
         loader.start();
     }
}

您应该进一步向代码中添加一些错误处理。