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我试图了解如何使用 URLClassLoader 访问/使 jar 文件可用。

首先,我正在加载 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();

     }
}

打印输出显示了 jgroups-3.4.2.Final.jar 中声明的字段,但随后会引发 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该类已加载但找不到?

谢谢艺术

4

2 回答 2

1

您的代码可能有几个问题。首先,您在 main 中实例化加载器 2 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.

此外,您已经将 JChannel 定义为loader之前的成员,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。我已将它替换为URLClassLoader您应该通过实例化的类加载的类start()

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();
     }
}

您应该在代码中进一步添加一些错误处理。

于 2014-01-12T12:08:26.147 回答
0
Exception in thread "main" java.lang.NoClassDefFoundError: org/jgroups/JChannel
    at myA.loader.start(loader.java:23)

代码在这一行失败:

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

JChannel类型对loaderClassLoader不可见。如果您尝试,这将是显而易见的:

       loader.class
             .getClassLoader()
             .loadClass("org.jgroups.JChannel");

您不应该对在运行时不在类型的类路径上的依赖项有任何编译时引用。

使用新的子ClassLoader 加载不会将该类型添加到某个全局类池中。加载器是分层的,具有子父关系。

于 2014-01-12T12:11:16.250 回答