java - 类加载器如何加载在方法中声明的扩展类的实例？(java.lang.NoClassDefFoundError)

Question

我有一个具有实用功能的类，可以按需调用。

重现步骤 ：

1. TestMainWebApp（这个项目有TestMainImp的依赖）
2. TestMainImpl (这个项目实现了TestMainInterface)
3. 测试主接口

TestMainWebApp > TestMainServlet.java

package com.main;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.test.util.Util;
/**
 * Servlet implementation class TestMainServlet
 */
@WebServlet("/TestMainServlet")
public class TestMainServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {   
         System.out.println("\nCurrent ClassLoader chain: "+JavaEETrainingUtil.getCurrentClassloaderDetail());
         Util prov = new Util();
        // prov.test();
        }
}

TestMainImpl > Util.java & TLSErrorRedirectListener.java

package com.test.util;
public class Util {
     private final static String CLAZZ = Util.class.getName();
    static {
            System.out.println("Classloading of "+CLAZZ+" in progress..."+JavaEETrainingUtil.getCurrentClassloaderDetail());

     }
        public boolean checkForTLSErrorRedirection(boolean b) {                         
            test.intf.ConfigurationListener listener = new com.listener.TLSErrorRedirectListener();   
            listener.valueChanged("test", "test");
            return true;

        }    

public void test() {  
        System.out.println(" test util");  
    }

}

TLSErrorRedirectionListener.java

package com.listener;
import com.test.util.JavaEETrainingUtil;
public class TLSErrorRedirectListener implements test.intf.ConfigurationListener {
     final static String CLAZZ = TLSErrorRedirectListener.class.getName();
     static {
            System.out.println("Classloading of "+CLAZZ+" in progress..."+JavaEETrainingUtil.getCurrentClassloaderDetail());
     }
    public void valueChanged(String key, String value) {
        switch(key){
        case "test1": 
        default : break;
        }
    }
}

TestMainInterface > ConfigurationListener.java

package test.intf;
public abstract interface ConfigurationListener
{
  public abstract void valueChanged(String paramString1, String paramString2);
}

案子 ：

1. TestMainInterface.jar 将在 TestMainImpl.jar 的类路径中（仅限@compiletime）
2. 在运行时（我不会有 TestMainInterface.jar）并且我不调用方法 "checkForTLSErrorRedirection()" 。我只调用 test() 方法。
3. 
   但是当我自己创建实例时，我得到了 java.lang.NoClassDefFoundError: test/intf/ConfigurationListener 。

你能帮忙找出根本原因吗？java如何加载在方法中声明的类？

注意： 用于调试目的的JavaEETrainingUtil.java

package noclassdef.example1;
import java.util.Stack;
import java.lang.ClassLoader;

/**
 * JavaEETrainingUtil
 * @author Pierre-Hugues Charbonneau
 *
 */
public class JavaEETrainingUtil {

        /**
         * getCurrentClassloaderDetail
         * @return
         */
        public static String getCurrentClassloaderDetail() {

               StringBuffer classLoaderDetail = new StringBuffer();       
               Stack<ClassLoader> classLoaderStack = new Stack<ClassLoader>();

               ClassLoader currentClassLoader = Thread.currentThread().getContextClassLoader();

               classLoaderDetail.append("\n-----------------------------------------------------------------\n");

               // Build a Stack of the current ClassLoader chain
               while (currentClassLoader != null) {

                       classLoaderStack.push(currentClassLoader);

                       currentClassLoader = currentClassLoader.getParent();
               }

               // Print ClassLoader parent chain
               while(classLoaderStack.size() > 0) {

                       ClassLoader classLoader = classLoaderStack.pop();

                       // Print current                     
                       classLoaderDetail.append(classLoader);

                       if (classLoaderStack.size() > 0) {
                              classLoaderDetail.append("\n--- delegation ---\n");           } else {
                              classLoaderDetail.append(" **Current ClassLoader**");
                       }
               }
               classLoaderDetail.append("\n-----\n");
               return classLoaderDetail.toString();
        }
}

Answer 1

这是不正确的。您不应该将abstract关键字添加到接口或其方法。不需要publicon 接口方法；他们必须是公开的。

这是对的：

package test.intf;
public interface ConfigurationListener {
  void valueChanged(String paramString1, String paramString2);
}

确保该test.intf.ConfigurationListener.class文件出现在您的部署中。如果没有，类加载器将永远找不到它。