java - 动态类加载: Exception in thread "main" java. lang.ClassFormatError

Question

我试图从 jar 文件执行动态类加载，不幸的是出现错误:

Exception in thread "main" java.lang.ClassFormatError: Incompatible magic value 1347093252 in class file com/life/Life
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:465)
    at gloria.MyClassLoader.loadClass(MyClassLoader.java:38)
    at gloria.Gloria.main(Gloria.java:9)
Java Result: 1

这是我的代码:

MainClass.java

public class MainClass {

    public static void main(String[] args) {
         try{
            ClassLoader parentClassLoader = MyClassLoader.class.getClassLoader();
            MyClassLoader classLoader = new MyClassLoader(parentClassLoader);
            Class myObjectClass = classLoader.loadClass("com.life.Life");

            //create new class loader so classes can be reloaded.
            classLoader = new MyClassLoader(parentClassLoader);
            myObjectClass = classLoader.loadClass("com.life.Life");

            Life life = (Life) myObjectClass.newInstance();
            System.out.println("Message: " + life.getMessage());

        }catch(Exception e){
            e.printStackTrace();
        }
    }

}

MyClassLoader.java

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;

public class MyClassLoader extends ClassLoader{

    public MyClassLoader(ClassLoader parent) {
         super(parent);
    }

    @Override
    public Class loadClass(String name) throws ClassNotFoundException {
        if(!"com.life.Life".equals(name))
               return super.loadClass(name);

        try {
            String url = "http://192.168.1.229:8081/downloads/Life.jar";
            URL myUrl = new URL(url);
            URLConnection connection = myUrl.openConnection();
            InputStream input = connection.getInputStream();
            ByteArrayOutputStream buffer = new ByteArrayOutputStream();
            int data = input.read();

            while(data != -1){
               buffer.write(data);
               data = input.read();
            }

            input.close();

            byte[] classData = buffer.toByteArray();

            return defineClass("com.life.Life", classData, 0, classData.length);

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }

}

Life.java

public interface Life {

    public String getMessage();

}

我在这里所做的是实例化一个来自 jar 文件的对象并在运行时加载它。我的代码有什么问题吗？有什么想法吗？

Answer 1

0xCAFEBABE 是 Java 文件通常的前 4 个字节。您的值 1347093252 是十六进制的 0x504B0304，这是 ZIP 文件的神奇值。因为 jar 也是一个 zip 文件。这意味着您的 jar 可能已损坏。尝试重新构建 jar。