类加载器不同导致类型强制转换报错:java.lang.ClassCastException

Posted 2021-10-15 master-dragon

直接看如下错误例子

com.hotload.Test cannot be cast to com.hotload.Test,因为com.hotload.Test的类加载器是MyComOtherClassLoader，而非默认的 sun.misc.Launcher$AppClassLoader

如下则正常

关于解决办法，比如可以通过序列化/反序列化解决转换，如下

但是，此时会发现test的classLoader不再是MyComOtherClassLoader了，如下图所示

---

附自定义的classloader

package com.hotload;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;


public class MyComOtherClassLoader extends ClassLoader {
    public String path;
    public String packageName;
    public String className;

    public MyComOtherClassLoader() {
        super(ClassLoader.getSystemClassLoader());
    }

    private String classNameToPath() {
        // 得到类文件的URL
        return path + "/" + packageName.replace('.', '/')
                + "/"
                + className + ".class";
    }

    @Override
    public Class loadClass(String name) throws ClassNotFoundException {
        // 非 com.hotload package下面的类，都用默认的加载，否则用自定义的加载方法
        if (!name.contains("com.hotload")) {
            // 是否已经被加载
            Class loadedClass = findLoadedClass(name);
            if (loadedClass == null) {
                // 用父类去加载该类
                loadedClass = getParent().loadClass(name);
                return loadedClass;
            } else {
                return loadedClass;
            }
        }


        int i = name.lastIndexOf('.');
        packageName = "";
        if (i != -1) {
            packageName = name.substring(0, i);
//            System.out.println("package: " + packageName);
            className = name.substring(i + 1);
//            System.out.println("class name: " + name);
            SecurityManager sm = System.getSecurityManager();
            if (sm != null) {
                sm.checkPackageAccess(packageName);
            }
        }
        //依然调用父类的方法
//        return super.loadClass(name);
        return this.findClass(name);
    }

    @Override
    public Class<?> findClass(String name) throws ClassNotFoundException {
        int i = name.lastIndexOf('.');
        packageName = "";
        if (i != -1) {
            packageName = name.substring(0, i);
//            System.out.println("package: " + packageName);
            className = name.substring(i + 1);
//            System.out.println("class name: " + name);
            SecurityManager sm = System.getSecurityManager();
            if (sm != null) {
                sm.checkPackageAccess(packageName);
            }
        }
        Class<?> clazz = this.findLoadedClass(name); // 父类已加载
        if(null != clazz){
            return clazz;
        }

//        System.out.println("findClass param name: " + name);
        byte [] b = this.getClassBytes();
//        System.out.println("b len:" + b.length);
        clazz=defineClass(null, b, 0, b.length);
        return clazz;
    }
    public byte[] getClassBytes() {
        String classPath = classNameToPath();
//        System.out.println("classPath:" + classPath);
        File file=new File(classPath);
        try(FileInputStream fis = new FileInputStream(file);ByteArrayOutputStream bos=new ByteArrayOutputStream();) {
            byte[] b=new byte[1024*2];
            int n;
            while((n=fis.read(b))!=-1){
                bos.write(b, 0, n);
            }
            return bos.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

}