自定义类加载器

男娘i 2022-02-19 23:13 470阅读 0赞

package com.jerry.demo.core;
    
    import com.jerry.demo.model.User;
    
    import java.io.ByteArrayInputStream;
    import java.io.ByteArrayOutputStream;
    import java.io.IOException;
    import java.lang.reflect.Method;
    import java.nio.file.Files;
    import java.nio.file.Path;
    import java.nio.file.Paths;
    
    public class CustomClassLoader extends  ClassLoader {
        private final static String DEFAULT_LOAD_PATH = "G:\\devOutput\\workspace\\jerry-paas-master\\demo\\target\\classes";
        private final String classDir ;
    
        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException {
           byte [] classByte =  readClassByte(name);
           if(null == classByte||classByte.length == 0){
               throw new ClassNotFoundException("cannot load this class:" + name);
           }
           return  this.defineClass(name,classByte,0,classByte.length);
           }
    
        public CustomClassLoader() {
            super();
            classDir = DEFAULT_LOAD_PATH;
    
        }
        public CustomClassLoader(String classDir) {
            super();
            this.classDir = classDir;
    
        }
    
        public CustomClassLoader(String classDir,ClassLoader parentClassParent) {
            super(parentClassParent);
            this.classDir = classDir;
    
        }
    
        private byte[] readClassByte(String className) throws ClassNotFoundException {
            className = className.replace(".","\\");
    //        className = classDir + className;
            Path classPath = Paths.get(classDir,className+".class");
            if(!classPath.toFile().exists()){
    
                throw new ClassNotFoundException();
    
            }
            ByteArrayOutputStream bao = new ByteArrayOutputStream();
            try {
                Files.copy(classPath, bao);
            return bao.toByteArray();
            }catch (IOException e){
    
                throw new ClassNotFoundException("加载类出错!");
    
            }
        }
    
        public static void main(String[] args) throws Exception {
            CustomClassLoader cl = new CustomClassLoader(DEFAULT_LOAD_PATH,CustomClassLoader.class.getClassLoader().getParent());
            Class clazz   = cl.findClass("com.jerry.demo.model.User");
            Method setName = clazz.getDeclaredMethod("setName", new Class[]{String.class});
            Object user = clazz.newInstance();
            setName.invoke(user,"jerry");
            System.out.println(user);
            System.out.println(user.getClass().getClassLoader());
            System.out.println(user.getClass().getClassLoader().getParent());
        }
    }