MyClassLoader 自定义类加载器

package demo;

import java.io.*;

public class MyClassLoader extends ClassLoader {
    private static final int BUFFER_SIZE = 8192;

    protected synchronized Class loadClass(String className, boolean resolve) 
                                 throws ClassNotFoundException {
        log("Loading class: " + className + ", resolve: " + resolve);

        // 1. is this class already loaded?
        Class cls = findLoadedClass(className);
        if (cls != null) {
            return cls;
        }

        // 2. get class file name from class name
        String clsFile = className.replace('.', '/') + ".class";
        
        // 3. get bytes for class
        byte[] classBytes = null;
        try {
            InputStream in = getResourceAsStream(clsFile);
            byte[] buffer = new byte[BUFFER_SIZE];
            ByteArrayOutputStream out = new ByteArrayOutputStream();
            int n = -1;
            while ((n = in.read(buffer, 0, BUFFER_SIZE)) != -1) {
                out.write(buffer, 0, n);
            }
            classBytes = out.toByteArray();
        }
        catch (IOException e) {
            log("ERROR loading class file: " + e);
        }

        if (classBytes == null) {
            throw new ClassNotFoundException("Cannot load class: " + className);
        }

        // 4. turn the byte array into a Class
        try {
            cls = defineClass(className, classBytes, 0, classBytes.length);
            if (resolve) {
                resolveClass(cls);
            }
        }
        catch (SecurityException e) { 
            // loading core java classes such as java.lang.String
            // is prohibited, throws java.lang.SecurityException.
            // delegate to parent if not allowed to load class
            cls = super.loadClass(className, resolve);
        }

        return cls;
    }

    private static void log(String s) {
        System.out.println(s);
    }
}

Test code:

package demo;

public class TestClassLoader {
    public static void main(String[] args) throws Exception {
        MyClassLoader loader1 = new MyClassLoader();
        
        // load demo.Base64
        Class clsB64 = Class.forName("demo.Base64", true, loader1);
        System.out.println("Base64 class: " + clsB64);

        if (Base64.class.equals(clsB64)) {
            System.out.println("Base64 loaded through custom loader is the same as that loaded by System loader.");
        }
        else {
            System.out.println("Base64 loaded through custom loader is NOT same as that loaded by System loader.");
        }

        // call the main method in Base64
        java.lang.reflect.Method main = clsB64.getMethod("main", 
                                                new Class[] {String[].class});
        main.invoke(null, new Object[]{ new String[]{} });
    }
}