自定义一个类加载器

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.nio.channels.WritableByteChannel;

/**
 * 自定义类加载器
 * @ClassName MyClassLoader
 * @Author xxx
 * @Version V1.0
 **/
public class MyClassLoader extends ClassLoader {

    private File fileObject;

    public void setFileObject(File fileObject) {
        this.fileObject = fileObject;
    }

    public File getFileObject() {
        return fileObject;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        try {
            byte[] data = getClassFileBytes(this.fileObject);
            return defineClass(name, data, 0, data.length);
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }

    }

    /**
     * 从文件中读取去class文件
     *
     * @throws Exception
     */
    private byte[] getClassFileBytes(File file) throws Exception {
        //采用NIO读取
        FileInputStream fis = new FileInputStream(file);
        FileChannel fileC = fis.getChannel();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        WritableByteChannel outC = Channels.newChannel(baos);
        ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
        while (true) {
            int i = fileC.read(buffer);
            if (i == 0 || i == -1) {
                break;
            }
            buffer.flip();
            outC.write(buffer);
            buffer.clear();
        }
        fis.close();
        return baos.toByteArray();
    }

}

重点：需要继承

ClassLoader 类，并重写

findClass()方法。注意：官方不建议重写loadClass()方法，可能会破坏双亲委托机制。

使用这个自定义类加载器时，先

MyClassLoader myClassLoader = new MyClassLoader();
myClassLoader.setFileObject(new File("此处填需要加载的class文件全路径"));
Class<?> test = myClassLoader.findClass("Test");

4 System.out.println(test.getClassLoader());

这个test的类加载器就是自定义的

MyClassLoader ！