package net.codingme.box.classloader;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
/**
 * <p>
 * 自定义 Java类加载器来实现Java 类的热加载
 *
 * @Author niujinpeng
 * @Date 2019/10/24 23:22
 */
public class MyClasslLoader extends ClassLoader {
    /** 要加载的 Java 类的 classpath 路径 */
    private String classpath;
    public MyClasslLoader(String classpath) {
        // 指定父加载器
        super(ClassLoader.getSystemClassLoader());
        this.classpath = classpath;
    }
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] data = this.loadClassData(name);
        return this.defineClass(name, data, 0, data.length);
    }
    /**
     * 加载 class 文件中的内容
     *
     * @param name
     * @return
     */
    private byte[] loadClassData(String name) {
        try {
            // 传进来是带包名的
            name = name.replace(".", "//");
            FileInputStream inputStream = new FileInputStream(new File(classpath + name + ".class"));
            // 定义字节数组输出流
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            int b = 0;
            while ((b = inputStream.read()) != -1) {
                baos.write(b);
            }
            inputStream.close();
            return baos.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

package net.codingme.box.classloader;
/**
 * <p>
 * 实现这个接口的子类，需要动态更新。也就是热加载
 *
 * @Author niujinpeng
 * @Date 2019/10/24 23:29
 */
public interface BaseManager {
    public void logic();
}

package net.codingme.box.classloader;
/**
 * <p>
 * 封装加载类的信息
 *
 * @Author niujinpeng
 * @Date 2019/10/24 23:32
 */
public class LoadInfo {
    /** 自定义的类加载器 */
    private MyClasslLoader myClasslLoader;
    /** 记录要加载的类的时间戳-->加载的时间 */
    private long loadTime;
    /** 需要被热加载的类 */
    private BaseManager manager;
    public LoadInfo(MyClasslLoader myClasslLoader, long loadTime) {
        this.myClasslLoader = myClasslLoader;
        this.loadTime = loadTime;
    }
    public MyClasslLoader getMyClasslLoader() {
        return myClasslLoader;
    }
    public void setMyClasslLoader(MyClasslLoader myClasslLoader) {
        this.myClasslLoader = myClasslLoader;
    }
    public long getLoadTime() {
        return loadTime;
    }
    public void setLoadTime(long loadTime) {
        this.loadTime = loadTime;
    }
    public BaseManager getManager() {
        return manager;
    }
    public void setManager(BaseManager manager) {
        this.manager = manager;
    }
}

package net.codingme.box.classloader;
import java.io.File;
import java.lang.reflect.InvocationTargetException;
import java.util.HashMap;
import java.util.Map;
/**
 * <p>
 * 加载 manager 的工厂
 *
 * @Author niujinpeng
 * @Date 2019/10/24 23:38
 */
public class ManagerFactory {
    /** 记录热加载类的加载信息 */
    private static final Map<String, LoadInfo> loadTimeMap = new HashMap<>();
    /** 要加载的类的 classpath */
    public static final String CLASS_PATH = "D:\\IdeaProjectMy\\lab-notes\\target\\classes\\";
    /** 实现热加载的类的全名称(包名+类名 ) */
    public static final String MY_MANAGER = "net.codingme.box.classloader.MyManager";
    public static BaseManager getManager(String className) {
        File loadFile = new File(CLASS_PATH + className.replaceAll("\\.", "/") + ".class");
        // 获取最后一次修改时间
        long lastModified = loadFile.lastModified();
        System.out.println("当前的类时间：" + lastModified);
        // loadTimeMap 不包含 ClassName 为 key 的信息，证明这个类没有被加载，要加载到 JVM
        if (loadTimeMap.get(className) == null) {
            load(className, lastModified);
        } // 加载类的时间戳变化了，我们同样要重新加载这个类到 JVM。
        else if (loadTimeMap.get(className).getLoadTime() != lastModified) {
            load(className, lastModified);
        }
        return loadTimeMap.get(className).getManager();
    }
    /**
     * 加载 class ，缓存到 loadTimeMap
     * 
     * @param className
     * @param lastModified
     */
    private static void load(String className, long lastModified) {
        MyClasslLoader myClasslLoader = new MyClasslLoader(className);
        Class loadClass = null;
        // 加载
        try {
            loadClass = myClasslLoader.loadClass(className);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        BaseManager manager = newInstance(loadClass);
        LoadInfo loadInfo = new LoadInfo(myClasslLoader, lastModified);
        loadInfo.setManager(manager);
        loadTimeMap.put(className, loadInfo);
    }
    /**
     * 以反射的方式创建 BaseManager 的子类对象
     * 
     * @param loadClass
     * @return
     */
    private static BaseManager newInstance(Class loadClass) {
        try {
            return (BaseManager)loadClass.getConstructor(new Class[] {}).newInstance(new Object[] {});
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
        return null;
    }
}

00:08:13.018: Java类的热加载
00:08:15.018: Java类的热加载