自定义类加载

红太狼 2022-03-20 03:20 310阅读 0赞

package com.lss;
    
    import java.io.ByteArrayOutputStream;
    import java.io.File;
    import java.io.FileInputStream;
    /**
     * 自定义类加载
     * @author SHOU SHEN LUAN
     *
     */
    public class MyClassLoader extends ClassLoader {
    	public static String CLASS_PATH = "D:\\";
    	public String name;
    
    	public MyClassLoader(String name) {
    		this.name = name;
    
    	}
    
    	public MyClassLoader(ClassLoader classLoader, String name) {
    		super(classLoader);
    		this.name = name;
    	}
    
    	@Override
    	public Class<?> findClass(String name) {
    		System.out.println("findClass:" + name);
    		byte[] data = loadClassData(name);
    		return this.defineClass(name, data, 0, data.length);
    	}
    	/**
    	 * 加载Class文件
    	 * @param name
    	 * @return
    	 */
    	public byte[] loadClassData(String name) {
    		try {
    			name = name.replace(".", "//");
    			FileInputStream is = new FileInputStream(new File(CLASS_PATH + name
    					+ ".class"));
    			ByteArrayOutputStream baos = new ByteArrayOutputStream();
    			int b = 0;
    			while ((b = is.read()) != -1) {
    				baos.write(b);
    			}
    			return baos.toByteArray();
    		} catch (Exception e) {
    			e.printStackTrace();
    		}
    		return null;
    	}
    	/**
    	 * 需要注意的是 如果当前项目指定目录中包含该Class文件的话，ClassLoader在调用findLoaderdClass时则可以加载或从类加载缓存中获取到该类对象，者不会调用findClass
    	 * @param args
    	 */
    	public static void main(String[] args) {
    		MyClassLoader classLoader = new MyClassLoader("myClassLoader");
    		try {
    			Class<?> clazz = classLoader.loadClass("com.lss.json.Pojo");
    			Object object = clazz.newInstance();
    			System.out.println(object.toString());
    			/*自定义ClassLoader类*/
    			System.out.println(object.getClass().getClassLoader());
    			//AppClassLoader JVM用此ClassLoader来加载启动参数中指定的ClassPath中Jar包及目录
    			System.out.println(object.getClass().getClassLoader().getParent());
    			//ExtClassLoader JVM 用此ClassLoader 来加载扩展功能的一些Jar包，例如SunJDK 中目录下有dns工具Jar包等，
    			System.out.println(object.getClass().getClassLoader().getParent().getParent());
    			//sun.misc.Launcher$ExtClassLoader的父类是{Bootstrap ClassLoader}使用C++实现，在代码中无法拿到该对象
    			//Sun JDK 启动时会初始化此ClassLoader，并由ClassLoader 完成 $JAVA_HOME中jre/lib/rt.jar里面所有Class文件的加载，Jar中包含了Javs规范定义的所有的接口实现
    			System.out.println(object.getClass().getClassLoader().getParent().getParent().getParent());
    			
    		} catch (ClassNotFoundException e) {
    			e.printStackTrace();
    		} catch (InstantiationException e) {
    			e.printStackTrace();
    		} catch (IllegalAccessException e) {
    			e.printStackTrace();
    		}
    	}
    }