【jvm系列-02】jvm的类加载子系统以及jclasslib的基本使用

public static int x = 10;  //在准备阶段赋值默认值为0，并且分配内存
public static void main(String[] args) {
    System.out.println(j);
}

public static int x = 10;  //当前阶段中此时x的值为10

//获取系统类加载器
ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
//sun.misc.Launcher$AppClassLoader@18b4aac2
System.out.println(systemClassLoader);
//获取上层扩展类加载器
ClassLoader extClassLoader = systemClassLoader.getParent();
//sun.misc.Launcher$ExtClassLoader@15615099
System.out.println(extClassLoader);
//获取上层引导类加载器
ClassLoader bootStrapClassLoader = extClassLoader.getParent();
//null 尝试获取失败，该类由c语言编写
System.out.println(bootStrapClassLoader);
//获取自定义类类加载器，以当前类为例
ClassLoader classLoader = ClassLoad.class.getClassLoader();
//sun.misc.Launcher$AppClassLoader@18b4aac2 
// 可以发现当前自定义类的了地价在其为系统类加载器
System.out.println(classLoader);

ClassLoader StringClassLoader = String.class.getClassLoader();
System.out.println(StringClassLoader);  //null

//获取引导类加载器可以加载的全部url
URL[] urLs = sun.misc.Launcher.getBootstrapClassPath().getURLs();
for (int i = 0; i < urLs.length; i++) {
    System.out.println(urLs[i]);
}

// 检查当前类加载器是否已经加载了该类
Class<?> c = findLoadedClass(name);
    if (c == null) {
        long t0 = System.nanoTime();
        try {
            if (parent != null) {  //如果当前加载器父加载器不为空则委托父加载器加载该类
                c = parent.loadClass(name, false);
            } else {  //如果当前加载器父加载器为空则委托引导类加载器加载该类
                c = findBootstrapClassOrNull(name);
            }
        } catch (ClassNotFoundException e) {
            // ClassNotFoundException thrown if class not found
            // from the non-null parent class loader
        }
    if (c == null) {
        // If still not found, then invoke findClass in order
        // to find the class.
        long t1 = System.nanoTime();
        //都会调用URLClassLoader的findClass方法在加载器的类路径里查找并加载该类
        c = findClass(name);

import java.io.FileInputStream;
import java.lang.reflect.Method;
/**
 * @Author: zhenghuisheng
 * @Date: 2023/3/16 23:09
 */
public class MyClassLoaderTest {
    static class MyClassLoader extends ClassLoader {
    private String classPath;
    public MyClassLoader(String classPath) {
            this.classPath = classPath;
    }
    private byte[] loadByte(String name) throws Exception {
        name = name.replaceAll("\\.", "/");
            FileInputStream fis = new FileInputStream(classPath + "/" + name + ".class");
            int len = fis.available();
            byte[] data = new byte[len];
            fis.read(data);
            fis.close();
            return data;
    }
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        try {
             byte[] data = loadByte(name);
             //defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节数组。
             return defineClass(name, data, 0, data.length);
             } catch (Exception e) {
             e.printStackTrace();
             throw new ClassNotFoundException();
            }
        }
    }
    /**
     * 下面的磁盘路径需要手动创建
     * @param args
     * @throws Exception
     */
    public static void main(String args[]) throws Exception {
        //初始化自定义类加载器，会先初始化父类ClassLoader，其中会把自定义类加载器的父加载器设置为应用程序类加载器AppClassLoader
        MyClassLoader classLoader = new MyClassLoader("D:/test");
        //D盘创建 test/com/zhenghuisheng/jvm 几级目录，将User类的复制类User.class丢入该目录
        //需要创建一个User类在这个路径下
        Class clazz = classLoader.loadClass("com.zhenghuisheng.jvm.User");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("sout", null);
        method.invoke(obj, null);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }
}