public class TestJDKClassLoader {

    public static void main(String[] args) {
        System.out.println(String.class.getClassLoader());
        System.out.println(com.sun.crypto.provider.DESKeyFactory.class.getClassLoader().getClass().getName());
        System.out.println(TestJDKClassLoader.class.getClassLoader().getClass().getName());
        System.out.println();
        ClassLoader appClassLoader = ClassLoader.getSystemClassLoader();
        ClassLoader extClassloader = appClassLoader.getParent();
        ClassLoader bootstrapLoader = extClassloader.getParent();
        System.out.println("the bootstrapLoader : " + bootstrapLoader);
        System.out.println("the extClassloader : " + extClassloader);
        System.out.println("the appClassLoader : " + appClassLoader);

        System.out.println();
        System.out.println("bootstrapLoader加載以下文件：");
        URL[] urls = Launcher.getBootstrapClassPath().getURLs();
        for (int i = 0; i < urls.length; i++) {
            System.out.println(urls[i]);
        }

        System.out.println();
        System.out.println("extClassloader加載以下文件：");
        System.out.println(System.getProperty("java.ext.dirs"));
        System.out.println();
        System.out.println("appClassLoader加載以下文件：");
        System.out.println(System.getProperty("java.class.path"));
    }
}

// 運(yùn)行結(jié)果：
null
sun.misc.Launcher$ExtClassLoader
sun.misc.Launcher$AppClassLoader

the bootstrapLoader : null
the extClassloader : sun.misc.Launcher$ExtClassLoader@330bedb4
the appClassLoader : sun.misc.Launcher$AppClassLoader@14dad5dc

bootstrapLoader加載以下文件：
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/resources.jar
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/rt.jar
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/sunrsasign.jar
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/jsse.jar
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/jce.jar
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/charsets.jar
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/lib/jfr.jar
file:/D:/ProgramFiles/jdk1.8.0_45_64bit/jre/classes

extClassloader加載以下文件：
D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext;C:\WINDOWS\Sun\Java\lib\ext

appClassLoader加載以下文件：
D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\charsets.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\deploy.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\access-bridge-64.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\cldrdata.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\dnsns.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\jaccess.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\jfxrt.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\localedata.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\nashorn.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunec.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunjce_provider.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunmscapi.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\sunpkcs11.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\ext\zipfs.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\javaws.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jce.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jfr.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jfxswt.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\jsse.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\management-agent.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\plugin.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\resources.jar;D:\ProgramFiles\jdk1.8.0_45_64bit\jre\lib\rt.jar;D:\Files\learn\tuling\jvm-demo\target\classes;D:\ProgramFiles\ideaIU-2021.3.3.win\lib\idea_rt.jar

Process finished with exit code 0

protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // First, check if the class has already been loaded
        // 檢查當(dāng)前類(lèi)加載器是否已經(jīng)找到
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                // 有無(wú)父類(lèi)加載器
                if (parent != null) {
                    c = parent.loadClass(name, false);
                } else {
                    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();
                c = findClass(name);

                // this is the defining class loader; record the stats
                sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                sun.misc.PerfCounter.getFindClasses().increment();
            }
        }
        if (resolve) {
            resolveClass(c);
        }
        return c;
    }
}

package java.lang;

public class String {

    public static void main(String[] args) {
        System.out.println("============自己的類(lèi)加載器====");
    }
}

// 執(zhí)行結(jié)果
錯(cuò)誤: 在類(lèi) java.lang.String 中找不到 main 方法, 請(qǐng)將 main 方法定義為:
   public static void main(String[] args)
否則 JavaFX 應(yīng)用程序類(lèi)必須擴(kuò)展javafx.application.Application

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將一個(gè)字節(jié)數(shù)組轉(zhuǎn)為Class對(duì)象，這個(gè)字節(jié)數(shù)組是class文件讀取后最終的字節(jié)數(shù)組。
                return defineClass(name, data, 0, data.length);
            } catch (Exception e) {
                e.printStackTrace();
                throw new ClassNotFoundException();
            }
        }
    }

    public static void main(String[] args) throws Exception {
        //初始化自定義類(lèi)加載器，會(huì)先初始化父類(lèi)ClassLoader，其中會(huì)把自定義類(lèi)加載器的父加載器設(shè)置為應(yīng)用程序類(lèi)加載器AppClassLoader
        MyClassLoader classLoader = new MyClassLoader("D:/test");
        //D盤(pán)創(chuàng)建 test/com/hyz/jvm 幾級(jí)目錄，將User類(lèi)的復(fù)制類(lèi)User1.class丟入該目錄
        Class clazz = classLoader.loadClass("com.hyz.jvm.User1");
//        Class clazz = classLoader.loadClass("java.lang.String");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("hello", null);
        method.invoke(obj, null);

        System.out.println(clazz.getClassLoader().getClass().getName());
    }
}

/**
 * 32 * 重寫(xiě)類(lèi)加載方法，實(shí)現(xiàn)自己的加載邏輯，不委派給雙親加載
 * 33 * @param name
 * 34 * @param resolve
 * 35 * @return
 * 36 * @throws ClassNotFoundException
 * 37
 */
protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException {
    synchronized (getClassLoadingLock(name)) {
        // First, check if the class has already been loaded
        Class<?> c = findLoadedClass(name);

        if (c == null) {
            // If still not found, then invoke findClass in order
            // to find the class.
            long t1 = System.nanoTime();
            if (!name.startsWith("com.hyz.jvm")) {
                c = this.getParent().loadClass(name);
            } else {
                c = findClass(name);
            }
            // this is the defining class loader; record the stats
            sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
            sun.misc.PerfCounter.getFindClasses().increment();
        }
        if (resolve) {
            resolveClass(c);
        }
        return c;
    }
}