类加载器与双亲委派机制一网打尽

@RestController
public class TestController {
    @RequestMapping(value = "/test",method = {RequestMethod.GET, RequestMethod.POST})
    public void testclass() {
        String name = "zj";
        int weight = 100;
        System.out.println("name:"+ name);
        System.out.println("weight: "+weight);
    }
}

String name = "ming";
int weight = 300;

public class Person {
    /**
     * 状态 or 属性
     */
    String name;//姓名
    String sex;//性别
    int height;//身高
    int weight;//体重
    /**
     * 行为
     */
    public void sleep(){
      System.out.println(this.name+"--"+ "睡觉");
  }
    public void eat(){
        System.out.println("吃饭");
    }
    public void Dance(){
        System.out.println("跳舞");
    }
}

#打开file文件
vim Person.class 
#在命令模式下输入.. 以16进制显示
 :%!xxd
#在命令模式下输入.. 切换回默认显示
:%!xxd -r

Person zhang = new Person();

static int value = 3;//类变量 初始化，设为默认值 0，不是 3哦 ！！！
int num = 4;//类成员变量，在这个阶段不初始化；在 new类，调用对应类的构造函数才进行初始化
final static valFin = 5;//这个比较特殊，在这个阶段也不会分配内存！！！

public class TestClassLoader {
    public static void main(String[] args) throws ClassNotFoundException {
        ClassLoader classLoader = TestClassLoader.class.getClassLoader();
        System.out.println(classLoader);
        System.out.println(classLoader.getParent());//获取其父类加载器
        System.out.println(classLoader.getParent().getParent());//获取父类的父类加载器
    }
}

public class Launcher {
    private static Launcher launcher = new Launcher();
    private static String bootClassPath =
        System.getProperty("sun.boot.class.path");
    public static Launcher getLauncher() {
        return launcher;
    }
    private ClassLoader loader;
    public Launcher() {
        // Create the extension class loader
        ClassLoader extcl;
        try {
            extcl = ExtClassLoader.getExtClassLoader(); //加载扩展类类加载器
        } catch (IOException e) {
            throw new InternalError(
                "Could not create extension class loader", e);
        }
        // Now create the class loader to use to launch the application
        try {
            loader = AppClassLoader.getAppClassLoader(extcl);//加载应用程序类加载器,并设置parent为extClassLoader
        } catch (IOException e) {
            throw new InternalError(
                "Could not create application class loader", e);
        }
        Thread.currentThread().setContextClassLoader(loader); //设置AppClassLoader为线程上下文类加载器
    }
    /*
     * Returns the class loader used to launch the main application.
     */
    public ClassLoader getClassLoader() {
        return loader;
    }
    /*
     * The class loader used for loading installed extensions.
     */
    static class ExtClassLoader extends URLClassLoader {}
/**
     * The class loader used for loading from java.class.path.
     * runs in a restricted security context.
     */
    static class AppClassLoader extends URLClassLoader {}

private ClassLoader(Void unused, ClassLoader parent) {
        this.parent = parent;//设置parent
        if (ParallelLoaders.isRegistered(this.getClass())) {
            parallelLockMap = new ConcurrentHashMap<>();
            package2certs = new ConcurrentHashMap<>();
            assertionLock = new Object();
        } else {
            // no finer-grained lock; lock on the classloader instance
            parallelLockMap = null;
            package2certs = new Hashtable<>();
            assertionLock = this;
        }
    }

public class String {
    static {
        System.out.println("自定义 String类");
    }
}

public class StringTest {
    public static void main(String[] args) {
        String str=new java.lang.String();
        System.out.println("start test-------");
    }
}

protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
    synchronized (getClassLoadingLock(name)) {
        // 一开始先 检查是否已经加载该类
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                // 如果未加载过类，则遵循 双亲委派机制，来加载类
                if (parent != null) {
                    c = parent.loadClass(name, false);
                } else {
                //如果父类是null就是BootstrapClassLoader，使用 启动类类加载器
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }
            if (c == null) {
                long t1 = System.nanoTime();
                // 如果还是没有加载成功，调用findClass(),让当前类加载器加载
                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;
    }
}
// 继承的子类得重写该方法
protected Class<?> findClass(String name) throws ClassNotFoundException {
    throw new ClassNotFoundException(name);
}

public class TestPerson {
    String name = "xiao ming";
    public void print(){
        System.out.println("hello my name is: "+ name);
    }
}

public class MyTestClassLoader extends ClassLoader  {
    final String classNameSpecify  = "TestPerson";
    public MyTestClassLoader() {
    }
    public MyTestClassLoader(ClassLoader parent)
    {
        super(parent);
    }
    protected Class<?> findClass(String name) throws ClassNotFoundException
    {
        File file = getClassFile(name);
        try
        {
            byte[] bytes = getClassBytes(file);
            Class<?> c = this.defineClass(name, bytes, 0, bytes.length);
            return c;
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        return super.findClass(name);
    }
    private File getClassFile(String name)
    {
        File file = new File("D:\\ideaProjects\\src\\main\\java\\com\\zj\\ideaprojects\\test2\\"+ classNameSpecify+ ".class");
        return file;
    }
    private byte[] getClassBytes(File file) throws Exception
    {
        // 这里要读入.class的字节，因此要使用字节流
        FileInputStream fis = new FileInputStream(file);
        FileChannel fc = fis.getChannel();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        WritableByteChannel wbc = Channels.newChannel(baos);
        ByteBuffer by = ByteBuffer.allocate(1024);
        while (true)
        {
            int i = fc.read(by);
            if (i == 0 || i == -1)
                break;
            by.flip();
            wbc.write(by);
            by.clear();
        }
        fis.close();
        return baos.toByteArray();
    }
    //我们这边要打破双亲委派模型，重写整个loadClass方法
    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        Class<?> c = findLoadedClass(name);
        if (c == null && name.contains(classNameSpecify)){//指定的类，不走双亲委派机制，自定义加载
            c = findClass(name);
            if (c != null){
                return c;
            }
        }
        return super.loadClass(name);
    }
}

@RestController
public class TestClassController {
    @RequestMapping(value = "testClass",method = {RequestMethod.GET, RequestMethod.POST})
    public void testClassLoader() throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
        MyTestClassLoader myTestClassLoader = new MyTestClassLoader();
        Class<?> c1 = Class.forName("com.zj.ideaprojects.test2.TestPerson", true, myTestClassLoader);
        Object obj = c1.newInstance();
        System.out.println("当前类加载器："+obj.getClass().getClassLoader());
        obj.getClass().getMethod("print").invoke(obj);
    }
}

public class TestPerson {
    String name = "xiao niu";
    public void print(){
        System.out.println("hello my name is: "+ name);
    }
}

public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();//important !
        return ServiceLoader.load(service, cl);
    }

public class App {
    String name = "webapp 1";
    public void print() {
        System.out.println("this is "+ name);
    }
}

public class MyTomcatClassloader extends ClassLoader {
    private String classPath;
    public MyTomcatClassloader(String classPath) {
        this.classPath = classPath;
    }
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException
    {
        File file = getClassFile(name);
        try
        {
            byte[] bytes = getClassBytes(file);
            Class<?> c = this.defineClass(name, bytes, 0, bytes.length);
            return c;
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        return super.findClass(name);
    }
    private File getClassFile(String name)
    {
        name = name.replaceAll("\\.", "/");
        File file = new File(classPath+ "/"+ name + ".class");//拼接路径，找到class文件
        return file;
    }
    private byte[] getClassBytes(File file) throws Exception
    {
        // 这里要读入.class的字节，因此要使用字节流
        FileInputStream fis = new FileInputStream(file);
        FileChannel fc = fis.getChannel();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        WritableByteChannel wbc = Channels.newChannel(baos);
        ByteBuffer by = ByteBuffer.allocate(1024);
        while (true)
        {
            int i = fc.read(by);
            if (i == 0 || i == -1) {
                break;
            }
            by.flip();
            wbc.write(by);
            by.clear();
        }
        fis.close();
        return baos.toByteArray();
    }
    //我们这边要打破双亲委派模型，重写整个loadClass方法
    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        Class<?> c = findLoadedClass(name);
        if (c == null && name.contains("tomcatTest")){//指定的目录下的类，不走双亲委派机制，自定义加载
            c = findClass(name);
            if (c != null){
                return c;
            }
        }
        return super.loadClass(name);
    }
}

@RestController
public class TestController {
    @RequestMapping(value = "/testTomcat",method = {RequestMethod.GET, RequestMethod.POST})
    public void testclass() throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
        MyTomcatClassloader myTomcatClassloader = new MyTomcatClassloader("D:\\GiteeProjects\\study-java\\demo-test\\src\\main\\java\\com\\zj\\demotest\\tomcatTest\\war1");
        Class cl = myTomcatClassloader.loadClass("com.zj.demotest.tomcatTest.App");
        Object obj = cl.newInstance();
        System.out.println("当前类加载器："+obj.getClass().getClassLoader());
        obj.getClass().getMethod("print").invoke(obj);
        MyTomcatClassloader myTomcatClassloader22 = new MyTomcatClassloader("D:\\GiteeProjects\\study-java\\demo-test\\src\\main\\java\\com\\zj\\demotest\\tomcatTest\\war2");
        Class cl22 = myTomcatClassloader22.loadClass("com.zj.demotest.tomcatTest.App");
        Object obj22 = cl22.newInstance();
        System.out.println("当前类加载器："+obj22.getClass().getClassLoader());
        obj22.getClass().getMethod("print").invoke(obj22);
    }
}

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());
        Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");
        return configurations;
    }