类加载机制和双亲委派

打印一个类加载器的示例。

import java.net.URL;
import sun.misc.Launcher;

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加载以下文件：");
        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加载以下文件： ");
        System.out.println(System.getProperty("java.ext.dirs"));


        System.out.println();
        System.out.println("appClassLoader加载以下文件： ");
        System.out.println(System.getProperty("java.class.path"));

    }
}

输出结果：

null
sun.misc.Launcher$ExtClassLoader
sun.misc.Launcher$AppClassLoader

the bootstrapLoader : null
the extClassloader : sun.misc.Launcher$ExtClassLoader@1554909b
the appClassLoader : sun.misc.Launcher$AppClassLoader@18b4aac2

bootstrapLoader加载以下文件：
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/resources.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/rt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/jsse.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/jce.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/charsets.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/jfr.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/classes
file:/Users/wq/Library/Caches/JetBrains/IntelliJIdea2024.2/captureAgent/debugger-agent.jar

extClassloader加载以下文件： 
/Users/wq/Library/Java/Extensions:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java

appClassLoader加载以下文件： 
/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home/jre/lib/rt.jar:/Users/wq/study/mianshi/jichu/demo/target/classes:/Applications/IntelliJ IDEA Ultimate.app/Contents/lib/idea_rt.jar:/Users/wq/Library/Caches/JetBrains/IntelliJIdea2024.2/captureAgent/debugger-agent.jar

类加载器

上面的类加载过程主要是通过类加载器来实现的，Java里有如下几种类加载器

引导类加载器：负责加载支撑JVM运行的位于JRE的lib目录下的核心类库，比如rt.jar、charsets.jar等。是最底层的类加载器，它使用本地代码实现，并且在Java中不可见（即通过 getParent() 方法无法获取到它

扩展类加载器：负责加载支撑JVM运行的位于JRE的lib目录下的ext扩展目录中的JAR类包

应用程序类加载器：负责加载ClassPath路径下的类包，主要就是加载你自己写的那些类

自定义加载器：负责加载用户自定义路径下的类包

双亲委派机制

在这里插入图片描述
找父亲加载，不行再由儿子自己加载

这里类加载其实就有一个双亲委派机制，加载某个类时会先委托父加载器寻找目标类，找不到再委托上层父加载器加载，如果所有父加载器在自己的加载类路径下都找不到目标类，则在自己的类加载路径中查找并载入目标类。

比如我们的Math类，最先会找应用程序类加载器加载，应用程序类加载器会先委托扩展类加载器加载，扩展类加载器再委托引导类加载器，顶层引导类加载器在自己的类加载路径里找了半天没找到Math类，则向下退回加载Math类的请求，扩展类加载器收到回复就自己加载，在自己的类加载路径里找了半天也没找到Math类，又向下退回Math类的加载请求给应用程序类加载器，应用程序类加载器于是在自己的类加载路径里找Math类，结果找到了就自己加载了。。

Math类–>应用程序类（有没有加载过）–>找扩展类加载（有没有加载过）–>引导类加载. 都找不到。
–> 引导类加载–>加载不了,扩展类加载–>加载不了,应用类加载器加载。

在这里插入图片描述

Class<?> c = findLoadedClass(name);
判断类是否加载过了，加载过了直接返回。
2.没有加载过，
找父加载器，为null则为引导类加载器。
不为空则为父类加载器去加载。
通过之前上面的打印的例子，知道ext类加载器的parent为null，app类加载器的parent为ext类加载器。
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClassOrNull(name);
}
3. ext去加载的时候再次判断类是否加载过，没有加载过，parent==null则交给bootstrap加载器加载。
4. bootstrap加载–>如果失败->扩展类加载–>如果失败–>应用类加载，成功则返回。

为什么要设计双亲委派机制？

沙箱安全机制：自己写的java.lang.String.class类不会被加载，这样便可以防止核心API库被随意篡改。

避免类的重复加载：当父亲已经加载了该类时，就没有必要子ClassLoader再加载一次，保证被加载类的唯一性。

全盘负责委托机制

“全盘负责”是指当一个ClassLoder装载一个类时，除非显示的使用另外一个ClassLoder，该类
所依赖及引用的类也由这个ClassLoder载入。

自定义类加载器实现


import java.io.FileInputStream;
import java.lang.reflect.Method;

public class MyClassLoaderTest {

    static class MyClassLoader extends ClassLoader {

        private String classPath;

        public MyClassLoader(String classPath) {
            this.classPath = classPath;
        }


        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException {

            try {
                byte[] data = loadByte(name);
                //name:类名  data:字节数组(字节码文件)
                return defineClass(name , data , 0 , data.length);
            } catch (Exception e) {
                e.printStackTrace();
                throw new ClassNotFoundException();
            }

        }


        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;

        }
    }

    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("/Users/wq/study/jvm");
         // /Users/wq/study/jvm下创建 com.wq.ha.User1几级目录，将User类的复制类User1.class丢入该目录
         Class clazz = classLoader.loadClass("com.wq.ha.User1");
         Object obj = clazz.newInstance();
         Method method = clazz.getDeclaredMethod("getAge" , null);
        Object invoke = method.invoke(obj, null);
        System.out.println(invoke);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }