类加载器实战

分手后的思念是犯贱 2022-09-05 12:47 68阅读 0赞

## 一 实战——显式加载和隐式加载 ##

public class UserTest {
        public static void main(String[] args) {
            User user = new User(); // 隐式加载
            try {
                Class clazz = Class.forName("chapter04.java.User"); // 显式加载
                ClassLoader.getSystemClassLoader().loadClass("chapter04.java.User");// 显式加载
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
    }

## 二 实战——类的加载器 ##

## 1 代码 ##

package chapter04.java;
    
    
    import java.io.ByteArrayOutputStream;
    import java.io.FileInputStream;
    import java.io.IOException;
    import java.io.InputStream;
    
    
    public class UserClassLoader extends ClassLoader {
        private String rootDir;
    
    
        public UserClassLoader(String rootDir) {
            this.rootDir = rootDir;
        }
    
    
        /**
         * 编写 findClass方法的逻辑
         */
        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException {
            // 获取类的class文件字节数组
            byte[] classData = getClassData(name);
            if (classData == null) {
                throw new ClassNotFoundException();
            } else {
                //直接生成class对象
                return defineClass(name, classData, 0, classData.length);
            }
        }
    
    
        /**
         * 编写获取class文件并转换为字节码流的逻辑 * @param className * @return
         */
        private byte[] getClassData(String className) {
            // 读取类文件的字节
            String path = classNameToPath(className);
            try {
                InputStream ins = new FileInputStream(path);
                ByteArrayOutputStream baos = new ByteArrayOutputStream();
                byte[] buffer = new byte[1024];
                int len = 0;
                // 读取类文件的字节码
                while ((len = ins.read(buffer)) != -1) {
                    baos.write(buffer, 0, len);
                }
                return baos.toByteArray();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }
    
    
        /**
         * 类文件的完全路径
         */
        private String classNameToPath(String className) {
            return rootDir + "/" + className.replace('.', '/') + ".class";
        }
    
    
        public static void main(String[] args) {
            String rootDir = "E:\\jvmp2\\";
            try {
                // 创建自定义的类的加载器1
                UserClassLoader loader1 = new UserClassLoader(rootDir);
                Class clazz1 = loader1.findClass("chapter04.java.User");
    
    
                // 创建自定义的类的加载器2
                UserClassLoader loader2 = new UserClassLoader(rootDir);
                Class clazz2 = loader2.findClass("chapter04.java.User");
    
    
                System.out.println(clazz1 == clazz2); // false clazz1与clazz2对应了不同的类模板结构。
                System.out.println(clazz1.getClassLoader());// chapter04.java.UserClassLoader@677327b6
                System.out.println(clazz2.getClassLoader());// chapter04.java.UserClassLoader@7f31245a
    
    
                //######################
                Class clazz3 = ClassLoader.getSystemClassLoader().loadClass("chapter04.java.User");
                System.out.println(clazz3.getClassLoader());//sun.misc.Launcher$AppClassLoader@18b4aac2
    
    
                //自定义类加载器的父类就是系统类加载器
                System.out.println(clazz1.getClassLoader().getParent());//sun.misc.Launcher$AppClassLoader@18b4aac2
    
    
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
    }

## 2 测试结果 ##

false
    chapter04.java.UserClassLoader@677327b6
    chapter04.java.UserClassLoader@7f31245a
    sun.misc.Launcher$AppClassLoader@18b4aac2
    sun.misc.Launcher$AppClassLoader@18b4aac2