UserClassLoader__类加载

Author: CPU-Code

JVM/src/com/cpucode/java/loader/UserClassLoader.java

@@ -0,0 +1,106 @@
+package com.cpucode.java.loader;
+
+import java.io.*;
+
+/**
+ * @author : cpucode
+ * @date : 2021/2/3
+ * @time : 16:47
+ * @github : https://github.com/CPU-Code
+ * @csdn : https://blog.csdn.net/qq_44226094
+ */
+public class UserClassLoader extends ClassLoader{
+    private String rootDir;
+
+    public static void main(String[] args) {
+        String rootDir = "D:\\Date\\github\\java\\JVM\\src\\";
+
+        try{
+            //创建自定义的类的加载器1
+            UserClassLoader loader1 = new UserClassLoader(rootDir);
+            Class clazz1 = loader1.findClass("com.cpucode.java.loader.User");
+
+            //创建自定义的类的加载器2
+            UserClassLoader loader2 = new UserClassLoader(rootDir);
+            Class clazz2 = loader2.findClass("com.cpucode.java.loader.User");
+
+            System.out.println(clazz1.getClassLoader());
+            System.out.println(clazz2.getClassLoader());
+
+            //clazz1与clazz2对应了不同的类模板结构
+            System.out.println(clazz1 == clazz2);
+
+            /*****************************************************/
+
+            Class clazz3 = ClassLoader.getSystemClassLoader().loadClass("com.cpucode.java.loader.User");
+            System.out.println(clazz3.getClassLoader());
+
+            System.out.println(clazz1.getClassLoader().getParent());
+        }catch (ClassNotFoundException e){
+            e.printStackTrace();
+        }
+
+        /**
+         * com.cpucode.java.loader.UserClassLoader@330bedb4
+         * com.cpucode.java.loader.UserClassLoader@4b67cf4d
+         * false
+         * sun.misc.Launcher$AppClassLoader@18b4aac2
+         * sun.misc.Launcher$AppClassLoader@18b4aac2
+         * */
+    }
+
+    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
+     * */
+    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";
+    }
+}

README.md

@@ -778,6 +778,7 @@ java编程基础 面向对象 javaAPI 集合 IO GUI JDBC 多线程 网络编程
 -----------------
 
 - [x] [UserTest__显隐加载](JVM/src/com/cpucode/java/loader/UserTest.java)
+- [x] [UserClassLoader__类加载](JVM/src/com/cpucode/java/loader/UserClassLoader.java)
 
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