✅ 【java.lang.ClassLoader】 自定义类加载器

LearningJDK.iml

@@ -4,6 +4,7 @@
     <exclude-output />
     <content url="file://$MODULE_DIR$">
       <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+      <excludeFolder url="file://$MODULE_DIR$/src/test/kang/classloader/other" />
       <excludeFolder url="file://$MODULE_DIR$/src/test/kang/packagee/exclude" />
     </content>
     <orderEntry type="inheritedJdk" />

src/test/kang/classloader/ClassLoaderTest03.java

@@ -0,0 +1,12 @@
+package test.kang.classloader;
+
+// 自定义类加载器
+public class ClassLoaderTest03 {
+    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
+        CustomClassLoader customClassLoader = new CustomClassLoader();
+
+        // 将编译好的User类放在{项目源码的src目录}/test/kang/classloader/other下面，然后就可以使用自定义类加载器加载了
+        Class<?> loadClass = customClassLoader.loadClass("test.kang.classloader.other.User");
+        loadClass.newInstance();
+    }
+}

src/test/kang/classloader/CustomClassLoader.java

@@ -0,0 +1,51 @@
+package test.kang.classloader;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.MalformedURLException;
+import java.net.URL;
+
+// 自定义类加载器，从指定的目录加载类或其他资源
+public class CustomClassLoader extends ClassLoader {
+
+    // 设置父级类加载器为app class loader，保持双亲委托模式
+    public CustomClassLoader() {
+        // 类加载器名称为custom
+        super("custom", ClassLoader.getSystemClassLoader());
+    }
+
+    @Override
+    protected Class<?> findClass(String className) {
+        // 包名+类名转换为路径
+        String cn = className.replace('.', '/') + ".class";
+
+        URL url = findResource(cn);
+
+        try {
+            InputStream inputStream = url.openStream();
+            byte[] bytes = new byte[10240]; // 假设一次就把类文件读完了
+            int len = inputStream.read(bytes);
+            return defineClass(className, bytes, 0, len);
+        } catch(IOException e) {
+            e.printStackTrace();
+        }
+
+        return null;
+    }
+
+    @Override
+    protected URL findResource(String resName) {
+        // 获取项目根目录
+        String basepath = new File("").getAbsolutePath();
+
+        URL url = null;
+        try {
+            // 这里约定从项目源码的src目录开始查找资源（不是类路径）
+            url = new URL("file:/" + basepath + "/src/" + resName);
+        } catch(MalformedURLException e) {
+            e.printStackTrace();
+        }
+        return url;
+    }
+}

src/test/kang/classloader/other/User.java

@@ -0,0 +1,8 @@
+package test.kang.classloader.other;
+
+public class User {
+    public User() {
+        ClassLoader classLoader = User.class.getClassLoader();
+        System.out.println("我是被 " + classLoader.getName() + " 加载的");
+    }
+}