自定义类加载器

朴灿烈づ我的快乐病毒、 2021-07-24 12:00 476阅读 0赞

# 一 点睛 #

**1 继承 ClassLoader**

**2 重写模板方法 findClass**

调用defineClass

**3 可以自定义类加载器加载自加密的 class**

可以防止反编译

可以方式篡改

# 二 核心源码 #

**1 loadClass**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

**2 ClassLoader**

关键是重写 findClass

如果是 AppClassLoader，首先会执行 URLClassLoader 的 findClass 方法

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1][]

# 三 怎样加载一个类 #

**1 代码**

package jvm;
    
    public class T005_LoadClassByHand {
        public static void main(String[] args) throws ClassNotFoundException {
            Class clazz = T005_LoadClassByHand.class.getClassLoader().loadClass("jvm.T002_ClassLoadLevel");
            System.out.println(clazz.getName());
        }
    }

**2 测试**

> jvm.T002\_ClassLoadLevel

# 四 自定义类加载器 #

**1 测试类**

package jvm;
    public class Log {
        public static void main(String[] args) {
            System.out.println("load Log class successfully");
        }
    }

**2 自定义类加载器**

package jvm;
    
    import java.io.ByteArrayOutputStream;
    import java.io.File;
    import java.io.FileInputStream;
    
    import java.nio.ByteBuffer;
    import java.nio.channels.Channels;
    import java.nio.channels.FileChannel;
    import java.nio.channels.WritableByteChannel;
    
    public class MyClassLoader extends ClassLoader
    {
        public MyClassLoader()
        {
    
    
        }
    
        public MyClassLoader(ClassLoader parent)
        {
            super(parent);
        }
    
        protected Class<?> findClass(String name) throws ClassNotFoundException
        {
            File file = new File("G:/test/Log.class");
            try{
                byte[] bytes = getClassBytes(file);
                //defineClass方法可以把二进制流字节组成的文件转换为一个java.lang.Class
                Class<?> c = this.defineClass(name, bytes, 0, bytes.length);
                return c;
            }
            catch (Exception e)
            {
                e.printStackTrace();
            }
            return super.findClass(name);
        }
    
        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();
        }
    
        public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
            MyClassLoader mcl = new MyClassLoader();
            Class<?> clazz = Class.forName("jvm.Log", true, mcl);
            Object obj = clazz.newInstance();
    
            System.out.println(obj);
            System.out.println(obj.getClass().getClassLoader());//打印出我们的自定义类加载器
        }
    }

**3 Log.class 的位置**

> G:\\test

**4 测试**

> jvm.Log@38af3868
> 
> jvm.MyClassLoader@1be6f5c3

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1]: /images/20210724/74c3b957ac744884a78de472685f203a.png