自定义加密的类加载器

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自定义加密的类加载器

1、加载器初识
JVM有有三种类加载器:
*BootStrap加载JRE\lib\rt.jar
*ExtClassLoader加载JRE/lib/ext/*.jar
*AppClassLoader加载classpath指定目录下的类
2、自定义加载器
1)被加载的类
public class ClassAttachment {

	public ClassAttachment() {
		System.out.println("Hello,World");
	}

}


2)加密class的加密器
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class Cypher {
	
	public static void main(String[] args) throws IOException {
		String srcPath =
              "D:\\Android\\reflect\\bin\\classloader\\ClassAttachment.class";
		String destPath = "cypherClass";
		String fileName = srcPath.substring(srcPath.lastIndexOf('\\') + 1);
		String destFilePath = destPath + "\\" + fileName;
		FileInputStream fis = new FileInputStream(srcPath);
		FileOutputStream fos = new FileOutputStream(destFilePath);
		cypher(fis, fos);
		fis.close();
		fos.close();
	}

	// 进行简单加密
	public static void cypher(InputStream is, OutputStream os) {
		int b;
		try {
			while ((b = is.read()) != -1) {
				os.write(b ^ 0xff);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}


3)自定义的加载器
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class MyClassLoader extends ClassLoader {
	
	// 指定存放加密的class的目录
	private String classDir;
	
	public MyClassLoader() {
	}
	
	public MyClassLoader(String classDir) {
		this.classDir = classDir;
	}

	@Override
	protected Class findClass(String name) throws ClassNotFoundException {
		// 生成加密class文件信息
		String classFileName =
      classDir + "\\" + name.substring(name.lastIndexOf('.') + 1) + ".class";
		
		try {
			System.out.println("-----through MyClassLoader-----");
			FileInputStream fis = new FileInputStream(classFileName);
			ByteArrayOutputStream baos = new ByteArrayOutputStream();
			// 解密
			Cypher.cypher(fis, baos);
			byte[] classByte = baos.toByteArray();
			fis.close();
			baos.close();
			
			// 根据byte[]生成Class
			return defineClass(null, classByte, 0, classByte.length);
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		
		return super.findClass(name);
	}
}

4)测试自定义的加载器
public class MyClassLoaderTest {

	public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {

		Class clazz =
   new MyClassLoader("cypherClass").loadClass("classloader.ClassAttachment");
		clazz.newInstance();
	}
}

测试方法:
新建一个cypherClass目录,通过加密器加密自动编译的ClassAttachment.class,用它覆盖掉原来的.class文件,由于,类加载的过程中,会先让上一级的加载器进行加载,如果,上一级的加载器没有找到要加载的class文件,才会让下一级的类加载器进行加载,由于AppClassLoader现在可以找到加密后的ClassAttachment.class文件,所以,会出现如下错误:
[color=red]
java.lang.ClassFormatError: Incompatible magic value 889275713 in class file classloader/ClassAttachment
[/color]
如果将原目录下的ClassAttachment.class文件删除掉,即让AppClassLoader找不到ClassAttachment.class文件,让下一级的类加载器进行加载,即自定义的加载器加载,程序运行成功:
运行结果:
-----through MyClassLoader-----
Hello,World

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