使用以下代码

package org.springframework.cloud.gateway.sample;

import org.springframework.util.Base64Utils;

import java.io.*;
import java.nio.charset.StandardCharsets;

public class EncodeShell {
   public static void main(String[] args){
      byte[] data = null;
      try {
         InputStream in = new FileInputStream("E:\\代审计划\\spring-cloud-gateway-3.0.6\\spring-cloud-gateway-sample\\target\\classes\\SpringRequestMappingMemshell.class");
         data = new byte[in.available()];
         in.read(data);
         in.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
      String shellStr = Base64Utils.encodeToString(data);
      System.out.println(shellStr);
      try {
         OutputStream out = new FileOutputStream("ShellStr.txt");
         out.write(shellStr.getBytes(StandardCharsets.UTF_8));
         out.flush();
         out.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

或者linux中

base64 /root/Desktop/NettyMemshell.class