关于struts2 验证码生成问题

问题：重复调用每次返回action的值相同

action:
================================================================
public class AccountAction extends ActionSupport {

/**
*生成验证图片的execute方法，
/*
public String validataImage(){
  createInputStream(ValidateImageService.Disturb_Type_Complex);//生成图片inputStream
  return SUCCESS;
}

/**
*生成图片的具体方法
/*
private String createInputStream(int disturbType) throws IOException{
        ByteArrayOutputStream bos =new ByteArrayOutputStream();  
        String validateCode = null;
        ValidateImageServiceImp validateImageService=new ValidateImageServiceImp();
        validateCode = validateImageService.createValidateCode(disturbType,18,   bos, 100, 30, null, 5);//把验证码图片放入bos
        inputStream = new ByteArrayInputStream(bos.toByteArray());  
        bos.close();  
        return validateCode;  
    }
}

struts.xml
========================================================================
<action name="validataImage" class="com.dearfashion.eshop.security.action.AccountAction" method="validataImage">       
    <result name="success" type="stream">
       <param name="contentType">image/jpeg</param>
       <param name="inputName">inputStream</param>
    </result>     
   </action>

jsp
====================================================
<img id="random" src="validataImage.action" />
          <a href="#" onclick="javascript:document.getElementById('random').src='validataImage.action'">看不清楚</a>


==========================================
点击 ‘看不清楚’每次返回的结果是同一张图片，或者重复调用
<img id="random" src="validataImage.action" />
<img id="random" src="validataImage.action" />
多次，也是同一张图片，并且方法AccountAction.validataImage()只会执行一次
分析原因，可能是存在缓冲，也可能其他原因，如果直接用response的方式就没有问题
如：outStream = response.getOutputStream();         
    JPEGImageEncoder encoder =JPEGCodec.createJPEGEncoder(outStream);
    encoder.encode(image);