用struts2写了一个action,在浏览器直接访问action 的地址，发现第一次访问execute方法被执行一个，第二次execute方法被执行3次！解决方法

public class MyCaptcha extends ActionSupport implements SessionAware {    @SuppressWarnings("rawtypes")    private Map session;    @SuppressWarnings("rawtypes")    public void setSession(Map session) {        this.session = session;    }    @Override    public String execute() throws Exception {        System.out.println("action被执行————————————————————————————————");        HttpServletResponse response = ServletActionContext.getResponse();        response.setContentType("image/png");        response.setHeader("Pragma", "No-cache");        response.setHeader("Cache-Control", "no-cache");        response.setDateHeader("Expires", 0);        ServletOutputStream outputStream = response.getOutputStream();        Captcha captcha = new Captcha();        captcha.generateImage(this.session, outputStream);        outputStream.close();        return NONE;    }}