Struts2 Getoutputstream () have already been called for this response exception resolution

Struts2 wrote a random verification code to generate the action class, the generated verification code picture output to the JSP page, the browser can display the verification code picture, but the background throws the following exception:

Getoutputstream () have already been called for this response

Online Search, if the code on the JSP, there is a workaround, the use of JSP built-in object out of some methods.

However, the code cannot use the JSP built-in object in action, and the above method is not valid.

The workaround for the Code in action is that the execution method in the normal action returns a string, and we return the return value to null and the background does not give an error. The principle is unknown ...