iReport + jasper report how to iterate List <Object>?

Scenario Description : The original report receiving an object as a parameter for the object ultimately generate the pdf; modify now , reports receiving a List , ultimately generate multiple object for the pdf.
Now I added a main report , so that it can receive the List , then the original report as a subreport.
the question: how will this List iteration to the subreport ?
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I use struts2 directly to the interface on OK. . .
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not understand
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this struts2 little right ? list passed to jasper , and how iterations it ?
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with ireporp do not control this problem, I would not pass over to jasport go inside. . .
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there is no known ... ?
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