export content to a text file using servlet with save dialog

my application using GWT, and I plan to use servlet to export the data in gwt to a text file, but I do not know how to export the data to a text file (with a prompt of save dialog) using servelt, anyone can help?
I see some article said need to use the URL class??
I do not know how to set the destination of outputstream, or just set the header, content type is ok?

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