Image processing with ImageIO.read()

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Java

Hi everyone!
I am working on processing image.
I use ImageIO class to input image data.
It work well when running on developer tools(eclipse).
but after export to executable jar it doesn't work.
I can't find reason.
Here is my code.

bigimg = ImageIO.read(new File("C:\\a.png"));

a.png is an existing file.

Help me~~!