Mocking file operations in Groovy during tests

filemock.groovy

def pipeInput = new PipedInputStream()
def outputStreamMock = new BufferedOutputStream(new PipedOutputStream(pipeInput))
writerMock = new StringWriter()
File.metaClass.withOutputStream = {Closure c ->
    c.call(outputStreamMock)
}
File.metaClass.withWriter = {Closure c ->
    c.call(writerMock)
}