I use @Grab annotation to load Java jar from maven center. Then I wrote java language in my groovy file.  I have some code pieces like the following:

while ((read = fInputStream.read(buffer)) != -1) {
    fOptStream.write(buffer, 0, read);              
}

I read a file and then save to another location, if the file ends, then return -1.

This code works fine on my local groovy 2.4.12. but when I execute the groovy file in Jenkins. I got endless loop in while. I printed the log and found the read value is correct(-1), but it won't break the while loop. It seems that the return value is the result of the assignment. So I modify the code as follows:

read = fInputStream.read(buffer)
while (read != -1) {
    fOptStream.write(buffer, 0, read);    
    read = fInputStream.read(buffer)          
}

It can work normally.

I guess there might be something wrong in Jenkins embedded groovy engine. 

It doesn't support an assignment within a while condition.