Jenkins scripted Pipeline: How to apply #NonCPS annotation in this specific case - jenkins

Question

I am working on a scripted Jenkins-Pipeline that needs to write a String with a certain encoding to a file as in the following example:
class Logger implements Closeable {
private final PrintWriter writer
[...]
Logger() {
FileWriter fw = new FileWriter(file, true)
BufferedWriter bw = new BufferedWriter(fw)
this.writer = new PrintWriter(bw)
}
def log(String msg) {
try {
writer.println(msg)
[...]
} catch (e) {
[...]
}
}
}
The above code doesn't work since PrintWriter ist not serializable so I know I got to prevent some of the code from being CPS-transformed. I don't have an idea on how to do so, though, since as far as I know the #NonCPS annotation can only be applied to methods.
I know that one solution would be to move all output-related code to log(msg) and annotate the method but this way I would have to create a new writer every time the method gets called.
Does someone have an idea on how I could fix my code instead?
Thanks in advance!

Answer 1

Here is a way to make this work using a log function that is defined in a shared library in vars\log.groovy:
import java.io.FileWriter
import java.io.BufferedWriter
import java.io.PrintWriter
// The annotated variable will become a private field of the script class.
#groovy.transform.Field
PrintWriter writer = null
void call( String msg ) {
if( ! writer ) {
def fw = new FileWriter(file, true)
def bw = new BufferedWriter(fw)
writer = new PrintWriter(bw)
}
try {
writer.println(msg)
[...]
} catch (e) {
[...]
}
}
After all, scripts in the vars folder are instanciated as singleton classes, which is perfectly suited for a logger. This works even without #NonCPS annotation.
Usage in pipeline is as simple as:
log 'some message'