def gerritcfg = readFile file:filename
def props =   readProperties text:gerritcfg
println props

node {
    filename= "pipeline-utility-steps-plugin/work/test.properties"
    def properties = readFile file:filename
    def props1 =   readProperties text:properties 
    def props2 =   readProperties file:filename 
    println props1
    println props2
}

$ file test.properties                                                                                                                                                                                                                                            
test.properties: UTF-8 Unicode text

the properties file looks like below：
$ cat test.properties                                                                                                                                                                                                                                             
prop1=abcabc123
prop2=中国中国

Started by user unknown or anonymous
Replayed #8
Running in Durability level: MAX_SURVIVABILITY
[Pipeline] node
Running on Jenkins in /work/brightma/github/pipeline-utility-steps-plugin/work/workspace/test_pipe
[Pipeline] {
[Pipeline] readFile
[Pipeline] readProperties
[Pipeline] readProperties
[Pipeline] echo
{prop2=中国中国, prop1=abcabc123}
[Pipeline] echo
{prop2=中国中国, prop1=abcabc123}
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Finished: SUCCESS