Logstash-forwarder-java is a log shipper program written in java. This is in fact a java version of logstash-forwarder by jordansissel. Here are a few features of this program :

compatible with Java 5 runtime
lightweight : requires only a few dependencies and few system resources
configuration compatible with logstash-forwarder
lumberjack output

Why ?

Logstash-forwarder is written in go. This programming language is not available on all platforms (for example AIX), that's why a java version is more portable.

Logstash runs on java and provides a lumberjack output, but the file input doesn't run on all plaforms (for example AIX) and logstash requires a recent JVM. Moreover Logstash is heavier : big package and more system resources.

So logstash-forwarder-java is a solution for those who want a portable, lightweight log-shipper for their ELK stack.