I have just install last Java 1.8 as suggested.
When launching the logstash application, I have no more error :
But when using log4j input, there is no entry in my ouput file (the file is not created at all whereas it is when using tcp input).
Nevertheless, by using netstat -a command, I can see that connections are UP on tcp/3518 with the monitored server (but can't explain why multiple) :
What can be the next step of investigation ?