Create a simple Java application that will exhibit concurrency concepts. Your application should create two threads that will act as counters. One thread should count up to 20. Once thread one reaches 20, then a second thread should be used to count down to 0. For your created code, please be provide a detailed analysis of appropriate concepts that could impact your application. Specifically, please address:

• Performance issues with concurrency
• Vulnerabilities exhibited with use of strings
• Security of the data types exhibited.