Java has been in service for decades and is, to this day, a reliable option when choosing the backbone of your stack. With AWS Lambda is no different as it makes a strong candidate for your functions.
Reliable and well -tested libraries. The libraries will make life easy for you through enhanced testability and maintainability of AWS Lambda tasks.
Predictive performance. While Java has slower spin uptime, you can easily predict the memory needs of your functions and to counteract those dreaded colds starts you can just up your memory allocation.
Tooling Support. Java has a wide range of tooling support which includes Eclipse, IntelliJ IDEA, Maven, and Gradle among others.