Thorough understanding of concurrency and transactions.
Strong understanding of distributed systems, file system/databases.
Deep knowledge of distributed systems software design with strong understanding of TCP fundamentals, computer architecture, storage and IO subsystems.
Understanding on security concepts and tooling including but not limited to PKI, Cryptography, along with how those apply to web concepts (HTTP(S), SSL/TLS, certificates etc)
Knowledge of JAX-RS
Experience with dependency injection frameworks (Spring, CDI, Guice, etc).
Experience with object-relational mapping technologies (Hibernate, JPA etc)