How would you improve performance of a Java application?

Pool valuable system resources like threads, database connections, socket connections etc. Emphasize on reuse of threads from a pool of threads. Creating new threads and discarding them after use can adversely
affect performance. Also consider using multi-threading in your single-threaded applications where possible to enhance performance. Optimize the pool sizes based on system and application specifications and requirements. Having too many threads in a pool also can result in performance and scalability problems due to consumption of memory stacks and CPU context switching (i.e. switching between threads as opposed to doing real computation.).

Minimize network overheads by retrieving several related items simultaneously in one remote invocation if possible. Remote method invocations involve a network round-trip, marshaling and unmarshaling of parameters, which can cause huge performance problems if the remote interface is poorly designed.

Most applications need to retrieve data from and save/update data into one or more databases. Database calls are remote calls over the network. In general data should be lazily loaded (i.e. load only when required as opposed to pre-loading from the database with a view that it can be used later) from a database to conserve
memory but there are use cases (i.e. need to make several database calls) where eagerly loading data and caching can improve performance by minimizing network trips to the database. Data can be eagerly loaded with a help of SQL scripts with complex joins or stored procedures and cached using third party frameworks or
building your own framework