Here is a review of some concurrency tips from Joshua Bloch, Brian Goetz and others.
 

Prefer immutable objects/data

Immutable objects do not change after construction. Immutable objects are simpler, safer, require no locks, and are thread safe. To make an object immutable don't provide setters/mutator methods, make fields private final, and prevent subclassing. If immutability is not an option, limit mutable state, less mutable state means less coordination.  Declare fields final wherever practical, final fields are simpler than mutable fields.
When threads share mutable data, each thread that reads or writes must coordinate access to the data. Failing to synchronize shared mutable data can lead to atomicity failures, race conditions, inconsistent state, and other forms of non-determinism. These erratic problems are among the most difficult to debug.

Limit concurrent interactions to well defined points, limit shared data, consider copying instead of sharing.
 

Threading risks for Web applications

A Servlet get, post, service method can be called for multiple clients at the same time. Multi-threaded Servlet Instance and Static variables are shared and therefore if mutable, access must be coordinated. Servlets are typically long-lived objects with a high thread load, if you over-synchronize performance suffers, try to either share immutable (final) data, or don