Concurrency Cheat Sheet

It's the mutable state, stupid.
All concurrency issues boil down to coordinating access to mutable state. The less mutable state, the easier it is to ensure thread safety.

Make fields final unless they need to be mutable.

Immutable objects are automatically thread-safe.
Immutable objects simplify concurrent programming tremendously. They are simpler and safer, and can be shared freely without locking or defensive copying.

Encapsulation makes it practical to manage the complexity.

Guard each mutable variable with a lock.

Guard all variables in an invariant with the same lock.

Hold locks for the duration of compound actions.

A program that accesses a mutable variable from multiple threads without synchronization is a broken program.

Don't rely on clever reasoning about why you don't need to synchronize.

Include thread safety in the design processor explicitly document that your class is not thread-safe.

Document your synchronization policy.