CheatSheetChapter2-5.md

• It’s the mutable state: 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: You could write a thread-safe program with all data stored in global variables, but why would you want to? Encapsulating data within objects makes it easier to preserve their invariants; encapsulating synchronization within objects makes it easier to comply with their synchronization policy.
• 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 process—or explicitly document that your class is not thread-safe.
• Document your synchronization policy.