Threading Safely in Java

A talk on learning the fundamentals of thread-safety in Java as presented on the java2days conference (https://2020.java2days.com/).

Examples & follow-up:
http://bit.ly/java2days2020-thread-safety

Transcript

1. Threading safely in Java P. Mihaylov Software Engineer, Tech. Trainer, Speaker preslav@pmihaylov.com Java2Days 2020, A random zoom room on the Internet
2. y’all
4. I like ‍ing, ing & (!)ing
5. Check out my work @ github.com/preslavmihaylov, pmihaylov.com
6. Why not a talk on concurrency?
7. Most courses/books are incomplete… except one which you’ll learn about at the end of the talk...
8. Not enough ❤ for thread-safety
9. Concurrency without thread-safety -> 200 mp/h without safety measures
10. Goals → 1) Learn the fundamentals 2) Get guidance on next steps
11. What’s covered 1. What does “thread-safety” mean? 2. Should you care about it at all? 3. Thread-safety problems & solutions a. When is there a potential thread-safety issue? b. Atomic access to shared state c. Delegating thread-safety d. Atomic compound actions e. Visibility & stale data 4. Where to go from here?
12. What’s not covered 1. Initialisation safety, Immutable & effectively immutable objects 2. Thread-confinement 3. Exploring commonly used thread-safe collections in Java 4. Managing threads with the Executors framework 5. Extending thread-safe classes 6. Managing thread lifecycle via cancellation/interrupts 7. Dealing with performance/liveness issues (i.e. deadlock, livelock, etc.) 8. Modern frameworks & libraries 9. Advanced concurrency tooling - atomic variables/collections, synchronizers, explicit locks 10. The Java Memory Model
13. What’s not covered 1. Initialisation safety, Immutable & effectively immutable objects 2. Thread-confinement 3. Exploring commonly used thread-safe collections in Java 4. Managing threads with the Executors framework 5. Extending thread-safe classes 6. Managing thread lifecycle via cancellation/interrupts 7. Dealing with performance/liveness issues (i.e. deadlock, livelock, etc.) 8. Modern frameworks & libraries 9. Advanced concurrency tooling - atomic variables/collections, synchronizers, explicit locks 10. The Java Memory Model
14. What does “thread-safety” mean?
15. Safe classes → behave correctly
16. Thread-safe classes → behave correctly while bombarded by a gazillion threads
17. Should you care about it at all?
18. “I don’t need thread-safe classes as I’m typically writing web apps without spawning any threads…” the average web dev
19. You’re already writing multi-threaded applications
21. Thread-safety problems & solutions
22. When is there a potential thread-safety issue?
24. Is it thread-safe? Join at slido.com #93849
25. Shared mutable state
26. Non-shared state Stack #1 Thread #1 Non-shared state #1 Non-shared state #2 Shared memory aka heap Shared state Non-shared state Stack #2 Thread #2
27. Atomic access to shared state
29. Is it thread-safe? Join at slido.com #93849
31. What can go wrong?
33. synchronized is Java’s default way to add critical sections/locks
34. Action point: 1) Create an example app with the two versions of the class 2) Start a dozen threads, invoking the methods concurrently 3) What’s the value after the threads finish?
35. Delegating thread-safety
38. Delegated thread-safety → relying on the thread-safetiness of objects you use
39. Action point: 1) Create an example app with the two versions of the class 2) Start a dozen threads, invoking the methods concurrently 3) Prove that one of the classes is thread-safe, the other is not
40. Atomic compound actions
42. Is it thread-safe? Join at slido.com #93849
44. Actions from the same invariant need to be synchronized together
45. Action point: 1) Create an example app with the two versions of the class 2) Start a dozen threads, invoking the methods concurrently 3) What’s the value after the threads finish?
46. Visibility & stale data
49. Is it thread-safe? Join at slido.com #93849
50. Thread-safety is not only about concurrent writes...
52. Action point: 1) Create an example app with the previous example 2) Start a dozen threads, invoking the methods concurrently 3) Verify that the application hangs forever due to visibility issues
53. Remember 3+1 things
55. Stateless methods → thread-safe methods
58. Synchronize both reads and writes...
61. Prefer delegation over explicit synchronization
63. 3+1
64. Be careful with compound actions...
67. Is that all I need to know?
68. What’s not covered 1. Initialisation safety, Immutable & effectively immutable objects 2. Thread-confinement 3. Exploring commonly used thread-safe collections in Java 4. Managing threads with the Executors framework 5. Extending thread-safe classes 6. Managing thread lifecycle via cancellation/interrupts 7. Dealing with performance/liveness issues (i.e. deadlock, livelock, etc.) 8. Modern frameworks & libraries 9. Advanced concurrency tooling - atomic variables/collections, synchronizers, explicit locks 10. The Java Memory Model
70. Action point: 1) Buy the book 2) Read every chapter at least twice. Focus on the first five ones 3) Take book notes along the way a) OR use mine - github.com/preslavmihaylov/booknotes 4) Finish the action points in this slide deck & continue doing that for all examples in the book
71. Get the mats → bit.ly/java2days2020-thread-safety
72. Threading safely in Java P. Mihaylov Software Engineer, Tech. Trainer, Speaker preslav@pmihaylov.com Java2Days 2020, A random zoom room on the Internet