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
   

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2. y’all
   

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4. I like ‍ing, ing & (!)ing
   

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5. Check out my work @
   github.com/preslavmihaylov,
   pmihaylov.com
   

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6. Why not a talk on concurrency?
   

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7. Most courses/books are incomplete…
   except one which you’ll learn about at the end of the talk...
   

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8. Not enough ❤ for thread-safety
   

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9. Concurrency without thread-safety ->
   200 mp/h without safety measures
   

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10. Goals →
    1) Learn the fundamentals
    2) Get guidance on next steps
    

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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?
    

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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
    

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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
    

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14. What does “thread-safety” mean?
    

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15. Safe classes →
    behave correctly
    

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16. Thread-safe classes →
    behave correctly
    while bombarded by
    a gazillion threads
    

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17. Should you care about it at all?
    

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18. “I don’t need
    thread-safe classes
    as I’m typically
    writing web apps
    without spawning
    any threads…”
    the average web dev
    

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19. You’re already writing
    multi-threaded applications
    

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21. Thread-safety problems & solutions
    

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22. When is there a potential
    thread-safety issue?
    

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24. Is it thread-safe?
    Join at slido.com #93849
    

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25. Shared mutable state
    

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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
    

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27. Atomic access to shared state
    

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29. Is it thread-safe?
    Join at slido.com #93849
    

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31. What can go wrong?
    

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33. synchronized is Java’s default way
    to add critical sections/locks
    

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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?
    

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35. Delegating thread-safety
    

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38. Delegated thread-safety →
    relying on the thread-safetiness of
    objects you use
    

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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
    

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40. Atomic compound actions
    

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42. Is it thread-safe?
    Join at slido.com #93849
    

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44. Actions from the
    same invariant need to be
    synchronized together
    

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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?
    

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46. Visibility & stale data
    

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49. Is it thread-safe?
    Join at slido.com #93849
    

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50. Thread-safety is not only about
    concurrent writes...
    

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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
    

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53. Remember 3+1 things
    

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55. Stateless methods →
    thread-safe methods
    

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58. Synchronize both reads and writes...
    

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61. Prefer delegation over explicit
    synchronization
    

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63. 3+1
    

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64. Be careful with compound actions...
    

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67. Is that all I need to know?
    

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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
    

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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
    

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71. Get the mats →
    bit.ly/java2days2020-thread-safety
    

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72. Threading safely in Java
    P. Mihaylov
    Software Engineer, Tech. Trainer, Speaker
    preslav@pmihaylov.com
    Java2Days 2020,
    A random zoom room on the Internet
    

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