Threading Safely in Java

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Threading safely in Java P. Mihaylov Software Engineer, Tech. Trainer, Speaker [email protected] Java2Days 2020, A random zoom room on the Internet

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y’all

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I like ‍ing, ing & (!)ing

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

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Why not a talk on concurrency?

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

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Not enough ❤ for thread-safety

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

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

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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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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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What does “thread-safety” mean?

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Safe classes → behave correctly

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

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Should you care about it at all?

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“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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You’re already writing multi-threaded applications

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Thread-safety problems & solutions

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When is there a potential thread-safety issue?

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

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Shared mutable state

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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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Atomic access to shared state

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

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What can go wrong?

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

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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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Delegating thread-safety

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

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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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Atomic compound actions

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

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Actions from the same invariant need to be synchronized together

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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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Visibility & stale data

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

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

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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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Remember 3+1 things

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Stateless methods → thread-safe methods

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Synchronize both reads and writes...

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Prefer delegation over explicit synchronization

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3+1

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Be careful with compound actions...

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Is that all I need to know?

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

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Threading safely in Java P. Mihaylov Software Engineer, Tech. Trainer, Speaker [email protected] Java2Days 2020, A random zoom room on the Internet