Engineering Atlas by Abhishek Goyal
Course contents

Safe publication and double-checked locking

What publishing an object means, how another thread can see a half-built object, the five safe publication idioms, and the whole singleton journey: synchronized, broken and fixed double-checked locking, the holder-class idiom and the enum singleton.

PremiumMust know for interviewsLesson25 min readAdvancedReviewed Oct 2026 · Java 21
On this page
  1. Picture it first: the key before the house is finished
  2. What "publishing" an object actually means
  3. How a reader sees a half-built object
  4. Under the hood: the lazy-init race you can see every time
  5. The five ways to publish safely
  6. The singleton journey: Naive, Solid, Staff
  7. Real world
  8. Advanced: what the spec says, what the hardware does
  9. How to say it in the interview

📦 What is safe publication?

Publishing an object means making it reachable by another thread: storing its reference in a shared field, a collection, or handing it to a task. Safe publication means the other thread is guaranteed to see the object fully built, with every field the constructor wrote. Without it, Java allows a reader to get a perfectly valid reference to an object whose fields still look like 0 and null. Double-checked locking is the famous lazy-singleton trick that got this wrong for years.

For this lesson, we'll work out what "publishing" really means, how a reader can end up holding a half-built object, the five ways to publish safely, and then walk the whole singleton journey: the slow-but-correct version, the broken double-checked locking, the fixed one with volatile (and why a boring local variable makes it faster), the holder-class idiom and the enum singleton. Interviewers love this topic because almost everyone has written a singleton, very few can explain why the classic version is broken, and the follow-up "so how would you write it?" has a clear right answer with real trade-offs.

What you’ll learn

  • Explain what publishing an object means and why instance = new T() is really allocate, construct, store that another thread may see out of order
  • Say honestly why a half-built object is hard to reproduce on a laptop, and cite jcstress results that show it on x86 and AArch64
  • List the five safe publication idioms: static initializer, volatile/AtomicReference, final field, lock on both sides, concurrent collection
  • Tell immutable, effectively immutable and mutable objects apart and what each needs after publication
  • Explain why classic double-checked locking is broken without volatile, and why Java 5 (JSR-133) made the volatile version correct
  • Write volatile DCL with a local variable and explain the one-volatile-read saving
  • Use the holder-class idiom (JLS 12.4.2 class-init lock) and the enum singleton, and know their trade-offs: no retry if init throws, no parameters
  • Ask the staff-level question: does this even need to be lazy?

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Keep reading with the Java Pass

The rest of this lesson is part of Java One-Stop Interview Prep: in-depth explanations, diagrams, model answers and how to say them in the interview.

  • Picture it first: the key before the house is finished
  • What "publishing" an object actually means
  • How a reader sees a half-built object
  • Under the hood: the lazy-init race you can see every time
  • The five ways to publish safely
  • The singleton journey: Naive, Solid, Staff
  • Real world
  • Advanced: what the spec says, what the hardware does
  • How to say it in the interview
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