🗂️ Heap Dumping Explained - LeakCanary's Bold Move

 

Introduction

In Part 2, we explored ObjectWatcher, the silent guardian that tracks destroyed objects. But what happens when ObjectWatcher suspects foul play? LeakCanary doesn’t just shrug — it goes nuclear: it dumps the heap.

Heap dumping is the most dramatic step in LeakCanary’s workflow. It freezes your app, writes a snapshot of memory to disk, and hands it over to Shark for analysis. Let’s break down why, how, and what happens next.

⚙️ What is a Heap Dump?

A heap dump is a binary snapshot of the JVM heap at a given moment. It contains:

  • All objects currently in memory.
  • Their classes and fields.
  • References between objects.
  • GC roots (entry points into the object graph).

Think of it as a crime scene photo: everything in memory is captured exactly as it is.

🧑‍💻 Triggering a Heap Dump

LeakCanary triggers a heap dump when retained objects exceed a threshold:

  • 5 retained objects when the app is visible.
  • 1 retained object when the app is backgrounded.

Code Example

HeapDumpTrigger.onRetainedObjectsDetected {
    heapDumper.dumpHeap()
}

This call freezes the app briefly, writes a .hprof file, and resumes execution.

📊 Diagram: Heap Dump Workflow

Retained Objects → Threshold Reached → Heap Dump Trigger → .hprof File → Shark Analysis

🚨 Trade‑offs

  • App freeze: Heap dumping pauses the app for a few seconds.
  • File size: Large heaps produce large .hprof files.
  • Performance impact: Best used in debug builds, not production.

LeakCanary balances these trade‑offs by only dumping when necessary.

🧠 Why Heap Dumps Are Worth It

  • They provide ground truth: no guesswork, just raw memory state.
  • They enable Shark’s analysis: shortest path finder, dominator tree, leak categorisation.
  • They empower developers to fix leaks before they hit production.

🔮 Coming Next

In Part 4, we’ll dive into Shark Heap Analysis — how Shark parses .hprof files, builds a heap graph, and generates actionable leak traces.

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