Preface

These are my notes from learning how hwui’s RenderThread works in Android 5.0. I haven’t covered every detail; I wanted to trace the main flow so that, next time I hit a problem, I would know where to look.

I captured the first RenderThread Draw during app startup with Systrace, as shown below. It helps show the execution order on that thread, but the end of a DrawFrame slice is not the moment pixels appear on screen: GPU work, buffer consumption, composition, and presentation still matter. A StartingWindow may appear during startup; this CPU slice alone does not establish when it disappears.

Finding a value in an unsorted array or linked list without an additional index can require scanning every element. A hash table uses a hash to locate a bucket and supports constant-time basic lookup when hashes are well distributed; it does not guarantee O(1) lookup for every input.

HashMap is an implementation of the Map interface based on a hash table. This implementation provides all optional mapping operations and permits null values and null keys. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

This article follows the source through the array-and-list structure, constructors, put and resizing, get, and the iterator checks using modCount. A final section covers changes in later JDKs.