--- title: "How we improved APM Java startup by encoding a prefix trie as a JVM constant" url: https://daily.dev/posts/how-we-improved-apm-java-startup-by-encoding-a-prefix-trie-as-a-jvm-constant-fogkaahmp source_url: https://www.datadoghq.com/blog/engineering/improving-apm-java-startup-with-a-prefix-trie type: article source: "Datadog" published: 2026-08-11T15:17:35.093Z updated: 2026-08-11T15:18:10.552Z tags: ["performance", "java", "jvm", "apm"] reading_time: 12 upvotes: 1 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # How we improved APM Java startup by encoding a prefix trie as a JVM constant **[Datadog](https://daily.dev/sources/datadog)** · 12 min read · 1 upvotes · 0 comments ## Summary Datadog's APM team reduced class-matching overhead by 30% over four years, with a key optimization being ClassNameTrie — a prefix trie encoded as a single JVM string constant. During JVM startup (premain phase), the JIT compiler is cold or absent, making conventional data structure construction expensive. By encoding the trie as a string constant loaded via a single ldc bytecode instruction, the team avoided I/O, resource lookups, and dependency loading. The encoding uses a compact node format with branch characters, value characters (leaf/bud/glob semantics), and jump offsets, all packed into a Java string's char sequence. Benchmarks show ClassNameTrie is nearly 5x faster than the old code-based approach on Java 8 during cold start, and faster than a classic radix trie due to cache locality. In a real Spring Boot application, class-name filtering reduced instrumented startup time by 20%, with ClassNameTrie adding another 1% and a follow-on known-types index adding 3%, totaling over 24% savings. The ClassNameTrie is open-sourced and now used in Datadog Live Debugger and CI Visibility as well. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why is code execution slower than loading constants during JVM premain startup? During the premain phase, the JIT compiler is cold or entirely absent (Java 8 does not start the JIT until after premain). This means all code runs interpreted and unoptimized. Loading a string constant via a single ldc bytecode instruction bypasses this problem entirely — the JVM loads the constant as part of class loading with no I/O, no resource lookup, and no object construction required. _Java engineers tuning agent startup overhead track JVM internals like this on daily.dev._ ### How does Datadog's ClassNameTrie encode a prefix trie as a Java string constant? Each node is packed into a char sequence: the first char gives the branch count, followed by sorted branch characters, then value chars (encoding leaf, bud, or segment-length via top bits, with an optional glob bit), then jump offsets for each branch. Inline segment strings follow nodes, and large jump offsets beyond 61,439 are stored in a separate long-jump table. The entire structure is embedded as a single JVM string constant loaded with one ldc instruction. _Developers building Java agents or instrumentation libraries find deep dives like this on daily.dev._ ### How much did switching to ClassNameTrie improve Java APM startup time compared to the old code-based approach? ClassNameTrie is nearly 5x faster than the old code-based approach on Java 8 during cold start, and faster than a classic radix trie due to its compact representation and cache locality. In a real Spring Boot application, class-name filtering by name reduced instrumented startup time by 20%, with ClassNameTrie contributing an additional 1% and a follow-on known-types index saving another 3%, totaling over 24% improvement. _Teams optimizing Java agent startup performance stay on top of techniques like this through daily.dev._ ## Similar posts on daily.dev - [How is Leyden improving Java Performance? Part 3 of 3](https://daily.dev/posts/how-is-leyden-improving-java-performance-part-3-of-3-hdbaiqpev) · Foojay.io · 0 upvotes · 0 comments --- Tags: [#performance](https://daily.dev/tags/performance), [#java](https://daily.dev/tags/java), [#jvm](https://daily.dev/tags/jvm), [#apm](https://daily.dev/tags/apm) [View this post on daily.dev](https://daily.dev/posts/how-we-improved-apm-java-startup-by-encoding-a-prefix-trie-as-a-jvm-constant-fogkaahmp)