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reflection: Method/Field/MethodHandle/VarHandle demos, module and final-field rules (JEP 500 on JDK 26), JMH call-cost numbers, contract tests

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Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b6NtPJHrkan4BXhCAM
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asmhatre
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co-authored by Claude Sonnet 5.5
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# reflection
Companion code for the ankurm.com post *"Java Reflection and MethodHandles in 2026: How They Work
and What They Cost."* One module in `java-core-examples`. Everything the post explains lives in
the post itself (accordions per section); this module is the runnable evidence.
## Versions this was built and tested against
| Component | Version | Notes |
|---|---|---|
| JDK | 25.0.5+8 (Temurin, LTS) | All demos and the JMH run. |
| JDK (comparison only) | 26.0.2.1+1 (Temurin) | Used for the JEP 500 final-field warning in `output/03`. |
| JMH | 1.37 | 3 warmup + 5 measurement iterations, 2 forks. |
| JUnit Jupiter | 5.11.0 | Contract tests - they fail if a documented behaviour changes. |
| Maven | 3.9.11 | |
| Hardware | 2 vCPU Intel Xeon @ 2.30 GHz VM | Shared sandbox: treat ns figures as relative, not absolute. |
## Quickstart
```bash
export JAVA_HOME=/path/to/jdk-25
mvn package # compiles, runs the contract tests, builds the JMH jar
java src/main/java/com/ankurm/reflection/QuickStart.java
java -cp target/classes com.ankurm.reflection.ReflectionBasicsDemo
java -cp target/classes com.ankurm.reflection.ModuleRulesDemo
java --add-opens java.base/java.lang=ALL-UNNAMED -cp target/classes com.ankurm.reflection.ModuleRulesDemo
java -cp target/classes com.ankurm.reflection.FinalFieldDemo
java -cp target/classes com.ankurm.reflection.MethodHandlesDemo
java -jar target/benchmarks.jar CallBenchmark
```
`scripts/run-all.sh` regenerates every file in `output/` (set `JDK26_HOME` to include the JDK 26 runs).
## What's in here
| File | What it shows |
|---|---|
| `Account.java` | The sample class: public API, private method, private/final fields, a throwing method. |
| `QuickStart.java` | Self-contained: one call by `Method`, one by `MethodHandle`. Run with `java QuickStart.java`. |
| `ReflectionBasicsDemo.java` | Inspect, construct, invoke; the five exceptions you will meet; extra stack frames. |
| `ModuleRulesDemo.java` | `setAccessible` on your class vs. a JDK internal; `--add-opens`. |
| `FinalFieldDemo.java` | Final instance / static / record fields; constant-inlining trap; JEP 500 on JDK 26. |
| `MethodHandlesDemo.java` | `invokeExact` vs `invoke`, composition, lookup-time access checks, `VarHandle`. |
| `CallBenchmark.java` | JMH: direct vs `Method` vs `MethodHandle` vs `Field` vs `VarHandle`, static-final vs instance field. |
| `ReflectionContractTest.java` | Eight tests pinning the contracts the post states. |
## Captured output
| File | From |
|---|---|
| `output/01-basics.txt` | `ReflectionBasicsDemo` |
| `output/02-module-rules.txt` | `ModuleRulesDemo`, without and with `--add-opens` |
| `output/03-final-fields.txt` | `FinalFieldDemo` on JDK 25, JDK 26, and JDK 26 with `--illegal-final-field-mutation=deny` |
| `output/04-method-handles.txt` | `MethodHandlesDemo` |
| `output/05-call-benchmark.txt` | `CallBenchmark` (non-deterministic; relative ordering is the result) |
| `output/06-quickstart.txt` | `QuickStart` |
Identity-hash suffixes like `unnamed module @6b95977` change on every run.