java-core-examples/reflection

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

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.