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

Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_017ic4b6NtPJHrkan4BXhCAM
2026-11-08 18:14:04 +00:00
parent d7d5689f05
commit f6237d4117
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+$ java -cp target/classes com.ankurm.reflection.ReflectionBasicsDemo
+
+== 1. Inspect the class ==
+class: com.ankurm.reflection.Account
+  public long balance[]
+  public void deposit[long]
+  public String owner[]
+  private String secret[]
+  public String[] stack[]
+  public String toString[]
+  public static int twice[int]
+
+== 2. Create and call ==
+created: Account[alice, 50]
+after deposit(25): Account[alice, 75]
+
+== 3. The exceptions you will meet ==
+wrong name        -> NoSuchMethodException: com.ankurm.reflection.Account.nope()
+wrong arg type    -> IllegalArgumentException: argument type mismatch
+method threw      -> InvocationTargetException wrapping java.lang.IllegalArgumentException: over limit: 5000
+private, no opt-in -> IllegalAccessException: class com.ankurm.reflection.ReflectionBasicsDemo cannot access a member of class com.ankurm.reflection.Account with modifiers "private"
+after setAccessible(true): secret-of-alice
+
+== 4. What reflection adds to the stack ==
+direct call:
+  com.ankurm.reflection.Account.stack
+  com.ankurm.reflection.ReflectionBasicsDemo.main
+through Method.invoke:
+  com.ankurm.reflection.Account.stack
+  jdk.internal.reflect.DirectMethodHandleAccessor.invoke
+  java.lang.reflect.Method.invoke
+  com.ankurm.reflection.ReflectionBasicsDemo.main
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+$ java -cp target/classes com.ankurm.reflection.ModuleRulesDemo
+
+== setAccessible on your own class ==
+Account.owner = carol
+
+== setAccessible on a JDK private field ==
+String lives in module : java.base
+we live in module      : unnamed module @6b95977 (unnamed=true)
+java.lang open to us   : false
+trySetAccessible       : false
+setAccessible threw    : InaccessibleObjectException
+  Unable to make field private final byte[] java.lang.String.value accessible: module java.base does not "opens java.lang" to unnamed module @6b95977
+
+$ java --add-opens java.base/java.lang=ALL-UNNAMED -cp target/classes com.ankurm.reflection.ModuleRulesDemo
+
+== setAccessible on your own class ==
+Account.owner = carol
+
+== setAccessible on a JDK private field ==
+String lives in module : java.base
+we live in module      : unnamed module @3764951d (unnamed=true)
+java.lang open to us   : true
+trySetAccessible       : true
+setAccessible worked, String.value is a byte[]
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+$ java -cp target/classes com.ankurm.reflection.FinalFieldDemo   # JDK 25
+
+java.version = 25.0.5
+
+== 1. final instance field ==
+owner() now: mallory
+
+== 2. static final field ==
+IllegalAccessException: Can not set static final int field com.ankurm.reflection.Account.MAX_DEPOSIT to java.lang.Integer
+
+== 3. record component ==
+IllegalAccessException: Can not set final int field com.ankurm.reflection.FinalFieldDemo$Point.x to java.lang.Integer
+
+== 4. constant inlining: the write 'works' but the getter never notices ==
+Field.get -> 99
+max()     -> 5
+
+$ java -cp target/classes com.ankurm.reflection.FinalFieldDemo   # JDK 26
+
+java.version = 26.0.2.1
+
+== 1. final instance field ==
+WARNING: Final field owner in class com.ankurm.reflection.Account has been mutated reflectively by class com.ankurm.reflection.FinalFieldDemo in unnamed module @18b4aac2 (file:/tmp/work/java-core-examples/reflection/target/classes/)
+WARNING: Use --enable-final-field-mutation=ALL-UNNAMED to avoid a warning
+WARNING: Mutating final fields will be blocked in a future release unless final field mutation is enabled
+owner() now: mallory
+
+== 2. static final field ==
+IllegalAccessException: Can not set static final int field com.ankurm.reflection.Account.MAX_DEPOSIT to java.lang.Integer
+
+== 3. record component ==
+IllegalAccessException: Can not set final int field com.ankurm.reflection.FinalFieldDemo$Point.x to java.lang.Integer
+
+== 4. constant inlining: the write 'works' but the getter never notices ==
+Field.get -> 99
+max()     -> 5
+
+$ java --illegal-final-field-mutation=deny -cp target/classes com.ankurm.reflection.FinalFieldDemo   # JDK 26
+
+java.version = 26.0.2.1
+
+== 1. final instance field ==
+Exception in thread "main" java.lang.IllegalAccessException: class com.ankurm.reflection.FinalFieldDemo (in unnamed module @18b4aac2) cannot set final field com.ankurm.reflection.Account.owner (in unnamed module @18b4aac2), unnamed module @18b4aac2 is not allowed to mutate final fields
+	at java.base/java.lang.reflect.Field.preSetFinal(Field.java:1504)
+	at java.base/java.lang.reflect.Field.setFinal(Field.java:1447)
+	at java.base/java.lang.reflect.Field.set(Field.java:909)
+	at com.ankurm.reflection.FinalFieldDemo.main(FinalFieldDemo.java:30)
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+$ java -cp target/classes com.ankurm.reflection.MethodHandlesDemo
+
+== 1. Look up once, call many times ==
+twice(21)        = 42
+balance()        = 50
+handle type      = (Account)long
+
+== 2. invokeExact is exact; invoke adapts ==
+invokeExact, wrong return type -> WrongMethodTypeException
+  handle's method type (Account)long but found (Account)Object
+invoke (boxing)  = 50 (Long)
+
+== 3. Handles compose ==
+bindTo(acct)     = 50
+after deposit10  = Account[erin, 60]
+balance as text  = "60"
+
+== 4. Access is checked when you look up, not when you call ==
+findVirtual(private) -> IllegalAccessException
+  no such method: com.ankurm.reflection.Account.secret()String/invokeVirtual
+privateLookupIn      -> secret-of-erin
+privateLookupIn(String) -> IllegalAccessException
+  module java.base does not open java.lang to unnamed module @135fbaa4
+
+== 5. VarHandle: typed, atomic field access ==
+get              = 60
+CAS(60 -> 100)   = true, balance now 100
+CAS(60 -> 200)   = false, balance now 100
+getAndAdd(5)     = was 100, now 105
+set on final field -> UnsupportedOperationException
+array element CAS-able: counts[1] = 7
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+$ java -jar target/benchmarks.jar CallBenchmark
+
+openjdk version "25.0.5" 2026-10-20 LTS
+
+Benchmark                                             Mode  Cnt   Score   Error  Units
+CallBenchmark.direct                                  avgt   10   0.516 ± 0.069  ns/op
+CallBenchmark.field_get                               avgt   10   0.899 ± 0.051  ns/op
+CallBenchmark.methodHandle_instanceField_invokeExact  avgt   10   2.600 ± 0.145  ns/op
+CallBenchmark.methodHandle_staticFinal_invokeBoxing   avgt   10   2.690 ± 0.262  ns/op
+CallBenchmark.methodHandle_staticFinal_invokeExact    avgt   10   0.476 ± 0.038  ns/op
+CallBenchmark.reflection_instanceFieldMethod          avgt   10   6.476 ± 1.952  ns/op
+CallBenchmark.reflection_lookupEveryCall              avgt   10  21.525 ± 1.756  ns/op
+CallBenchmark.reflection_staticFinalMethod            avgt   10   1.208 ± 0.152  ns/op
+CallBenchmark.varHandle_staticFinal_get               avgt   10   0.519 ± 0.076  ns/op
+
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+$ java src/main/java/com/ankurm/reflection/QuickStart.java
+
+reflection   : Hello, Ankur
+MethodHandle : Hello, Ankur