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feat(jdk26): add demo for JEP 500 #413

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60 changes: 31 additions & 29 deletions src/main/java/org/javademos/init/Java26DemoLoader.java
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package org.javademos.init;

import java.util.Map;

import org.javademos.commons.IDemo;
import org.javademos.commons.IDemoLoader;

import org.javademos.java26.jep517.Http3Demo;
import org.javademos.java26.jep522.ReduceSyncDemo;
import org.javademos.java26.jep524.PemEncodingsDemo;
import org.javademos.java26.jep525.StructuredConcurrencyDemo;
import org.javademos.java26.jep526.LazyConstantsDemo;
import org.javademos.java26.jep529.VectorAPIDemo;

/**
* Loads demos for Java 26.
*/
public class Java26DemoLoader implements IDemoLoader {

@Override
public void loadDemos(Map<Integer, IDemo> demos) {
demos.put(517, new Http3Demo());
demos.put(522, new ReduceSyncDemo());
demos.put(524, new PemEncodingsDemo());
demos.put(525, new StructuredConcurrencyDemo());
demos.put(526, new LazyConstantsDemo());
demos.put(529, new VectorAPIDemo());
}
}
package org.javademos.init;

import java.util.Map;

import org.javademos.commons.IDemo;
import org.javademos.commons.IDemoLoader;

import org.javademos.java26.jep500.FinalMeanFinalDemo;
import org.javademos.java26.jep517.Http3Demo;
import org.javademos.java26.jep522.ReduceSyncDemo;
import org.javademos.java26.jep524.PemEncodingsDemo;
import org.javademos.java26.jep525.StructuredConcurrencyDemo;
import org.javademos.java26.jep526.LazyConstantsDemo;
import org.javademos.java26.jep529.VectorAPIDemo;

/**
* Loads demos for Java 26.
*/
public class Java26DemoLoader implements IDemoLoader {

@Override
public void loadDemos(Map<Integer, IDemo> demos) {
demos.put(500, new FinalMeanFinalDemo());
demos.put(517, new Http3Demo());
demos.put(522, new ReduceSyncDemo());
demos.put(524, new PemEncodingsDemo());
demos.put(525, new StructuredConcurrencyDemo());
demos.put(526, new LazyConstantsDemo());
demos.put(529, new VectorAPIDemo());
}
}
94 changes: 94 additions & 0 deletions src/main/java/org/javademos/java26/jep500/FinalMeanFinalDemo.java
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package org.javademos.java26.jep500;

import java.lang.reflect.Field;

import org.javademos.commons.IDemo;

/// Demo for JDK 26 feature JEP 500 - Prepare to Make Final Mean Final.
///
/// Since JDK 5, deep reflection (`Field.setAccessible` + `Field.set`) can reassign `final` fields.
/// JDK 26 starts the process of disallowing that - by default, such a mutation still succeeds,
/// but the JVM prints a warning. A future release will throw `IllegalAccessException` instead.
///
/// Relevant JVM options:
/// - `--enable-final-field-mutation=ALL-UNNAMED` (or `=M1,M2` for named modules)
/// explicitly allows the mutation for the selected code and silences the warning
/// (executable JARs may use the `Enable-Final-Field-Mutation: ALL-UNNAMED` manifest entry)
/// - `--illegal-final-field-mutation=allow|warn|debug|deny` controls what happens
/// on an illegal mutation (`warn` is the default in JDK 26, `deny` will be the future default)
///
/// ### JEP history
/// - JDK 26: [JEP 500 - Prepare to Make Final Mean Final](https://openjdk.org/jeps/500)
///
/// ### Further reading
/// - [Integrity by Default](https://openjdk.org/jeps/8305968)
/// - [Field.set Javadoc](https://docs.oracle.com/en/java/javase/26/docs/api/java.base/java/lang/reflect/Field.html#set(java.lang.Object,java.lang.Object))
///
/// @author Alois Sečkár <ellrohir@seznam.cz>
public class FinalMeanFinalDemo implements IDemo {

@Override
public void demo() {
info(500);

// 1) a final field in a normal class
// since JDK 5 deep reflection was allowed to reassign it
// in JDK 26 it still works, but the JVM issues a warning to the standard error stream:
// WARNING: Final field value in ...Holder has been mutated reflectively by class ...FinalMeanFinalDemo ...
// WARNING: Use --enable-final-field-mutation=ALL-UNNAMED to avoid a warning
// (the warning is printed at most once per module, so it may be missing if other code
// in the unnamed module already triggered it before)
// with `--illegal-final-field-mutation=deny`, `Field.set` throws `IllegalAccessException`
var holder = new Holder(100);
System.out.println("Holder value before mutation: " + holder.value);
try {
Field field = Holder.class.getDeclaredField("value");
field.setAccessible(true); // the behavior of `setAccessible` itself is unchanged
field.set(holder, 200); // this is where JDK 26 checks the new restrictions
System.out.println("Holder value after mutation: " + holder.value + " (check stderr for the warning)");
} catch (ReflectiveOperationException ex) {
System.out.println("Final field mutation denied: " + ex);
}

// 2) record components were never mutable via deep reflection (since JDK 16)
// JEP 500 aims to align final fields in normal classes with this behavior
var point = new Point(1, 2);
try {
Field field = Point.class.getDeclaredField("x");
field.setAccessible(true);
field.set(point, 42);
} catch (ReflectiveOperationException ex) {
System.out.println("Record field mutation denied: " + ex.getClass().getSimpleName());
}

// 3) the recommended way - do not mutate, create a new instance with the desired value instead
// (e.g. via constructor, static factory or "wither" method)
var updated = holder.withValue(300);
System.out.println("New Holder created instead of mutation: " + updated.value);

// Notes:
// - to find the code mutating final fields, run with `--illegal-final-field-mutation=debug`
// (prints a stack trace for every mutation) or record a JFR file and look for
// `jdk.FinalFieldMutation` events: `jfr print --events jdk.FinalFieldMutation recording.jfr`
// - serialization libraries should use `sun.reflect.ReflectionFactory` instead of deep reflection
// - `MethodHandles.Lookup.unreflectSetter` on a final field is restricted the same way as `Field.set`
// - mutating final fields via JNI `Set<type>Field` is undefined behavior; `-Xcheck:jni` now warns about it
// - once final means final, the JVM may trust final fields and apply optimizations like constant folding

System.out.println();
}

static class Holder {
final int value;

Holder(int value) {
this.value = value;
}

Holder withValue(int newValue) {
return new Holder(newValue);
}
}

record Point(int x, int y) {}
}
106 changes: 57 additions & 49 deletions src/main/resources/JDK26Info.json
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[
{
"jep": 517,
"jdk": 26,
"name": "JEP 517 - HTTP/3 for the HTTP Client API",
"dscr": "Update the HTTP Client API to support the HTTP/3 protocol, so that libraries and applications can interact with HTTP/3 servers with minimal code change.",
"link": false,
"code": true
},
{
"jep": 522,
"jdk": 26,
"name":"JEP 522 - G1 GC: Improve Throughput by Reducing Synchronization",
"dscr": "Improve latency and throughput by reducing the amount of synchronization required between application threads and GC threads",
"link": false,
"code": false
},
{
"jep": 524,
"jdk": 26,
"name": "JEP 524 - PEM Encodings of Cryptographic Objects (Second Preview)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep538.PemEncodingsDemo`.",
"link": true,
"code": false
},
{
"jep": 525,
"jdk": 26,
"name": "JEP 525 - Structured Concurrency (Sixth Preview)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep533.StructuredConcurrencyDemo`.",
"link": true,
"code": false
},
{
"jep": 526,
"jdk": 26,
"name": "JEP 526 - Lazy Constants (Preview)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep531.LazyConstantsDemo`.",
"link": true,
"code": false
},
{
"jep": 529,
"jdk": 26,
"name": "JEP 529 - Vector API (Eleventh Incubator)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep537.VectorAPIDemo`.",
"link": true,
"code": false
}
[
{
"jep": 500,
"jdk": 26,
"name": "JEP 500 - Prepare to Make Final Mean Final",
"dscr": "Issue warnings about uses of deep reflection to mutate final fields to prepare for a future release that restricts final field mutation by default.",
"link": false,
"code": true
},
{
"jep": 517,
"jdk": 26,
"name": "JEP 517 - HTTP/3 for the HTTP Client API",
"dscr": "Update the HTTP Client API to support the HTTP/3 protocol, so that libraries and applications can interact with HTTP/3 servers with minimal code change.",
"link": false,
"code": true
},
{
"jep": 522,
"jdk": 26,
"name":"JEP 522 - G1 GC: Improve Throughput by Reducing Synchronization",
"dscr": "Improve latency and throughput by reducing the amount of synchronization required between application threads and GC threads",
"link": false,
"code": false
},
{
"jep": 524,
"jdk": 26,
"name": "JEP 524 - PEM Encodings of Cryptographic Objects (Second Preview)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep538.PemEncodingsDemo`.",
"link": true,
"code": false
},
{
"jep": 525,
"jdk": 26,
"name": "JEP 525 - Structured Concurrency (Sixth Preview)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep533.StructuredConcurrencyDemo`.",
"link": true,
"code": false
},
{
"jep": 526,
"jdk": 26,
"name": "JEP 526 - Lazy Constants (Preview)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep531.LazyConstantsDemo`.",
"link": true,
"code": false
},
{
"jep": 529,
"jdk": 26,
"name": "JEP 529 - Vector API (Eleventh Incubator)",
"dscr": "Up-to-date demo in `org.javademos.java27.jep537.VectorAPIDemo`.",
"link": true,
"code": false
}
]
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