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+---
+applyTo: ['*']
+description: "Comprehensive best practices for adopting new Java 25 features since the release of Java 21."
+---
+
+# GitHub Copilot Instructions: Java 21 to Java 25 Upgrade Guide
+
+These instructions help GitHub Copilot assist developers in upgrading Java projects from JDK 21 to JDK 25, focusing on new language features, API changes, and best practices.
+
+## Language Features and API Changes in JDK 22-25
+
+### Pattern Matching Enhancements (JEP 455/488 - Preview in 23)
+
+**Primitive Types in Patterns, instanceof, and switch**
+
+When working with pattern matching:
+- Suggest using primitive type patterns in switch expressions and instanceof checks
+- Example upgrade from traditional switch:
+```java
+// Old approach (Java 21)
+switch (x.getStatus()) {
+    case 0 -> "okay";
+    case 1 -> "warning"; 
+    case 2 -> "error";
+    default -> "unknown status: " + x.getStatus();
+}
+
+// New approach (Java 25 Preview)
+switch (x.getStatus()) {
+    case 0 -> "okay";
+    case 1 -> "warning";
+    case 2 -> "error"; 
+    case int i -> "unknown status: " + i;
+}
+```
+
+- Enable preview features with `--enable-preview` flag
+- Suggest guard patterns for more complex conditions:
+```java
+switch (x.getYearlyFlights()) {
+    case 0 -> ...;
+    case int i when i >= 100 -> issueGoldCard();
+    case int i -> ... // handle 1-99 range
+}
+```
+
+### Class-File API (JEP 466/484 - Second Preview in 23, Standard in 25)
+
+**Replacing ASM with Standard API**
+
+When detecting bytecode manipulation or class file processing:
+- Suggest migrating from ASM library to the standard Class-File API
+- Use `java.lang.classfile` package instead of `org.objectweb.asm`
+- Example migration pattern:
+```java
+// Old ASM approach
+ClassReader reader = new ClassReader(classBytes);
+ClassWriter writer = new ClassWriter(reader, 0);
+// ... ASM manipulation
+
+// New Class-File API approach
+ClassModel classModel = ClassFile.of().parse(classBytes);
+byte[] newBytes = ClassFile.of().transform(classModel, 
+    ClassTransform.transformingMethods(methodTransform));
+```
+
+### Markdown Documentation Comments (JEP 467 - Standard in 23)
+
+**JavaDoc Modernization**
+
+When working with JavaDoc comments:
+- Suggest converting HTML-heavy JavaDoc to Markdown syntax
+- Use `///` for Markdown documentation comments
+- Example conversion:
+```java
+// Old HTML JavaDoc
+/**
+ * Returns the <b>absolute</b> value of an {@code int} value.
+ * <p>
+ * If the argument is not negative, return the argument.
+ * If the argument is negative, return the negation of the argument.
+ * 
+ * @param a the argument whose absolute value is to be determined
+ * @return the absolute value of the argument
+ */
+
+// New Markdown JavaDoc  
+/// Returns the **absolute** value of an `int` value.
+///
+/// If the argument is not negative, return the argument.
+/// If the argument is negative, return the negation of the argument.
+/// 
+/// @param a the argument whose absolute value is to be determined
+/// @return the absolute value of the argument
+```
+
+### Derived Record Creation (JEP 468 - Preview in 23)
+
+**Record Enhancement**
+
+When working with records:
+- Suggest using `with` expressions for creating derived records
+- Enable preview features for derived record creation
+- Example pattern:
+```java
+// Instead of manual record copying
+public record Person(String name, int age, String email) {
+    public Person withAge(int newAge) {
+        return new Person(name, newAge, email);
+    }
+}
+
+// Use derived record creation (Preview)
+Person updated = person with { age = 30; };
+```
+
+### Stream Gatherers (JEP 473/485 - Second Preview in 23, Standard in 25)
+
+**Enhanced Stream Processing**
+
+When working with complex stream operations:
+- Suggest using `Stream.gather()` for custom intermediate operations
+- Import `java.util.stream.Gatherers` for built-in gatherers
+- Example usage:
+```java
+// Custom windowing operations
+List<List<String>> windows = stream
+    .gather(Gatherers.windowSliding(3))
+    .toList();
+
+// Custom filtering with state
+List<Integer> filtered = numbers.stream()
+    .gather(Gatherers.fold(0, (state, element) -> {
+        // Custom stateful logic
+        return state + element > threshold ? element : null;
+    }))
+    .filter(Objects::nonNull)
+    .toList();
+```
+
+## Migration Warnings and Deprecations
+
+### sun.misc.Unsafe Memory Access Methods (JEP 471 - Deprecated in 23)
+
+When detecting `sun.misc.Unsafe` usage:
+- Warn about deprecated memory-access methods
+- Suggest migration to standard alternatives:
+```java
+// Deprecated: sun.misc.Unsafe memory access
+Unsafe unsafe = Unsafe.getUnsafe();
+unsafe.getInt(object, offset);
+
+// Preferred: VarHandle API
+VarHandle vh = MethodHandles.lookup()
+    .findVarHandle(MyClass.class, "fieldName", int.class);
+int value = (int) vh.get(object);
+
+// Or for off-heap: Foreign Function & Memory API
+MemorySegment segment = MemorySegment.ofArray(new int[10]);
+int value = segment.get(ValueLayout.JAVA_INT, offset);
+```
+
+### JNI Usage Warnings (JEP 472 - Warnings in 24)
+
+When detecting JNI usage:
+- Warn about upcoming restrictions on JNI usage
+- Suggest adding `--enable-native-access` flag for applications using JNI
+- Recommend migration to Foreign Function & Memory API where possible
+- Add module-info.java entries for native access:
+```java
+module com.example.app {
+    requires jdk.unsupported; // for remaining JNI usage
+}
+```
+
+## Garbage Collection Updates
+
+### ZGC Generational Mode (JEP 474 - Default in 23)
+
+When configuring garbage collection:
+- Default ZGC now uses generational mode
+- Update JVM flags if explicitly using non-generational ZGC:
+```bash
+# Explicit non-generational mode (will show deprecation warning)
+-XX:+UseZGC -XX:-ZGenerational
+
+# Default generational mode
+-XX:+UseZGC
+```
+
+### G1 Improvements (JEP 475 - Implemented in 24)
+
+When using G1GC:
+- No code changes required - internal JVM optimization
+- May see improved compilation performance with C2 compiler
+
+## Vector API (JEP 469 - Eighth Incubator in 25)
+
+When working with numerical computations:
+- Suggest Vector API for SIMD operations (still incubating)
+- Add `--add-modules jdk.incubator.vector`
+- Example usage:
+```java
+import jdk.incubator.vector.*;
+
+// Traditional scalar computation
+for (int i = 0; i < a.length; i++) {
+    c[i] = a[i] + b[i];
+}
+
+// Vectorized computation
+var species = IntVector.SPECIES_PREFERRED;
+for (int i = 0; i < a.length; i += species.length()) {
+    var va = IntVector.fromArray(species, a, i);
+    var vb = IntVector.fromArray(species, b, i);
+    var vc = va.add(vb);
+    vc.intoArray(c, i);
+}
+```
+
+## Compilation and Build Configuration
+
+### Preview Features
+
+For projects using preview features:
+- Add `--enable-preview` to compiler arguments
+- Add `--enable-preview` to runtime arguments
+- Maven configuration:
+```xml
+<plugin>
+    <groupId>org.apache.maven.plugins</groupId>
+    <artifactId>maven-compiler-plugin</artifactId>
+    <configuration>
+        <release>25</release>
+        <compilerArgs>
+            <arg>--enable-preview</arg>
+        </compilerArgs>
+    </configuration>
+</plugin>
+
+<plugin>
+    <groupId>org.apache.maven.plugins</groupId>
+    <artifactId>maven-surefire-plugin</artifactId>
+    <configuration>
+        <argLine>--enable-preview</argLine>
+    </configuration>
+</plugin>
+```
+
+- Gradle configuration:
+```kotlin
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(25)
+    }
+}
+
+tasks.withType<JavaCompile> {
+    options.compilerArgs.add("--enable-preview")
+}
+
+tasks.withType<Test> {
+    jvmArgs("--enable-preview")
+}
+```
+
+## Migration Strategy
+
+### Step-by-Step Upgrade Process
+
+1. **Update Build Tools**: Ensure Maven/Gradle supports JDK 25
+2. **Update Dependencies**: Check for JDK 25 compatibility
+3. **Handle Warnings**: Address deprecation warnings from JEPs 471/472
+4. **Enable Preview Features**: If using pattern matching or other preview features
+5. **Test Thoroughly**: Especially for applications using JNI or sun.misc.Unsafe
+6. **Performance Testing**: Verify GC behavior with new ZGC defaults
+
+### Code Review Checklist
+
+When reviewing code for Java 25 upgrade:
+- [ ] Replace ASM usage with Class-File API
+- [ ] Convert complex HTML JavaDoc to Markdown
+- [ ] Use primitive patterns in switch expressions where applicable
+- [ ] Replace sun.misc.Unsafe with VarHandle or FFM API
+- [ ] Add native-access permissions for JNI usage
+- [ ] Use Stream gatherers for complex stream operations
+- [ ] Update build configuration for preview features
+
+### Testing Considerations
+
+- Test with `--enable-preview` flag for preview features
+- Verify JNI applications work with native access warnings
+- Performance test with new ZGC generational mode
+- Validate JavaDoc generation with Markdown comments
+
+## Common Pitfalls
+
+1. **Preview Feature Dependencies**: Don't use preview features in library code without clear documentation
+2. **Native Access**: Applications using JNI directly or indirectly may need `--enable-native-access` configuration
+3. **Unsafe Migration**: Don't delay migrating from sun.misc.Unsafe - deprecation warnings indicate future removal
+4. **Pattern Matching Scope**: Primitive patterns work with all primitive types, not just int
+5. **Record Enhancement**: Derived record creation requires preview flag in Java 23
+
+## Performance Considerations
+
+- ZGC generational mode may improve performance for most workloads
+- Class-File API reduces ASM-related overhead
+- Stream gatherers provide better performance for complex stream operations
+- G1GC improvements reduce JIT compilation overhead
+
+Remember to test thoroughly in staging environments before deploying Java 25 upgrades to production systems.