java-developer

name	java-developer
description	Guide for developing on the Java 21 baseline, including modern features, best practices, and integration with Groovy/Grails projects Use when this capability is needed.
metadata	{"author":"apache"}

What I Do

• Provide guidance on Java 21 syntax, features, and APIs for use in Grails/Groovy projects.
• Assist with code generation, refactoring, and debugging using modern Java features like records, sealed classes, pattern matching, and text blocks.
• Recommend best practices for Java code that interoperates with Groovy in mixed-language projects.
• Guide on tooling: Gradle builds, JDK setup, testing with JUnit 5/Spock, and profiling.

When to Use Me

Use this skill when working on Java code within this repository, especially for:

• Writing Java classes that will be used alongside Groovy code.
• Implementing features using records, sealed classes, or pattern matching for instanceof.
• Migrating older Java code (e.g., Java 8/11/17) to Java 21 idioms.
• Performance optimization, security hardening, or module system (JPMS) questions.
• Understanding how Java code integrates with Groovy's dynamic features.

Java 21 Baseline Features

Records (JEP 395)

Immutable data carriers with auto-generated constructors, accessors, equals, hashCode, and toString:

public record Book(String title, String author, int year) {
    // Compact constructor for validation
    public Book {
        if (title == null || title.isBlank()) {
            throw new IllegalArgumentException("Title cannot be blank");
        }
    }
}

Sealed Classes (JEP 409)

Restrict which classes can extend or implement a type:

public sealed interface Shape permits Circle, Rectangle, Triangle {
    double area();
}

public final class Circle implements Shape {
    private final double radius;
    public Circle(double radius) { this.radius = radius; }
    public double area() { return Math.PI * radius * radius; }
}

public non-sealed class Rectangle implements Shape {
    // Can be further extended
}

Pattern Matching for instanceof (JEP 394)

Eliminate redundant casts:

// Before pattern matching
if (obj instanceof String) {
    String s = (String) obj;
    System.out.println(s.length());
}

// Modern Java
if (obj instanceof String s) {
    System.out.println(s.length());
}

Text Blocks (JEP 378)

Multi-line string literals with proper formatting:

String json = """
    {
        "name": "Grails",
        "version": "7.0"
    }
    """;

String sql = """
    SELECT id, name, created_at
    FROM users
    WHERE status = 'ACTIVE'
    ORDER BY created_at DESC
    """;

Switch Expressions (JEP 361)

Use switch as an expression with arrow syntax:

String result = switch (day) {
    case MONDAY, FRIDAY, SUNDAY -> "Relaxed";
    case TUESDAY -> "Productive";
    case THURSDAY, SATURDAY -> "Moderate";
    case WEDNESDAY -> {
        var temp = calculateWorkload();
        yield temp > 5 ? "Busy" : "Normal";
    }
};

Enhanced NullPointerException Messages (JEP 358)

Detailed NPE messages showing exactly which variable was null:

Cannot invoke "String.length()" because "user.getAddress().getCity()" is null

Helpful Stream and Collection APIs

// Stream.toList() - unmodifiable list
List<String> names = users.stream()
    .map(User::getName)
    .toList();

// Collectors improvements
Map<Status, List<User>> byStatus = users.stream()
    .collect(Collectors.groupingBy(User::getStatus));

Java/Groovy Interoperability

Calling Java from Groovy

Groovy seamlessly calls Java code:

// Java record used in Groovy
def book = new Book("Grails Guide", "Author", 2024)
println book.title()  // Groovy can use property syntax too
println book.title    // Also works

Calling Groovy from Java

// Groovy classes are just Java classes
GroovyService service = new GroovyService();
service.process(data);

// Working with Groovy closures in Java
Closure<String> closure = ...;
String result = closure.call("input");

Best Practices for Mixed Projects

• Use Java for performance-critical code with @CompileStatic equivalent behavior.
• Use Java records for DTOs shared between Java and Groovy code.
• Prefer Java interfaces that Groovy classes implement.
• Avoid Groovy-specific features (like categories) in APIs consumed by Java.

Build and Testing

Gradle Configuration

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(17)
    }
}

tasks.withType(JavaCompile).configureEach {
    options.encoding = 'UTF-8'
    options.compilerArgs += ['-parameters']  // Preserve parameter names
}

Testing with JUnit 5

@Test
@DisplayName("Book record should validate title")
void bookShouldValidateTitle() {
    assertThrows(IllegalArgumentException.class, () ->
        new Book("", "Author", 2024)
    );
}

@ParameterizedTest
@ValueSource(strings = {"", " ", "  "})
void blankTitlesShouldBeRejected(String title) {
    assertThrows(IllegalArgumentException.class, () ->
        new Book(title, "Author", 2024)
    );
}

Testing with Spock (from Groovy)

def "Java record should work in Spock tests"() {
    when:
    def book = new Book("Test", "Author", 2024)

    then:
    book.title() == "Test"
    book.author() == "Author"
}

Performance and Profiling

JVM Options for Java 21

# Recommended GC for most workloads
-XX:+UseG1GC

# For low-latency requirements
-XX:+UseZGC

# Memory settings
-Xms512m -Xmx2g

# Enable JFR for profiling
-XX:StartFlightRecording=duration=60s,filename=recording.jfr

Profiling Tools

• JDK Flight Recorder (JFR): Built-in low-overhead profiler
• VisualVM: GUI-based monitoring and profiling
• async-profiler: Low-overhead sampling profiler

Common Patterns

Null Handling with Optional

public Optional<User> findById(Long id) {
    return Optional.ofNullable(repository.get(id));
}

// Usage
String name = findById(id)
    .map(User::getName)
    .orElse("Unknown");

Resource Management with try-with-resources

try (var reader = new BufferedReader(new FileReader(path));
     var writer = new BufferedWriter(new FileWriter(output))) {
    reader.lines()
        .map(String::toUpperCase)
        .forEach(line -> writer.write(line + "\n"));
}

Immutable Collections

// Create immutable collections
List<String> list = List.of("a", "b", "c");
Set<String> set = Set.of("x", "y", "z");
Map<String, Integer> map = Map.of("one", 1, "two", 2);

// Copy to immutable
List<String> copy = List.copyOf(mutableList);

Code Style Guidelines

• Follow existing patterns in the codebase.
• Use var for local variables when the type is obvious from context.
• Prefer records for simple data carriers.
• Use sealed classes to model restricted type hierarchies.
• Add @Override annotation when overriding methods.
• Use meaningful parameter names (preserved with -parameters flag).

Resources

• Java 21 Documentation: https://docs.oracle.com/en/java/javase/21/
• Java Language Updates: https://docs.oracle.com/en/java/javase/21/language/
• JEP Index: https://openjdk.org/jeps/0

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Source: apache/grails-core — distributed by TomeVault.