# Java Developer

> Guide for developing on the Java 21 baseline, including modern features, best practices, and integration with Groovy/Grails projects

- Skill: `apache/java-developer` (Agent Skill)
- Install (CLI): `npx skillmds@latest add apache/java-developer`
- Raw SKILL.md: https://api.skillmd.com/api/skills/apache/java-developer/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- License: Apache-2.0
- Author: apache (https://skillmd.com/u/apache)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/apache/java-developer

---

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SPDX-License-Identifier: Apache-2.0

Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements; and to You under the Apache License, Version 2.0. 
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## 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:
```java
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:
```java
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:
```java
// 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:
```java
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:
```java
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
```java
// 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:
```groovy
// 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
```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
```groovy
java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(17)
    }
}

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

### Testing with JUnit 5
```java
@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)
```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
```bash
# 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
```java
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
```java
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
```java
// 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