Java Development

GitHubOfficialFirst committed by jduboisDistributed by GitHub

Skill
Tool integration
Software development
MIT
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From the author

Day-to-day plain Java development without a framework: Maven and Gradle usage and wrappers, project layout, dependency and vulnerability management, modern Java idioms by JDK version (records, sealed types, pattern matching, virtual threads), SLF4J logging, error and null handling, immutability, concurrency basics, static analysis and formatting (Spotless, Error Prone, -Xlint), packaging (jar, jlink, jpackage) and GitHub Actions CI. Use when writing, building, reviewing or maintaining ordinary Java code.

— the description by jdubois

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides comprehensive support for standard Java development, focusing on build tool management (Maven/Gradle), dependency hygiene, and the application of modern Java idioms based on the project's specific JDK version. It emphasizes best practices for code quality, static analysis, and packaging, ensuring that development remains consistent with the project's existing configuration and standards.

What it does

Manages Maven and Gradle build configurations, wrappers, and multi-module layouts.
Provides version-specific Java idiom guidance from JDK 8 through 25.
Handles dependency management, vulnerability scanning, and conflict resolution.
Configures static analysis tools like Spotless, Error Prone, and -Xlint.
Supports packaging applications into jars, jlink images, or jpackage installers.
Maintains CI pipelines using GitHub Actions.

When to use it

When writing, building, or refactoring plain Java code without a framework.
When upgrading or aligning code to match a project's specific Java release.
When diagnosing build failures, dependency conflicts, or vulnerability alerts.
When setting up or maintaining project-wide formatting and static analysis rules.
When preparing a Java application for deployment or distribution.

Advantages

Ensures strict adherence to project-specific Java versions and build configurations.
Promotes clean, idiomatic code using the latest features supported by the project.
Reduces technical debt through proactive dependency and vulnerability management.
Standardizes build processes and CI/CD workflows.
Prevents common pitfalls related to JDK-specific changes and preview features.

Usage examples

“How do I implement a record-based switch expression in this project?”: The agent checks the project's Java release; if it is 21 or higher, it provides a code snippet using record patterns and exhaustive switch expressions.
“Check for dependency updates and resolve a conflict in my Maven build.”: The agent runs dependency analysis commands, identifies the conflicting versions, and suggests a fix using dependencyManagement in the pom.xml.
“Add a GitHub Actions workflow to run my Gradle build.”: The agent generates a standard CI file using actions/setup-java and the project's wrapper, ensuring the build is verified on every push.
“How can I use virtual threads for my I/O tasks?”: The agent verifies the project is on JDK 21+, explains the use of Executors.newVirtualThreadPerTaskExecutor, and warns about blocking synchronization.

Install

Download the skill as a ZIP:
https://autogineer.com/agent-skills/github/java-development/download

Unzip it into the cross-client skills folder of your project:

mkdir -p .agents/skills && unzip java-development.zip -d .agents/skills

Documentation by jdubois

Java Development (plain Java)

Everyday guidance for Java projects that are not tied to a framework. If the project uses Spring Boot, Quarkus, Helidon or similar, prefer that framework's skills for framework concerns and use this one for the plain-Java parts.

Rules that always apply

Detect before you write. Read pom.xml/build.gradle*, .mvn/, gradle/, .java-version/.sdkmanrc, .editorconfig, CI files, and a few source files. Match the build tool, Java release, formatter, naming, logging and test libraries already in use.
Never upgrade implicitly. Do not change the Java release, build-tool version, plugin or dependency versions, or add a formatter/linter/static analyzer the project lacks, as a side effect of another task. Suggest it and ask. Version upgrades are their own task (java-upgrade, if installed).
Use the wrapper (./mvnw, ./gradlew) when present; otherwise the installed mvn/gradle. Do not commit a wrapper unprompted.
Verify with a build. After any change run the project's own verification (./mvnw -B verify or ./gradlew build) and report the result. Run a narrower command first while iterating. If you could not run it, say so.
Ask the user before: changing public APIs or serialized/DB/wire formats, raising the Java release, adding runtime dependencies, adding tools to the build, deleting or weakening tests, changing CI, or when two reasonable designs have different trade-offs.
Prefer the standard library and small, explicit code over new dependencies. No Lombok unless already used.

Pick idioms by the project's Java release

Check maven.compiler.release/<release>, java.toolchain, or sourceCompatibility; never use a feature newer than that release. Summary (details and snippets in references/modern-java.md):

Release	You can use
8	lambdas, streams, `Optional`, `java.time`
11	`var`, `String.isBlank/strip/lines`, `java.net.http.HttpClient`, `List.of`/`Map.of` (9)
17	records, sealed classes, text blocks, switch expressions, `instanceof` patterns
21	pattern matching for `switch`, record patterns, virtual threads, `SequencedCollection` (`getFirst/getLast/reversed`)
22-24	unnamed variables `_` (22), `Stream.gather` (24)
25	scoped values, instance `main`/compact source files, module import declarations (all final)

Structured concurrency (StructuredTaskScope) is still a preview API in 25 and 27 (verified: needs --enable-preview); do not use it in production code unless the user opts in to preview features.

Build tools in practice

Maven: ./mvnw -B verify (compile, unit tests, integration tests, packaging), -pl <module> -am for one module, dependency:tree, -DskipTests only when the user asks. Gradle: ./gradlew build, test --tests 'pkg.Class', dependencies, dependencyInsight --dependency <name>.
Standard layout: src/main/java, src/main/resources, src/test/java, src/test/resources; package by feature, one top-level type per file, no split packages.
Set the Java version explicitly (maven.compiler.release / Gradle toolchain with options.release) and UTF-8 encoding; do not rely on the JDK that happens to run the build.
Wrapper, BOM and dependency:tree/versions: usage, version catalogs and multi-module layout: references/build-and-dependencies.md.

Dependencies and vulnerabilities

Import BOMs (junit-bom, testcontainers-bom, the framework's BOM) instead of repeating versions; one version per library across modules.
Inspect before changing: mvn dependency:tree -Dincludes=group:artifact / gradlew dependencyInsight. Resolve conflicts with dependencyManagement or Gradle constraints, not by excluding randomly.
Check updates with mvn versions:display-dependency-updates (add -Dmaven.version.ignore to hide alpha/beta/RC) and report them; do not apply major upgrades without asking.
Vulnerabilities: use what the project has (Dependabot/Renovate alerts, OWASP dependency-check, OSV-Scanner, GitHub dependency review). Suggest one if none exists; do not add it silently. Keep secrets out of scanner config; NVD-based scanners need an API key to be practical.

Version-dependent failures (verified)

JDK 23+ no longer runs annotation processors implicitly (Lombok, MapStruct, Dagger...): -proc:none is the default unless -processor, --processor-path or --processor-module-path is given. Declare the processor path in the build (Maven annotationProcessorPaths, Gradle annotationProcessor) or pass -proc:full. JDK 21+ prints a warning when it detects implicit use; -proc:full is also backported to 17.0.11, 11.0.23 and 8u411. Source: https://inside.java/2024/06/18/quality-heads-up/.
Virtual threads and synchronized: on JDK 21-23 a virtual thread that blocks inside synchronized is pinned to its carrier; JEP 491 removes nearly all such pinning in JDK 24+. On 21-23 prefer ReentrantLock around blocking calls. Source: https://openjdk.org/jeps/491.
Error Prone on JDK 16+ needs the --add-exports/--add-opens javac flags, and on JDK 21 -XDaddTypeAnnotationsToSymbol=true (see the quality reference).
Never use a language feature newer than the project's release (see the table above).

Static analysis and formatting (only if already present, or the user agrees)

Respect what is configured: if the project uses Spotless/Checkstyle/SpotBugs/Error Prone, run them and fix findings; if not, do not add them. When asked to introduce them, start with -Xlint:all and Spotless (formatter only), then Error Prone. Verified configs for Maven and Gradle on JDK 21 and 25 (including the JDK-21-only -XDaddTypeAnnotationsToSymbol=true requirement and Maven .mvn/jvm.config flags) are in references/quality-and-packaging.md. Never reformat the whole codebase in an unrelated change.

Packaging and CI

Plain library jar: mvn package/gradlew jar. Runnable app: shaded/fat jar or lib/ directory plus manifest Main-Class; custom runtime with jlink; native installer/app-image with jpackage (verified on macOS, see references/quality-and-packaging.md).
GitHub Actions: actions/checkout, actions/setup-java with distribution: temurin, an explicit java-version, built-in dependency cache, then ./mvnw -B verify or ./gradlew build. Snippet and notes in references/quality-and-packaging.md. Do not change CI without asking.

java-junit (unit tests), java-testcontainers (integration tests with Docker), java-hibernate-jpa (persistence), java-debugging (failures and stack traces), java-diagnostics (performance, memory, threads, JFR), java-upgrade (JDK/framework upgrades), lsp-setup (Java language server for navigation and refactoring).

References

references/modern-java.md: idioms and verified snippets (records, sealed types, switch patterns, text blocks, virtual threads, SequencedCollection, unnamed variables), preview status, pitfalls.
references/build-and-dependencies.md: Maven/Gradle commands, wrappers, BOMs, conflict diagnosis, update checking, Dependabot/Renovate, vulnerability scanning.
references/quality-and-packaging.md: logging setup, -Xlint, Spotless, Error Prone, Checkstyle/SpotBugs notes, jar/jlink/jpackage, GitHub Actions.

Files

ATTRIBUTION.mdAdded by AutoGineer863 B
LICENSELicence file added by AutoGineer1.0 KiB
SKILL.md7.7 KiB
references/build-and-dependencies.md5.3 KiB
references/modern-java.md5.3 KiB
references/quality-and-packaging.md8.7 KiB

references/modern-java.md

Modern Java idioms by release

Snippets in this file compiled and ran on JDK 21 and 25 (-Xlint:all -Werror) unless marked. Use a feature only if the project's configured release supports it; do not raise the release to use one.

Records, sealed types, pattern matching (17 / 21)

public sealed interface Shape permits Circle, Rect, Group {}

public record Circle(double r) implements Shape {
    public Circle {                                   // compact constructor: validate
        if (r <= 0) throw new IllegalArgumentException("r must be > 0");
    }
}
public record Rect(double w, double h) implements Shape {}
public record Group(List<Shape> items) implements Shape {
    public Group { items = List.copyOf(items); }      // defensive immutable copy
}

static double area(Shape s) {
    return switch (s) {                                // exhaustive: no default, compiler checks
        case Circle(double r) -> Math.PI * r * r;     // record pattern (21)
        case Rect(double w, double h) -> w * h;
        case Group(List<Shape> items) -> items.stream().mapToDouble(Main::area).sum();
    };
}

static String describe(Object o) {
    return switch (o) {
        case null -> "null";
        case Integer i when i > 10 -> "big int " + i; // guard
        case Integer i -> "int " + i;
        case String str -> "string " + str.length();
        default -> "other";
    };
}

Records are shallowly immutable: copy mutable components (List.copyOf). They are good DTOs/value types, not JPA entities (see java-hibernate-jpa).
Prefer a sealed hierarchy and an exhaustive switch without default, so adding a subtype breaks compilation where handling is missing.
Pre-21 (17): if (o instanceof String s && !s.isBlank()) {...} and arrow-form switch expressions work; case Integer i -> patterns in switch do not.

Text blocks (15)

String json = """
    {
      "name": "demo"
    }
    """;

Indentation is stripped relative to the closing delimiter; use .formatted(...) for values; never build SQL/JSON by concatenating untrusted input.

Virtual threads (21, final)

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    List<Future<Integer>> futures = new ArrayList<>();
    for (int i = 0; i < 1_000; i++) {
        int n = i;
        futures.add(executor.submit(() -> { Thread.sleep(Duration.ofMillis(10)); return n; }));
    }
    for (var f : futures) f.get();
}
Thread.ofVirtual().name("worker-", 0).start(task);   // single thread

Verified: 1,000 tasks, each sleeping 10 ms, run through newVirtualThreadPerTaskExecutor on JDK 21 and 25; Thread.currentThread().isVirtual() is true inside tasks.

Use for blocking I/O workloads (HTTP, JDBC); no benefit for CPU-bound work.
Do not pool them or cache them; create one per task. Limit concurrency to a downstream resource with a Semaphore, not a pool size.
Avoid heavy ThreadLocal state per task. On JDK 21-23 a virtual thread blocked inside synchronized pins its carrier; JEP 491 (JDK 24) removes most of this pinning. Use ReentrantLock on 21 for long blocking critical sections.
Frameworks may switch to virtual threads by configuration (e.g. a Spring Boot property); that is framework territory, not this skill.

Structured concurrency and scoped values

ScopedValue: preview on 21, final in 25 (verified: compiles and runs on 25 without --enable-preview, rejected on 21 as a preview API).
StructuredTaskScope: preview on 25 and on 27 (verified: "is a preview API and is disabled by default"). Its API changed between 21 and 25 (25 uses StructuredTaskScope.open()). Do not use without explicit user approval to enable --enable-preview for compile and run; Maven needs <compilerArgs>--enable-preview</compilerArgs> and Surefire argLine.

SequencedCollection (21)

static <T> Optional<T> last(SequencedCollection<T> c) {
    return c.isEmpty() ? Optional.empty() : Optional.ofNullable(c.getLast()); // Null-permitting collections
}

getFirst/getLast/addFirst/removeLast/reversed() are available on List, Deque, LinkedHashSet, SortedSet; getFirst() on an empty list throws NoSuchElementException. SequencedCollection implementations may permit null elements (for example, LinkedList and LinkedHashSet), so use Optional.ofNullable when the result can be null.

Later features

Feature	Status (verified on JDK 25 / 27 unless noted)
Unnamed variables `_`	Final in 22; on 21 the compiler says "unnamed variables are a preview feature"
`Stream.gather` / `Gatherers.windowFixed(2)`	Works on 25 and 27; not present on 21
Instance `void main()` (compact source file)	Works without flags on 25 and 27
`import module java.base;`	Works without flags on 25 and 27
`ScopedValue`	Works without flags on 25 and 27
`StructuredTaskScope`	Preview on 25 and 27

Verify the status of any other feature with java <File>.java on the target JDK before recommending it; JEP statuses change every six months.

Collection and API pitfalls

stream().toList() (16+) returns an unmodifiable list; Collectors.toList() makes no guarantee.
Map.of/List.of reject nulls and Map.of has undefined iteration order; use LinkedHashMap when order matters.
Do not use finalize, Thread.stop, new Integer(...), Date/Calendar, SecurityManager.

Requirements and compatibility

Written for: GitHub Copilot

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Needs an API key

Security

No issues found by our static scan· No issues found

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

First committed by

jdubois

Publisher

GitHub (Official)

Distributed by

GitHub

Source

github/awesome-copilot · skills/java-development · commit 797337f

Licence

MIT License (redistribution allowed)

LICENSE (added by AutoGineer from LICENSE)
ATTRIBUTION.md (written by AutoGineer)

Synced

2026-11-09

Contributors