---

name: java-reviewer description: | WHEN: Java code review, Java idioms, exception handling, collections, streams, Java 17+ features WHAT: Exception patterns + resource management + null safety + stream usage + modern Java features WHEN NOT: Spring specific → spring-boot-reviewer, Android → kotlin-android-reviewer, Kotlin → kotlin-spring-reviewer

Java Reviewer Skill

Purpose

Reviews Java code for idiomatic patterns, exception handling, resource management, null safety, and modern Java (17+) feature adoption.

When to Use

• Java code review requests
• "Java patterns", "exception handling", "Java best practices" mentions
• Legacy Java modernization
• Projects with pom.xml or build.gradle
• src/main/java/**/*.java files

Project Detection

• pom.xml (Maven) or build.gradle (Gradle)
• src/main/java/ directory structure
• .java source files
• java.version property in build config

Workflow

Step 1: Analyze Project

**Java Version**: 17 / 21
**Build Tool**: Maven / Gradle
**Testing**: JUnit 5
**Key Dependencies**:
  - Lombok
  - MapStruct
  - Jakarta EE

Step 2: Select Review Areas

AskUserQuestion:

"Which Java areas to review?"
Options:
- Full Java idiom check (recommended)
- Exception handling
- Resource management (try-with-resources)
- Null safety (Optional, annotations)
- Stream/Collection patterns
- Modern Java features (17+)
multiSelect: true

Detection Rules

Critical: Empty Catch Blocks

Pattern	Issue	Severity
Empty catch block	Silent failure	CRITICAL
Catch all Exception	Over-broad handling	HIGH
Catch Throwable	Catching errors	CRITICAL
Missing logging in catch	Lost error info	HIGH

// BAD: Empty catch block
try {
    parseFile(path);
} catch (IOException e) {
    // Silent failure - CRITICAL
}

// BAD: Catch all Exception
try {
    process();
} catch (Exception e) {
    log.error("Error", e);  // Too broad
}

// GOOD: Specific exception handling
try {
    parseFile(path);
} catch (FileNotFoundException e) {
    log.warn("File not found: {}", path);
    return Optional.empty();
} catch (IOException e) {
    log.error("Failed to read file: {}", path, e);
    throw new ProcessingException("File read error", e);
}

// BAD: Catching Throwable
try {
    compute();
} catch (Throwable t) {  // Catches OutOfMemoryError, StackOverflowError
    log.error("Error", t);
}

// GOOD: Catch Exception, let Errors propagate
try {
    compute();
} catch (Exception e) {
    log.error("Processing failed", e);
    throw new ServiceException(e);
}

Critical: Mutable Static Fields

Pattern	Issue	Severity
public static non-final	Thread-unsafe	CRITICAL
Static mutable collection	Race condition	CRITICAL
Static DateFormat	Not thread-safe	CRITICAL

// BAD: Mutable static field
public class Config {
    public static String apiUrl;  // CRITICAL: Thread-unsafe
    public static List<String> allowedHosts = new ArrayList<>();  // CRITICAL
}

// GOOD: Immutable static
public class Config {
    public static final String API_URL = "https://api.example.com";
    public static final List<String> ALLOWED_HOSTS = List.of("a.com", "b.com");
}

// BAD: Static DateFormat (not thread-safe)
public class DateUtils {
    private static final SimpleDateFormat FORMAT = new SimpleDateFormat("yyyy-MM-dd");
}

// GOOD: ThreadLocal or DateTimeFormatter
public class DateUtils {
    private static final DateTimeFormatter FORMAT = DateTimeFormatter.ofPattern("yyyy-MM-dd");
}

High: Raw Types (No Generics)

Pattern	Issue	Severity
List instead of List<T>	Type safety lost	MEDIUM
Map instead of Map<K, V>	Unchecked operations	MEDIUM
Class instead of Class<?>	Wildcard missing	LOW

// BAD: Raw types
List users = new ArrayList();
users.add("string");  // No compile error, runtime issue
User user = (User) users.get(0);  // ClassCastException

// GOOD: Generics
List<User> users = new ArrayList<>();
users.add(new User("John"));  // Type-safe
User user = users.get(0);  // No cast needed

// BAD: Raw Map
Map config = new HashMap();
config.put("key", 123);
String value = (String) config.get("key");  // ClassCastException

// GOOD: Typed Map
Map<String, Object> config = new HashMap<>();
// Or better:
Map<String, String> stringConfig = new HashMap<>();

High: Resource Management

Pattern	Issue	Severity
Manual close()	Leak on exception	HIGH
Finally without null check	NPE risk	MEDIUM
Missing try-with-resources	Resource leak	HIGH

// BAD: Manual resource management
InputStream is = null;
try {
    is = new FileInputStream(file);
    process(is);
} finally {
    is.close();  // NPE if exception in constructor
}

// GOOD: try-with-resources
try (InputStream is = new FileInputStream(file)) {
    process(is);
}  // Auto-closed, even on exception

// GOOD: Multiple resources
try (
    Connection conn = dataSource.getConnection();
    PreparedStatement stmt = conn.prepareStatement(sql);
    ResultSet rs = stmt.executeQuery()
) {
    while (rs.next()) {
        process(rs);
    }
}

// Java 9+: Effectively final variables
InputStream is = new FileInputStream(file);
try (is) {  // Can use existing variable
    process(is);
}

High: Collection Null Returns

Pattern	Issue	Severity
Return null for empty	NPE risk	HIGH
null collection parameter	Hidden bug	MEDIUM
No defensive copy	Mutation leak	MEDIUM

// BAD: Return null for empty
public List<User> findUsers(String query) {
    List<User> results = repository.search(query);
    if (results.isEmpty()) {
        return null;  // Client must null-check
    }
    return results;
}

// GOOD: Return empty collection
public List<User> findUsers(String query) {
    List<User> results = repository.search(query);
    return results != null ? results : Collections.emptyList();
}

// BETTER: Use Optional for single values
public Optional<User> findUser(String id) {
    return Optional.ofNullable(repository.findById(id));
}

// BAD: No defensive copy
public List<String> getItems() {
    return items;  // Caller can modify
}

// GOOD: Defensive copy or unmodifiable
public List<String> getItems() {
    return List.copyOf(items);  // Java 10+
    // or: return Collections.unmodifiableList(items);
}

Medium: String Concatenation in Loops

Pattern	Issue	Severity
+ in loop	O(n^2) complexity	MEDIUM
String.format in loop	Performance hit	LOW

// BAD: String concatenation in loop
String result = "";
for (String item : items) {
    result += item + ",";  // Creates new String each iteration
}

// GOOD: StringBuilder
StringBuilder sb = new StringBuilder();
for (String item : items) {
    sb.append(item).append(",");
}
String result = sb.toString();

// BETTER: String.join (Java 8+)
String result = String.join(",", items);

// BEST: Collectors.joining (streams)
String result = items.stream()
    .collect(Collectors.joining(","));

Medium: Stream Anti-patterns

Pattern	Issue	Severity
forEach with side effects	Not functional	MEDIUM
Nested streams	Performance	MEDIUM
Stream in loop	Create once	LOW
Parallel without need	Overhead	LOW

// BAD: forEach with mutation
List<User> activeUsers = new ArrayList<>();
users.stream()
    .filter(User::isActive)
    .forEach(activeUsers::add);  // Side effect

// GOOD: Collect
List<User> activeUsers = users.stream()
    .filter(User::isActive)
    .collect(Collectors.toList());

// Java 16+: toList()
List<User> activeUsers = users.stream()
    .filter(User::isActive)
    .toList();

// BAD: Unnecessary parallel
users.parallelStream()  // Only 10 users
    .map(User::getName)
    .collect(Collectors.toList());

// GOOD: Sequential for small collections
users.stream()
    .map(User::getName)
    .collect(Collectors.toList());

Low: Missing @Override

Pattern	Issue	Severity
No @Override on override	Typo not caught	LOW
Missing equals/hashCode	Collection issues	MEDIUM

// BAD: Missing @Override
public class User {
    public boolean equals(Object obj) {  // Typo not caught
        // ...
    }
}

// GOOD: With @Override
public class User {
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        User user = (User) obj;
        return Objects.equals(id, user.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

Modern Java Features (17+)

Old Pattern	Modern Alternative	Since
instanceof + cast	Pattern matching	Java 16
Switch statement	Switch expression	Java 14
Verbose records	record	Java 16
Text blocks (\n)	"""	Java 15
Sealed classes	sealed, permits	Java 17

// OLD: instanceof + cast
if (obj instanceof User) {
    User user = (User) obj;
    process(user.getName());
}

// NEW: Pattern matching (Java 16+)
if (obj instanceof User user) {
    process(user.getName());
}

// OLD: Switch statement
String status;
switch (code) {
    case 200:
        status = "OK";
        break;
    case 404:
        status = "Not Found";
        break;
    default:
        status = "Unknown";
}

// NEW: Switch expression (Java 14+)
String status = switch (code) {
    case 200 -> "OK";
    case 404 -> "Not Found";
    default -> "Unknown";
};

// OLD: Verbose DTO
public class UserDto {
    private final String name;
    private final int age;
    // constructor, getters, equals, hashCode, toString
}

// NEW: Record (Java 16+)
public record UserDto(String name, int age) {}

// Text blocks (Java 15+)
String json = """
    {
        "name": "John",
        "age": 30
    }
    """;

Response Template

## Java Code Review Results

**Project**: [name]
**Java Version**: 17 / 21
**Build Tool**: Maven / Gradle

### Exception Handling

#### CRITICAL
| File | Line | Issue |
|------|------|-------|
| Service.java | 45 | Empty catch block |
| Utils.java | 78 | Catching Throwable |

#### HIGH
| File | Line | Issue |
|------|------|-------|
| Dao.java | 23 | Catching generic Exception |
| Config.java | 12 | Mutable static field |

### Resource Management
| File | Line | Issue |
|------|------|-------|
| FileService.java | 56 | Missing try-with-resources |

### Modern Java Adoption
- [ ] Use pattern matching instanceof (Java 16+)
- [ ] Convert DTOs to records
- [ ] Use switch expressions
- [ ] Use text blocks for multiline strings

### Recommendations
1. [ ] Replace empty catch blocks with proper handling
2. [ ] Use try-with-resources for all IO
3. [ ] Return empty collections instead of null
4. [ ] Enable preview features if using Java 21

### Positive Patterns
- Good use of Optional for nullable returns
- Proper use of streams for collection processing

Best Practices

1. Fail Fast: Validate inputs early
2. Immutability: Prefer final fields, records
3. Null Safety: Optional, @Nullable/@NonNull
4. Modern APIs: java.time, List.of, Map.of
5. Checked Exceptions: Use sparingly, wrap in runtime

Integration

• spring-boot-reviewer skill: Spring-specific patterns
• orm-reviewer skill: JPA/Hibernate patterns
• code-reviewer skill: General quality
• security-scanner skill: Security audit

Notes

• Based on Java 17+ best practices
• Encourages adoption of modern Java features
• Compatible with Maven and Gradle projects
• Works with Lombok and other code generators