# Java 21 Features and Modernization Guide

## Java 21 Language Features

### Pattern Matching for Switch

- **Description**: Enhanced switch expressions with pattern matching
- **Use Case**: Simplify conditional logic and type checking
- **Example**:
  ```java
  switch (obj) {
      case String s -> System.out.println("String: " + s);
      case Integer i -> System.out.println("Integer: " + i);
      case null -> System.out.println("Null value");
      default -> System.out.println("Unknown type");
  }
  ```

### Records

- **Description**: Immutable data classes with automatic methods
- **Use Case**: Replace traditional POJOs and DTOs
- **Example**:
  ```java
  public record User(String name, String email, int age) {
      public User {
          if (age < 0) throw new IllegalArgumentException("Age cannot be negative");
      }
  }
  ```

### Sealed Classes

- **Description**: Restrict which classes can extend a class
- **Use Case**: Model domain hierarchies and improve type safety
- **Example**:

  ```java
  public sealed class Shape permits Circle, Rectangle, Triangle {
      public abstract double area();
  }

  public final class Circle extends Shape {
      private final double radius;
      public double area() { return Math.PI * radius * radius; }
  }
  ```

### Text Blocks

- **Description**: Multi-line string literals
- **Use Case**: SQL queries, JSON, HTML templates
- **Example**:
  ```java
  String sql = """
      SELECT u.id, u.name, u.email
      FROM users u
      WHERE u.active = true
      AND u.created_date > ?
      """;
  ```

### Virtual Threads (Project Loom)

- **Description**: Lightweight threads for high-concurrency applications
- **Use Case**: I/O-bound applications, microservices
- **Example**:
  ```java
  try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
      executor.submit(() -> {
          // I/O operation
          return processData();
      });
  }
  ```

## Performance Improvements

### Garbage Collection Enhancements

- **ZGC**: Low-latency garbage collector
- **G1GC**: Improved performance and predictability
- **SerialGC**: Better performance for small applications
- **ParallelGC**: Enhanced throughput

### JVM Optimizations

- **AOT Compilation**: Ahead-of-time compilation support
- **JIT Improvements**: Better just-in-time compilation
- **Memory Management**: Improved memory allocation and deallocation
- **Threading**: Enhanced thread management

## Security Enhancements

### Cryptographic Updates

- **TLS 1.3**: Enhanced security protocols
- **Cryptographic Algorithms**: Updated and improved algorithms
- **Key Management**: Better key handling and storage
- **Secure Random**: Improved random number generation

### Security Headers

- **HTTP Security Headers**: Enhanced security header support
- **Content Security Policy**: Better CSP implementation
- **CORS**: Improved Cross-Origin Resource Sharing
- **Authentication**: Enhanced authentication mechanisms

## API Improvements

### Collections Framework

- **Immutable Collections**: Factory methods for immutable collections
- **Stream API**: Enhanced stream operations
- **Optional**: Improved Optional handling
- **CompletableFuture**: Better async programming support

### I/O Improvements

- **NIO.2**: Enhanced file system operations
- **HTTP Client**: Built-in HTTP client
- **Process API**: Improved process management
- **File I/O**: Better file handling and operations

## Migration Strategies

### From Java 8 to Java 21

1. **Update Dependencies**: Check compatibility with Java 21
2. **Modernize Code**: Use new language features
3. **Update Build Tools**: Maven/Gradle updates
4. **Test Thoroughly**: Comprehensive testing
5. **Performance Testing**: Validate performance improvements

### From Java 11 to Java 21

1. **Language Features**: Adopt new language features
2. **Performance Optimization**: Leverage performance improvements
3. **Security Updates**: Implement security enhancements
4. **API Updates**: Use improved APIs
5. **Testing**: Validate all changes

### From Java 17 to Java 21

1. **New Features**: Implement new language features
2. **Performance Tuning**: Optimize for Java 21
3. **Security Hardening**: Implement security improvements
4. **Monitoring**: Update monitoring and observability
5. **Documentation**: Update documentation and guides

## Best Practices

### Code Modernization

- **Use Records**: Replace POJOs with records
- **Pattern Matching**: Simplify conditional logic
- **Text Blocks**: Use for multi-line strings
- **Sealed Classes**: Model domain hierarchies
- **Virtual Threads**: Use for I/O-bound operations

### Performance Optimization

- **JVM Tuning**: Optimize JVM parameters
- **Garbage Collection**: Choose appropriate GC
- **Memory Management**: Optimize memory usage
- **Threading**: Use virtual threads for I/O
- **Caching**: Implement effective caching strategies

### Security Implementation

- **Encryption**: Use updated cryptographic algorithms
- **Authentication**: Implement secure authentication
- **Authorization**: Use proper authorization mechanisms
- **Input Validation**: Validate all inputs
- **Security Headers**: Implement security headers

### Testing Strategies

- **Unit Testing**: Comprehensive unit test coverage
- **Integration Testing**: Test all integrations
- **Performance Testing**: Validate performance improvements
- **Security Testing**: Test security implementations
- **Compatibility Testing**: Test compatibility with dependencies

## Migration Checklist

### Pre-Migration

- [ ] Analyze current Java version and features
- [ ] Check dependency compatibility
- [ ] Plan migration strategy
- [ ] Set up testing environment
- [ ] Create rollback plan

### During Migration

- [ ] Update Java version
- [ ] Update dependencies
- [ ] Modernize code
- [ ] Update build configuration
- [ ] Test thoroughly

### Post-Migration

- [ ] Validate functionality
- [ ] Performance testing
- [ ] Security testing
- [ ] Documentation updates
- [ ] Team training

## Common Issues and Solutions

### Compatibility Issues

- **Dependency Conflicts**: Resolve version conflicts
- **API Changes**: Update deprecated APIs
- **Build Tool Issues**: Update Maven/Gradle
- **IDE Issues**: Update IDE and plugins

### Performance Issues

- **Memory Usage**: Optimize memory allocation
- **Garbage Collection**: Tune GC parameters
- **Threading**: Optimize thread usage
- **I/O Operations**: Optimize I/O operations

### Security Issues

- **Cryptographic Updates**: Update crypto algorithms
- **Security Headers**: Implement security headers
- **Authentication**: Update authentication mechanisms
- **Authorization**: Implement proper authorization