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# Configuration Overrides
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Runtime configuration modification system enabling test-specific settings including random port assignment and property overrides without modifying configuration files. Essential for creating isolated test environments.
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## Capabilities
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### ConfigOverride
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Abstract base class providing configuration override functionality for modifying Dropwizard application settings at runtime during testing.
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```java { .api }
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/**
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 * Abstract base class for runtime configuration overrides
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 */
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public abstract class ConfigOverride {
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    // Static factory methods for simple key-value overrides
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    /**
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     * Create a simple configuration override with default prefix
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     * @param key Configuration property key (will be prefixed with "dw.")
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     * @param value Configuration property value
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     * @return ConfigOverride instance for the key-value pair
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     */
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    public static ConfigOverride config(String key, String value);
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    /**
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     * Create a configuration override with custom prefix
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     * @param propertyPrefix Custom prefix for the configuration property
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     * @param key Configuration property key
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     * @param value Configuration property value
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     * @return ConfigOverride instance for the prefixed key-value pair
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     */
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    public static ConfigOverride config(String propertyPrefix, String key, String value);
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    // Static factory methods for lazy-evaluated overrides
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    /**
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     * Create a configuration override with lazy-evaluated value
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     * @param key Configuration property key (will be prefixed with "dw.")
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     * @param value Supplier providing the configuration value when needed
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     * @return ConfigOverride instance with lazy evaluation
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     */
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    public static ConfigOverride config(String key, Supplier<String> value);
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    /**
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     * Create a configuration override with custom prefix and lazy evaluation
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     * @param propertyPrefix Custom prefix for the configuration property
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     * @param key Configuration property key
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     * @param value Supplier providing the configuration value when needed
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     * @return ConfigOverride instance with custom prefix and lazy evaluation
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     */
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    public static ConfigOverride config(String propertyPrefix, String key, Supplier<String> value);
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    // Static factory methods for random port assignment
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    /**
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     * Create random port assignments for server.applicationConnectors[0].port and 
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     * server.adminConnectors[0].port using default prefix
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     * @return ConfigOverride instance for random port assignment
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     */
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    public static ConfigOverride randomPorts();
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    /**
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     * Create random port assignments with custom property prefix
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     * @param propertyPrefix Custom prefix for port configuration properties
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     * @return ConfigOverride instance for random port assignment with custom prefix
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     */
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    public static ConfigOverride randomPorts(String propertyPrefix);
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    // Abstract lifecycle methods
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    /**
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     * Apply the configuration override to system properties
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     * Called before application startup
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     */
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    public abstract void addToSystemProperties();
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    /**
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     * Remove the configuration override from system properties
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     * Called after application shutdown for cleanup
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     */
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    public abstract void removeFromSystemProperties();
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    // Constants
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    /**
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     * Default property prefix used by Dropwizard configuration system
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     */
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    public static final String DEFAULT_PREFIX = "dw.";
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}
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```
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**Usage Examples:**
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```java
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// Simple property override
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ConfigOverride dbOverride = ConfigOverride.config("database.url", "jdbc:h2:mem:test");
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// Custom prefix override
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ConfigOverride customOverride = ConfigOverride.config("myapp.", "feature.enabled", "true");
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// Lazy evaluation for dynamic values
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ConfigOverride dynamicOverride = ConfigOverride.config("timestamp", 
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    () -> String.valueOf(System.currentTimeMillis()));
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// Random port assignment (most common use case)
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ConfigOverride randomPorts = ConfigOverride.randomPorts();
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// Random ports with custom prefix
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ConfigOverride customRandomPorts = ConfigOverride.randomPorts("myapp.");
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// Use with DropwizardTestSupport
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DropwizardTestSupport<MyConfiguration> testSupport = 
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    new DropwizardTestSupport<>(
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        MyApplication.class,
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        "test-config.yml",
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        ConfigOverride.randomPorts(),
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        ConfigOverride.config("database.url", "jdbc:h2:mem:test"),
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        ConfigOverride.config("redis.enabled", "false")
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    );
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// Multiple overrides for complex test scenarios
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ConfigOverride[] overrides = {
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    ConfigOverride.randomPorts(),
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    ConfigOverride.config("database.url", "jdbc:h2:mem:test"),
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    ConfigOverride.config("database.user", "sa"),
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    ConfigOverride.config("database.password", ""),
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    ConfigOverride.config("logging.level", "DEBUG"),
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    ConfigOverride.config("metrics.enabled", "false")
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};
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DropwizardTestSupport<MyConfiguration> testSupport = 
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    new DropwizardTestSupport<>(MyApplication.class, "test-config.yml", overrides);
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```
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### ConfigOverrideValue
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Concrete implementation of ConfigOverride for specific key-value configuration overrides. This class is package-private but is created by the ConfigOverride.config() factory methods.
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```java { .api }
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/**
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 * Concrete implementation for key-value configuration overrides
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 * Created by ConfigOverride.config() factory methods
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 */
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class ConfigOverrideValue extends ConfigOverride {
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    // Package-private implementation class
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    // Use ConfigOverride.config() factory methods to create instances
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}
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```
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### ConfigOverrideRandomPorts
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Concrete implementation of ConfigOverride for automatic random port assignment. This class is package-private but is created by the ConfigOverride.randomPorts() factory methods.
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```java { .api }
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/**
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 * Concrete implementation for random port assignment
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 * Created by ConfigOverride.randomPorts() factory methods
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 */
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class ConfigOverrideRandomPorts extends ConfigOverride {
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    // Package-private implementation class
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    // Use ConfigOverride.randomPorts() factory methods to create instances
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}
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```
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## Common Configuration Override Patterns
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### Database Configuration
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```java
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// H2 in-memory database for testing
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ConfigOverride.config("database.url", "jdbc:h2:mem:test"),
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ConfigOverride.config("database.user", "sa"),
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ConfigOverride.config("database.password", ""),
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ConfigOverride.config("database.driverClass", "org.h2.Driver")
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// PostgreSQL test database
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ConfigOverride.config("database.url", "jdbc:postgresql://localhost:5432/test_db"),
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ConfigOverride.config("database.user", "test_user"),
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ConfigOverride.config("database.password", "test_password")
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```
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### Logging Configuration
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```java
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// Enable debug logging for tests
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ConfigOverride.config("logging.level", "DEBUG"),
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ConfigOverride.config("logging.loggers.com.mycompany", "TRACE")
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// Disable logging for performance tests
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ConfigOverride.config("logging.level", "OFF")
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```
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### Feature Toggles
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```java
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// Disable external service integrations
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ConfigOverride.config("externalServices.enabled", "false"),
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ConfigOverride.config("caching.enabled", "false"),
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ConfigOverride.config("metrics.enabled", "false")
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// Enable test-specific features
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ConfigOverride.config("testMode.enabled", "true"),
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ConfigOverride.config("testData.autoLoad", "true")
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```
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### Network Configuration
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```java
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// Random ports (most common)
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ConfigOverride.randomPorts()
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// Specific ports for integration tests
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ConfigOverride.config("server.applicationConnectors[0].port", "8080"),
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ConfigOverride.config("server.adminConnectors[0].port", "8081")
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// Custom connector configurations
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ConfigOverride.config("server.applicationConnectors[0].type", "http"),
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ConfigOverride.config("server.applicationConnectors[0].bindHost", "localhost")
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```
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### Security Configuration
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```java
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// Disable authentication for testing
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ConfigOverride.config("auth.enabled", "false"),
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ConfigOverride.config("csrf.enabled", "false")
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// Test-specific authentication
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ConfigOverride.config("auth.testMode", "true"),
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ConfigOverride.config("auth.testUser", "test@example.com")
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```