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# Kafka Helper
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The `org.testcontainers.kafka.KafkaHelper` class provides shared constants and utility methods for modern Kafka containers (`KafkaContainer` and `ConfluentKafkaContainer`). This class is package-private and used internally by the container implementations.
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## Constants
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### KAFKA_PORT
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Default Kafka port used by modern containers.
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```java { .api }
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static final int KAFKA_PORT = 9092;
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```
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### STARTER_SCRIPT
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Path to the startup script used internally by containers.
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```java { .api }
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static final String STARTER_SCRIPT = "/tmp/testcontainers_start.sh";
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```
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### COMMAND
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Default command array used to start containers.
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```java { .api }
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static final String[] COMMAND = {
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    "sh",
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    "-c", 
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    "while [ ! -f " + STARTER_SCRIPT + " ]; do sleep 0.1; done; " + STARTER_SCRIPT
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};
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```
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### WAIT_STRATEGY
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Default wait strategy for container readiness.
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```java { .api }
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static final WaitStrategy WAIT_STRATEGY = Wait.forLogMessage(".*Transitioning from RECOVERY to RUNNING.*", 1);
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```
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## Utility Methods
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### envVars()
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Returns the default environment variables map for KRaft mode configuration.
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```java { .api }
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static Map<String, String> envVars();
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```
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**Returns**: Map containing default Kafka environment variables including:
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- `CLUSTER_ID`: Default cluster ID for KRaft mode
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- `KAFKA_LISTENERS`: Default listener configuration
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- `KAFKA_LISTENER_SECURITY_PROTOCOL_MAP`: Protocol mapping
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- `KAFKA_INTER_BROKER_LISTENER_NAME`: Inter-broker communication listener
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- `KAFKA_PROCESS_ROLES`: Process roles (broker,controller)
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- `KAFKA_CONTROLLER_LISTENER_NAMES`: Controller listener names
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- `KAFKA_NODE_ID`: Node identifier
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- `KAFKA_CONTROLLER_QUORUM_VOTERS`: Quorum voter configuration
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- Various replication and topic configuration settings
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### resolveListeners(GenericContainer<?> kafkaContainer, Set<String> listenersSuppliers)
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Resolves custom listeners and updates container configuration.
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```java { .api }
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static void resolveListeners(GenericContainer<?> kafkaContainer, Set<String> listenersSuppliers);
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```
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**Parameters**:
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- `kafkaContainer` (GenericContainer<?>): The Kafka container instance
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- `listenersSuppliers` (Set<String>): Set of listener strings in format "host:port"
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This method processes custom listeners, adds network aliases, and updates the container's `KAFKA_LISTENERS` and `KAFKA_LISTENER_SECURITY_PROTOCOL_MAP` environment variables.
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### resolveAdvertisedListeners(Set<Supplier<String>> listenerSuppliers)
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Resolves advertised listeners from supplier functions.
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```java { .api }
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static List<String> resolveAdvertisedListeners(Set<Supplier<String>> listenerSuppliers);
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```
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**Parameters**:
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- `listenerSuppliers` (Set<Supplier<String>>): Set of supplier functions providing listener strings
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**Returns**: List of advertised listener strings with protocol prefixes
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## Default Configuration
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The KafkaHelper provides these default configurations for modern containers:
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- **Default Cluster ID**: `"4L6g3nShT-eMCtK--X86sw"`
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- **Default Listeners**:
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  - `PLAINTEXT://0.0.0.0:9092` - External connections
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  - `BROKER://0.0.0.0:9093` - Inter-broker communication
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  - `CONTROLLER://0.0.0.0:9094` - KRaft controller
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- **Protocol Mapping**: Maps PLAINTEXT, BROKER, and CONTROLLER protocols
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- **Single Node Setup**: Configured for single-node testing scenarios with appropriate replication factors
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- **KRaft Mode**: Enabled by default with process roles set to "broker,controller"
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## Internal Usage
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This class is used internally by:
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- `org.testcontainers.kafka.KafkaContainer` - Apache Kafka containers
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- `org.testcontainers.kafka.ConfluentKafkaContainer` - Confluent Platform containers
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The helper ensures consistent configuration and behavior across both modern container implementations while providing shared functionality for listener resolution and environment variable setup.