Value Injection

The light-4j framework provides a powerful mechanism to inject values into configuration files from external sources, such as environment variables or a centralized values.yml file. This allows for dynamic and flexible configuration management without modifying the application code.

Basic Usage

Property injection uses the ${key:defaultValue} pattern. When a configuration file is loaded, any property with this pattern will be replaced by:

1. Environment Variable: If an environment variable matching the key (often converted to uppercase with underscores) exists, its value is used.
2. values.yml: If the key is defined in the values.yml file, its value is used.
3. Default Value: If neither is found, the defaultValue is used.

Example:

server.port: ${PORT:8080}

Centralized values.yml

The values.yml file (also supports .yaml or .json) acts as a central repository for all injected values. It is typically located in the same config directory as other configuration files.

When using values.yml, you should prefix keys with the name of the configuration file they correspond to, although this is not strictly required if you use generic keys.

Example values.yml:

server.serviceId: com.networknt.gateway-1.0.0
router.maxRequestTime: 5000
db.user: root
db.password: ${DB_PASS:password}

Self-Injection in values.yml

A recent enhancement allows values.yml to support property injection from its own entries. This enables you to reuse values and maintain consistency within the centralized configuration file.

Implementation Details

To support self-injection, the ConfigInjection class follows a two-pass initialization strategy:

1. Exclusion: values.yml is explicitly excluded from the standard injection process during its initial load. This ensures it is loaded as a raw map, preventing any “cannot be expanded” exceptions during startup.
2. Pass 1: The raw values are loaded into the internal decryptedValueMap and undecryptedValueMap.
3. Pass 2: Once the maps are assigned to the static fields, the system performs a second pass using CentralizedManagement.mergeMap. During this pass, ${key} patterns are resolved against the internal maps, allowing for recursive resolution of self-references and environment variables.

Example of Self-Injection

# values.yml
db.host: localhost
db.user: root
db.url: jdbc:mysql://${db.host}:3306/mydb?user=${db.user}

In this example, db.url correctly resolves to jdbc:mysql://localhost:3306/mydb?user=root during application startup.

Exclusion List

If you want to prevent certain configuration files from being processed for injection, you can add them to the exclusionConfigFileList in config.yml.

# config.yml
exclusionConfigFileList:
  - openapi
  - values

Note

values.yml and config.yml are automatically excluded from the initial injection pass by the framework to maintain system stability.

Decryption

Value injection also supports automatic decryption of values. If a value is encrypted (usually prefixed with CRYPT:), it will be decrypted using the configured Decryptor before being injected.