Externalized Properties

A lightweight and extensible library to resolve application properties from various external sources.

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Twelve Factor Methodology

Externalized Properties was inspired by the The Twelve Factor Methodology's section III. Config.

The goal of this library is to make it easy for applications to implement configuration best practices by providing easy-to-use APIs as well as providing the flexibility to choose where to store their configurations/properties.

🛠️ Installation

Gradle

implementation "io.github.joel-jeremy.externalized-properties:externalized-properties-core:${version}"
// Optional/additional resolvers
implementation "io.github.joel-jeremy.externalized-properties:externalized-properties-database:${version}"
implementation "io.github.joel-jeremy.externalized-properties:externalized-properties-git:${version}"

Maven

<dependency>
  <groupId>io.github.joel-jeremy.externalized-properties</groupId>
  <artifactId>externalized-properties-core</artifactId>
  <version>${version}</version>
</dependency>
<!-- Optional/additional resolvers -->
<dependency>
  <groupId>io.github.joel-jeremy.externalized-properties</groupId>
  <artifactId>externalized-properties-database</artifactId>
  <version>${version}</version>
</dependency>
<dependency>
  <groupId>io.github.joel-jeremy.externalized-properties</groupId>
  <artifactId>externalized-properties-git</artifactId>
  <version>${version}</version>
</dependency>

🧩 Java 9 Module Names

Externalized Properties jars are published with Automatic-Module-Name manifest attribute:

• Core - io.github.joeljeremy.externalizedproperties.core
• Database Resolver - io.github.joeljeremy.externalizedproperties.database
• Git Resolver - io.github.joeljeremy.externalizedproperties.git

Module authors can use above module names in their module-info.java:

module foo.bar {
  requires io.github.joeljeremy.externalizedproperties.core;
  requires io.github.joeljeremy.externalizedproperties.database;
  requires io.github.joeljeremy.externalizedproperties.git;
}

🌟 Features

Externalized Properties takes full advantage of Java's Dynamic Proxies (Why Dynamic Proxies?).

✔️ Property Resolution

• Map Properties to (Java Dynamic Proxy) Interface Methods
• Default/Fallback Values
• Support for Property Names Known at Runtime
• Support for Various Configuration File/Resource Formats
• Caching
• Eager Loading
• Custom Resolvers

✔️ Conversion

• Automatic Property Conversion
• Conversion to Generic Types
• Conversion of Arbitrary Values
• Custom Converters

✔️ Variable Expansion

• Automatic Variable Expansion in Property Names
• Automatic Variable Expansion in Properties
• Variable Expansion in Arbitrary Values

✔️ Processing

• Targeted Processing of Properties
• Custom Processors

✔️ Profiles

• Profile-Specific Configurations

✔️ Ordinal Components

• Ordinal Resolvers
• Ordinal Converters

🏎️ Quick Start

Properties are mapped to proxy interface methods by using the @ExternalizedProperty annotation.

(For more advanced scenarios, please see the feature documentations.)

Given an interface:

public interface ApplicationProperties {
  @ExternalizedProperty("java.home")
  String javaHome();

  @ExternalizedProperty("encrypted.property")
  @Decrypt("MyDecryptor")
  String encryptedProperty();

  @ExternalizedProperty("java.version")
  int javaVersion();
}

We can initialize and start resolving external configurations/properties by:

public static void main(String[] args) {
  // 1. Configure and build the ExternalizedProperties instance.
  ExternalizedProperties externalizedProperties = buildExternalizedProperties();

  // 2. Initialize a proxy using the ExternalizedProperties.
  ApplicationProperties props = externalizedProperties.initialize(ApplicationProperties.class);

  // 3. Resolve the properties.
  String javaHome = props.javaHome();
  String encryptedProperty = props.encryptedProperty();
  int javaVersion = props.javaVersion();
}

private static ExternalizedProperties buildExternalizedProperties() {
  return ExternalizedProperties.builder()
      .defaults() 
      .resolvers(...)
      .processors(...)
      .converters(...) 
      .build();
}

🧪 Samples

More sample can be found here: https://github.com/joel-jeremy/externalized-properties-samples