How Properties Work in Spring Boot

How Properties Work in Spring Boot | Generated by AI

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Let’s explore how properties work in Spring Boot, including how to use YAML or .properties files, how to override them with command-line arguments like -D, the various ways to initialize properties, and how to test them effectively.

How Properties Work in Spring Boot

Spring Boot provides a flexible system for managing application configuration through properties, which are key-value pairs. These properties can be defined in multiple sources, and Spring Boot resolves them based on a specific order of precedence. This allows you to customize your application for different environments or deployment scenarios. Properties are loaded into the Spring Environment, making them accessible throughout your application.

The main sources of properties include:

Configuration files (e.g., application.properties or application.yml)
Command-line arguments (e.g., --server.port=8081)
System properties (e.g., -Dserver.port=8081)
Environment variables
Java code (e.g., via @Value or @ConfigurationProperties)

Using YAML or Properties Files

Spring Boot supports two primary formats for configuration files, both typically placed in src/main/resources:

1. `.properties` Files

This is a simple, flat key-value format:

server.port=8080
spring.datasource.url=jdbc:mysql://localhost:3306/mydb

2. `.yml` or `.yaml` Files

This is a structured, hierarchical format using indentation:

server:
  port: 8080
spring:
  datasource:
    url: jdbc:mysql://localhost:3306/mydb

Key Points:

Use .properties for simple configurations and .yml for nested or complex setups.
Profile-specific files (e.g., application-dev.yml) can be used for environment-specific settings.
Example: Setting server.port=8080 changes the port your Spring Boot application runs on.

Using Command-Line Arguments to Override Properties

You can override properties defined in configuration files using command-line arguments in two ways:

1. Using `--` for Spring Boot Properties

Pass properties directly when running the application:

java -jar myapp.jar --server.port=8081 --spring.datasource.url=jdbc:mysql://localhost:3306/testdb

These take precedence over configuration files.

2. Using `-D` for System Properties

Set system properties with -D, which Spring Boot also recognizes:

java -Dserver.port=8081 -Dspring.datasource.url=jdbc:mysql://localhost:3306/testdb -jar myapp.jar

System properties override configuration file values as well.

Different Ways to Initialize Properties

Spring Boot offers several methods to define or initialize properties beyond files and command-line arguments:

1. Environment Variables

Properties can be set via environment variables. For example:

Set SERVER_PORT=8081 in your environment, and Spring Boot maps it to server.port.
Naming Convention: Convert property names to uppercase and replace dots (.) with underscores (_), e.g., spring.datasource.url becomes SPRING_DATASOURCE_URL.

2. Java Code

You can initialize properties programmatically:

Using @Value: Inject a specific property into a field.
```
@Value("${server.port}")
private int port;
```

Using @ConfigurationProperties: Bind a group of properties to a Java object.

@Component
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private String name;
    // getters and setters
}

This binds properties like app.name to the name field.

3. Default Values

Provide fallback values if a property isn’t defined:

In @Value: @Value("${server.port:8080}") uses 8080 if server.port is unset.
In configuration files: Set defaults in application.properties or YAML.

Property Precedence

Spring Boot resolves properties from multiple sources in this order (higher precedence overrides lower):

Command-line arguments (--property=value)
System properties (-Dproperty=value)
Environment variables
Configuration files (application.properties or application.yml)
Default values in code

Example: If server.port=8080 is in application.properties but you run java -jar myapp.jar --server.port=8081, the port will be 8081.

How to Test Properties

Testing properties ensures your application behaves as expected. Here are common approaches:

1. Using `@TestPropertySource`

Override properties in unit tests:

@SpringBootTest
@TestPropertySource(properties = {"server.port=9090"})
public class MyTest {
    // Test code
}

2. Using `application-test.properties`

Place a test-specific file in src/test/resources:

server.port=9090

Spring Boot loads this automatically during tests.

3. Programmatic Overrides

Set properties in test setup:

@SpringBootTest
public class MyTest {
    @BeforeEach
    public void setup() {
        System.setProperty("server.port", "9090");
    }
}

4. Using `@ActiveProfiles`

Activate profile-specific properties:

Define application-test.yml with test settings.

Use:

@SpringBootTest
@ActiveProfiles("test")
public class MyTest {
    // Test code
}

Summary

Configuration Files: Use application.properties or application.yml for default settings.
Command-Line Arguments: Override with --property=value or -Dproperty=value.
Initialization Methods: Leverage environment variables, Java code (@Value, @ConfigurationProperties), or defaults.
Testing: Use @TestPropertySource, test-specific files, programmatic overrides, or profiles.

This covers the essentials of working with properties in Spring Boot. Let me know if you’d like more details!

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