Mastering JSON Configuration in Spring Boot 3: From CLI to Custom Property Sources

Environment: Spring Boot 3.4.0

1. Introduction

Spring Boot can load JSON configuration files via @ConfigurationProperties, binding a JSON file to a configuration class. It also supports providing JSON data through the spring.application.json property or SPRING_APPLICATION_JSON environment variable, giving developers and operators flexible configuration options.

2. Practical Cases

2.1 Load JSON from the command line

Any property named spring.application.json or the SPRING_APPLICATION_JSON environment variable is parsed and added to the application environment at startup.

$ SPRING_APPLICATION_JSON="{\"pack\":{\"title\":\"xxxooo\"}}" java -jar myapp.jar

The above makes pack.title=xxxooo available in the Spring environment.

Loading JSON via system property:

java -Dspring.application.json="{\"pack\":{\"title\":\"xxxooo\"}}" -jar load_json-1.0.0.jar

Loading JSON via command‑line argument:

java -jar load_json-1.0.0.jar --spring.application.json="{\"pack\":{\"title\":\"xxxooo\"}}"

These simple methods work but lack scalability for large JSON payloads.

2.2 Load JSON with @PropertySource

Define a configuration class:

public class JsonProperties {
    private Integer port;
    private String host;
    // getters, setters
}

Create a JSON file configprops.json:

{
  "port": 6666,
  "host": "127.0.0.1"
}

Annotate the class:

@Component
@PropertySource("classpath:configprops.json")
@ConfigurationProperties
public class JsonProperties {}

When the application starts, an error occurs due to an extra comma. To fix it, implement a custom PropertySourceFactory:

public class JsonPropertySourceFactory implements PropertySourceFactory {
    @Override
    public PropertySource<?> createPropertySource(String name, EncodedResource resource) throws IOException {
        Map readValue = new ObjectMapper().readValue(resource.getInputStream(), Map.class);
        return new MapPropertySource("json-property", readValue);
    }
}

Update the annotation:

@Component
@PropertySource(value = "classpath:configprops.json", factory = JsonPropertySourceFactory.class)
@ConfigurationProperties
public class JsonProperties {}

Application output shows the JSON file was read successfully.

2.3 Nested JSON structures

For nested JSON, map the nested object to a Map field:

@Component
@PropertySource(value = "classpath:configprops.json", factory = JsonPropertySourceFactory.class)
@ConfigurationProperties
public class JsonProperties {
    private Integer port;
    private String host;
    private LinkedHashMap<String, ?> pack;
}

The pack node is bound to the LinkedHashMap.

2.4 Use ApplicationContextInitializer

For full control, implement a custom ApplicationContextInitializer that reads the JSON file and adds each entry as a property with a custom. prefix.

public class JsonPropertyContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {
    private static final String CUSTOM_PREFIX = "custom.";
    @Override
    public void initialize(ConfigurableApplicationContext ctx) {
        try {
            Resource resource = ctx.getResource("classpath:configprops.json");
            Map<String, Object> map = new ObjectMapper().readValue(resource.getInputStream(), Map.class);
            map.forEach((k, v) -> ctx.getEnvironment().getPropertySources()
                .addFirst(new MapPropertySource(CUSTOM_PREFIX + k, Collections.singletonMap(CUSTOM_PREFIX + k, v))));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

Inject the properties with @Value("${custom.port}") and @Value("${custom.host}").

2.5 Direct Bean definition

Read the JSON file with ObjectMapper in a @Bean method:

@Configuration
public class AppConfig {
    @Bean
    public AppProperties appProperties() throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        return mapper.readValue(new ClassPathResource("configprops.json").getInputStream(), AppProperties.class);
    }
}

This completes all methods for loading JSON configuration in Spring Boot.