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1. Overview

When writing a Spring Boot application, it's helpful to map configuration properties onto Java beans. What's the best way to document these properties, though?

In this tutorial, we’ll explore the Spring Boot Configuration Processor and the associated JSON metadata files that document each property's meaning, constraints, and so on.

2. Configuration Metadata

Most of the applications we work on as developers must be configurable to some extent. However, usually, we don’t really understand what a configuration parameter does, if it has a default value, if it's deprecated, and at times, we don't even know the property exists.

To help us out, Spring Boot generates configuration metadata in a JSON file, which gives us useful information on how to use the properties. So, the configuration metadata is a descriptive file which contains the necessary information for interaction with the configuration properties.

The really nice thing about this file is that IDEs can read it, too, giving us autocomplete of Spring properties, as well as other configuration hints.

3. Dependencies

In order to generate this configuration metadata, we’ll use the configuration processor from the spring-boot-configuration-processor dependency.

So, let’s go ahead and add the dependency as optional:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-configuration-processor</artifactId>
    <version>2.1.7.RELEASE</version>
    <optional>true</optional>
</dependency>

This dependency will provide us with a Java annotation processor invoked when we build our project. We’ll talk in detail about this later on.

It’s a best practice to add a dependency as optional in Maven in order to prevent @ConfigurationProperties from being applied to other modules that our project uses.

4. Configuration Properties Example

To see the processor in action, let’s imagine we have a few properties that we need to include in our Spring Boot application via a Java bean:

@Configuration
@ConfigurationProperties(prefix = "database")
public class DatabaseProperties {
	
    public static class Server {

        private String ip;
        private int port;

        // standard getters and setters
    }
	
    private String username;
    private String password;
    private Server server;
	
    // standard getters and setters
}

To do this, we'd use the @ConfigurationProperties annotation. The configuration processor scans for classes and methods with this annotation to access the configuration parameters and generate configuration metadata.

Let's add a couple of these properties into a properties file. In this case, we'll call it databaseproperties-test.properties:

#Simple Properties
database.username=baeldung
database.password=password

And, just to be sure, we'll also add a test to make sure that we are all lined up:

@RunWith(SpringRunner.class)
@SpringBootTest(classes = AnnotationProcessorApplication.class)
@TestPropertySource("classpath:databaseproperties-test.properties")
public class DatabasePropertiesIntegrationTest {

    @Autowired
    private DatabaseProperties databaseProperties;

    @Test
    public void whenSimplePropertyQueriedThenReturnsPropertyValue() 
      throws Exception {
        Assert.assertEquals("Incorrectly bound Username property", 
          "baeldung", databaseProperties.getUsername());
        Assert.assertEquals("Incorrectly bound Password property", 
          "password", databaseProperties.getPassword());
    }
    
}

We’ve also added the nested properties database.server.id and database.server.port via the inner class Server. We should add the inner class Server as well as a field server with its own getter and setter.

In our test, let’s do a quick check to make sure we can set and read successfully nested properties as well:

@Test
public void whenNestedPropertyQueriedThenReturnsPropertyValue() 
  throws Exception {
    Assert.assertEquals("Incorrectly bound Server IP nested property",
      "127.0.0.1", databaseProperties.getServer().getIp());
    Assert.assertEquals("Incorrectly bound Server Port nested property", 
      3306, databaseProperties.getServer().getPort());
}

Okay, now we're ready to use the processor.

5. Generating Configuration Metadata

We mentioned earlier that the configuration processor generates a file – it does this uses annotation processing.

So, after compiling our project, we'll see a file called spring-configuration-metadata.json inside target/classes/META-INF:

{
  "groups": [
    {
      "name": "database",
      "type": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties"
    },
    {
      "name": "database.server",
      "type": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties$Server",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties",
      "sourceMethod": "getServer()"
    }
  ],
  "properties": [
    {
      "name": "database.password",
      "type": "java.lang.String",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties"
    },
    {
      "name": "database.server.ip",
      "type": "java.lang.String",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties$Server"
    },
    {
      "name": "database.server.port",
      "type": "java.lang.Integer",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties$Server",
      "defaultValue": 0
    },
    {
      "name": "database.username",
      "type": "java.lang.String",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties"
    }
  ],
  "hints": []
}

Next, let's see how changing annotations on our Java beans affect the metadata.

5.1. Additional Information on Configuration Metadata

First, let’s add JavaDoc comments on Server.

Second, let’s give a default value to the database.server.port field and finally add the @Min and @Max annotations:

public static class Server {

    /**
     * The IP of the database server
     */
    private String ip;

    /**
     * The Port of the database server.
     * The Default value is 443.
     * The allowed values are in the range 400-4000.
     */
    @Min(400)
    @Max(800)
    private int port = 443;

    // standard getters and setters
}

If we check the spring-configuration-metadata.json file now, we'll see this extra information reflected:

{
  "groups": [
    {
      "name": "database",
      "type": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties"
    },
    {
      "name": "database.server",
      "type": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties$Server",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties",
      "sourceMethod": "getServer()"
    }
  ],
  "properties": [
    {
      "name": "database.password",
      "type": "java.lang.String",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties"
    },
    {
      "name": "database.server.ip",
      "type": "java.lang.String",
      "description": "The IP of the database server",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties$Server"
    },
    {
      "name": "database.server.port",
      "type": "java.lang.Integer",
      "description": "The Port of the database server. The Default value is 443.
        The allowed values are in the range 400-4000",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties$Server",
      "defaultValue": 443
    },
    {
      "name": "database.username",
      "type": "java.lang.String",
      "sourceType": "com.baeldung.autoconfiguration.annotationprocessor.DatabaseProperties"
    }
  ],
  "hints": []
}

We can check the differences with the database.server.ip and database.server.port fields. Indeed, the extra information is quite helpful. As a result, it's much easier for developers and IDEs to understand what each property does.

We should also make sure we trigger the build to get the updated file. In Eclipse, if we check the Build Automatically option, each save action will trigger a build. In IntelliJ, we should trigger the build manually.

5.2. Understanding the Metadata Format

Let’s have a closer look at the JSON metadata file and discuss its components.

Groups are higher-level items used to group other properties, without specifying a value itself. In our example, we have the database group, which is also the prefix of the configuration properties. We also have a server group, which we created via an inner class and groups ip and port properties.

Properties are configuration items for which we can specify a value. These properties are set in .properties or .yml files and can have extra information, like default values and validations, as we saw in the example above.

Hints are additional information to help the user set the property value. For example, if we have a set of allowed value for a property, we can provide a description of what each of them does. The IDE will provide auto-competition help for these hints.

Each component on the configuration metadata has its own attributes to explain in finer details the configuration properties.

6. Conclusion

In this article, we looked at the Spring Boot Configuration Processor and its ability to create configuration metadata. Using this metadata makes it a lot easier to interact with our configuration parameters.

We gave an example of a generated configuration metadata and explained in details its format and components.

We also saw how helpful the autocomplete support on our IDE can be.

As always, all of the code snippets mentioned in this article can be found on our GitHub repository.

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>> CHECK OUT THE COURSE
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