Configure swagger with spring boot

If you haven’t starting working with spring boot yet, you will quickly find that it pulls out all the common configuration from across your applications. Recently I helped in an effort to modularize configuration and worked on creating a spring-boot-swagger starter project. This project, like other boot modules, would be included in the pom and you would enjoy the fruits of auto configuration.

If you aren't familiar with swagger, swagger is a "specification and complete framework implementation for describing, producing, consuming, and visualizing RESTful web services". It has various integration points with java back end technologies (JAX-RS, scala, php, nodejs, etc) but even better it has separated the front end into a project called swagger-ui. So as long as your service complies to the spec you can have a pretty UI on top of your rest services. It does lack official support spring MVC though there are various projects on github that attempt to integrate the two, even I started a project. Also there has been talk in spring’s JIRA about swagger support.

We went with swagger-springmc simply due to the number of watchers/stars and it fit in pretty good with little effort. It can be integrated with traditional spring XML or through java config both ways requiring two properties to be set to specify the API version and the base path. With the direction of spring-boot, we wanted to abstract the dependency and defaults so consuming apps would just have to plug it in to the pom.xml. It lead us down a few options which we described below and in the end landed on #3 of Injecting ConfigurableApplicationContext in one of our auto configuration classes.

Option 1 - PropertyPlaceholderConfigurer

As martypitt / swagger-springmvc states in the documentation you can configure the properties programmatically with the PropertyPlaceholderConfigurer. We need the properties in spring's Environment class, a class that manages profiles and properties, but PropertyPlaceholderConfigurer is not able to add properties to environment. PropertySourcesPlaceholderConfigurer uses Environment and not vice versa. Also, we wanted this in the spring boot module, not all consuming rest applications. In case this works for you, this is what the configuration would look like:

 @Bean
public static PropertyPlaceholderConfigurer swaggerProperties() throws UnknownHostException {

    // Swagger expects these to property values to be replaced. We don't want to propagate these to consumers of
    // this configuration, so we derive reasonable defaults here and configure the properties programmatically.
    Properties properties = new Properties();
    properties.setProperty("documentation.services.basePath", servletContext.getContextPath());
    // this property will be overridden at runtime, so the value here doesn't matter
    properties.setProperty("documentation.services.version", "REPLACE-ME");

    PropertyPlaceholderConfigurer configurer = new PropertyPlaceholderConfigurer();
    configurer.setProperties(properties);
    configurer.setIgnoreUnresolvablePlaceholders(true);
    return configurer;
}

Option 2 - Implementing ApplicationContextInitializer

ApplicationContextInitializer is a call back interface for initializing ConfigurableApplicationContext prior to being refreshed typically used within web applications that require some programmatic initialization of the application context. So this would work great, on start up we can set the two properties but what it doesn't give us is the ability to get hold of a ServletContext through ServletContextAware because it fires to early on in the spring lifecycle.

So if your application will deploy to a single environment or if you want to require each of your applications to specify the properties required in the application.properties file, this solution might work for you. It would look something like this:

In your spring.factories within your boot starter project you would need to add:

org.springframework.context.ApplicationContextInitializer=com.levelup.SomeInitializer

Then add the following class

public class SwaggerApplicationContextInitializer implements
        ApplicationContextInitializer<ConfigurableApplicationContext> {

    @Override
    public void initialize(ConfigurableApplicationContext applicationContext) {

        MutablePropertySources propertySources = applicationContext
                .getEnvironment().getPropertySources();
        
        Map<String, Object> propertyMap = new HashMap<String, Object>();
        propertyMap.put("documentation.services.basePath",
                "http://localhost:8080/participant");
        propertyMap.put("documentation.services.version", "REPLACE-ME");

        propertySources.addFirst(new MapPropertySource(
                "documentation.services", propertyMap));

    }
}

Option 3 - Injecting ConfigurableApplicationContext

At the end of the day this solution may not be spring certified and there probably another way to do it but worked. ConfigurableApplicationContext class provides facilities to configure an application context in addition to the application context client methods. So we created a class that implements InitializingBean that allows us to react once all their properties have been set. It allows us to configure the properties need and then add them to the environment so when the DocumentationConfig looks for them, they are available.

@Configuration
@AutoConfigureBefore(SwaggerAutoConfiguration.class)
@EnableConfigurationProperties({ SwaggerBeanProperties.class })
public class SwaggerPropertiesAutoConfiguration implements InitializingBean,
        ServletContextAware {

    private ServletContext servletContext;

    @Autowired 
    ConfigurableApplicationContext applicationContext;
    
    @Autowired
    SwaggerBeanProperties swaggerBeanProperties;

    @Override
    public void afterPropertiesSet() throws Exception {

        MutablePropertySources propertySources = applicationContext
                .getEnvironment().getPropertySources();

        Map<String, Object> propertyMap = new HashMap<String, Object>();
        propertyMap.put("documentation.services.basePath",
                getDocumentBasePath());

        propertyMap.put("documentation.services.version", getDocumentVersion());

        propertySources.addFirst(new MapPropertySource(
                "documentation.services", propertyMap));

    }

    private String getDocumentBasePath() {
        
        if (swaggerBeanProperties.getBasePath() != null) {
            return swaggerBeanProperties.getBasePath();
        } else {
            return servletContext.getContextPath();
        }
    }

    private String getDocumentVersion() {

        if (swaggerBeanProperties.getVersion() != null) {
            return swaggerBeanProperties.getVersion();
        } else {
            return "1.0";
        }
    }

    @Override
    public void setServletContext(ServletContext servletContext) {
        this.servletContext = servletContext;
    }
}

We all have heard it before, "we don't want the base configuration" so we created a @ConfigurationProperties class that will look for the properties and will be injected into SwaggerPropertiesAutoConfiguration above. If properties are set in the application we will apply them, otherwise we will look for the defaults in hopes to create consistency across all apps.

@AutoConfigureBefore(SwaggerPropertiesAutoConfiguration.class)
@ConfigurationProperties(name = "documentation.services", ignoreUnknownFields = true)
public class SwaggerBeanProperties  {
    
    private String basePath;
    private String version;
    
    public String getBasePath() {
        return basePath;
    }
    public void setBasePath(String basePath) {
        this.basePath = basePath;
    }
    public String getVersion() {
        return version;
    }
    public void setVersion(String version) {
        this.version = version;
    }
}

The last class was simply to import the DocumentationConfig.class

@Configuration
@Import(DocumentationConfig.class)
public class SwaggerAutoConfiguration  {
}

Finally, the spring.factories

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
boot.web.swagger.autoconfigure.SwaggerAutoConfiguration,\
boot.web.swagger.autoconfigure.SwaggerPropertiesAutoConfiguration

org.springframework.context.ApplicationContextInitializer=

Spring-boot was made to solve the "we have ever which way to configure an application" syndrome but occasionally you still might have get dirty. It might not be perfect but it works for now. If you have suggestions or other ways this could be handled, let me know.