76. Spring Boot Application

This section includes topics relating directly to Spring Boot applications.

76.1 Create Your Own FailureAnalyzer

FailureAnalyzer is a great way to intercept an exception on startup and turn it into a human-readable message, wrapped in a FailureAnalysis. Spring Boot provides such an analyzer for application-context-related exceptions, JSR-303 validations, and more. You can also create your own.

AbstractFailureAnalyzer is a convenient extension of FailureAnalyzer that checks the presence of a specified exception type in the exception to handle. You can extend from that so that your implementation gets a chance to handle the exception only when it is actually present. If, for whatever reason, you cannot handle the exception, return null to give another implementation a chance to handle the exception.

FailureAnalyzer implementations must be registered in META-INF/spring.factories . The following example registers ProjectConstraintViolationFailureAnalyzer :

org.springframework.boot.diagnostics.FailureAnalyzer=\
com.example.ProjectConstraintViolationFailureAnalyzer

Note

If you need access to the BeanFactory or the Environment , your FailureAnalyzer can simply implement BeanFactoryAware or EnvironmentAware respectively.

76.2 Troubleshoot Auto-configuration

The Spring Boot auto-configuration tries its best to “do the right thing”, but sometimes things fail, and it can be hard to tell why.

There is a really useful ConditionEvaluationReport available in any Spring Boot ApplicationContext . You can see it if you enable DEBUG logging output. If you use the spring-boot-actuator (see the Actuator chapter), there is also a conditions endpoint that renders the report in JSON. Use that endpoint to debug the application and see what features have been added (and which have not been added) by Spring Boot at runtime.

Many more questions can be answered by looking at the source code and the Javadoc. When reading the code, remember the following rules of thumb:

  • Look for classes called *AutoConfiguration and read their sources. Pay special attention to the @Conditional* annotations to find out what features they enable and when. Add --debug to the command line or a System property -Ddebug to get a log on the console of all the auto-configuration decisions that were made in your app. In a running Actuator app, look at the conditions endpoint ( /actuator/conditions or the JMX equivalent) for the same information.

  • Look for classes that are @ConfigurationProperties (such as ServerProperties) and read from there the available external configuration options. The @ConfigurationProperties annotation has a name attribute that acts as a prefix to external properties. Thus, ServerProperties has prefix="server" and its configuration properties are server.port , server.address , and others. In a running Actuator app, look at the configprops endpoint.

  • Look for uses of the bind method on the Binder to pull configuration values explicitly out of the Environment in a relaxed manner. It is often used with a prefix.

  • Look for @Value annotations that bind directly to the Environment .

  • Look for @ConditionalOnExpression annotations that switch features on and off in response to SpEL expressions, normally evaluated with placeholders resolved from the Environment .

76.3 Customize the Environment or ApplicationContext Before It Starts

A SpringApplication has ApplicationListeners and ApplicationContextInitializers that are used to apply customizations to the context or environment. Spring Boot loads a number of such customizations for use internally from META-INF/spring.factories . There is more than one way to register additional customizations:

  • Programmatically, per application, by calling the addListeners and addInitializers methods on SpringApplication before you run it.

  • Declaratively, per application, by setting the context.initializer.classes or context.listener.classes properties.

  • Declaratively, for all applications, by adding a META-INF/spring.factories and packaging a jar file that the applications all use as a library.

The SpringApplication sends some special ApplicationEvents to the listeners (some even before the context is created) and then registers the listeners for events published by the ApplicationContext as well. See “Section 23.5, “Application Events and Listeners”” in the ‘Spring Boot features’ section for a complete list.

It is also possible to customize the Environment before the application context is refreshed by using EnvironmentPostProcessor . Each implementation should be registered in META-INF/spring.factories , as shown in the following example:

org.springframework.boot.env.EnvironmentPostProcessor=com.example.YourEnvironmentPostProcessor

The implementation can load arbitrary files and add them to the Environment . For instance, the following example loads a YAML configuration file from the classpath:

public class EnvironmentPostProcessorExample implements EnvironmentPostProcessor {

	private final YamlPropertySourceLoader loader = new YamlPropertySourceLoader();

	@Override
	public void postProcessEnvironment(ConfigurableEnvironment environment,
			SpringApplication application) {
		Resource path = new ClassPathResource("com/example/myapp/config.yml");
		PropertySource<?> propertySource = loadYaml(path);
		environment.getPropertySources().addLast(propertySource);
	}

	private PropertySource<?> loadYaml(Resource path) {
		if (!path.exists()) {
			throw new IllegalArgumentException("Resource " + path + " does not exist");
		}
		try {
			return this.loader.load("custom-resource", path).get(0);
		}
		catch (IOException ex) {
			throw new IllegalStateException(
					"Failed to load yaml configuration from " + path, ex);
		}
	}

}

Tip

The Environment has already been prepared with all the usual property sources that Spring Boot loads by default. It is therefore possible to get the location of the file from the environment. The preceding example adds the custom-resource property source at the end of the list so that a key defined in any of the usual other locations takes precedence. A custom implementation may define another order.

Warning

While using @PropertySource on your @SpringBootApplication may seem to be a convenient and easy way to load a custom resource in the Environment , we do not recommend it, because Spring Boot prepares the Environment before the ApplicationContext is refreshed. Any key defined with @PropertySource is loaded too late to have any effect on auto-configuration.

76.4 Build an ApplicationContext Hierarchy (Adding a Parent or Root Context)

You can use the ApplicationBuilder class to create parent/child ApplicationContext hierarchies. See “Section 23.4, “Fluent Builder API”” in the ‘Spring Boot features’ section for more information.

76.5 Create a Non-web Application

Not all Spring applications have to be web applications (or web services). If you want to execute some code in a main method but also bootstrap a Spring application to set up the infrastructure to use, you can use the SpringApplication features of Spring Boot. A SpringApplication changes its ApplicationContext class, depending on whether it thinks it needs a web application or not. The first thing you can do to help it is to leave server-related dependencies (e.g. servlet API) off the classpath. If you cannot do that (for example, you run two applications from the same code base) then you can explicitly call setWebApplicationType(WebApplicationType.NONE) on your SpringApplication instance or set the applicationContextClass property (through the Java API or with external properties). Application code that you want to run as your business logic can be implemented as a CommandLineRunner and dropped into the context as a @Bean definition.