And most important, what's the reason behind the "Auto" prefix? Classes annotated with #Configuration rather than #AutoConfiguration are less automatic or something?
#Configuration is a spring framework annotation and not strictly bound to spring-boot. It was introduced when spring started to allow programmatic creation of spring-beans as to move forward from xml definitions of beans.
#AutoConfiguration is a spring-boot specific annotation not commonly available in spring framework. The reason it exists, is for external providers that cooperate with spring-boot to be able to mark some classes in some libraries they provide with this annotation as to inform spring-boot that those classes could be parsed and make some initializations during start up of spring application automatically.
So if some regular programmer that develops some application happens to have kafka in dependencies then some beans will automatically be created and added in application context and will be ready for the programmer to use, although he has not defined any configuration for them. Spring-boot already knows this as the kafka provider has already informed by marking some class in the jar they provide with the annotation #AutoConfiguration.
For this reason #AutoConfiguration has some more powerful configurations available as before, after, beforeName, afterName as to allow the provider to specify when the configuration is applied during application startup if some order is necessary.
So this annotation is not to be used from some regular programmer that develops an application using spring-boot. It is for someone that develops a library that other users might use with spring-boot. One such example is kafka library.
For this to work in a spring-boot project #EnableAutoConfiguration is needed as well, to enable auto configuration.
From spring documentation
Spring Boot auto-configuration attempts to automatically configure
your Spring application based on the jar dependencies that you have
added. For example, if HSQLDB is on your classpath, and you have not
manually configured any database connection beans, then Spring Boot
auto-configures an in-memory database.
#Configuration instead is to be used from some regular programmer that develops an application using spring-boot or spring-framework as to inform the framework for which beans should be created and how.
#AutoConfiguration was introduced in 2.7 with the idea to mark all auto-configurations with its dedicated annotation and move away from spring.factories for auto-configuration imports in 3.0 as described in Github issue.
According to Spring documentation:
[#AutoConfiguration] indicates that a class provides configuration that can be
automatically applied by Spring Boot. Auto-configuration classes are
regular #Configuration with the exception that
Configuration#proxyBeanMethods() proxyBeanMethods is always false.
Usually, #AutoConfiguration classes automatically configure an application based on the dependencies that are present on the classpath. Those classes are generally marked as #ConditionalOnClass and #ConditionalOnMissingBean annotations that detect the presence or absence of specific classes.
Additionally, if a configuration needs to be applied in a specific order, you can use the before, beforeName, after, and afterName attributes on the #AutoConfiguration, unlike #Configuration which doesn't provide those attributes.
Related
I'm totally new to Kafka and terribly confused by this:
https://docs.spring.io/spring-kafka/reference/html/#with-java-configuration-no-spring-boot
I don't understand what that even means. What does "no spring boot mean" because that example sure as hell uses spring boot. I'm so confused....
EDIT
if I'm using SpringBoot and spring-kafka, should I have to manually create #Bean ConcurrentKafkaListenerContainerFactory as shown here. Most of the examples in the docs for setting up filtering / config / etc seem to use the "manual" configuration using #Bean. Is that "normal"? The docs are very confusing to me...especially this warning:
Spring for Apache Kafka is designed to be used in a Spring Application Context. For example, if you create the listener container yourself outside of a Spring context, not all functions will work unless you satisfy all of the …Aware interfaces that the container implements.
It's referring to the autowired configuration, as compared to putting each property in the config via HashMap/Properties in-code.
Also, it does not use #SpringBootApplication or SpringApplication.run, it just calls a regular main method using a hard-coded Config class.
Spring boot contains the functionality of AutoConfiguration
What this means is that spring boot when discovers some specific jar dependencies it knows, in the project, it automatically configures them to work on a basic level. This does not exist in simple Spring project where even if you add the dependency you have to also provide the configuration as to how it should work in your application.
This is also happening here with dependencies of Kafka. Therefore the documentation explains what more you have to configure if you don't have spring-boot with auto-configuration to make kafka work in a spring project.
Another question asked in comment is what happens in case you want some complex custom configuration instead of the automatic configuration provided while you are in a spring-boot app.
As documented
Auto-configuration tries to be as intelligent as possible and will
back-away as you define more of your own configuration. You can always
manually exclude() any configuration that you never want to apply (use
excludeName() if you don't have access to them). You can also exclude
them via the spring.autoconfigure.exclude property.
So if you want to have some complex configuration which is not automatically provided by spring-boot through some other mechanism like a spring-boot specific application property, then you can make your own configuration with your custom bean and then either automatic configuration from spring-boot for that class will back of as spring does several intelligent checks during application context set up or you will have to exclude the class from auto configuration manually.
In that case you could probably take as an example reference of how to register manually your complex configurations in spring boot what is documented on how to be done in non spring boot app. doc
Im trying to connect to Pivotal Cloud Cache server Regions.
I'm using #EnableClusterAware and trying to configure the client Regions via ClientRegionFactoryBean as below:
#Bean("clientRegion")
ClientRegionFactoryBean someClientRegion(GemFireCache gemfire) {
// ...
}
Either GemFireCache or ClientCache beans are not available with the annotation #EnableClusterAware. It's available only with #ClientCacheApplication.
Is there any annotations that needs to be used in conjunction with #EnableClusterAware so that GemFireCache gets injected?
Please help.
When Spring Boot for Apache Geode, or alternatively GemFire, (SBDG) is on the classpath of your Spring Boot application (see here), then SBDG auto-configuration will automatically create and configure a ClientCache instance for you (see here).
TIP: You can also review the Getting Stated Sample Guide and Source to see this behavior in action, for yourself. The Guide also talks about the use of the #EnableClusterAware annotation, here.
The #EnableClusterAware annotation is simply a development-time, SBDG annotation that enables you to switch between environments (e.g. local vs. managed, such as when pushing your Spring Boot application up to run in PCF, connected to PCC, and then testing locally in your IDE) without having to change any configuration (hence the goals).
The #EnableClusterAware annotation does not create any GemFire/Geode cache instances for you. Only SBDG's auto-configuration, or declaring an explicit SDG annotation (e.g. #ClientCacheApplication) will do that for you. Still, when using SBDG auto-configuration, you do not need an explicit SDG cache application annotation, like #ClientCacheApplication, since SBDG auto-configuration (again) creates and configures a ClientCache instance by default.
TIP: See the documentation on the #EnableClusterAware annotation for more details.
If ClientCache is not provided (auto-configured), then you are not:
Using SBDG, or rather do not have SBDG on the Spring Boot application classpath (see here)
Have not bootstrapped or configured and ran your Spring application with Spring Boot
Your Spring Boot application configuration is incorrect
You have explicitly disabled or overrode the SBDG auto-configuration
Etc.
One or more of these have to be true.
I have been using Spring Boot and Spring Cloud for a while, and Spring core for even longer, yet I cannot find a clear rule on which technique to use when creating a framework :
Spring Boot starter + spring.factories file
Spring Boot starter + #EnableXxx
both
For example :
the Eureka client starter requires users to import spring-cloud-starter-eureka-client AND to add #EnableEurekaClient on some configuration class
On the other hand, adding spring-boot-starter-jdbc on the classpath is enough to trigger data source auto configuration. There is no such thing as #EnableDataSourceAutoConfiguration
Somebody who wants to configure caching through configuration properties also needs to add #EnableCaching manually
Sometimes the #EnableXxx annotated class is included in the code brought by a starter, sometimes not.
I know #Enable... is the "pre Spring-Boot" method (technically a good-looking #Import), but the fact that #EnableEurekaClient exists is proof that it's not deprecated by any mean.
What's the rule there ? I can't find it in the docs.
There's no hard and fast rule (at least from the Spring Cloud perspective).
Dave Syer said:
A candidate rule of thumb is "if something is on the classpath but
provides multiple features, you need a way to switch on a subset"
I prefer to think of it as "revealing a conscious choice"
as opposed to "relying on magic"
I've kind of looked at as starting servers or doing work (like Discovery Client)
I work on a corporate Spring Boot extension that autoconfigures its own RabbitMQ clients. This extension (a starter) replaces the Spring Boot RabbitAutoConfiguration.
I know there are many ways to disable the RabbitAutoConfiguration :
in each Application (main) class with #EnableAutoConfiguration(exclude = RabbitAutoConfiguration.class) or #SpringBootApplication(exclude = RabbitAutoConfiguration.class)
in the application.properties or yml file (externalized or within the jar), with spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration
I wonder if there is a way that the presence of my new starter disables the Spring Boot RabbitAutoConfiguration.
I tried some dirty things, such as putting an application.properties with the exclude property in the corporate starter module, but as Spring Boot only reads one in the classpath, it can be easily overriden by one used in a client application. I do not want to impose some restrictions.
I do not like the idea of each application adding the same exclusion one way or the other (properties or annotation).
Any ideas ?
EDIT
I need to configure several RabbitMQ ConnectionFactory and RestTemplate within the same application.
If you want to truly replace the standard RabbitAutoConfiguration by yours, you just need to add #AutoconfigureBefore(RabbitAutoConfiguration.class) on your own auto-configuration to teach Spring Boot to process yours before the standard one.
If it is a replacement, yours will register beans that the standard auto-configuration will detect and it will back-off the same way as if you would have defined them manually.
Having said that, why are you doing this? I'd rather complement the existing auto-configuration rather than replacing the standard one. Is there a problem with the standard one? If so, we'd love to hear about it and adapt the code so that you don't have to fully replace it.
I have a web project, and I depoly it on tomcat easily. Infact I have a WebAppInitializer class that implements WebApplicationInitializer (this class it's really fat), as you know every application server that supports servlet 3.0, it can easily detect it and try to boot it. Now I wonder that it could be possible to use spring boot starter and without any further configuration, I pass my WebAppInitializer to it and spring boot based on my WebAppInitializer boots my project?
I just want to use the approach of spring-boot to deploy application on Tomcat and I don't want to use other spring-boot's facilities.
Yes, it's an old question. But I do not see an accepted answer and the one closest to a working one only has a link to an external resource. So here it is.
https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-embedded-container-context-initializer
28.4.2 Servlet Context Initialization
Embedded servlet containers do not directly execute the Servlet 3.0+
javax.servlet.ServletContainerInitializer interface or Spring’s
org.springframework.web.WebApplicationInitializer interface. This is
an intentional design decision intended to reduce the risk that third
party libraries designed to run inside a war may break Spring Boot
applications.
If you need to perform servlet context initialization in a Spring Boot
application, you should register a bean that implements the
org.springframework.boot.web.servlet.ServletContextInitializer
interface. The single onStartup method provides access to the
ServletContext and, if necessary, can easily be used as an adapter to
an existing WebApplicationInitializer.
Scanning for Servlets, Filters, and listeners
When using an embedded
container, automatic registration of classes annotated with
#WebServlet, #WebFilter, and #WebListener can be enabled by using
#ServletComponentScan.
[Tip] #ServletComponentScan has no effect in a standalone container,
where the container’s built-in discovery mechanisms are used instead.
I've tried it. It works
In my use case I have project containing a few dozens of webapps, designed to run on Tomcat as WAR. Lots of logics was neatly crafted into WebApplicationInitializers and it seemed there should be an easier way to reuse all this. Adding implements ServletContextInitializer to those initializers and exposing them as beans through #Configuration classes lit my webservers up with SpringBoot's embedded Tomcat.
http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto-create-a-deployable-war-file describes precisely how to do it
From the Spring Boot docs:
Servlet 3.0+ applications might translate pretty easily if they already use the Spring Servlet 3.0+ initializer support classes. Normally all the code from an existing WebApplicationInitializer can be moved into a SpringBootServletInitializer. If your existing application has more than one ApplicationContext (e.g. if it uses AbstractDispatcherServletInitializer) then you might be able to squash all your context sources into a single SpringApplication. The main complication you might encounter is if that doesn’t work and you need to maintain the context hierarchy. See the entry on building a hierarchy for examples. An existing parent context that contains web-specific features will usually need to be broken up so that all the ServletContextAware components are in the child context.
So yes, it's possible but you need to convert it to SpringBootServletInitializer, which seems to be quite similar.