Minimum version of spring for spring-boot v 1.3 - spring

Question

I have an existing spring application built using spring framework version 3.1.2. I am trying to create a spring-boot application out of this existing application, but getting some dependency issues. So just wondering, what is the spring framework version, that is supported by spring-boot v 1.3.0.
Or to put it in another words, is it possible to have a spring-boot application from a spring 3.1.2 based application?

Answer 1

Spring boot has hard dependencies on classes in Spring 4 and could not be configured to work with Spring 3. If you are really interested in using Spring Boot the only way you can do this is to follow a migration path to Spring 4 and then add Spring Boot to your application.
It is worth mentioning that the "boot" in Spring Boot is meant to be short for bootstrapping, as in initial setup of an application. I'm not saying there would be zero benefits from migrating from Spring 4 vanilla to Spring Boot. But make sure you are migrating for the right reasons the main purpose of Spring Boot is easy bootstrapping of applications but here are some other features which might be worth making the move.
Spring Boot dev-tools (Auto restart on code changes)
Awesome spring boot plugins for maven and gradle to ease upgrading spring in the future (hint it upgrades many other dependencies for you)
Bootstrapping new features such as MongoDb through auto-configuration.
Migration from 3.1 to 3.2
https://docs.spring.io/spring/docs/3.2.x/spring-framework-reference/html/migration-3.2.html
Migration from Spring 3 to Spring 4.
https://spring.io/blog/2014/01/30/migrating-from-spring-framework-3-2-to-4-0-1

Answer 2

There are many features in spring boot that are dependent upon new features added to Spring 4. One primary example is the new list of annotations added to Spring 4 that allow conditional wiring/loading of beans. Which is the primary method of wiring configurations in a plugin-like way.
For example lets see the AutoConfiguration class for the H2 console
https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/h2/H2ConsoleAutoConfiguration.java
The first thing we see is it's wired to be a Configuration class. It will only load if WebServlet.class is on the classpath and if the property spring.h2.console is = true. It is also configured to load SecurityAutoConfiguration first as this is a dependency at least for securing the h2 console page.
#Configuration
#ConditionalOnWebApplication
#ConditionalOnClass(WebServlet.class)
#ConditionalOnProperty(prefix = "spring.h2.console", name = "enabled", havingValue = "true", matchIfMissing = false)
#EnableConfigurationProperties(H2ConsoleProperties.class)
#AutoConfigureAfter(SecurityAutoConfiguration.class)
public class H2ConsoleAutoConfiguration {
When this Configuration is loaded it will check these conditions and upon all conditions being true then and only then will it load in the beans defined in the class. In this case it wires the h2console servlet.
#Bean
public ServletRegistrationBean h2Console() {
String path = this.properties.getPath();
String urlMapping = (path.endsWith("/") ? path + "*" : path + "/*");
return new ServletRegistrationBean(new WebServlet(), urlMapping);
}
There is also the security configuration in that class which introduces one more concept of conditionally loading a configuration based on another class being loaded into the context. These annotations do not always need to be on a Configuration level but can also apply to the bean level.
These concepts are core to how Spring Boot is implemented and therefore could not work with Spring 3.
Spring 3 list of annotations
http://docs.spring.io/spring/docs/3.0.x/javadoc-api/org/springframework/context/annotation/
Spring 4 Conditional Annotations
https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/context/annotation/

Answer 3

Thanks Zergleb for posting detailed answer. I found a way to run the spring 3 app as an independent jar by created an uber jar with a little instrumentation to bootstrap spring through a java class.
It is explained nicely in a short post at https://mihhaillapushkin.wordpress.com/2013/02/18/spring-3-for-standalone-applications