What is the exact meaning of invasive? and what makes Spring non-invasive?

check http://forum.springsource.org/showthread.php?27846-Spring-is-non-invasive

What does it really means?

"You are not forced to import or extend any Spring APIs, example Struts forces you to extend Action".

Of course in some areas, such as the web framework, it's impossible to avoid aplication code depending on the framework. But Spring consistently attempts to reach the non-invasive ideal where configuration management is concerned

Most of the java frameworks force you to extend one of their classes or implement one of their interfaces. Example of such an invasive programming model was EJB 2-era stateless session beans, earlier versions of Struts, WebWork, Tapestry.

Spring avoids littering your code with its API. Spring never forces you to implement a Spring-specific interface or extend a Spring-specific class. Instead, the classes often don’t have any indication that they’re being used by Spring.

public class HelloWorld {
    public String hello() {
      return "Hello World";
    }
}

This is a simple, Java class(POJO). Nothing indicates that it's a Spring component. Spring is non-invasive means this class could function equally well in a Spring application as it could in a non-Spring application. Although POJOs are simple they can be powerful.

One of the ways POJOs can become powerful in Spring is by using Dependency Injection.

Spring can be invasive and non-invasive, it's only up to you.

Non-invansive spring doesn't use annotations (for example @Autowired) nor its own classes (for example JdbcTemplate), it only uses configuration to wire your beans (simple POJO) together (you still have to initialize spring somehow in your code, which is a little bit invasive anyway). But you are free to drop spring without any significant code changes. Invasive spring, on the other hand, provides you template classes (for persistence, web services,...), annotations and other stuff you cannot simply leave behind without refactoring (if you use them).

EDIT: Some say spring is not invasive, because it doesn't force you to implement interfaces nor extend classes. For me a framework is not invasive, if it can be easily replaced.

If an IoC container is invasive, it means your code needs to be explicitly aware of dependency injection. For example, in Guice you use the @Inject annotation (and others). These annotations are more standardized than they used to be, which is good - it means with a single set of annotations you can (at least in theory) make your code available for use with various different invasive IoC containers.

With a non-invasive container, you can write your code with no reference to IoC at all... everything is just determined by reflection over members and annotations which would be present even if you weren't using IoC.

There are pros and cons of both invasive and non-invasive containers - being more specific in code can give you more control over some of the details of binding, for example - but it's worth being aware of the difference.