When people mention that Spring is a lightweight containter compared to other frameworks, do they mean? That it occupies less memory in the system or it does not have the operations like start stop that we have for EJB containers and it doesn't use a special container?
What makes Spring a lightweight container?
Whether it is "lightweight" or "heavyweight", it is all about comparison. We consider Spring to be lightweight when we are comparing to normal J2EE container. It is lightweight in the sense of extra memory footprint for the facilities provided (e.g. Transaction Control, Life Cycle, Component dependency management)
However, there are sometimes other criteria to compare for the "weight" of a container, e.g. intrusiveness in design and implementation; facilities provided etc.
Ironically, Spring is sometimes treated as heavy weight container when compared to other POJO-based container, like Guice and Plexus.
Spring calls itself 'lightweight' because you don't need all of Spring to use part of it. For example, you can use Spring JDBC without Spring MVC.
Spring provides various modules for different purposes; you can just inject dependencies according to your required module. That is, you don't need to download or inject all dependencies or all JARs to use a particular module.
If you want to run a Java EE application, you can't just create a small application that will run on its own. You will need a Java EE application server to run your application, such as Glassfish, WebLogic or WebSphere. Most application servers are big and complex pieces of software, that are not trivial to install or configure.
You don't need such a thing with Spring. You can use Spring dependency injection, for example, in any small, standalone program.
I think "lightweight" is mostly a buzz-word. It's meaning is highly subjective and based on context. It can mean "low memory footprint", it can be low execution overhead, low start-up overhead. People also use it to differentiate between some perceived level of complexity and/or learning-curve. In any case, it's assuredly relative as there is no defined point on any scale where "light" becomes "heavy" in terms of "weight".
I personally think it's a dangerous word since it has no real, quantifiable meaning. It's something people throw into architecture proposals to beef up the "pro" section of a certain framework they want to use anyway. If you see or hear it being used in any such situation, it's a perfect opportunity to ask "what does that mean?". If you get an angry or frustrated response (combined with rolling of eyes and shaking of head), it means that the person has decided on a certain architecture, but hasn't managed to formulate coherent or objective reasons for it.
EDIT: not sure I would categorize spring as a "container" either, but that's a similar apples and oranges discussion. I'd call it a framework.
Spring is light weight becouse other J2ee container especially EJB2.1 require more configuration, It can have lot of do nothing code to ,it have complex directory structure for packing applications, overall it took extra memory;on other hand spring minimizes all this things.so it light weight.
I think one can also say that spring is light weight because it uses POJO(Plain old java object) .POJO class does not require to implement,extends technologies specific API(Interfaces,Classes) or it is not bounded to any technology specific API
