Put simply, Guice alleviates the need for factories and the use of new in your Java code. Think of Guice's @Inject as the new new. You will still need to write factories in some cases, but your code will not depend directly on them. Your code will be easier to change, unit test and reuse in other contexts.

Guice embraces Java's type safe nature, especially when it comes to features introduced in Java 5 such as generics and annotations. You might think of Guice as filling in missing features for core Java. Ideally, the language itself would provide most of the same features, but until such a language comes along, we have Guice.

PicoContainer is a highly embeddable, full-service, Inversion of Control (IoC) container for components honor the Dependency Injection pattern. The project started in 2003 and pioneered Constructor Injection auto-wiring. It is also Open Source and therefore free to use. The license is BSD and thus you can safely use this with commercial or other open source software.

Spring is built on an Inversion of Control (IoC) container that enables Java components to be centrally configured and wired together—resulting in code that is more portable, reusable, testable and maintainable.

Spring is a popular and widely deployed open source framework that helps developers build high quality applications faster. Spring provides a consistent programming and configuration model that is well understood and used by millions of developers worldwide. Unlike the traditional Java EE platform, Spring provides a range of capabilities for creating enterprise Java, rich web, and enterprise integration applications that can be consumed in a lightweight, a-la-carte manner.