Google Guice vs. PicoContainer for Dependency Injection
You may want to include Spring in your list of Dependency Injection frameworks you are considering. Here are some answers to your questions:
Coupling to the framework
Pico - Pico tends to discourage setter injection but other than that, your classes don't need to know about Pico. It's only the wiring that needs to know (true for all DI frameworks).
Guice - Guice now supports the standard JSR 330 annotations, so you do not need Guice specific annotations in your code anymore. Spring also supports these standard annotations. The argument that the Guice guys use is that without a Guice annotation processor running, these shouldn't have an impact if you decide to use a different framework.
Spring - Spring aims to allow you to avoid any mention of the Spring framework in your code. Because they do have a lot of other helpers / utilities etc. the temptation is pretty strong to depend on Spring code, though.
Performance
Pico - I'm not too familiar with the speed characteristics of Pico
Guice - Guice was designed to be fast and the comparison mentioned in the reference has some numbers. Certainly if speed is a primary consideration either using Guice or wiring by hand should be considered
Spring - Spring can be slow. There has been work to make it faster and using the JavaConfig library should speed things up.
Ease of use
Pico - Simple to configure. Pico can make some autowire decisions for you. Not clear how it scales to very large projects.
Guice - Simple to configure, you just add annotations and inherit from AbstractModule to bind things together. Scales well to large projects as configuration is kept to a minimum.
Spring - Relatively easy to configure but most examples use Spring XML as the method for configuration. Spring XML files can become very large and complex over time and take time to load. Consider using a mix of Spring and hand cranked Dependency Injection to overcome this.
Community Size
Pico - Small
Guice - Medium
Spring - Large
Experience
Pico - I haven't had much experience with Pico but it is not a widely used framework so it will be harder finding resources.
Guice - Guice is a popular framework and its focus on speed is welcome when you've got a large project that you're restarting a lot in development. I have a concern about the distributed nature of the configuration i.e. it's not easy to see how our whole application is put together. It's a bit like AOP in this respect.
Spring - Spring is usually my default choice. That said, the XML can become cumbersome and the resulting slowdown annoying. I often end up using a combination of hand crafted Dependency Injection and Spring. When you actually need XML based configuration, Spring XML is quite good. Spring also put a lot of effort into making other frameworks more Dependency Injection friendly which can be useful because they often use best practice when doing so (JMS, ORM, OXM, MVC etc.).
References
The answer put up by jamie.mccrindle is actually pretty good, but I'm left confused why Spring is the default choice when it's pretty clear that superior alternatives (both Pico and Guice) are available. IMO Spring's popularity has reached it's peak and now it's currently living off the generated hype (along with all the other "me too" Spring sub projects looking to ride the Spring bandwagon).
Spring's only real advantage is community size (and quite frankly, due to the size and complexity, it's needed), but Pico and Guice don't need a huge community because their solution is much cleaner, more organized, and more elegant. Pico seems more flexible than Guice (you can use annotations in Pico, or not--it's extremely efficient).(Edit: Meant to say it's extremely flexible, not that it isn't also efficient.)
Pico's tiny size and lack of dependencies is a MAJOR win which shouldn't be understated. How many megs do you need to download to use Spring now? It's a kludgy-mess of huge jar files, with all it's dependencies. Intuitively thinking, such an efficient and "small" solution should scale and perform better than something like Spring. Is Spring's bloat really going to make it scale better? Is this bizarro world? I wouldn't make assumptions that Spring is "more scalable" until that's proven (and explained).
Sometimes creating something good (Pico/Guice) and then keeping your HANDS OFF of it instead of adding bloat and kitchen sink features with endless new versions really does work out...
NOTE: This is more of a comment/rant than an answer
PicoContainer is great. I'd switch back to it if they'd just fix their web sites. It's really confusing now:
- http://picocontainer.com which is the most recent and is up and running with content of v2.15 last updated 2014, but many pages have formatting issues and a few pages don't work at all. It looks like the pages were auto-converted from the older content. (Note, this may not have been the case any longer in 2014).
- https://web.archive.org/web/20150516190836/http://picocontainer.codehaus.org/ Which seems frozen in time at version 2.10.2 - It would be really nice if the pages said something like "hey, you're looking at an old web site!"
- https://web.archive.org/web/20150516190836/http://picocontainer.codehaus.org/ - A confluence wiki that documents v 1.x, but it doesn't say that anywhere on the pages!
I'm using Guice 2.x now, even though it's bigger, and it has fewer features. It was just much easier to find the documentation, and it's user group is very active. However, if the direction of Guice 3 is any indication, it looks like Guice is starting to bloat, just like Spring did way back in the early days.
Update: I posted a comment to the Pico Container folks and they've made some improvements to the web site. Much better now!