I want to make a java application that supports plug ins. Now my core will use jars for certain processes. If my plug ins where to also use these jars, do the plug ins of my application need to configure their build path to include the jars they would also use or is their a way so that the jars can be imported similar to how I import packages from the main application
Guice and Spring are tools for dependency injection, which means that creating objects is easier with them because they take care of instantiating objects and placing them into other objects that depends on them.
Now, when we talk about plugins, we usually are talking too about dynamically loading new classes into a running app. Think on eclipse IDE. Its architecture was designed from the beginning to be "pluggable", like, you can download jars and eclipse will add them to the running application without the need of application restart.
In this case, if you want to build pluggable apps, in a sense of dynamic classloading, I'd recommend you not to go through this path, but to research subjects such as OSGI. One popular OSGI framework is http://felix.apache.org/
Another approach for application extension (we may call this pluggable too, somehow, I guess), depending on how your app is organized and what it does, is to develop a DSL (http://en.wikipedia.org/wiki/Domain-specific_language) for it and extend it letting people adding scripts to it. Isn't something like this when a browser let you add pieces of funcionality written in javascript? Groovy makes DSL easier in some aspects, for java programmers. (see http://docs.codehaus.org/display/GROOVY/Writing+Domain-Specific+Languages)
If you want dynamic plugable systems OSGI can give you this, but OSGI its IMMO a over-complicated technology, use only if you are really sure that needs this dynamic plug-ability.
Other option for builds extensible systems its use de ServiceProvider mechanism, this is a core java mechanism, for example its the one that JDBC implementations use, you can put a JDBC driver in your classpath and the application can find it and use it without needing that you explicitly import the driver classes in your code.
This is an example of using ServiceProvider in your owns applications: http://docs.oracle.com/javase/tutorial/ext/basics/spi.html#limitations-of-the-service-loader-api
Its of course more limited than OSGI, but its very easy to use when you get the idea, and you don't need any external library because its a java core mechanism.
EDIT: about the libraries.
In runtime: With ServiceProvicer there is no separate classloaders (you can implement off course, but by default, in OSGI its implemented this separation), in runtime if your plugin need X class and this class is in the classpath all is ok, the limitation its that the main application and all the plugins use this version of the dependency (guice 3 for example) and you cannot have one plugin using X version and other plugin using X+2 version if this version are not compatible. (this is the famous hell .jar, and one of the principal motivations behind jigsaw project for example).
In compile time, include the dependency in your pom, ant build file, gradle build file or whatever build system your use as usual.
We want to integrate DROOLS with my current web Application which is based on struts 2. Is there a sample Application which could be used as reference?
Generally we are seeing all application use Spring+ Drools.
Also later on can it be possible to integrate Guvnor for a GUI of the rules created?
Yes it is possible. Drools is not tier specific, you can plug it into you Java application however you see fit. As a general rule you would incorporate it into your service tier, where all the heavy lifting is done.
Drools needs very little configuration (in many scenarios it needs none at all). Simply drop the applicable JAR files into your library folder and reference them in your classpath.
I actually built a prototype application for a client using Yahoo UI, Struts and Drools. It works like a charm (can't share the source unfortunately). To wit, you are definitely not tied to Spring.
As far as your second question, note that using Guvnor to manage rules and accessing those rules from your app logic are two totally separate things. The Guvnor governance application is bundled as a web app that you deploy on a server. Once deployed it provides a very nice interface that you can use for managing a rules repository. To use those managed rules in your application you need to include the appropriate JAR files in your application and do some configuration.
I would recommend standing up a simple application first that simple executes some rules in an embedded DRL, before attempting anything more complex like integrating with Guvnor.
i have an API that is being written for a large group of 40 or so applications to share.
my problem is currently they plan on having the API as a simple library included in each war file for each program. the problem thats going to occur is when two apps are running on the same instance with different versions of the api library. ive had a lot of problems in the past with this.
i seem to remember a while ago something where i can wrap my library into an ear file or something and deploy it to tomcat to make it global. simply including it in the lib folder won't work because it will include hibernate systems that have to be deployed to allow the api methods to access the database. then in each application i would have an interface i can implement that allows me to call those api methods. very similar to local EJB3 but not as complex and didn't require an enterprise level server to implement.
anyone else remember something like this or was it a bad dream on my part?
You will have problems if you use a single jar shared by all the webapps, since it will then be impossible for two apps to use a different version of a library. But if each webapp has its own version of the library in its WEB-INF/lib, the container shouldn't have any problem: each webapp has its own classloader, which doesn't see the libraries of other webapps.
I'm working with very large JSF/Facelets applications which use Spring for DI/bean management.
My applications have modular structure and I'm currently looking for approaches to standardize the modularization.
My goal is to compose a web application from a number of modules (possibly depending on each other). Each module may contain the following:
Classes;
Static resources (images, CSS, scripts);
Facelet templates;
Managed beans - Spring application contexts, with request, session and application-scoped beans (alternative is JSF managed beans);
Servlet API stuff - servlets, filters, listeners (this is optional).
What I'd like to avoid (almost at all costs) is the need to copy or extract module resources (like Facelets templates) to the WAR or to extend the web.xml for module's servlets, filters, etc. It must be enough to add the module (JAR, bundle, artifact, ...) to the web application (WEB-INF/lib, bundles, plugins, ...) to extend the web application with this module.
Currently I solve this task with a custom modularization solution which is heavily based on using classpath resources:
Special resources servlet serves static resources from classpath resources (JARs).
Special Facelets resource resolver allows loading Facelet templates from classpath resources.
Spring loads application contexts via the pattern classpath*:com/acme/foo/module/applicationContext.xml - this loads application contexts defined in module JARs.
Finally, a pair of delegating servlets and filters delegate request processing to the servlets and filters configured in Spring application contexts from modules.
Last days I read a lot about OSGi and I was considering, how (and if) I could use OSGi as a standardized modularization approach. I was thinking about how individual tasks could be solved with OSGi:
Static resources - OSGi bundles which want to export static resources register a ResourceLoader instances with the bundle context. A central ResourceServlet uses these resource loaders to load resources from bundles.
Facelet templates - similar to above, a central ResourceResolver uses services registered by bundles.
Managed beans - I have no idea how to use an expression like #{myBean.property} if myBean is defined in one of the bundles.
Servlet API stuff - use something like WebExtender/Pax Web to register servlets, filters and so on.
My questions are:
Am I inventing a bicycle here? Are there standard solutions for that? I've found a mentioning of Spring Slices but could not find much documentation about it.
Do you think OSGi is the right technology for the described task?
Is my sketch of OSGI application more or less correct?
How should managed beans (especially request/session scope) be handled?
I'd be generally grateful for your comments.
What you're aiming to do sounds doable, with a few caveats:
The View Layer: First, your view layer sounds a little overstuffed. There are other ways to modularize JSF components by using custom components that will avoid the headaches involved with trying to create something as dramatic as late-binding managed beans.
The Modules Themselves: Second, your modules don't seem particularly modular. Your first bullet-list makes it sound as if you're trying to create interoperable web apps, rather than modules per se. My idea of a module is that each component has a well-defined, and more or less discrete, purpose. Like how ex underlies vi. If you're going down the OSGi route, then we should define modular like this: Modular, for the sake of this discussion, means that components are hot-swappable -- that is, they can be added and removed without breaking the app.
Dependencies: I'm a little concerned by your description of the modules as "possibly depending on each other." You probably (I hope) already know this, but your dependencies ought to form a directed acyclic graph. Once you introduce a circular dependency, you're asking for a world of hurt in terms of the app's eventual maintainability. One of the biggest weaknesses of OSGi is that it doesn't prevent circular dependencies, so it's up to you to enforce this. Otherwise your dependencies will grow like kudzu and gradually choke the rest of your system's ecosystem.
Servlets: Fuhgeddaboudit. You can't late-bind servlets into a web app, not until the Servlet 3.0 spec is in production (as Pascal pointed out). To launch a separate utility servlet, you'll need to put it into its own app.
OK, so much for the caveats. Let's think about how this might work:
You've defined your own JSF module to do... what, exactly? Let's give it a defined, fairly trivial purpose: a login screen. So you create your login screen, late-bind it using OSGi into your app and... then what? How does the app know the login functionality is there, if you haven't defined it in your .jspx page? How does the app know to navigate to something it can't know is there?
There are ways to get around this using conditional includes and the like (e.g., <c:if #{loginBean.notEmpty}>), but, like you said, things get a little hairy when your managed loginBean exists in another module that may not have even been introduced to the app yet. In fact, you'll get a servlet exception unless that loginBean exists. So what do you do?
You define an API in one of your modules. All the managed beans that you intend to share between modules must be specified as interfaces in this API layer. And all your modules must have default implementations of any of these interfaces that they intend to use. And this API must be shared between all interoperable modules. Then you can use OSGi and Spring to wire together the specified beans with their implementation.
I need to take a moment to point out that this is not how I would approach this problem. Not at all. Given something like as simple as a login page, or even as complicated as a stock chart, I'd personally prefer to create a custom JSF component. But if the requirement is "I want my managed beans to be modular (i.e., hot-swappable, etc)," this is the only way I know to make it work. And I'm not even entirely sure it will work. This email exchange suggests that it's a problem that JSF developers have only just started to work on.
I normally consider managed beans to be part of the view layer, and as such I use them only for view logic, and delegate everything else to the service layer. Making managed beans late-binding is, to my mind, promoting them out of the view layer and into the business logic. There's a reason why all those tutorials are so focused on services: because most of the time you want to consider what it would take for your app to run "headless," and how easy it would be to "skin" your view if, for instance, you wanted it to run, with all its functionality, on an Android phone.
But it sounds like a lot of what you're working with is itself view logic -- for instance, the need to swap in a different view template. OSGi/Spring should be able to help, but you'll need something in your app to choose between available implementations: pretty much what OSGi's Service Registry was built to do.
That leaves static resources. You can modularize these, but remember, you'll need to define an interface to retrieve these resources, and you'll need to provide a default implementation so your app doesn't choke if they're absent. If i18n is a consideration, this could be a good way to go. If you wanted to be really adventurous, then you could push your static resources into JNDI. That would make them completely hot-swappable, and save you the pain of trying to resolve which implementation to use programmatically, but there are some downsides: any failed lookup will cause your app to throw a NamingException. And it's overkill. JNDI is normally used in web apps for app configuration.
As for your remaining questions:
Am I inventing a bicycle here? Are there standard solutions for that?
You are, a little. I've seen apps that do this kind of thing, but you seem to have stumbled into a fairly unique set of requirements.
Do you think OSGi is the right technology for the described task?
If you need the modules to be hot-swappable, then your choices are OSGi and the lighter-weight ServiceLocator interface.
Is my sketch of OSGI application more or less correct?
I can't really tell without knowing more about where your component boundaries are. At the moment, it sounds like you may be pushing OSGi to do more than it is capable of doing.
But don't take my word for it. I found other reading material in these places.
And since you ask about Spring Slices, this should be enough to get you started. You'll need a Git client, and it looks like you'll be training yourself on the app by looking through the source code. And it's very early prototype code.
I am facing the same problems in my current project. In my opinion, OSGi is the best and cleanest solution in terms of standards and future support, but currently you may hit some problems if you try using it in a web application:
there is no well integrated solution between a Web Container and the OSGi platform yet.
OSGi may be too much for a custom build web application that is just searching for a simple modularized architecture. I would consider OSGi if my project needs to support third party extensions that are not 100% under our control, if the project needs hot redeployments, strict access rules between plugins, etc.
A custom solution based on class loaders and resource filters seems very appropriate for me.
As an example you can study the Hudson source code or Java Plug-in Framework (JPF) Project(http://jpf.sourceforge.net/).
As about extending the web.xml, we may be lucky with the Servlet 3.0 specification(http://today.java.net/pub/a/today/2008/10/14/introduction-to-servlet-3.html#pluggability-and-extensibility).
The "web module deployment descriptor fragment" (aka web-fragment.xml) introduced by the Servlet 3.0 specification would be nice here. The specification defines it as:
A web fragment is a logical
partitioning of the web app in such a
way that the frameworks being used
within the web app can define all the
artifacts without asking devlopers to
edit or add information in the
web.xml.
Java EE 6 is maybe not an option for you right now though. Still, it would to be the standardized solution.
Enterprise OSGi is a fairly new domain so dont think you will get a solution that directly satisfies your need. That said one of the things I found missing from Equinox (osgi engine behind eclipse and hence one with largest user base!) is a consistent configuration / DI service. In my project recently we had some similar needs and ended building a simple configuration osgi service.
One of the problems which will be inherent to modular applications would be around DI, as the module visibility could prevent class access in some cases. We got around this using a registered-buddy policy, which is not too ideal but works.
Other than configuration, you can take a look at the recently released Equinox book for guidance on using OSGi as base for creating modular applications. The examples may be specific to Equinox, but the principles would work with any OSGi framework. Link - http://equinoxosgi.org/
You should look into Spring DM Server (it's being transitioned to Eclipse Virgo but that's not been released yet). There's a lot of good things in the recent OSGi enterprise spec which has also just been released.
Some of the Spring DM tutorials will help, I'd imagine. But yes, it's possible to have both resources and classes loaded from outside the web bundle using standard modularity. In that, it's a good fit.
As for the session context - it gets handled as you would expect in a session. However, you might run into problems with sharing that session between web bundles to the extent that in not sure if it's even possible.
You could also look to have a single web bundle and then use e.g. the Eclipse extension registry to extend the capabilities of you web app.
I am looking for something very close to an application server with these features:
it should handle a series of threads/daemons, allowing the user to start-stop-reload each one without affecting the others
it should keep libraries separated between different threads/daemons
it should allow to share some libraries
Currently we have some legacy code reinventing the wheel... and not a perflectly round-shaped one at that!
I thought to use Tomcat, but I don't need a web server, except maybe for the simple backoffice user interface (/manager/html).
Any suggestion? Is there a non-web application server, or is there a better alternative to Tomcat (more lightweight, for example, or easier to configure)? Thanks in advance.
Have you looked at OSGi ? You can load/unload bundles (basically .jar files with metadata) independently of each other, and optionally define dependencies between these (with a software lifecycle defined such that bundles are aware of other bundles being loaded/unloaded).
I have found the Jetty "contexts" concept very useful in handling applications (packaged as WAR's and with servlet context listeners), where the xml-file placed in contexts/ describe fully what you want to have started. When you remove the xml-file again, the thing described is stopped.
If you do not start a server connector you will just have a start-stop thing which sounds like what you are looking for.
Jetty can be made very small so the overhead is not bad.
You could consider Spring dmServer. It's a rather non-traditional appserver, with a very lightweight OSGi core (the web container is optional, for example), but it gives you classloader isolation and basic container services. It's not a JavaEE container, but comes with plug-in modules that are.
You're stlll going to have to do a lot of work yourself, but the basics of dmServer are very sound.
No one stops you from sending binary and text data instead of HTML-pages using http protocol. That is whats servlets are for. So I would use the tomcat server.