tech oriented notes to self and lessons learned
Implementing Jersey 2 Spring integration
2014-02-08Posted by on
Jersey is the excellent Java JAX-RS specification reference implementation from Oracle. Last year, when we were starting to build RESTful backend web services for a high-volume website, we chose to use the JAX-RS API as our REST framework and Spring framework for dependency injection. Jersey was our JAX-RS implementation of choice.
When the project was started JAX-RS API 2.0 specification was not yet released, and neither was Jersey 2.0. Since we didn’t see any fundamental deficiencies with JAX-RS 1.1, and because a stable Spring integration module existed for Jersey 1.1, we decided to go with the tried-and-true version instead of taking on the bleeding edge.
Still, I was curious to learn what could be gained by adopting the newer version, so I started looking at the JAX-RS 2 API on my free time and doing some prototyping with Jersey 2. I noticed that Jersey 2 lacked Spring framework integration that was available for the previous version. Studying the issue further, I found that the old Spring integration module would not be directly portable to Jersey 2. The reason was that Jersey 1 builds on a custom internal dependency injection framework while Jersey 2 had switched to HK2 for dependency injection. (HK2 is an interesting, light-weight dependency injection framework used in GlassFish.)
My original goals for Jersey-Spring integration were fairly simple:
inject Spring beans declared in application context XML into JAX-RS resource classes (using @Autowired annotation or XML configuration)
So, I thought I’d dig a bit deeper and started looking into Jersey source code. I was happy to notice that Jersey development was being done in an open and approachable manner. The source code was hosted on GitHub and updated frequently. After a while of digging, a high-level design for Jersey Spring integration started to take shape. It took quite some experimenting and many iterations before the first working prototype. At that point, being an optimist, I hoped I was nearly done and contacted the jersey-users mailing list to get feedback on the design and implementation. The feedback: add more use cases, provide sample code, implement test automation, sign Oracle Contributor Agreement :-) (The feedback, of course, was very reasonable from the Jersey software product point of view). So, while it wasn’t quite back to the drawing board, but at this point I realized the last mile was to be considerably longer than I had hoped for.
Eventually, though, the Jersey-Spring integration got merged in Jersey 2 code base in Jersey v2.2 release. The integration API is based on annotations and supports the following features:
- inject Spring beans into Jersey managed JAX-RS resource classes (using org.springframework.beans.factory.annotation.Autowired or javax.inject.Inject). @Qualifier and @Named annotations can be used to further qualify the injected instance.
- allow JAX-RS resource class instance lifecycle to be managed by Spring instead of Jersey (org.springframework.stereotype.Component)
- support different Spring bean injection scopes: singleton, request, prototype. Bean scope is declared in applicationContext.xml.
Source code for the Jersey-Spring integration can be found in the main Jersey source repository:
Jersey-Spring integration consists of the following implementation classes:
This ComponentProvider implementation is registered with Jersey SPI extension mechanism and it’s responsible for bootstrapping Jersey 2 Spring integration. It makes Jersey skip JAX-RS life-cycle management for Spring components. Otherwise, Jersey would bind these classes to HK2 ServiceLocator with Jersey default scope without respecting the scope declared for the Spring component. This class also initializes HK2 spring-bridge and registers Spring @Autowired annotation handler with HK2 ServiceLocator. When being run outside of servlet context, a custom org.springframework.web.context.request.RequestScope implementation is configured to implement request scope for beans.
HK2 injection resolver that injects dependencies declared using Spring framework @Autowired annotation. HK2 invokes this resolver and asks it to resolve dependencies annotated using @Autowired.
Handles container lifecycle events. Refreshes Spring context on reload and close it on shutdown.
A convenience class that helps the user avoid having to configure Spring ContextLoaderListener and RequestContextListener in web.xml. Alternatively the user can configure these in web application web.xml.
The JAX-RS specification defines its own dependency injection API. Additionally, Jersey supports JSR 330 style injection not mandated by the JAX-RS specification. Jersey-Spring integration adds support for Spring style injection. Both JAX-RS injection and Spring integration provide a mechanism for binding objects into a registry, so that objects can later be looked up and injected. If you’re using a full Java EE application server, such as Glassfish, you also have the option of binding objects via the CDI API. On non-Java EE environments it’s possible to use CDI by embedding a container implementation such as Weld. Yet another binding method is to use Jersey specific API. The test code includes a JAX-RS application class that demonstrates how this can be done.
If you want to work on Jersey-Spring, you need to check out Jersey 2 code base and build it. That process is rather easy and well documented:
You simply need to clone the repository and build the source. The build system is Maven based. You can also easily import the code base into your IDE of choice (tried it with IDEA 12, Eclipse 4.3 and NetBeans 8.0 beta) using its Maven plugin. I noticed, however, that some integration tests failed with Maven 3.0, and I had to upgrade to 3.1, but apart from that there weren’t any issues.
After building Jersey 2 you can modify the Spring integration module, and build only the changed modules to save time.
Jersey-Spring integration tests have been built using Jersey test framework and they’re run under the control of maven-failsafe-plugin. Integration tests consist of actual test code and a JAX-RS backend webapp that the tests exercise. The backend gets deployed into an external Jetty servlet container using jetty-maven-plugin. Jersey-Spring tests can be executed separately from the rest of the tests. Integration tests can be found in a separate Maven submodule here:
In addition to demonstrating the basic features of Jersey-Spring, the tests show how to use different Spring bean scopes: singleton, request, prototype. The tests also exhibit using a JAX-RS application class for registering your own dependencies in the container, in different scopes.
I think the JAX-RS 2.0 API provides a nice and clean way of implementing RESTful interfaces in Java. Development of the Jersey JAX-RS reference implementation is being conducted in an open and transparent manner. Jersey also has a large and active user community.
As noted by Frederick Brooks, Jr.: “All programmers are optimists”. It’s often easy to underestimate the amount of work required to integrate code with a relative large and complex code base, and in particular when you need to mediate between multiple different frameworks (in this case Jersey, HK2, Spring framework). Also, though Jersey has pretty good user documentation, I missed high-level architectural documentation on the design and implementation. A lot of poking around was needed to be able to identify the correct integration points. Fortunately, the Jersey build system is pretty easy to use and allows building only selected parts, which makes experimenting and the change-build-test cycle relatively fast.
Both Jersey and Spring framework provide a rich set of features and you can use them together in a multitude of ways. Jersey-Spring integration in it’s current form covers a couple of basic integration scenarios between the two. If you find that your particular scenario isn’t supported, join the jersey-users mailing list to discuss it. You can also just check out the code, implement your changes and contribute them by submitting a pull request on GitHub.