Manual Dependency Injection with Jersey and embedded Jetty

I wrote a little application demonstrating how to manually (through constructors) inject dependencies into Jersey resources in an embedded container like Jetty. 

The benefits are:

- lightweightness (no need to use Spring, Guice, etc.),

- more control over your application (less magic behind the scenes),

- controlling dependencies in tests (via Dependency Injection)

- running a web app with a simple Java main method (via embedded Jetty)

The example application is called Time Expert. It exposes the current time via a RESTful web service, so when I run it in the production (via Main.java) I can use it as following:

	curl -v -H "Accept: text/plain" http://localhost:6666/time
	HTTP/1.1 200 OK
	Content-Type: text/plain
	Content-Length: 21
	Server: Jetty(9.0.5.v20130815)
	It's currently 20:59

view raw gistfile1.txt hosted with ❤ by GitHub

Here's how I test it (TimeAcceptanceTest.java) :

	@Test
	public void saysTheCurrentTime() {
	givenTheCurrentTimeIs("20:15");
	whenAUserChecksTheTime();
	thenTheUserSees("It's currently 20:15!");
	}

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However,  I cannot rely here on the real time because it changes every time I run tests . The problem can be easily solved with the Clock pattern. My tests require a FixedClock which allows me to set the current time to any value:

	public class FixedClock implements Clock {
	private Date fixedNow;
	@Override
	public Date now() {
	return fixedNow;
	}
	public void setNow(Date fixedNow) {
	this.fixedNow = fixedNow;
	}
	}

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 So I start my server with a fixed clock:

	@Before
	public void startApplication() {
	clock = new FixedClock();
	timeExpert = new TimeExpertServer(clock);
	timeExpert.start();
	}

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... and then set it to, say, 20:15:

	@Test
	public void saysTheCurrentTime() throws Exception {
	givenTheCurrentTimeIs("20:15");
	whenAUserChecksTheTime();
	thenTheUserSees("It's currently 20:15!");
	}
	private void givenTheCurrentTimeIs(String currentTime) throws ParseException {
	Date currentTimeAsDate = new SimpleDateFormat("HH:mm").parse(currentTime);
	clock.setNow(currentTimeAsDate);
	}
	private void whenAUserChecksTheTime() {
	actualApplicationTime = timeExpertUser.checkCurrentTime();
	}
	private void thenTheUserSees(String expectedApplicationTime) {
	assertEquals(expectedApplicationTime, actualApplicationTime);
	}

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Since my Jersey resource class is only aware of the clock interface, it will display 20:15:

	@Path("time")
	public class TimeResource {
	private final Clock clock;
	public TimeResource(Clock clock) {
	this.clock = clock;
	}
	@GET
	@Produces(MediaType.TEXT_PLAIN)
	public String getCurrentTime() {
	return String.format("It's currently %s!", format(clock.now()));
	}
	private String format(Date now) {
	return new SimpleDateFormat("HH:mm").format(now);
	}
	}

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In order to create your Jersey resources with manually injected dependencies you have to register org.glassfish.jersey.servlet.ServletContainer with a customized org.glassfish.jersey.server.ResourceConfig. In that config Jersey resources can be newed up with their dependencies:

	public void start() {
	server = new Server(6666);
	ServletContextHandler handler = new ServletContextHandler();
	handler.setContextPath("");
	// adds Jersey Servlet with a customized ResourceConfig
	handler.addServlet(new ServletHolder(new ServletContainer(resourceConfig())), "/*");
	server.setHandler(handler);
	try {
	server.start();
	} catch (Exception e) {
	throw new RuntimeException("Could not start the server", e);
	}
	}
	private ResourceConfig resourceConfig() {
	// manually injecting dependencies (clock) to Jersey resource classes
	return new ResourceConfig().register(new TimeResource(clock));
	}

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Of course the production provides a real clock (which returns new Date()):

	public class Main {
	public static void main(String[] args) {
	new Main().startTimeExpert();
	}
	private void startTimeExpert() {
	final TimeExpertServer timeExpert = new TimeExpertServer(new RealClock());
	try {
	timeExpert.start();
	timeExpert.join();
	} finally {
	timeExpert.stop();
	}
	}
	}

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Code is available under: https://github.com/unclejamal/TimeMaster

Enjoy!

Tested using:

- Java 1.7,

- Gradle 1.7 (easily convertible to Maven :)),

- Jersey 2.2,

- Jetty 9.0.5.