HTTP Server is a very simple embeddable HTTP server written in Java with very few dependencies. Its original intent is to be used in JUnit tests so you can verify your code properly made HTTP requests and that the request was formatted properly.

I wrote a simple SlackNotification class for work, but I had no easy way to test that it was working properly. At first, I sent a message on Slack telling everyone I was testing and to ignore the next 50 messages or so. That works, but a better test wouldn't bother all of my coworkers and could be run as many times as I wanted, preferably when the rest of our stuff is built. Besides, I wanted to make sure my code worked as a unit test. Actually hitting Slack during testing is more of an integration level test.

HTTP Server implements Closeable. Use a try-with-resources statement to keeps things simple and tidy. After start() is called, HTTP server binds to an open port by default. Then call your code to be tested passing in "localhost" and the port that HTTP Server bound to. After your code under test is finished, you can inspect the requests and responses to make sure things were put together correctly.

@Test
void testSlackNotification() throws SlackException {
	try ( HTTPServer server = HTTPServer.always200OK() ) {

		// call some code you wrote that makes an HTTP call
		SlackNotification slack = new SlackNotification();
		slack.setUrl( "http://localhost:" + server.getPort() );
		slack.setMessage( "Hello world" );
		slack.setEmoji( ":tada:" );
		
		// more setup
		
		slack.send();
		
		// now assert everything looks like it should
		List<HTTPRequest> requests = server.getRequests();
		Assert.assertFalse( requests.isEmpty() );
		Assert.assertTrue( requests.get(0).getBodyAsString().contains( "Hello world" ) );
		
		// more asserts
	}

}

Using TestNG so you can run concurrent tests? Run as many of these servers as you like simultaneously as long as you have enough memory and ports open (there are 65,535 of them).

Don't want to disturb your coworkers or have to clean up a website after your unit tests run? Test against this server.

Life isn't always 200 OK. Eventually something is going to blow up. Why not test for it? Implement the HTTPRequestHandler interface so your test server responds exactly like you want.

For example, what if you want to test how your code handles errors from the server?

@Test
void testInvalidSlackToken() {
	try ( HTTPServer server = new HTTPServer() ) {
	
		// make the server tell us that we sent bad data
		server.setHTTPRequestHandler( (request, response) -> response.setStatus( 400 ) );
		server.start();
	
		// call some code you wrote that makes an HTTP call
		SlackNotification slack = new SlackNotification();
		slack.setUrl( "http://localhost:" + server.getPort() );
		slack.setToken( "obviously not a real slack token" );
		slack.setMessage( "Hello world" );
		slack.setEmoji( ":tada:" );

		assertThrows( SlackException.class, () -> slack.send() );
	}

}