Writing and using Hamcrest Matchers

Hamcrest Matchers
assertThat(audience, is(payingAttention()));

Writing and using them for
assertions, mocking and behavioral
verification

What is Hamcrest?
According to the project homepage,
[Hamcrest] provides a library of matcher objects (also known
as constraints or predicates) allowing 'match' rules to be
defined declaratively, to be used in other frameworks. Typical
scenarios include testing frameworks, mocking libraries and
UI validation rules.

Hamcrest is not a testing library: it just happens that
matchers are very useful for testing.

Typical usage example
Actual value
assertThat(
someString,
is(equalTo(someOtherString))
);
Expectation on the
value, represented as
a Matcher

Wait – but what’s wrong with
assertEquals?
assertEquals(someString, someOtherString)
This looks just as good, doesn’t it?

assertEquals(
cat.getName(),
otherCat.getName())

Not so bad, either

However,
What about collections?

assertEquals(someKitten,
cat.getKittens().iterator().next())

This works if our kitten is the first element in the
collection, but what about asserting that the kitten
exists anywhere in the collection?

Well, we can do this:
boolean found = false;

for (Kitten kitten : cat.getKittens()) {
if (kitten.equals(someKitten)) found = true;
}

assertTrue(found);

But don’t you prefer this?
Iterable<Kitten>
assertThat(
cat.getKittens(),
hasItem(someKitten))

Matcher on
Iterable<Kitten> that
accepts a Matcher<Kitten>

Basic Matchers
A Matcher is initialized with the expected
values, which are compared against the
actual object we’re matching against when
invoking the matcher.

IsEqual Matcher
class IsEqual<T> extends BaseMatcher<T> {
private final Object object; // c’tor omitted for readability

public boolean matches(Object arg) {
return object.equals(arg);
}
}

StringEndsWith Matcher
class StringEndsWith <T> extends BaseMatcher<T> {
private final String suffix; // c’tor omitted for readability

public boolean matches(String s) {
return s.endsWith(suffix);
}
}

Mockito in one slide
CatShop catShop = mock(CatShop.class);
when(catShop.purchase(somePurchaseRequest
).thenReturn(someCat) Creating the mock

... do some work Stubbing the mock

verify(catShop) Behavior verification

.purchase(expectedPurchaseRequest)

Without Hamcrest
CatPurchase catPurchase = new CatPurchase();
catPurchase.setBreed(“Brittish”);

when(catShop.purchase(catPurchase))
).thenReturn(somePreviouslyStubbedCat)
However, this will force us to set all other fields of the
CatPurchase class, since Mockito will perform an exact match
comparison between our instance and the actual one

Of course, you could do this:
when(
catShop.purchase(
any(CatPurchaseDTO.class))

This works, but lacks the benefit of asserting that our
operation is only valid for the expected input

The solution: use argThat()
Mockito helper that creates
an argument matcher from
when( a Hamcrest matcher

catShop.purchase(argThat(
hasPropertyWithValue(
“breed”,
startsWith(“Brittish”))))
Hamcrest matcher that
accepts a Java Bean
property name and a
nested value matcher

Using for behavioral verification
CatDao catDao = mock(CatDao.class);
CatStore catStore = new CatStore (catDao);with a Catcall to
Verify that there was a
CatDao.update() instance,
catStore.saveOrUpdate(existingCat); the ‘name’ property is
for which
“felix” and the ‘kittens’ property is an
verify(catDao).update(argThat( Iterable containing two kittens,
kitten1 and kitten2
allOf(
hasPropertyWithValue(“name”, “felix”),
hasPropertyWithValue(“kittens”,
hasItems(kitten1, kitten2)))));

Writing your own matchers
In the previous examples, we used the
hasPropertyWithValue() matcher, which, while
allowing for fluent assertions or stubbing, has
the disadvantage of not being type-safe.

This is where writing custom matchers becomes
useful (or, as some would say, necessary).

The Matcher<T> hierarchy
abstract class TypeSafeMatcher<T> extends BaseMatcher<T> {
boolean matchesSafely(T item);
}

interface Matcher<T> extends SelfDescribing {
boolean matches(Object item);
}

interface SelfDescribing {
void describeTo(Description description);
}

Dissecting some Wix matchers
class HostMatcher extends TypeSafeMatcher<WixUrl> {
private final Matcher<String> host; // c’tor omitted for readability

public boolean matchesSafely(WixUrl url) {
return host.matches(url.host); Nested matcher that
} will be replayed on the
Actual value being
actual value
matched against
public void describeTo(Description description) {
description.appendText("Host that matches ").appendValue(host); we write a
Here
} readable description of
our expected value

public static HostMatcher hasHost(Matcher<String> host)A utility factory method for
{
return new HostMatcher(host); fluently creating this
} matcher. Not mandatory
} but very convenient.

Using our matcher
WixUrl url =
new WixUrl(“http://www.wix.com/some/path”);

assertThat(url, hasHost(is(“www.wix.com”))); ✔
assertThat(url, hasHost(endsWith(“wix.com”))); ✔
assertThat(url, hasHost(is(“google.com”))); ✗
java.lang.AssertionError:
Expected: Host that matches <is ”google.com">
got: <http://www.wix.com/some/path>

Another URL matcher
class WixUrlParamMatcher extends TypeSafeMatcher<WixUrl> {
private final Matcher<String> name; // c’tor omitted for readability
url.params is a Map<String, String>, so
private final Matcher<String> value;
we create a matcher for a map entry
around our name and value matchers
public boolean matchesSafely(WixUrl url) { replay it against the actual value
and
return hasEntry(name, value).matches(url.params);
}

description
.appendText("Param with name ").appendValue(name)
.appendText(" and value ").appendValue(value);
}
}

Using the two matchers together
String s = “www.wix.com?p=v1&p=v2&p3=v3”;
WixUrl url = new WixUrl(s);

assertThat(url, allOf(
hasHost(is(“www.wix.com”)),
hasParam(is(“p”), anyOf(is(“v1”), is(“v2”))),
hasParam(is(“p3”), startsWith(“v”))));

But wait – my URL is a String!
Sometimes you’ll have matchers that accept a specific
type, such as WixUrl or XML Document. For this
purpose, use a wrapping matcher that performs the
conversion for you:
class StringAsUrlMatcher extends TypeSafeMatcher<String> {
private final Matcher<WixUrl> urlMatcher;
public boolean matchesSafely(String urlString) {
return matcher.matches(new WixUrl(urlString));
}
description.appendText("Url that matches ")
.appendDescriptionOf(urlMatcher);
}
}

Ad-Hoc matchers for readable
tests

Consider the following class

class Proxy {
private final HttpClient httpClient;
private String targetUrl;

public String handle (String path) {
httpClient.execute(// some HttpGet);
}
}

The HttpClient interface
public HttpResponse execute(HttpGet get);

Our class under test is expected to replace the domain
in path with targetUrl, thus serving as an HTTP Proxy.

We would like to stub and verify the HttpGet parameter
to make sure it builds the proxy URL properly.

My test looks something like this
HttpClient client = mock(HttpClient.class);
String url = “http://www.example.com/”;
{…} handler = new Proxy(client, url);
when(client.execute({www.a.com/path}))
.thenReturn(someResponse);

handler.handle(“www.a.com/path”);

verify(client).execute({www.example.com/path});

The solution
Matcher<HttpGet> HttpGet(final Matcher<String> urlMatcher) {
return new TypeSafeMatcher<HttpGet>() {
public boolean matchesSafely(HttpGet httpGet) {
return urlMatcher.matches(httpGet.getURI().toString()));
}

description.appendText("HttpGet with url ")
.appendDescriptionOf(urlMatcher);
}
};
}

Usage of the HttpGet matcher
when(handler.execute(argThat(
is(HttpGet(startsWith(“http://www.a.com”))))))
.thenReturn(response);

handler.handle(“http://www.a.com/some/path”);

verify(client).execute(argThat(is(HttpGet(
is(“http://www.example.com/some/path”)))));

The plot thickens
Moments after triumphantly running the test I
realized that in addition to verifying that the
request went to the appropriate URL, I had to
verify that some – but not all – HTTP headers
were copied to the proxy request and some new
ones were added to it.

Ad-hoc matchers to the rescue
1) Add the following parameter to the HttpGet
method:
final Matcher<Header[]> headersMatcher

2) Change the matchesSafetly() method:
public boolean matchesSafely(HttpGet httpGet) {
return urlMatcher.matches(httpGet.getURI().toString())
&& headersMatcher.matches(httpGet.getAllHeaders());
}

Ad-hoc matchers to the rescue
3) Write a matcher for the Header class:
Matcher<Header> Header(
final Matcher<String> name, final Matcher<String> value) {
return new TypeSafeMatcher<Header>() {
public boolean matchesSafely(Header header) {
return name.matches(header.getName())
&& value.matches(header.getValue());
}
}
}

Putting it all together
verify(client).execute(argThat(is(HttpGet( that the X-Wix-Base-Uri header
Asserts
contains the expected value (using the
is({URL matcher omitted for readability}),
WixUrl matchers we’ve seen before).
allOf(
hasItemInArray(
Header(
is("X-Wix-Base-Uri"),
Asserts that there’s no header by
isUrlThat( the name of X-Seen-By, no matter
what value it has
hasHost(“www.wix.com”), hasPath(myPath)))),
not(hasItemInArray(
Header(is("X-Seen-By"), any(String.class))))
)))));

Questions?

shaiy@wix.com
http://il.linkedin.com/in/electricmonk
http://twitter.com/shaiyallin

Writing and using Hamcrest Matchers

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