DDevelopment & Design

Vavr, Collections, and Java Stream API Collectors

Vavr is now a must-have for every modern Java 8+ project. It encourages writing code in a functional manner by providing a new persistent Collections API along with a set of new Functional Interfaces and monadic tools like Option, Try, Either, etc.

You can read more about it here.

Vavr’s Persistent Collections API

To provide useable immutable data structures, the whole Collections API needed to be redesigned from scratch.

The standard java.util.Collection interface contains methods that discourage immutability such as:

boolean add(E e);
boolean remove(Object o);
boolean addAll(Collection<? extends E> c);
boolean removeAll(Collection<?> c);

One might think that the problem is that those methods allow modifications of the particular collection instance, but this is not entirely true – with immutable data structures, each mutating operation needs to derive a new collection from the existing one. Simply put, each of those methods should be able to return a new instance of the collection.

Here, the whole collections hierarchy is restricted to returning boolean or void from mutating methods – which makes them suitable only for mutable implementations.

Of course, immutable implementations of java.util.Collection exist, but above-mentioned methods are simply forbidden. That’s how it looks like in the com.google.common.collect.ImmutableList:

/**
 * Guaranteed to throw an exception and leave the list unmodified.
 *
 * @throws UnsupportedOperationException always
 * @deprecated Unsupported operation.
 */
@Deprecated
@Override
public final void add(int index, E element) {
  throw new UnsupportedOperationException();
}

And this is far from perfect – even the simplest add() operation becomes a ceremony:

ImmutableList<Integer> original = ImmutableList.of(1);

List<Integer> modified = new ImmutableList.Builder<Integer>()
  .addAll(original)
  .add(2)
  .build();

A major redesign made it possible to interact with immutable collections more naturally and add some new exciting features:

import io.vavr.collection.List;
// ...

List<Integer> original = List.of(1);
List<Integer> modified = original.append(2);

modified.dropWhile(i -> i < 42);
modified.combinations();
modified.foldLeft(0 , Integer::sum)

Collecting Vavr’s Collections

One of the key features of the Java Stream API was the collect() API that made it possible to take elements from Stream and apply the provided strategy to them – in most cases that would be simply placing all elements in some collection.

Vavr’s collections have a method that provides the similar(but limited) functionality but it’s not being used often because almost all operations that were available only using Stream API, are available on the collection level in Vavr.

But… one of the method signatures of Vavr’s collect() is especially intriguing:

<R, A> R collect(java.util.stream.Collector<? super T, A, R> collector)

As you can see, Vavr’s collections are fully compatible with Stream API Collectors and we can use our favourite Collectors easily:

list.collect(Collectors.toList());
list.collect(Collectors.groupingBy(Integer::byteValue));

That might not be super useful for everyday use-cases because the most common operations are accessible without using Collectors but it’s comforting to know that Vavr’s functionality is a superset of Stream API’s (at least in terms of collect() semantics)

Collecting Everything

The interesting realization happens when we decide to investigate the type hierarchy in Vavr:

source: http://www.vavr.io/vavr-docs/

We can notice here that the Value resides on top collections hierarchy and this is where the collect() method mentioned above is defined.

If we look closer, it’s clear that classes like Option, Try, Either, Future, Lazy also implement the Value interface. The reasoning behind this is that they are all essentially containers for values – containers that can hold max up to one element.  

This makes them compatible with Stream API Collectors, as well:

Option.of(42)
  .collect(Collectors.toList());

Try.of(() -> URI.create("4comprehension.com"))
  .collect(Collectors.partitioningBy(URI::isAbsolute));

Summary

The redesign of the Collections API allowed the introduction of cool new methods, as well as achieving full interoperability with Java Stream API Collectors – which can also be applied to Vavr’s functional control structures like Option, Try, Either, Future, or Lazy.

The examples above use:

<dependency>
    <groupId>io.vavr</groupId>
    <artifactId>vavr-test</artifactId>
    <version>0.9.0</version>
</dependency>

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Using Kotlin extensions in Groovy

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DDevelopment & Design

Using WsLite in practice

TL;DR

There is a example working GitHub project which covers unit testing and request/response logging when using WsLite.

Why Groovy WsLite ?

I’m a huge fan of Groovy WsLite project for calling SOAP web services. Yes, in a real world you have to deal with those - big companies have huge amount of “legacy” code and are crazy about homogeneous architecture - only SOAP, Java, Oracle, AIX…

But I also never been comfortable with XFire/CXF approach of web service client code generation. I wrote a bit about other posibilites in this post. With JAXB you can also experience some freaky classloading errors - as Tomek described on his blog. In a large commercial project the “the less code the better” principle is significant. And the code generated from XSD could look kinda ugly - especially more complicated structures like sequences, choices, anys etc.

Using WsLite with native Groovy concepts like XmlSlurper could be a great choice. But since it’s a dynamic approach you have to be really careful - write good unit tests and log requests. Below are my few hints for using WsLite in practice.

Unit testing

Suppose you have some invocation of WsLite SOAPClient (original WsLite example):

def getMothersDay(long _year) {
    def response = client.send(SOAPAction: action) {
       body {
           GetMothersDay('xmlns':'http://www.27seconds.com/Holidays/US/Dates/') {
              year(_year)
           }
       }
    }
    response.GetMothersDayResponse.GetMothersDayResult.text()
}

How can the unit test like? My suggestion is to mock SOAPClient and write a simple helper to test that builded XML is correct. Example using great SpockFramework: 

void setup() {
   client = Mock(SOAPClient)
   service.client = client
}

def "should pass year to GetMothersDay and return date"() {
  given:
      def year = 2013
  when:
      def date = service.getMothersDay(year)
  then:
      1 * client.send(_, _) >> { Map params, Closure requestBuilder ->
            Document doc = buildAndParseXml(requestBuilder)
            assertXpathEvaluatesTo("$year", '//ns:GetMothersDay/ns:year', doc)
            return mockResponse(Responses.mothersDay)
      }
      date == "2013-05-12T00:00:00"
}

This uses a real cool feature of Spock - even when you mock the invocation with “any mark” (_), you are able to get actual arguments. So we can build XML that would be passed to SOAPClient's send method and check that specific XPaths are correct:

void setup() {
    engine = XMLUnit.newXpathEngine()
    engine.setNamespaceContext(new SimpleNamespaceContext(namespaces()))
}

protected Document buildAndParseXml(Closure xmlBuilder) {
    def writer = new StringWriter()
    def builder = new MarkupBuilder(writer)
    builder.xml(xmlBuilder)
    return XMLUnit.buildControlDocument(writer.toString())
}

protected void assertXpathEvaluatesTo(String expectedValue,
                                      String xpathExpression, Document doc) throws XpathException {
    Assert.assertEquals(expectedValue,
            engine.evaluate(xpathExpression, doc))
}

protected Map namespaces() {
    return [ns: 'http://www.27seconds.com/Holidays/US/Dates/']
}

The XMLUnit library is used just for XpathEngine, but it is much more powerful for comparing XML documents. The NamespaceContext is needed to use correct prefixes (e.g. ns:GetMothersDay) in your Xpath expressions.

Finally - the mock returns SOAPResponse instance filled with envelope parsed from some constant XML:

protected SOAPResponse mockResponse(String resp) {
    def envelope = new XmlSlurper().parseText(resp)
    new SOAPResponse(envelope: envelope)
}

Request and response logging

The WsLite itself doesn’t use any logging framework. We usually handle it by adding own sendWithLogging method:

private SOAPResponse sendWithLogging(String action, Closure cl) {
    SOAPResponse response = client.send(SOAPAction: action, cl)
    log(response?.httpRequest, response?.httpResponse)
    return response
}

private void log(HTTPRequest request, HTTPResponse response) {
    log.debug("HTTPRequest $request with content:\n${request?.contentAsString}")
    log.debug("HTTPResponse $response with content:\n${response?.contentAsString}")
}

This logs the actual request and response send through SOAPClient. But it logs only when invocation is successful and errors are much more interesting… So here goes withExceptionHandler method:

private SOAPResponse withExceptionHandler(Closure cl) {
    try {
        cl.call()
    } catch (SOAPFaultException soapEx) {
        log(soapEx.httpRequest, soapEx.httpResponse)
        def message = soapEx.hasFault() ? soapEx.fault.text() : soapEx.message
        throw new InfrastructureException(message)
    } catch (HTTPClientException httpEx) {
        log(httpEx.request, httpEx.response)
        throw new InfrastructureException(httpEx.message)
    }
}
def send(String action, Closure cl) {
    withExceptionHandler {
        sendWithLogging(action, cl)
    }
}

XmlSlurper gotchas

Working with XML document with XmlSlurper is generally great fun, but is some cases could introduce some problems. A trivial example is parsing an id with a number to Long value:

def id = Long.valueOf(edit.'@id' as String)

The Attribute class (which edit.'@id' evaluates to) can be converted to String using as operator, but converting to Long requires using valueOf.

The second example is a bit more complicated. Consider following XML fragment:

<edit id="3">
   <params>
      <param value="label1" name="label"/>
      <param value="2" name="param2"/>
   </params>
   <value>123</value>
</edit>
<edit id="6">
   <params>
      <param value="label2" name="label"/>
      <param value="2" name="param2"/>
   </params>
   <value>456</value>
</edit>

We want to find id of edit whose label is label1. The simplest solution seems to be:

def param = doc.edit.params.param.find { it['@value'] == 'label1' }
def edit = params.parent().parent()

But it doesn’t work! The parent method returns multiple edits, not only the one that is parent of given param…

Here’s the correct solution:

doc.edit.find { edit ->
    edit.params.param.find { it['@value'] == 'label1' }
}

Example

The example working project covering those hints could be found on GitHub.