DDevelopment & Design

Modular Web Application using Eclipse Snaps

Few days ago, new milestone 3.0.0.M03 of Eclipse Virgo was released. Final release of this well prepared OSGi-based application Server getting closer so I decided to take a look at how to use it to write simple modular web application.

What does it mean a modular web application in OSGi environment? I imagine that this application should:

1) Change their look after installation of some extensions.

2) Also we should be able to write new request handlers for our application or modify flow control of existed.

First idea which occurred to me how to resolve first mentioned problem is to use fragment bundles mechanism. Fragment bundles give opportunity to create bundles with resources and classes which will be accessible from host bundle classloader. A key use case of its was “providing translation files for different locales“. So we can do few fragments with jsp files which should be able to include in index servlet page. Also we can provide some configuration files with class names of new request handlers which could be created using java reflection.

But fragment bundles have some disadvantages. How we can read in specification: “The new content of the updated fragment must not be allowed to attach to the host bundles until the Framework is restarted or the host bundle is refreshed“. This mean that we cannot do installation of extention without refreshing host bundle. Also new jsps must be compiled as a part of host application’s jsps so it is second reason why host bundle should be restarted. The last thing is that fragment bundles can’t have Bundle Activator which is also useful in many cases.

Eclipse Virgo gives component which resolves this problems – Virgo Snaps. It is easy in use extension which helps create modular web application framework. Sources are available here

After build of project and copy artifacts to virgo:

cd build-snaps

ant jar

cd ..

cp org.eclipse.virgo.snaps.api/target/artifacts/org.eclipse.virgo.snaps.api.jar ${virgo.home}/repository/usr/

cp org.eclipse.virgo.snaps.core/target/artifacts/org.eclipse.virgo.snaps.core.jar ${virgo.home}/repository/usr/

… and restart of server, we are ready to use snaps.

In samples dir there is example menu-bar showing idea of Snaps. In host bundle – animal.menu.bar we can see in top.jsp usage of taglib snaps:

  • “> ${snap.properties[‘link.text’]}

 

 

It is simple way to iterate through snaps associated with this host. Only thing which we must to do is to add this snippet to MANIFEST.MF:

Snap-Host: animal.menu.bar;version="[1.0, 2.0)"

Snap-ContextPath: /cat

In first line we are manifesting that this snap will be used as a part of host animal.menu.bar. In the second that all servlets will be deployed in this subcontext of context of host.

In this example also were used properties from file snap.properties which are available in session as attribute properties of snap.

After install host bundle at url: http://localhost:8080/animal-menu-bar we can see page without menu items. After installation of new items are become available.

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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.