Posts Tagged ‘tag’
  • Discover Spring authoring

    In this article, I will describe a useful but much underused feature of Spring, the definition of custom tags in the Spring beans definition files.

    Spring namespaces

    I will begin with a simple example taken from Spring’s documentation. Before version 2.0, only a single XML schema was available. So, in order to make a constant available as a bean, and thus inject it in other beans, you had to define the following:

    <bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
      class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean" />

    Spring made it possible, but realize it’s only a trick to expose the constant as a bean. However, since Spring 2.0, the framework let you use the util namespace so that the previous example becomes:

    <util:constant static-field="java.sql.Connection.TRANSACTION_SERIALIZABLE"/>

    In fact, there are many namespaces now available:

    Prefix Namespace Description
    bean Original bean schema
    util Utilities: constants, property paths and collections
    jee JNDI lookup
    lang Use of other languages
    tx Transactions
    aop AOP
    context ApplicationContext manipulation

    Each of these is meant to reduce verbosity and increase readibility like the first example showed.


    What is still unknown by many is that this feature is extensible that is Spring API provides you with the mean to write your own. In fact, many framework providers should take advantage of this and provide their own namespaces so that integrating thier product with Spring should be easier. Some already do: CXF with its many namespaces comes to mind but there should be others I don’t know of.

    Creating you own namespace is a 4 steps process: 2 steps about the XML validation, the other two for creating the bean itself. In order to illustrate the process, I will use a simple example: I will create a schema for EhCache, the Hibernate’s default caching engine.

    The underlying bean factory will be the existing EhCacheFactoryBean. As such, it won’t be as useful as a real feature but it will let us focus on the true authoring plumbing rather than EhCache implementation details.

    Creating the schema

    Creating the schema is about describing XML syntax and more importantly restrictions. I want my XML to look something like the following:

    <ehcache:cache id="myCache" eternal="true" cacheName="foo"
    maxElementsInMemory="5" maxElementsOnDisk="2" overflowToDisk="false"
    diskExpiryThreadIntervalSeconds="18" diskPersistent="true" timeToIdle="25" timeToLive="50"
      <ehcache:manager ref="someManagerRef" />

    Since I won’t presume to teach anyone about XML, here’s the schema. Just notice the namespace declaration:

    <xsd:schema xmlns:xsd=""
      targetNamespace="" xmlns=""
      <xsd:complexType name="cacheType">
        <xsd:sequence maxOccurs="1" minOccurs="0">
          <xsd:element name="manager" type="managerType" />
        <xsd:attribute name="id" type="xsd:string" />
        <xsd:attribute name="cacheName" type="xsd:string" />
        <xsd:attribute name="diskExpiryThreadIntervalSeconds" type="xsd:int" />
        <xsd:attribute name="diskPersistent" type="xsd:boolean" />
        <xsd:attribute name="eternal" type="xsd:boolean" />
        <xsd:attribute name="maxElementsInMemory" type="xsd:int" />
        <xsd:attribute name="maxElementsOnDisk" type="xsd:int" />
        <xsd:attribute name="overflowToDisk" type="xsd:boolean" />
        <xsd:attribute name="timeToLive" type="xsd:int" />
        <xsd:attribute name="timeToIdle" type="xsd:int" />
        <xsd:attribute name="memoryStoreEvictionPolicy" type="memoryStoreEvictionPolicyType" />
      <xsd:simpleType name="memoryStoreEvictionPolicyType">
        <xsd:restriction base="xsd:string">
          <xsd:enumeration value="LRU" />
          <xsd:enumeration value="LFU" />
          <xsd:enumeration value="FIFO" />
      <xsd:complexType name="managerType">
        <xsd:attribute name="ref" type="xsd:string" />
      <xsd:element name="cache" type="cacheType" />

    And for those, like me, that prefer a graphic display:

    Mapping the schema

    The schema creation is only the first part. Now, we have to make Spring aware of it. Create the file META-INF/spring.schemas and write in the following line is enough:


    Just take care to insert the backslash, otherwise it won’t work. It maps the schema declaration in the XML to the real file that will be used from the jar!

    Before going further, and for the more curious, just notice that in spring-beans.jar (v3.0), there’s such a file. Here is it’s content:


    It brings some remarks:

    • Spring eat their own dogfood (that’s nice to know)
    • I didn’t look into the code but I think that’s why XML validation of Spring’s bean files never complain about not finding the schema over the Internet (a real pain in production environment because of firewall security issues). That’s because the XSD are looked inside the jar
    • If you don’t specify the version of the Spring schema you use (2.0, 2.5, 3.0, etc.), Spring will automatically upgrade it for you with each major/minor version of the jar. If you want this behaviour, fine, if not, you’ll have to specify version

    Creating the parser

    The previous steps are only meant to validate the XML so that the eternal attribute will take a boolean value, for example. We still did not wire our namespace into Spring factory. This is the goal of this step.

    The first thing to do is create a class that implement org.springframework.beans.factory.xml.BeanDefinitionParser. Looking at its hierarchy, it seems that the org.springframework.beans.factory.xml.AbstractSimpleBeanDefinitionParser is a good entry point since:

    • the XML is not overly complex
    • there will be a single bean definition

    Here’s the code:

    public class EhCacheBeanDefinitionParser extends AbstractSimpleBeanDefinitionParser {
      private static final List&amp;amp;amp;amp;amp;lt;String&amp;amp;amp;amp;amp;gt; PROP_TAG_NAMES;
      static {
      PROP_TAG_NAMES = new ArrayList();
      protected Class getBeanClass(Element element) {
        return EhCacheFactoryBean.class;
      protected boolean shouldGenerateIdAsFallback() {
        return true;
      protected void doParse(Element element, ParserContext parserContext, BeanDefinitionBuilder builder) {
        for (String name : PROP_TAG_NAMES) {
          String value = element.getAttribute(name);
          if (StringUtils.hasText(value)) {
            builder.addPropertyValue(name, value);
        NodeList nodes = element.getElementsByTagNameNS("", "manager");
        if (nodes.getLength() > 0) {
        String msep = element.getAttribute("memoryStoreEvictionPolicy");
        if (StringUtils.hasText(msep)) {
          MemoryStoreEvictionPolicy policy = MemoryStoreEvictionPolicy.fromString(msep);
          builder.addPropertyValue("memoryStoreEvictionPolicy", policy);

    That deserves some explanations. The static block fills in what attributes are valid. The getBeanClass() method returns what class will be used, either directly as a bean or as a factory. The shouldGenerateIdAsFallback() method is used to tell Spring that when no id is supplied in the XML, it should generate one. That makes it possible to create pseudo-anonymous beans (no bean is really anonymous in the Spring factory).

    The real magic happen in the doParse() method: it just adds every simple property it finds in the builder. There are two interesting properties though: cacheManager and memoryStoreEvictionPolicy.

    The former, should it exist, is a reference on another bean. Therefore, it should be added to the builder not as a value but as a reference. Of course, the code doesn’t check if the developer declared the cache manager as an anonymous bean inside the ehcache but the schema validation took already care of that.

    The latter just uses the string value to get the real object behind and add it as a property to the builder. Likewise, since the value was enumerated on the schema, exceptions caused by a bad syntax cannot happen.

    Registering the parser

    The last step is to register the parser in Spring. First, you just have to create a class that extends org.springframework.beans.factory.xml.NamespaceHandlerSupport and register the handler under the XML tag name in its init() method:

    public class EhCacheNamespaceHandler extends NamespaceHandlerSupport {
      public void init() {
        registerBeanDefinitionParser("cache", new EhCacheBeanDefinitionParser());

    Should you have more parsers, just register them in the same method under each tag name.

    Second, just map the formerly created namespace to the newly created handler in a file META-INF/spring.handlers:


    Notice that you map the declared schema file to the real schema but the namespace to the handler.


    Now, when faced by overly verbose bean configuration, you have the option to use this nifty 4-steps techniques to simplify it. This technique is of course more oriented toward product providers but can be used by projects, provided the time taken to author a namespace is a real time gain over normal bean definitions.

    You will find the sources for this articlehere in Maven/Eclipse format.

    To go further:

    • Spring authoring: version 2.0 is enough since nothing changed much (at all?) with following versions
    • Spring's Javadoc relative to authoring
  • VIT: Very Important Taglib

    Display taglib

    Every application needs to display tabulated data. Well, web applications are no different. So, as an architect, your job is to provide a mechanism to handle these tables gracefully and equally in your whole application.

    First thing that comes to mind is the reuse of an existing mechanism to do so. Well, you’re lucky. If you don’t happen to have to use JSF, there’s a neat tag library that handles the job like a pro. For those that don’t remember what a taglib is, please feel free to take a look there, I’m waiting for you so take your time. If you don’t have time to read the tutorial, just know that taglibs are jars, and as such, can be dropped into webapps and used as such.

    The Display tag library provides the followings options out-of-the-box:

    • paging: the ability to display only a subset of a list and to navigate from subset to subset in a ordered way,
    • sorting: the capacity to order a subset of a list, either from the subset or from the entire list,
    • grouping: the aptitude to display an information but only if it is new,
    • export: Excel, CSV, XML are provided but a framework mechanism enables you to write more,
    • formatting: dates come to mind but what about currency, numbers, ...,
    • alternating color lines: a classic one,
    • hyperlinks,
    • internationalization.

    With these few lines of code:

    <display:table name="test" export="true" sort="list" pagesize="8">
              <display:column property="city" title="CITY" group="1" sortable="true" headerClass="sortable" /> 
              <display:column property="project" title="PROJECT" group="2" sortable="true" headerClass="sortable" /> 
              <display:column property="amount" title="HOURS" /> 
              <display:column property="task" title="TASK" /> 

    you get this :

    Display tag example

    Not bad, eh ? Well, this treasure is currently available in version 1.1.1.

    Those of you that have a little experience in the use of external libraries are well aware that, under normal circumstances, this would hint that it won’t fit your every needs and it will be a nightmare to integrate. In this case, it is not the case (really !). The taglib is highly configurable thanks to

    • property files for labels,
    • css for presentation.

    Plus, if this doesn’t cover all your needs (aren’t you the demanding type ?), the API is clear and concise enough to let you develop your own classes. For example, if you have an higly personal class to present in your table, the API lets you do it the precise way you want with the help of the API and the Decorator pattern.

    Icing on the cake: It is brought to you with the open source artistic license. From now on, you have no reason to ignore this little jewel.

    I welcome your comments if you did already used this taglib: was it as nice for you as for me ?

    Categories: JavaEE Tags: displaylibrarytabletabulatedtagtaglib