Wednesday, November 21, 2012

Sitemesh, my favourite web development companion

I've always like Sitemesh even those days before I was introduced to WebWork. I guess after that I just like it even more. Funny thing is not much people seems to be using Sitemesh out there though. Guess more people are familiar with Tiles thanks to Struts I guess.
This is what i know about Sitemesh and what works for me when setting up Sitemesh

#1 Declare sitemesh filters in web.xml

<web ...="...">

# 2 add sitemesh.xml and decorator.xml in '/WEB-INF/' directory

sitemesh.xml by default exists in sitemesh jar file and the copy in sitemesh jar file will be used if one cannot be found in /WEB-INF directory. It's really useful to have a custimizable copy of sitemesh.xml lying in your /WEB-INF directory so you can tweak and perhaps add custom parsers, mappers etc. Following is a copy of a typical sitemesh.xml file
  <property name="decorators-file" value="/WEB-INF/decorators.xml">
  <excludes file="${decorators-file}">
    <parser class="com.opensymphony.module.sitemesh.parser.HTMLPageParser" content-type="text/html">
    <mapper class="com.opensymphony.module.sitemesh.mapper.PageDecoratorMapper">
      <param name="property.1" value="meta.decorator" />
      <param name="property.2" value="decorator" />
    <mapper class="com.opensymphony.module.sitemesh.mapper.FrameSetDecoratorMapper">
    <mapper class="com.opensymphony.module.sitemesh.mapper.PrintableDecoratorMapper">
      <param name="decorator" value="printable" />
      <param name="" value="printable" />
      <param name="parameter.value" value="true" />
    <mapper class="com.opensymphony.module.sitemesh.mapper.FileDecoratorMapper">
    <mapper class="com.opensymphony.module.sitemesh.mapper.ConfigDecoratorMapper">
      <param name="config" value="${decorators-file}" />
Couple of important things :-


These are sitemesh parser parsing html into sitemesh internal Page object to be used by mappers, decorators etc., we'd rarely need to ever change this

<property> tag

These are tags so you can later refer to the property value through ${...} syntax.

<excludes> tag

Exclude files from being parsed, we wanna exclude ${decorator-file} which points to /WEB-INF/ cause we have a mapper that specifically parse this file later.

<mapper> tag

mappers map an incoming request to a decorator which decorates in incoming request and producing a decorated request ultimately returned to the browser. They are executed in order and ConfigDecoratorMapper should always be the last cause it is a 'catch all' mapper. Following is a typical copy of decorators.xml
  <decorator name="error_page" page="/WEB-INF/decorators/error_page_layout.jsp">
  <decorator name="information_panel" page="/WEB-INF/decorators/information_panel_layout.jsp">

With the above example
  • '/bootstrap/*' will not be decorated
  • '/' will be decorated by '/WEB-INF/decorators/error_page_layout.jsp'
In the layout jsp, we could use the following tags :-
  <%@taglib prefix="sitemesh-decorator"  uri="" %>
  <%@taglib prefix="sitemesh-page"  uri="" %>

  <sitemesh-decorator:title />
  <sitemesh-decorator:head />
  <sitemesh-decorator:body />
  <sitemesh-decorator:getProperty property="..." default="[default value if no property found]" writeEntireProperty="[yes/no]" />
  <sitemesh-decorator:usePage id="..." />

  <sitemesh-page:applyDecorator name="..." title="..." page="..."/>

<sitemesh-decorator:title/> tag

Get the content inside html <title> tag of our original html page

<sitemesh-decorator:head/> tag

Stick in the <head> tag content of our original html page, only the content not the enclosing tags though.

<sitemesh-decorator:body/> tag

Stick in the <body> content of our original html page, everything in it.

<sitemesh-decorator:usePage id="..." />

Stick sitemesh Page object into request scope under the variable name given by 'id' attribute.

<sitemesh-page:applyDecorator page="..."/>

Apply the decorator given by 'page' attribute against the body of this tag. In other words, the body of this tag is going to be decorated by the decorator given in page attribute.
   Apply decorator named 'myDecorator' on the body of this tag.
 <sitemesh-page:applydecorator name="myDecorator">

     Apply decorator located at '/WEB-INF/decorators/myDecorator' on the body 
     of this tag overriding the title (if exists) in the body of this tag.
  <sitemesh-page:applydecorator page="/WEB-INF/decorators/myDecorator" title="...">

<sitemesh-decorator:getProperty /> tag

Stick in the parsed original html page's bits and pieces Eg. <html> tag attributes are sticked in as properties without any prefix <title> tag content are sticked in as 'title' <body> tag attributes are sticked in with prefix 'body' <meta> tag 'content' attributes content are sticed in with prefix 'meta' Eg.
with :-
<html myAttribute="test">
    <title>my title</title>
    <meta name="meta-name" content="meta-content" />
  <body onload="alert('ready');">

  This will gives us "test" from  tag's attribute
<sitemesh-decorator:getProperty property="myAttribute" />

  This will gives us 'my title' from the content of <title> tag
<sitemesh-decorator:getProperty property="title" />

  This will gives us 'meta-content' from  tag with name 'meta-name'
<sitemesh-decorator:getProperty property="meta.meta-name" />

  This will gives us 'alert('ready');' from onload attribute in <body> tag
<sitemesh-decorator:getProperty property="body.onload" />

Chao ^_^

Tuesday, November 6, 2012

Do we need equals(...) and hashcode(...) overrides in JPA domain objects ?

The idea of overriding equals(...) and hashcode(...) is such that an object can be uniquely identified through some more meaningful semantics instead of the default semantics which basically says that "object of the same class are equals if they are in the same memory location".

With JPA implementations, the concerns we have when leaving equals(...) and hashcode(...) as defaults are :-

  1. Composite primary Key will not work
  2. Issues with detaching and merging of domain objects
  3. Multiple copies of objects that are semantically similar can exists in our collection object
  4. entityManager.persist(...)

#1 Composite Key will not work

If we are not using composite key at all, then we should be fine. Is composite key a good thing to used compared to just say running number generated off sequence by the database itself is another topic of itself. I suppose if we are doing it for an existing database with tons of data already populated with some natural composite key being already in place, we'll just have to live with it. If it's a green field project, do we really want to use natural keys as our composite key?
  • Natural composite primary key takes up more indexing space compared to just incremental number as primary key
  • Database might take up more time when storing natural composite key assuming the index is a BTREE, where it needs to find the slot to stick in the composite key. Incremental number is just more predictable to the db in this case and I guess most db will be coded to take advantage of this.

#2 Issues with detaching and merging

This is a major concern. I guess the question is do me really need to use the merging feature of JPA? Following are some bits we want to take into account when doing a merge.
  • lazy-loaded relationships aren't going to be merge even if CascadeType is MERGE unless they are triggered before detach
  • merging across a relationship that is being removed will caused an exception
  • merging across a relationship that doesn't exists in persistence context will have undefined consequences except if the CascadeType across that relationship is MERGE
  • accidently 'null'ing out a detached non-lazily loaded or a triggered lazily loaded relationship will null out its counterpart in the persistence context when merging

It's much more convenient, in my humble opinion, to just do an DIY merge rather then relying on JPA's merging mechanisms (this comes from a web developer perspective). Say, we have forms on web pages that upon submit gets to a Spring controller where the values from the form are being populated in to a command object. We could just do a DIY merge into the domain object we do an 'entityManager.find(...)'.

 private EntityManager em;

 public String submission(Command command, BindingResult bindingResult, Model model){
      MyDomainObject domainObject = em.find(...);
But doesn't that means I'll lost my Optimistic Locking check if i have a '@Version' property in my entity?

Unfortunately I guess to a certain extend yes. So we'd probably wanna do that bit of logic ourselves.

#3 Multiple copies of objects that are semantically similar can exists in our collection object

If we do
   Set set = ... ;
   set.add(new MyDomainObject("jack"));
   set.add(new MyDomainObject("jack"));
both will be treated as different entity and will result in 2 records in the set itself. However this is arguably controllable in our application, surely we'll have some sort of validation be it in the controller or service that restrict addition that do not make any business sense.

#4 entityManager.persist(...)

So if we have an auto-generated primary key eg.
  private long id;
the id will only be valid after entityManager.persist(...) is invoked, else it'd be zero (the default value for any long primitive type). Since we did not overrides equals(...) and hashcode(...) to based on 'id' for equality, we are safe to use our domain object after entityManager.persist(...) is called.

Just me 2 cents. If you have a natural business key for your domain objects that uniquely identify itself, feel free to override equals(...) and hashcode(...). If you decide not to and if you are ok with working around some restrictions, that you should be fine as well.