Raible Designs

[DJUG] JMX and JDO 2.0

Last night I attended the local Denver JUG meeting. Chris Huston gave a presentation on JMX and Dion gave a presentation on JDO 2.0. Below are my notes from the event.
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Java Management Extensions one of the first JSRs and now included in to JDK 5. Monitors health of the system. Alter sys-admin of error conditions. Provides capability of to view and modify runtime parameters.

MBeans are deployed in an MBeanServer and their capabilities are exposed through Connectors. Then tools talk to these connectors to get their information.

JMX is covered by two separate specifications: JSR-3 (MBeans) and JSR-160 (Connectors). BTW, Chris has some good slide-making capabilities - I'll have to get some tips from him for my next preso. For example, he shows an entire class and its methods, then he fades it and zooms in on certain methods and highlights them to explain what they do.

Four types of MBeans: Standard MBeans, Dynamic MBeans, Model MBeans and Open MBeans. More info on these Mbeans can be found at http://docs.sun.com.

Standard MBean: All you need to do create an interface for your class your class name with "MBean" appended to it.

In order to deploy, you need to create and configure an MBeanServer. You can create an MBeanServer using javax.management.MBeanServerFactory.createMBeanServer(). MBeanServers keep track of MBeans using a javax.management.ObjectName. After creating and configuring an MBeanServer, you simply need to register your MBean using an Agent. All pretty simple so far, right? Now you have to use a Connector to expose your MBean to the outside world.

"Connectors are the fly in the ointment." MX4J is an open source project that has good Connectors: i.e. RMIAdaptor and HTTPAdaptor. There's also SNMP Adapters from a number of commercial vendors. In order to solve the current lack of good Connectors we have JSR-160: Distributed Services Specification. This JSR defines one connector: JMXConnector (part of JDK 5).

Chris is now showing us a counter widget deployed as a Swing app and managed using MX4J's web interfaces. Very cool stuff - the ability manipulate runtime parameters. As Chris stated - the real power of JMX will come from the tools. I wonder if there's MBeans available for monitoring webapps - seems like there'd be quite a market for management beans you could just drop in and deploy.

Notifications are another feature of MBeans and can be sent by an MBean. Two important interfaces: NotificationEmitter and NotificationListener.

Chris gave a good introduction to JMX, but I don't see myself using it anytime soon. It seems to be something that framework and container developers might use, but not many application developers. Maybe I'm wrong - are you deploying MBeans as part of your web applications? Chris mentioned that Hibernate and Spring have MBeans as part of their distribution - anyone using these? If nothing else, it's great to know that JMX is available if I ever do need to monitor and control applications.

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Now Dion's up after a 1/2 hour break with announcements. Dion is talking about JDO 2.0. I think JDO would be a lot more popular if it wasn't for Hibernate. Hibernate seems to have done JDO better and faster.

What is JDO?

• Allows for object persistence
• "I have objects, some need to be persisted"
• "Keep state and behavior together. That is the OO way."

JDO Main Objectives:

• To enable pluggable implemenentations of data stores into application servers
• To provide applicaiton programmers a Java-centric view of persistent information, including enterprise data and locally stored data.

Other Objectives:

• Transparent Persistence
• Persistence-by-Reachability
• OO framework on top of potentially non-OO stores.
  • RDBMS (98% of the time)
  • Files
  • TP Monitors
  • Legacy
  • Properietary
• Integrate with existing technologies
• Java compatible - no need to learn new language
• Usable in managed and non-managed environments
• Define easy semantics for caching objects
• Ability to run in all Java platforms (J2SE, J2EE, J2ME)

When JDO first started, a bad thing happened - the object database guys were heavily involved. JDO 1.0 had 80% of its Expert Group from object database vendors. JDO 2.0 got object guys off the committee and real-world (relational) folks onto it.

JDO 1.0: Most of it is not very new - just a standard way of implementing persistence. Other proprietary tools already accomplish most of what JDO is.

JDO 2.0 Strategic Goals:

• Easy to Use
  • natural programming model
  • configuration and deployment
• In a Wide Variety of Environments
  • J2SE (rich clients)
  • application, web servers

JDO 2 Status: First Early Draft Review is out there. Soon there's going to be a Community Review - then things will start rolling after that.

JDO 2.0 Goals:

• Maintain JDO 1.0 Compatibility (big thing with Sun)
• Standardize Mapping to Relational DB (was vendor extension in 1.0)
• Multi-tier Development Improvements
• Usability Improvement
• Better Object Modelling
• Richer Queries - JDOQL, SQL and Named Queries
• More Vendor Support - try to get Hibernate and Oracle on the bandwagon for political reasons

Dion's take on politics between EJB 3 and JDO 2

• Let's get a standard around transpanent persistence first
• Let's have a standard that can be shared between various groups: J2EE, J2SE, J2ME
• Why have more than one O/R mapping specification?
• Specifications such as EJB could be an umbrella spec for the cross cutting concerns

Why are people even arguing about it? Money. Because companies have invested a lot of time and money into X technology and therefore they bash the other. This problem might be solved by an O/R Mapping JSR that both EJB and JDO have to adhere to.

JDBC vs. JDO

JDO will use all J2EE services inside a container.

Deployment time scalability - database becomes bottleneck as number of users increases. JDO middle tier can relieve this load on the databse. JDO's configuration can be altered at deploy time to handle better scalability through caching.

JDO 2.0 Multi-tier Development - big thing is disconnected objects through attach/detach. Very simple API for detaching and re-attaching objects. Internally there's a version that's used to determine if the object is the same one.

JDO and EJB
As opposed to Entity bean, JDO instances...

• May have subclasses
• May exist outside a container
• Has 1 optional interface to implement
• Accessible as transient objects

And as of JDO 2 (new and improved object modeling!)

• Persistent Abstract Class Support
  • map abstract classes to tables
  • map properties, fields to columns
  • create new implementation instances
• Persistence Interface Support
  • map interfaces to tables
  • map properties to columns
  • more, but I missed them

Some argue that JDO is what Entity Beans should have been!

• JDO is "purer" way to use OO
• Since Entities should be used via local interfaces, why not just do Session Facade -> JDO objects
• CMP in Sun AppServer uses JDO

Some argue that JDO complements Entity Beans because Entity beans can use JDO.

EJB3 is totally moving directorys with regards to Entity Beans, and they are much more like JDO.

EJB3 EntityManager == JDO PersistenceManager == Hibernate Session

JDO and Connector Architecture - JDO is NOT a framework, but plays well with others!

The JDO architecture uses the J2EE Connector architecture for application servers. The JDO PersistenceManager is a caching manager as defined by the J2EE Connector architecture.

A PersistenceManager may define the following Connector components:

• ResourceAdapter
• ConnectionManager
• ManagedConnectionFactory
• ManagedConnection
• LocalTransaction

Persistence Contract (behind the scenes)
- To achieve transparence, JDO:

• Defines a PersistenceCapable interface
• Used too mandate binary compatibility among all vendors (optional in JDO 2)
• Uses tools to make a class persistence capable

- Declares any fields available for persistence storeage

- Defines methods that allow JDO instance management

• jdoIsDirty()
• jdoIsDeleted()
• jdoMakeDirty()

JDO has a number of callbacks that you can implement: jdoPostLoad(), jdoPreStore(), jdoPreClear(), jdoPreDelete().

In JDO 1: one interface: InstanceCallbacks
In JDO 2: InstanceCallbacks purely extend interfaces: LoadCallback, StoreCallback, etc.

Ways to make classes persistence capable: byte code enhancement, annotations (maybe for JDO 2.1).

JDO Interfaces: PersistenceManagerFactory, PersistenceManager, Transaction (new set/getRollbackOnly() in JDO 2) and JDOHelper (class with static methods, for use by persistence-aware classes), Query, Extent. You'll probably never use an Extent; like "select * from table". Represents all instances of a class or subclasses.

A persistence cabable class is just a POJO - you could use the same class for JDO and/or Hibernate. A default contructor is required in JDO 1, but not in 2.

JDO 2.0 has "use cases" for lazy-loading. This enables you to configure fetching based on certain situations. Sounds pretty cool.

Extent is usually optimized for better performance. If you're selecting all records from a table, you're probably better of using Extent vs. Query in order to avoid OutOfMemory errors.

New in JDO 2: ability to specify query filters in other languages - i.e. SQL or HQL or user-defined. These languages are up to the JDO vendor to implement.
JDO can use stored procedures if you map it in your metadata.

JDO 2.0's Query API looks very rich. It allows you to limit your results to certain properties/columns, as well as set what type of to return.

JDO 2.0 supports obtaining a java.sql.Connection from PersistenceManager. However, this may reduce portability since not all SQL is portable b/w databases.

Overall, I thought this was a great talk by Dion. If you're looking to start working with JDO 2.0, I'd recommend JPOX (also see JPOX with Spring or Tapestry). Unfortunately, their implementation still requires you to "enhance" your classes at build-time, but hopefully they'll have a transparent mechanism for manipulation (i.e. cglib) soon.

Commons Validator with WebWork?

I'm in the midst of integrating WebWork into AppFuse. I started wiring validation last night. This morning, I realized that I could probably use Commons Validator to make validation a lot easier. Here's why:

• WebWork validation rules are defined per-action, rather than per-object. This makes it difficult to define the validation rules with XDoclet. I can't just put tags in the model object to generate the validation rules. This works with Struts and Spring quite nicely.
• Using a custom LabelTag, I've been able to hook into Commons Validator and mark fields as required. With WebWork's validation, it seems that this can only be accomplished by adding a required="true" attribute to my <ww:textfield>. I prefer the transparency of the custom tag.
• I've never been able to get WebWork's client-side validation working. Commons Validator has excellent client-side validation support.
• Spring has support classes for Commons Validator - leading me to believe that I can configure validation in Spring and re-use its JavascriptValidatorTag.

Now I just need to figure out 1) how to hook WebWork custom tags (or my own custom tag) into the validator to mark fields required and 2) how to write a validation interceptor that uses Commons Validator. Whaddya think? Are there advantages to using WW's validation over Commons Validator? Any tips for writing an interceptor or modifying the custom tags?

No G5 PowerBooks Anytime Soon

Mac Rumors: No G5 PowerBooks Anytime Soon. If I had a vote, I'd say it's time to give up on the G5 for the PowerBook and make OS X run on Intel or AMD chips - then produce an Intel/AMD version of the PowerBook. Apple makes great hardware, but their laptops are much slower than their Intel counterparts (for Java at least). All I want is a PowerBook that can keep up with an Intel-based laptop - is that too much to ask?

Going to ApacheCon?

Are you heading to ApacheCon this year? I am.

Thank you for your recent session proposals for ApacheCon US 2004.

The following session has been accepted and scheduled.

  (1010) 'Comparing Web Frameworks: Struts, Spring MVC, WebWork, Tapestry & JSF'

You will receive more information about this soon,
if you haven't already.

Vegas baby.

Eclipse tip o' the day

By default, Eclipse expects all its plugins to be in $ECLIPSE_HOME/plugins. This doesn't work too well for upgrading since you have to copy all of your installed plugins to Eclipse everytime you upgrade. To solve this, you can place all your plugins in an external direct and point to them using a link file. To do this, create a links directory in $ECLIPSE_HOME and create a file inside it that points to an alternate location. The name of this file doesn't seem to matter, but it must end with a ".link" extension. In this file, put a path:

path=C:\\Tools\\myplugins

Then in c:\Tools\myplugins, create an Eclipse-like directory structure so you end up with c:\Tools\myplugins\eclipse\plugins. Then drop your plugins in that directory. If you choose to use an alternate workspace as well, upgrading Eclipse will be as easy as copying in your links folder. BTW, here's more information on upgrading Eclipse.

How do I customize Eclipse on-the-fly?

My task for this week is to figure out how to customize Eclipse on-the-fly? By this, I mean that we're going to install Eclipse and then programmatically add plugins and configure them. Is this possible? We'd also like to add projects to the workbench and have them be there when the user first opens Eclipse. My current technique is to do diffs on directories after I've configured/added stuff, but that's probably not a good long-term solution.

Just take her with you

With my hectic schedule lately, it's been tough to get a workout in and not piss off Julie or Abbie. When I'm done working, Abbie wants to play like a crazy lady. I don't blame her - we have a lot of fun together. If I finish the day and walk into the living room to tell them I'm heading out to get some exercise, both of them get mad. Julie because I've been working all day and she wants a break; Abbie because she's ready for some fun.

About 2 weeks ago, we were all a little huffy at each other as I wanted to head out for a run before DJUG. Julie was trying to convince me to skip it so I could spend some time with Abbie. Abbie saw I was leaving - put on her shoes and ran to the door to go with me. Everyone was tense and Julie says "Just take her with you!" I laugh b/c our stroller isn't a jogging stroller, but then I realize it wasn't a bad idea.

Abbie usually can't sit still in the stroller for more than a few minutes, so even though I thought it would be a disaster, I took her anyway. I jogged and pushed her around our local park - and to my amazement - she loved it. She didn't want to get out once - she just giggled and laughed and pointed out all the dogs and airplanes. Since then, we've gone a couple times.

Getting bold with this new "take her with you" mantra, yesterday I took Abbie on a hike. Rather than a simple walk around the lake, I wanted to take her on a real hike. So we trekked on out to the foothills, strapped her in the backpack and headed up Mt. Falcon. This is a beautiful trail that I used to mountain bike a fair amount. I expected an hour trip, but it ended up being an hour and 40 minutes. How did Abbie do? She loved it. Sure we had to stop a few times, and of course she didn't want to get in the backpack after doing so, but I had lots of joose and gummy bears, so she was happy. It was something I'll remember for the rest of my life - especially the last 20 minutes where she made me sing to her the whole time. A true father-daughter experience - I'm looking forward to many more. I can't wait until Raible #2 arrives, I'm taking a month off and hope to enjoy a lot more days like yesterday.

My Tiles to SiteMesh Migration

I spent a few hours last night replacing Tiles with SiteMesh in AppFuse. I had the Struts version done in 2 hours, and most of the Spring version done in an additional hour. Then I spent another 3 hours today twiddling with things and getting it just right. During this process, I discovered a few things I thought I'd share. Keep in mind that I'm pretty much a SiteMesh rookie. Hopefully, implementing and using it in AppFuse will help me to fully understand its power.

• SiteMesh lacks injection. Tiles allows you to inject JSP fragments into your base layout on a per-page basis. I didn't use this feature much, but I did use it to determine which menu should show up on what pages. For example, the login page would get a "projects used" menu, the signup page wouldn't have a menu, and all other pages would get the standard site menu. With SiteMesh, all of this seems to be done best with JSP includes and some <c:if> logic in your decorator. I ended up using <c:import> in my login.jsp for the login menu, importing nothing for signup, and using <c:if> statements in my decorator to see if the user was logged in. If they were, then I import the site menu and its necessary scripts and styles. In the end, this worked fine and it's probably easier for new AppFuse users to understand - so that's a good thing.
• Injecting scripts and stylesheets. With Tiles, I was able to easily control which scripts and stylesheets were shown on each page. With SiteMesh, this is pretty easy to do by putting them in the <head> element of your page. However, what I would've really liked to see is the ability to put these in an included JSP and have them end up in the <head> of the final document. I know it's virtually impossible, but it would be cool.
• Headings. Adding a <title> title element per-page works great using the <title> tag. However, if you want to add a heading (which I specify with an <h1> tag), you have to use a <content> tag. It would be cool if I could somehow use <h1> or <heading> in my page to indicate a heading. I ended up going with the following in each page to specify the title and heading:

  <title><fmt:message key="mainMenu.title"/></title> <content tag="heading"><fmt:message key="mainMenu.heading"/></content>

  There's probably an easier way to do this, but this works for now.
• Injecting <body> ids. Using body ids to set styles on a per-page basis is a great way to control CSS. With Tiles, I set this as an attribute in tiles-config.xml and then grabbed/used it with the following JSTL code:

  <c:set var="bodyId" scope="request">         <tiles:getAsString name="body.id" ignore="true"/>     </c:set> <body<c:if test="${not empty bodyId}"> id="<c:out value="${bodyId}"/>"</c:if>>

  Yeah, it's ugly, but it works. With SiteMesh, you can easily set a body id by using <body id="name">. In fact, you don't even need to wrap your content with it, you can simpley do <body id="name"/>. I'm grabbing and using this in my decorator with the following code. This works, but it would be cool if I could check for the existence of the attribute first - so I could eliminate id="" when no body id is set.

  <body id="<decorator:getProperty property='body.id'/>">

• Inheritance. With Tiles, you could extend page definitions and override attributes. This feature seems to be completely lacking in SiteMesh. I don't know how you could implement it, but it would be nice to have something where you could specify the parent - for instance, to use the same <head> content.
• Error page decoration. SiteMesh seems to be incapable of decorating error pages (i.e. 404) in Tomcat 5.0.x (even if I add <dispatcher>ERROR</dispatcher> to the filter-mapping). A workaround is to use wrap the error page with a <page:applyDecorator> tag. This works, but if you specify elements in <head>, they will end up in the body of page, rather than in the decorator's header.
• SiteMesh simplifies. Switching from Tiles to SiteMesh allowed me to delete somewhere around 8 JSPs. AppFuse has 34 after committing everything. Most of these were JSPs that sat in the root folder and had a one-liner to pull in a Tiles definition. However, it also eliminated 11 JSPs from the Spring MVC install - allowing reduction of duplication. The Spring MVC install now has 17 JSPs.

So there you have it. AppFuse now uses SiteMesh instead of Tiles! I'm sure the implementation will get cleaner and more refined as more folks use it. I'm looking forward to deleting some chunks out of AppFuse's tutorials because SiteMesh makes page development so much easier. The hardest part of SiteMesh is setting up the infrastructure. Once you're got that done, you hardly ever touch it again.

Next task: WebWork integration.

One thing's for sure

The Java Community certainly does care about JSF. Regardless of whether folks like it or hate it - there's definitely a lot of opinions! Who woulda thunk a simple blog entry would get so much traffic? Wierd.

You know it's a late night when Erik posts before you go to bed...

FindBugs

I ran FindBugs on AppFuse last night and found/fixed a number of issues as a result. I'm now down to only a handful left - most of them being "Class is Serializable, but doesn't define serialVersionUID". I tried to generate one using serialver, but I couldn't it to work after numerous attempts. My issues seemed to be classpath related: it wanted the servlet api in my classpath, and once I'd add that, it could find my class. I'll have to try the SerialVer Ant Tasks. Fixing this issue would be nice, but I doubt it's really affecting appfuse-based applications. The other bug is "Inconsistent synchronization" in UserCounterListener.contextInitialized() method. Any tips on solving this one are appreciated.