Using Log4J in Spring: Effective Logging

Spring supports logging in its usual plug and use format. To use Apache's log4j in a Spring-Maven environments:

1. Add the log4j dependency to the pom
2. In web.xml of the application define spring listener for log4j and its config location
3. In the config location put in the appropriate properties file defining your loggers

Log4J Config

The log4j.properties at the above link is fully commented to help you(and me) understand how to use a properties file to split your logs at a finer level.

I have realized that in order to utilize the full power of log4j it is important to understand and use log4j.properties. This can help you to isolate logs from different part of of your application- something that can be really handy when you are trying to pin-point the source of trouble in a large system. The bigger and more distributed your system becomes, more important and useful logging appears.

As usual, I have focussed only on the practical aspects. For knowing more about log4j, see:

Log4J Website @ Apache

Log4J on Wiki

Log4J API 

PostgreSQL Vs MySQL

My team decided to use PostgreSQL instead of MySQL. This got me interested in finding out why. Found these articles that helped me understand:

WikiVs on MySQL vs PostgreSQL

(a recent comparison, subtly votes in favor of PostgreSQL)

Comparison by Fermilab

(this is the most concise comparison but is a little old, very nicely tabulated)

Comparison by Ian Gilfillan

(a very old comparison to give some prespective)

In a nutshell, both are equally good in the current scenario; though before the 8th release of PostgreSQL,  MySQL held a distinct advantage. Currently PostgreSQL offers a wider range of features and on the whole MySQL is a bit faster. However with every release MySQL is inching towards more features and PostGreSQL towards more speed. MySQL is currently more widespread, perhaps because it is older and is preferred by the widespread PHP community. Another difference is the licensing. Both are open-source but MySQL is under GNU while PostgreSQL is under BSD.  

Using JAXB in a Spring-Maven Environment for WebServices

In a Spring application you can consume a webservice using JaxRpcPortProxyFactorybean or XFireClientFactoryBean (in conjunction with XFire). However, there is another way which can prove to be extremely useful and convenient if you are using Spring and Maven: by using Spring and Maven in conjunction with JAXB. As usual, I will focus on using JAXB and not about JAXB. To learn more about JAXB see:

JAXB Reference at Sun

In this article we will cover

1. Setting up JAXB in Spring environment to generate stubs for the webservice

2. Using it in an external project or the same project.

As per the default maven build structure, you need to make a package called "wsdl" under src/main and place a copy of wsdl of the webservice you wish to use. Then configure your pom file with the right dependencies:

POM config for JAXB

Two things that you should notice in the pom besides JAXB dependencies are the build plugins maven-source-plugin and maven-jaxb2-plugin. They will generate stubs for your webservice in the "target" directory under "generated-sources" when you compile the project/ generate resources using maven. After that your webservice should be ready to used. Almost :D.

The generated source will contain stubs that correspond to exposed functions, responses and the various objects used. The name of the stubs will correspond to name in wsdl. In the same project you can use it by importing stubs from the package specified in com.saveenkumar.myApp.someWebService.types.  For an external project, you just need to import the project as a dependency.

Let us now see how to use these stubs. You will be able to use the webservice in your Spring app with the help of org.springframework.ws.client.core.WebServiceTemplate. You can see the bean config that you can use to inject it here:

WebServiceTemplate config as a spring bean

Note that the "contextPaths" property of the JAXB marshaller is same as the  property above. If you want to dig still deeper, have a look about using WebServiceTemplate  at:

WebServiceTemplate Reference

and

WebServiceTemplate API

It may be a good idea to hide the WebServiceTemplate behind a nice interface to abstract away the user from the generated JAXB classes.

The generated classes will always have a class called "ObjectFactory". There will also be objects corresponding to every operation, every response and every object used in these transactions. Let us say our webservice exposes the following operations:

List getAllStockInfo();

List getStocksByVolatilityRange(float low, float high);

StockInfo getStockInfo(String ticker);

May generate following objects:

ObjectFactory

ArrayOfStockInfo

StockInfo

GetAllStockInfo

GetAllStockInfoResponse

GetStockInfo

GetStockInfoResponse

GetStocksByVolatilityRange

GetStocksByVolatilityRangeResponse

Let us say we want to use GetStockInfo. The code would look something like:

Sample JAXB Usage

I would also recommend using SoapUI to test your webservices. 

@Resource and @Autowired

In September I wrote an article on using @Autowired for testing RMI services in Spring. Recently when trying to test a webservice consumer, I ran into trouble with @Autowired. Instead using @Resource (javax.annotation.Resource in Java 1.6) seemed to work fine. Digging I found this article in Spring forum:

Spring Forum Article on @Resource and @Autowired

which pointed to this in spring reference:

Spring Reference to Using @Autowired

Seems the two are pretty similar. Using @Qualifier (org.springframework.beans.factory.annotation.Qualifier) with @Autowired solved the problem and in the end I could use either. 

Serialization

Serialization is not just about implementing the java.io.Serializable interface, I learned. Thanks to this article pointed by my colleague, Ben. Hope it helps you too.

Click here to see the article

Retreiving Stored Objects Using Hibernate

This one is relatively simple. 

1. Use org.springframework.orm.hibernate3.support.HibernateDaoSupport to manage data (add or retrieve)
   
2. Refer to the official hibernate documentation to see how you can use Criteria to narrow your result set by clicking here
   
3. use getSession() from HibernateDaoSupport to get the hibernate session and fire away using what you learned in step 2

Persisting A List of Custom Objects Using Hibernate in a Spring Env.

The prospect of persisting an object using hibernate into a database and then reading it back did not seem challenging with good old hibernate taking care of most things. 

Piece of cake? Not quite actually as I discovered while trying to do it for a List of custom objects. Let us see what were the challenges and how I found the solutions. Let us assume I wish to persist an object that contains the current situation of all stocks trading in the market. Our simplistic object has an id and a list of stocks: 

StockInfoHolder POJO

The list of stocks is made of a custom object:

StockInfo POJO

And since we refuse to live without spring, we need some bean config:

Spring Bean Config

With spring so near how can we forget maven and the pom dependencies:

POM Dependencies

Phew! Sometimes I forget how much these XML config make our lives easier (?).  Now, had it been my own custom object, I would have used @Embedded. But what do I do with a java collection. I was tempted to mark it with @Lob and it compiled fine. But when I actually tried to persist it, it gave me one big, fat ClassCastException: java.util.ArrayList cannot be cast to java.sql.Blob. OK, so objects don't turn into byte arrays overnight and that attempt was dumb and desperate. 

Then what? Referring "Java Persistence with Hibernate" (Bauer and King,2006) and official hibernate docs helped my to make my classes look like this:

Hibernate Ready StockInfoHolder

Hibernate Ready StockInfo

Now this works like a charm: at least the part about starting your app, seeing your tables created perfectly and storing your objects at the right place with some class like:

HibernateDao 

In our next post we will see how we read this information back and use it if I face any difficulty with it as well!