Strange things do happen

This a about a basic error that did. I got this error saying “class not found” in runtime. Let me explain the situation. I was calling a method from another osgi bundle staying in another osgi bundle. If I go in to more specific level I was working in usage agent component and I needed to call a method (a static method) that was in the rss-manager component.

In rss manager,

public static Queue <DBStatisticsEntry> getStats() {
return stats;
}

In Usage agent

dbStats = DBSizeChecker.getStats();

For a Java user it don't have any errors. But remember that this did give the error, “class not found” so before getting to know that method, it even don't know the class. And IDE don't give any errors, means that it is not a syntax error. And it don't give any warnings, no best practice violations.

My guesses of what has went wrong..

1. rss-manager jar inside the server do not have the DBSizeChecker class in it. As this class was added lately by me, I thought for a second what it the jar inside the server is not the current one that I am using? But opening the jar showed that class is there.
2. Problem with IDE, there might be a syntax issue but IDE won't show it due to some problem within it. And my server don't through any exceptions, only IDE gives me that exception while debugging.
3. Is this the right way to use static methods? I was confident that it was the way, but as I didn't have clue on what went wrong, I even did a search on “how to call a static method from another class in java”
4. rss manager might not have started, I checked it using osgi console and it was active and running.

And what was the real problem.

DBSizeChecker class was in the 'internal' folder in the OSGI bundle. If you don't know what that is, in OSGI we do not expose internal classes to the outside. Thats is why we didn't get a error from IDE but from runtime. In runtime OSGI comes in to action. This is basics in OSGI, but those matters.

This problem showed me how important is it to have someone to help when you get errors. This error is due to something simple but still I didn't get it, but when I showed it to a senior (kasun) he nailed the error in a minute.

Using OSGi console to debug things

This OSGi console was very helpful to me in finding out problems relevant to OSGi bundles. You can have lot of information about this in the Internet. In here what I am going to write is about how I used it in the debugging.

First start the server with following option

-DosgiConsole. This will start the server with OSGi console. This is very helpful in situations like, checking whether all bundles that you put in to droppings have activated. When the server starts. Put ss in console. This will give a list of all bundles.

Use it with 'like' modifier to get the relevant bundles only. Ex ss like student

Now you can see state of all bundles. With the installation of a bundle in the OSGi runtime this bundle is persisted in a local bundle cache. The OSGi runtime then tries to resolve all dependencies of the bundle.If all required dependencies are resolved the bundle is in the status RESOLVED otherwise it is in the status INSTALLED.

If several bundles exist which would satisfy the dependency, then the bundle with the highest version is used. If the versions are the same, then the bundle with the lowest ID will be used. If the bundle is started, its status is STARTING. Afterwards it gets the ACTIVE status. - (http://www.vogella.com/articles/OSGi/article.html#osgiarch_bundles good reference)

You have to have all your bundles acticvated. If not check what is wrong with them using 'bundle <bundle number>' command. If there is no error, try starting it manually using 'start <bundle number>'

OSGi Services

Full system(carbon core + products) is built as a combination of OSGi bundles. Some of theses bundles expose services at OSGi level. I have to use them to get the tenant usage plan.

There is a service in manager which gives the usage plan of the tenant. To use it first I have to catch them, unluckily those manager component that is needed for the service it not there in the DSS, so we have to add them to DSS temporary as a feature. core and mgt are placed in DSS as a feature. First we thought is putting those to into droppings. But it didn't work as mgt package tried to start before core which is not possible as it requires core. Because of that we have to make it as a feature.

After putting them we have catch that service. Those are normally catches at serviceComponents(one class with 'activate' method). Below is how I did it. There are some methods that is used to bind and unbind the service objects we have to have them where we catch the service

I added the osgi comments as below  

/** 
 * @scr.component name="org.wso2.carbon.rssmanager" immediate="true" 
 * @scr.reference name="default.tenant.billing.service" 
 *                interface="org.wso2.carbon.stratos.common.TenantBillingService" 
 *                cardinality="1..1" policy="dynamic" 
 *                bind="setTenantBillingService" 
 *                unbind="unsetTenantBillingService" 
 * @scr.reference name="user.realmservice.default" 
 *                  interface="org.wso2.carbon.user.core.service.RealmService" 
 *                  cardinality="1..1" policy="dynamic" 
 *                  bind="setRealmService" 
 *                  unbind="unsetRealmService" 
 */ 

then I added following methods

/** 
     * osgi bind method for RealmService 
     * @param tenantBillingService 
     */ 
    protected void setRealmService(RealmService realmService) { 
        this.realmService = realmService; 
    } 

    /** 
     * osgi unbind method for RealmService 
     * @param tenantBillingService 
     */ 
    protected void unsetRealmService(RealmService realmService) { 
        this.realmService =null; 
    } 
   
    /** 
     * returns a RealmService objects that is used to get the tenant list. 
     * @param tenantBillingService 
     */ 
    private RealmService getRealmService() { 
        return realmService; 
    }

After catching it I can use it as below,

if(realmService!=null){ 
        TenantManager tenantManager = getRealmService().getTenantManager(); 
        Tenant[] tenants; 
        try { 
            tenants = (Tenant[]) tenantManager.getAllTenants(); 
            for (int i = 0; i < tenants.length; i++) { 
               // check for each tenant 
            } 
        } catch (UserStoreException e1) { 
            // TODO Auto-generated catch block 
            e1.printStackTrace(); 
        } 
        }else{ 
            System.out.println("realmService NULL"); 
        }

Problems came across Service was not available in DSS First I tried to catch this in my own java class (not in RSSManagerServiceComponent) which was OK but was not the standard. Standard is to catch it in RSSManagerServiceComponent. Anyway it should work, on matter how we do it. In my case it didn't work for tenant.billing.service . Problem was same service (tenant.billing.service not realmservice) was caught in RSSManagerServiceComponent before coming in to my class. This is where I figured out the importance of that standard. Had to include lot of other jar files in droppings.

Stating the WSO2 internship

In the first week we are asked to get familiar with the technologies that we will find in the future. These technologies are relevant to anyone who is in this filed (don't have to work in WSO2). Below I have provided what I went through the first week with the references that they use.(Thank goes to Selvaratnam Uthaiyashankar for providing these materials for us)

XML

=====

1. What is XML? -

* http://www.ibm.com/developerworks/edu/x-dw-xmlintro-i.html?S_TACT=105AGX06&S_CMP=HP

(Introduction to XML developerWorks, Doug Tidwell

(dtidwell@us.ibm.com), XML Evangelist, IBM　). It covers pretty much

everything. Do not worry about DTD, just XML Schema would do.

* http://www.ibm.com/developerworks/xml/newto/

* http://www.xml.com/lpt/a/316

2. XML Parsing, DOM, SAX and Pull - Learn about DOM, and SAX, pull

parsing you can learn later.

* SAX/DOM -http://www.ibm.com/developerworks/xml/library/x-jaxp/

* SAX - http://www.ibm.com/developerworks/xml/library/x-saxapi/

* Validation -

http://www.ibm.com/developerworks/xml/library/x-jaxpval.html?S_TACT=105AGX06&S_CMP=EDU

* StAX'ing up XML, Part 1: An introduction to Streaming API

for XML (StAX) -

http://www.ibm.com/developerworks/xml/library/x-stax1.html - there are

three articles in the StAX'ing up XML, read them later. We do not need

Stax initially for our work.

3. XML Schema - http://www.w3schools.com/Schema/default.asp

4. XML Beans -

http://xmlbeans.apache.org/documentation/tutorial_getstarted.html

Web Services

============

Learn about Axis2 as much as possible. Follow the

Axis2 user guide A to Z,

(1) http://axis.apache.org/axis2/java/core/docs/userguide.html

(2) Apache Axis2 Web Services, 2nd Edition

1. Download and Install Axis2, read user/ developer Guide

2. Invoke a default service. Basically learn all the options, both

POJO and code generate a client and use the client

3. Write a new service and deploy it, and invoke it using your client.