Random Thoughts

The Simple Network Management Protocol (SNMP) is heavily used to manage devices on a network. SNMP4J is an open source SNMP implementation that allows Java programs to create, send and receive SNMP messages. In this post, I will show a simple example to read the raw idle cpu time from a server.

In order to use SNMP4J API, you need to have the following jars in the project classpath:

• snmp-1.3.jar
• SNMP4J.jar
• log4j.jar

The first step is to create a new communication object to connect to the remote host.

InetAddress hostAddress = InetAddress.getByName("remote_host_name");
SNMPv1CommunicationInterface communication = new SNMPv1CommunicationInterface(0, hostAddress, "community_name", 151);

The next step is to query the Management Information Base for the raw idle cpu time.

SNMPVarBindList list = comm.getMIBEntry("1.3.6.1.4.1.2021.11.53.0"); 
Here 1.3.6.1.4.1.2021.11.53.0 is the OID for the raw idle CPU time. 

The final step is to retrieve the value from the returned value. Since we know that the returned list should have only one value, we look up the first item.

SNMPSequence sequence = (SNMPSequence) list.getSNMPObjectAt(0);
String idleCPUTime =  ((SNMPInteger)sequence.getSNMPObjectAt(1)).toString();

Putting it all together:

import java.net.InetAddress;
import snmp.SNMPInteger;
import snmp.SNMPSequence;
import snmp.SNMPVarBindList;
import snmp.SNMPv1CommunicationInterface;

public class SnmpLookup 
{
    public String getIdleCpuTime(String name, int port,String community)
    {
        String idleCPUTime = null;
        try
        {
            InetAddress hostAddress = InetAddress.getByName(name);
            SNMPv1CommunicationInterface comm = new SNMPv1CommunicationInterface(0, hostAddress, community, port);
            SNMPVarBindList list = comm.getMIBEntry("1.3.6.1.4.1.2021.11.53.0");
            SNMPSequence sequence = (SNMPSequence) list.getSNMPObjectAt(0);
            idleCPUTime =  ((SNMPInteger)sequence.getSNMPObjectAt(1)).toString();
        }
        catch(Exception e)
        {
           // Purposefully swalloning the exception
        }   	
        return idleCPUTime;
    }   
}

Recently, when trying to execute a Sybase stored procedure using SimpleJdbcCall, I found that the Spring framework could not read the database metadata. SimpleJdbcCall uses the metadata to intelligently figure out the parameters and their types and just makes stored procedure invocation painless.

After going through Spring’s source code, I found that Spring uses the database product name to determine how the metadata should be loaded. Turns out that the commonDatabaseName method in the JdbcUtils that is used to get the product name does a string comparison against “sql server” and our Sybase server is named “SQL Server”.

I submitted a patch to do this comparison with out taking considering the case. Hopefully this gets considered in the 3.0 release.

Today, I ran into a really cool eclipse plugin called Mousefeed. Once installed, this plugin will popup a reminder with shortcut information every time a button or a menu item is clicked using mouse.

I find this a great way to quickly learn Eclipse shortcuts and becoming productive with the IDE.

I feel that an important advantage of using JQuery is its ability to separate behavior from structure. Consider the case where you want to trigger a JavaScript method when a link is clicked. A common implementation would be:

Click Me

Even though this mixing of behavior (JavaScript) and structure (HTML) looks straight forward, it can become a maintenance issue in large projects. This is where JQuery shines. Using JQuery, the above code can be rewritten in a cleaner way:

Click Me

Now, consider a slightly trickier case where context specific parameters need to be passed to the JavaScript method:

Click Me 1
Click Me 2
Click Me 3
Click Me 4
Click Me 5

Here is the refactored version using JQuery:

Click Me 1
Click Me 2
Click Me 3
Click Me 4
Click Me 5

1. Selectors
   Before an element can be manipulated using JQuery, it needs to be identified. JQuery provides a powerful syntax for selecting a one or more elements. Here are some simple selectors (notice similarity to CSS element selection):
   • Selecting by id: $(‘#yourelementid’)
   • Selecting by class name: $(‘.yourclassname’)
   • Selecting by tag name: $(‘p’) selects all the

     elements in the page

   Other useful selectors:

   • div > a will select all the links present directly inside a div tag
   • tag[attribute=value] will select all the elements of type tag with the given attribute name and value. For example input[type=’text’] will select all the text tags
   • :checkbox, :radio, :file are shortcuts to select checkboxes, radio buttons and file elements.
   • :hidden selects all the elements that are hidden (such as head and base tags). This DOES NOT select all the form hidden fields.
2. Reading and Modifying attribute values
   Once a element is selected, its value can be read and modified using attr method, Here are examples:
   • $(‘base’).attr(“href”) reads the href attribute of the base tag
   • $(“#formTextId”).attr(“value”, ‘Test Value’);
3. Document ready handler
   The document ready handler provides a convenient location to place all the document’s initialization code. JQuery executes this handler right after the browser finishes converting HTML into DOM tree. Here is the syntax for handler:

   $(document).ready(function() {
      // Your code
   });
   

   A shortcut syntax for this:

    $(function() {
         // Your code
    });
   

   It is possible to chain the several handlers and JQuery will execute them in the order in which they are declared.

4. live() method
   JQuery’s live() method binds a handler to an event for current and future matched elements. This is extremely useful when elements are loaded dynamically (using AJAX for example) and need to have their behavior controlled by JQuery. For example the following code will display a “Print” window when a link of class printLink is clicked.

    $(".printLink").live("click", function(){
   	window.print();
   	return false;	
   });
   

   A JQuery plugin called Live Query provides a similar but more robust functionality

5. load() method
   Provides a simple and convenient way to inject html content into an element (or set of elements) using Ajax. For example the following code injects html code of a

   element with id ‘paragraphId’ inside test.html page into div with id ‘someDivId’.
   $(‘#someDivId’).load(‘/test.html #paragraphId’);

JQuery’s form plugin provides several convenient methods for clearing/resetting form element values. However, the clearForm method does not reset hidden fields. Here is a simple implementation for clearing hidden fields:

jQuery.fn.clearHiddenFields = function() {
	return this.each(function(){
		$("input[type='hidden']", this).each(function(){
			this.value = '';
		});
	});		
};

Invoking this method is very straightforward:

$("#yourformid").clearHiddenFields();

or simply:

$("form").clearHiddenFields();

In this post I will share how to cache method results using Ehcache and Spring AOP. This is based on the post

The first step is to add all the required jars to the project. At minimum, the following jars should be present:

Spring and related jars (using 2.5.6 version here):

• spring-2.5.6.jar
• commons-logging.jar
• commons-collections.jar
• aspectjrt.jar
• aspectjweaver.jar
• jsr250-api-1.0.jar

Ehcache and its dependencies (using version 1.5.0):

• ehcache-1.5.0.jar
• backport-util-concurrent-3.0.jar
• jsr107cache-1.0.jar

For the sake of this post, I will be using a DemoService with a method whose results need to be cached.

package com.inflinx.blog.service;
import java.util.List;
public interface DemoService
{
	public List getCarTypes(String filter);
}

Here is a dummy implementation of the above service:

package com.inflinx.blog.service;
import java.util.ArrayList;
import java.util.List;
import org.springframework.stereotype.Service;

@Service("demoService")
public class DemoServiceImpl implements DemoService
{

	public List getCarTypes(String filter)
	{
		List carTypes = new ArrayList();
		carTypes.add("Mini");
		carTypes.add("Economy");
		carTypes.add("Sport Utility");
		carTypes.add("Convertible");
		carTypes.add("Standard");
		
		return carTypes;
	}
}

And the application context with bean scanning turned on:

The next step in the process is to add ehcache.xml to the classpath. Details on this configuration file can be found at Ehcache web site

Spring makes integration with Ehcache easy with EhCacheManagerFactoryBean and EhCacheFactoryBean classes. The EhCacheManagerFactoryBean provides access to Ehcache’s CacheManager and the EhCacheFactoryBean exposes caches defined in ehcache.xml.

To make use of these beans, lets add the following to the application context file:

The next step is to create an AOP Aspect that would provide caching transparently:

package com.inflinx.blog.aspect;

import javax.annotation.Resource;
import net.sf.ehcache.Cache;
import net.sf.ehcache.Element;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;

@Component("cacheAspect")
@Aspect
@Order(2)
public class CacheAspect
{
	@Resource(name="carTypesCache")
	private Cache cache;
	
	@Around("execution(* com.inflinx.blog.service.DemoService.getCarTypes(java.lang.String))")
	public Object cacheCarTypes(ProceedingJoinPoint joinPoint) throws Throwable
	{
		Object[] arguments = joinPoint.getArgs();
		String location = (String)arguments[0];
		Element element = cache.get(location);
		if(null == element)
		{
			Object result = joinPoint.proceed();
			if(null != result)
			{
				element = new Element(location, result);
				cache.put(element);
			}
		}
		return element != null ? element.getValue() : null;
	}
}

Finally, declare the newly created aspect in the application context file:

Spring Modules provides a great alternative for declarative caching with @Cacheable annotation. As of this post, Spring Modules is still in “draft” status.

You can download the source code used in this post here