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MSAccess and a really weird behaviour with distinct
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Loading ...Recently I started working with MS Access databases, I know not the best choice. I ran into a really weird problem with the database, which I didn’t expect.
I have a relativly simple select query which contains a distinct. Something like “select distinct id, content from table”. Nothing fancy so you’d expect the content of the table excluding all duplicates.
But this is also where the weirdness begins with MS Access. I was using this type of query to select data for my search engine. But some data wasn’t included for some reason.
After a lot of debugging my queries and code it turned out that MS Access ODBC was only returning the first 255 characters of a MEMO field. So I thought the driver was to blame, until I ran the same query in MS Access itself. As it turns out Access was the one truncating MEMO fields to VARCHAR(255) fields when using a DISTINCT in a query.
Controlling Tomcat using Ant scripting
As a Java webdeveloper you are faced with a lot of different aspects of development. One is testing and debugging code, to do this you need an application server like Tomcat. One of the issues I recently encountered was the need to control various Tomcat instances with a single Ant build script. Below are some of the solutions I’ve used to manipulate Tomcat.
Note: some parts of this script rely on the ant-contrib library. Download this library and include it in the tomcat.xml with the following code:
<taskdef resource="net/sf/antcontrib/antcontrib.properties"> <classpath> <pathelement location="./ant-contrib-1.0b3.jar"/> </classpath> </taskdef>
Starting tomcat
Though starting tomcat may seem easy. You could just use the development IDE (like Netbeans) to start and stop Tomcat, but what this lacks is the ability to control several instances with easy shortcuts. So I defined the following Ant macrodef in a file called tomcat.xml:
<macrodef name="tomcat-start">
<sequential>
<trycatch>
<try>
<if>
<not><http url="http://localhost"/></not>
<then>
<java classname="org.apache.catalina.startup.Bootstrap"
fork="yes"
dir="${tomcat.dir}"
spawn="true"
jvm="${tomcat.java.home}/bin/java">
<jvmarg value="-Dcatalina.home=${tomcat.dir}"/>
<jvmarg value="-Dcatalina.base=${tomcat.dir}"/>
<jvmarg value="-Djava.io.tmpdir=${tomcat.dir}/temp"/>
<jvmarg value="-Djava.endorsed.dirs=${tomcat.dir}/common/endorsed"/>
<jvmarg value="-Xdebug"/>
<jvmarg value="-Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n"/>
<classpath>
<pathelement location="${tomcat.java.home}/lib/tools.jar"/>
<pathelement location="${tomcat.dir}/bin/bootstrap.jar"/>
</classpath>
<arg line="start" />
</java>
<waitfor maxwait="10" maxwaitunit="second" checkevery="5000">
<http url="http://localhost"/>
</waitfor>
<echo message="Tomcat started"/>
</then>
<else>
<echo message="Tomcat already started..." />
</else>
</if>
</try>
<catch>
<echo message="Unable to start tomcat"/>
</catch>
</trycatch>
</sequential>
</macrodef>
This macro is really easy. First it starts of by checking if tomcat is not already running (well actually it just checks to see if there is something running on port 80). If nothing is running then tomcat is started using the java defined by the ant property tomcat.java.home. This macro also depends on some other properties that are set by a different macro, which will be a bit further on in the post. Tomcat is started by using the Bootloader class provided by the server, to make sure it loads correctly the classpath is set.
The last step in the macro is to wait for tomcat to start, this has to be done since we start tomcat in a seperate java instance (the spawn option).
Stopping Tomat
Off course you also want to be able to stop tomcat to load some new data, or an entire web-app. I use the following macro to stop Tomcat:
<macrodef name="tomcat-stop">
<sequential>
<trycatch>
<try>
<java classname="org.apache.catalina.startup.Bootstrap"
fork="yes"
dir="${tomcat.dir}"
spawn="true"
jvm="${java.home}/bin/java">
<jvmarg value="-Dcatalina.home=${tomcat.dir}"/>
<jvmarg value="-Dcatalina.base=${tomcat.dir}"/>
<jvmarg value="-Djava.io.tmpdir=${tomcat.dir}/temp"/>
<classpath>
<pathelement location="${tomcat.java.home}/lib/tools.jar"/>
<pathelement location="${tomcat.dir}/bin/bootstrap.jar"/>
</classpath>
<arg line="stop" />
</java>
<kill-java name="Bootstrap" />
<echo message="Tomcat stopped" />
</try>
<catch>
<echo message="Unable to stop tomcat forcing shutdown....."/>
<kill-java name="Bootstrap" />
</catch>
</trycatch>
</sequential>
</macrodef>
Just like with the starting of Tomcat I use the provided Bootloader class to instruct tomcat to stop. Again I use the java that is set in tomcat.java.home as well as some other properties loaded by another macro. Since Tomcat sometimes fails to stop gracefully (due to poorly designed webapps) you also have to kill the Java process of Tomcat. This is done with the call to kill-java.
Killing any java process
As you saw in the tomcat-stop macro I used a macro called kill-java to make sure that Tomcat is really killed and no longer running in the background. The macro is as follows:
<macrodef name="kill-java"
description="Forcefully stop tomcat....">
<attribute name="name"/>
<sequential>
<!-- Execute the jps and check for any Java process with the provided @{name} attribute -->
<exec executable="${tomcat.java.home}/bin/jps" output="pid.out.file" />
<!-- Load in the name / pid file and strip all information except the PID -->
<loadfile srcfile="pid.out.file" property="pid.out">
<filterchain>
<linecontains>
<contains value="@{name}"/>
</linecontains>
<tokenfilter>
<deletecharacters chars="@{name}"/>
<trim/>
<ignoreblank/>
</tokenfilter>
<striplinebreaks/>
</filterchain>
</loadfile>
<echo message="Killing java process with pid ${pid.out}"/>
<!-- Kill the process, warning this only Works on Windows -->
<exec spawn="true" executable="taskkill">
<arg line="/PID ${pid.out}" />
<arg line="/F" />
</exec>
<delete file="pid.out.file" />
</sequential>
</macrodef>
This macro is really simple and relies on the jps application provided by Java. This application returns a list of all running processes with name and process id (PID). All we need to do is get the line containing the process name provided and strip everything except the PID.
Please note: the task killing the proces is designed for Windows, you could change this with kill in Linux.
Rounding it up
Though you now have all you need to manipulate tomcat there is one last macro that we are tegenkant upon. That is the initializer of the various Tomcat properties. I use the following macro:
<macrodef name="tomcat-init">
<attribute name="from"/>
<sequential>
<property name="tomcat.dir" value="${tomcat.@{from}.dir}"/>
<property name="tomcat.server" value="${tomcat.@{from}.server}"/>
<property name="tomcat.port" value="${tomcat.@{from}.port}" />
<property name="j2ee.server.type" value="${tomcat.server}" />
<taskdef name="webapp-stop"
classname="org.apache.catalina.ant.StopTask"
classpath="${tomcat.dir}/server/lib/catalina-ant.jar"/>
<taskdef name="webapp-start"
classname="org.apache.catalina.ant.StartTask"
classpath="${tomcat.dir}/server/lib/catalina-ant.jar"/>
</sequential>
</macrodef>
This little macro will initialize tomcat for a specific environment (for exemple DEV). Of course none of this works without the loading of some preset properties in a property file tomcat.properties. Which contains the following data:
tomcat.java.home=C:/jdk1.5.0_22_32b tomcat.DEV.dir=d:/jakarta/tomcat5.0 tomcat.DEV.server=tomcat55-DEV tomcat.DEV.port=8000 tomcat.MAIN.dir=d:/jakarta/tomcat5.0-MAIN tomcat.MAIN.server=tomcat55-MAIN tomcat.MAIN.port=8000
To load the property file just include the following instruction in the build script:
<property file="./tomcat.properties" />
Netbeans 6.5 Final Release
A short while back I wrote a quick review on the beta of Netbeans 6.5 and that it had some issues. Since the final version was released some time ago I thought it was time for a new review. This time of the final version.
The reason for me not to write one straight away was simple, I first wanna play around a little to find all the pleasant features and those less pleasant ones.
Bigger, better, best
The first thing you expect is this new version to be bigger, better and the best one yet. Well it is bigger in size, with 190MiB for just Java development. So this means a lot more time downloading, but hey that’s a one time thing.
So lets move onto something more relevant. Is it better then Netbeans 6.1. Lets summorize some of the issues I had with the previous version and see if the problem is solved:
- It crashed when working on larger projects. This issue has been fixed, netbeans won’t crash or hang. Though on occasion the parser may crash. Disabling autocomplete and syntax checking.
- For some reason previous versions had difficulty displaying the names of parameters when using Java libraries. For as far as I’ve been able to determine this has been solved.
- The autocomplete completely fails in some more complicated web projects. Well I’m sorry to say this is still true. For some projects not only does autocomplete fail, but so does the syntax checker, both for the JSP files as well as the Java Source files.
So are there any other changes that would make you consider moving from Netbeans 5.5 to 6.5?
New stuff you will like in Netbeans 6.5
Well have there been any changes that would make you wanna switch to the new version.
- The new version actually has some basic debugging integrated of JavaScript. Though I haven’t got it working yet.
- PHP development is now support. Which is a big plus, but it still kinda feels like a cheap addon that doesn’t work perfectly yet. (Maybe in the next release)
- Thread debugging has been improved and made easier
Unfortunattely not all is good in the world of Netbeans. There are also some new bugs, some of which are really annoying.
- For some unexplainable reason adding of libraries by right clicking the ‘Libraries’ in the project view doesn’t work all the time.
- Like I stated earlier the code completion fails in some of the older Java project I’m working on. Not only does code completion fail, but the syntax checker tells me every single line is wrong. Even though the project compiles beautifully
As a final note: is it worth upgrading to this version. Then I’d have to say hell yeah, it’s a lot better then previous versions. If you are a PHP developer however I suggest not using it, for the moment. Especially when you are already using Zend-Studio, which is still a lot better.
Netbeans 6.5 Beta
Since I have to use Netbeans at my work, I always try and find the best version for me. In my case this means a version that runs quickly and is sorta stable. Since the new Netbeans 6.5 beta was released I decided to give it a shot.
First things I was seriously hoping was that they fixed the bug with Netbeans not recognizing the names of function parameters of libraries. This is a problem which got me so pissed that I switched back from Netbeans 6 to 5.5, which did not have the problem. Both Netbeans 6.0 and Netbeans 6.1 had the same problem with function parameters.
Let’s just say that on this they did not disapoint. The new Beta version has got no issues recognizing the names of function parameters. So that’s a serious improvement of the previous releases. I also like the new style for debugging and code editing.
The only real issues so far are the fact that with large web applications Netbeans 6.5 constantly hangs and stops responding. Another issue is that the debugger does not highlight the line that it’s on. The latter is probably a simple fix, but it does make debugging slightly difficult.
So lets wait and see if some of these issues get resolved in the final release.
How To Get An Accurate Session Count With Tomcat
Recently we started to notice that even after disabling a server in our loadbalancer we still get traffic on it after two months. At least for as far as we count it. We currenlty count traffic by using the SessionListener to get notification of people leaving and coming, but somehow our counter gets stuck at about 500 sessions 2 months after switching the server of in the load balancer.
Some time back I already wrote about the setup we use in counting sessions. Read more on that in the ‘Cleaning Up Java Sessions’ post. We put live this change some time back. But I’m still having some issues with the sessions.
What we do is something like this. When a user comes in through a servlet we look if a specific object is set in the session. If not we invalidate the session and add the object. In a seperate class we catch all session starts and destructions.
Upon the session start function in HttpSessionListener we increase the counter. Upon the destruction of the session and if the our session object is set we decrease the counter. But for some reason we have some sessions that do not get cleaned up by Tomcat. Or there are some users that simply stay online for over 2 months.
To get a semi accurate count of the active session I looked into getting the amount of active sessions from the Tomcat StandardManager class. In theory you would be able to obtain this through the tomcat implementation of the HttpSession object. But for some reason these objects are package protected.
There has to be an easier way of getting an accurate count of the amount of active sessions through the tomcat API.
