Java DNS Cache

This one is for the record - an issue I had faced and meant to document, so here goes.

From our app (Java version used is 1.4.2_19), we connect to a GSS load balanced url which operates in a round-robin fashion. For e.g., if the url is http://www.twitter.com, which maps to 128.242.240.148/116/20, the app should get a different IP each time or at least most of the time. But it sticks to the first IP it discovers. The reason is Java's DNS cache.

Java caches DNS settings after the app accesses a url so that any subsequent operation uses the cached setting - in our case, if www.twitter.com resolves to 128.242.240.20 the first time, that value is stored in the DNS cache and everytime twitter is accessed, the IP it resolves to remains the same.

Sounds daft, right. Not quite. This cache is maintained in the InetAddress class - there is one each for successful and unsuccessful host name resolutions. The positive caching is there to guard against DNS spoofing attacks; while the negative caching is used to improve performance. By default, the result of positive host name resolutions are cached forever, because there is no general rule to decide when it is safe to remove cache entries. The result of unsuccessful host name resolution is cached for a very short period of time (10 seconds) to improve performance. Refer the javadoc for more on the above.

In cases like ours, where we don't want DNS caching forever, the solution is to set the TTL value for positive caching to a very low value. This much is well documented. There are 3 ways to set this.
- Set the value from command line using the setting -Dnetworkaddress.cache.ttl=x where x is the number of seconds for which the value is to be cached.
- Use the sun property setting. Works the same as above, only use -Dsun.net.inetaddr.ttl=x in lieu of -Dnetworkaddress.cache.ttl
- Modify the java.security file (Path is $JAVA_HOME/jre/lib/security/java.security) to set the value of networkaddress.cache.ttl to a low value. By default the setting is -1 (cache forever). Note that this setting would affect any application which uses this JDK.

Tried Option 1 - added the -Dnetworkaddress.cache.ttl=60 setting to the weblogic startup file, which should have cleared the cache every 60 seconds. But it did not have the intended effect. There is a related bug 6247501 reported, but that's Windows specific and we were on Unix, so shouldn't have affected us. Looked through the InetAddress caching mechanism, but could not quite figure out why it failed. Gave up after an hour.

Modified the JVM to use the sun setting -Dsun.net.inetaddr.ttl=60 and restarted the domain. Worked like a charm.

Can only conclude that -Dnetworkaddress.cache.ttl does not work from the command line but only from within the java.security file. If you do not wish to change the java.security setting, which would be the case if multiple domains reference the same JDK, a safer option is to use the -Dsun.net.inetaddr.ttl setting from the command line.

Happy caching.

Eclipse Memory Issues

JDK 5 was the default Java version in use, upgraded to JDK1.6.0_21-b06 today and Eclipse started tanking. Starts up fine but after a few minutes would fail with the error 'Internal plug-in action delegate error on creation. PermGen space'. Right, seemed to indicate that I wasn't allocating enough memory for permgen. But the same program works fine with the very same memory settings under JDK 5, so this was definitely an upgrade problem. So much for my support of auto updates :-|

The default setting in eclipse.ini was
--launcher.XXMaxPermSize
256m
-vmargs
-Dosgi.requiredJavaVersion=1.5
-Xms40m
-Xmx256m

And 256m is generally sufficient for permgen, so what gives. Checked the configuration file to confim if the settings had taken effect (Click on Help -> About Eclipse -> Installation Details -> Configuration). Shows up the Xms and Xmx, but no MaxPermGen, hmmm... interesting.
eclipse.vmargs=-Dosgi.requiredJavaVersion=1.5
-Xms40m
-Xmx256m

Googled a bit. The eclipse wiki indicated that it was a bug with Oracle/Sun JDK 1.6.0_21 (had to be my version, duhhh), being tracked as 319514. The bug link is a pretty interesting read.

Apparently, as part of Oracle's rebranding of Sun's products, the Company Name property of the java.exe file, the executable file containing Oracle's JRE for Windows, was updated from "Sun Microsystems" to "Oracle" in Java SE 6u21. Now on Windows, Sun VM is identified using the GetFileVersionInfo API, which reads the company name (present under version details) from jvm.dll or the java executable and compares it against the string "Sun Microsystems". Post update 21, the company name was "Oracle" and the launcher does not recognize this string, hence the -XX:MaxPermSize setting is not honoured.

The workarounds are
- Switch back to '1.6.0_20'
- Change the commandline for launching or add the following line after "-vmargs" to your Eclipse.ini file:-XX:MaxPermSize=256m
- For 32-bit Helios, download the fixed eclipse_1308.dll and place it into (eclipse_home)/plugins/org.eclipse.equinox.launcher.win32.win32.x86_1.1.0.v20100503
- Download and install any of the upgraded versions i.e. version 1.6.0_21-b07 or higher from the java site (alternative link is http://java.sun.com/javase/downloads/index.jsp). Make sure you have b07 or higher by running java -version.

Went with the last option and so far life's good. The MaxPermSize setting shows up under vm options
eclipse.vmargs=-Dosgi.requiredJavaVersion=1.5
-Xms40m
-Xmx256m
-XX:MaxPermSize=256m

But got this bad feeling that when I move to JDK 7, there'll be a newer set of problems cropping up (From the Oracle site: In consideration to Eclipse and other potentially affected users, Oracle has restored the Windows Company Name property value to "Sun Microsystems" in further JDK 6 updates. This value will be changed back to "Oracle" in JDK 7.) Sigh, so much for compatibility.

Google CL rocks

Refer Isaac Truet's post Setup GoogleCL on WinXP if you plan to setup GoogleCL on Windows. Tried it on XP, yet to check on Windows 7.

This post is just to indulge my fantasy of posting a blog from the command line :-)

Command used to post
google blogger post --blog "TechRamblings" --title "Google CL rocks" --tags "python, googlecl, development" googlecl.html

Eclipse Decompiler Plugins

JADClipse plugin worked fine with Eclipse Galileo (3.5) on my old laptop, but failed on the Dell. The funny thing is JAD works fine from the command line, but cuts no ice with Eclipse. Tried by pointing Eclipse to the JADClipse update url, it downloads the jars yes but decompile it does not. Tried an eclipse -clean, still no luck.

All I can assume is JADClipse does't work on 64 bit machines, weird yes, but true.

An option you can choose to go with is JD-Eclipse plugin. Use Eclipse Update Manager to download JD and change the .class association to the JD Class File Editor.

BTW, if interested, steps to follow for setting up JAD with Eclipse on win32 machines.

SyntaxHighlighter on Blogger

If you intend to post code snippets on your blog, SyntaxHighlighter is the best option available.

Steps to enable it in your blogpost
1. Download the latest version and unzip in a local folder.
2. Upload all the files to either your domain or to sites.google.com.
3. Edit your template (In blogger, click on Design -> Edit HTML) and paste the following snippet under the closing div tag.

4. And finally, at the start of your code snippet, add the <pre class="brush: js"> tag. Close the code snippet with the corresponding </pre> tag.

Eclipse Maven Integration - Settings file does not exist

If you're trying to integrate Maven 2x with Eclipse and the following error pops up when installing a Maven project, read on. The error is 'cos Eclipse isn't aware of the Maven setup on your machine, so we set M2_HOME and add that under the classpath variables made available to Eclipse. For good measure, we also setup a local repository for Maven (that's unrelated to the fault below, just a good to have)

[ERROR] Error executing Maven.
[ERROR] The specified user settings file does not exist: C:\Users\xxxxxx\.m2\settings.xml

1. Navigate to the directory where Maven is installed - e.g. F:\apache-maven-2.2.1. Copy the absolute path. Create a new environment variable M2_HOME and modify the PATH variable

M2_Home=F:\apache-maven-2.2.1
PATH=%M2_HOME%\bin;...

2. Create a new folder 'repository' under M2_HOME.

3. Edit the file settings.xml present under M2_HOME\conf directory. The localRepository tag element would be commented by default. Uncomment this and modify the value to point to the absolute path of the repository folder u just created ( When a jar file is referenced, Maven first looks for it in the local repository, if not found, it downloads the same from the online repository. )

F:\apache-maven-2.2.1\repository

4. Next Eclipse needs to be told the path to the local maven repository. Select Window -> Preferences. On the LHS pane of the popup, select Java -> Build Path -> Classpath Variables. Check if the value under M2_REPO is the same as the localRepository defined in settings.xml. If not, from the LHS pane, select Maven -> User Settings and change the path to the settings.xml under M2_HOME i.e. F:\apache-maven-2.2.1\conf\settings.xml

That's it.

Buzz bookmarklet for Chrome

Got tired of copying urls and posting them in Buzz or using Tweetdeck or the Reader as an intermediary, so started checking out bookmarklets. This one was created by taking the delicious bookmarket as base and the testing has been cursory at best. Ensure that the 'Always show Bookmarks bar' is checked, open Bookmark Manager and create a new bookmark, paste the code below. That's it, good 2 go.

javascript:void(window.open('http://www.google.com/buzz/post?url='+encodeURIComponent(window.location.href)+'&title='+ encodeURIComponent(document.title),'buzzwindow','location=yes, links=no,scrollbars=no,toolbar=no,width=750,height=450'));