The other day I was reading the Grails docs and I came across a useful GSP tag: grep. I have been using Grails for over 3 years now but just recently got to see this new tag which has eased my life a bit in situations where the list of objects have to be filtered and iterated at the same time to perform some operations on them. The GSP ‘grep’ tag (equivalent to the ‘grep’ method in Groovy) filters the list on a given condition and iterates over the filtered list allowing you to do some operations on the objects (or whatever you want to do with the filtered objects).
I will give you a small example: Suppose we have a list of Person objects. You want to display the names starting with A, B, C and so on in different color schemes. How would you do it?
One implementation is to send different lists from the controllers (this would mean sending 26 lists i.e. one list for each character in the English alphabet) and render them in different color schemes. The other way would be to iterate over the Persons list in the GSP and have different <g:if>…<:g:elseif> statements (for each character).
There is another smarter way: you could also use the ‘findAll‘ method of the List in the GSP <g:each> tag’s ‘in’ attribute i.e <g:each in=${personList.findAll {it.name.startsWith(‘A’)}} var=”person”> … </g:each>.
Similarly, we can use ‘grep’ tag here. We can filter out the list like this: <g:grep in=”${personList.name}” filter=”~/^A.*/”> … </g:grep>.
Notice the use of ‘name‘ field of the Person object in the ‘in‘ attribute of the grep tag. Also, the attribute ‘filter’ takes many forms of filter options; here we are using regex to filter the persons with the name starting with ‘A’.
I am not saying that this tag provides some new functionality which we could not have achieved by any other means but this tag makes it easier to filter out the lists on some specific parameters.
You can read more about this tag here. Also you can read about the Groovy GDK’s grep method usage here to see how and in what scenarios you can use this tag.
Hope this helps !!
- Abhishek Tejpaul
abhishek@intelligrape.com
Abhishek Tejpaul
An open-source and Agile enthusiast.
Posts by Abhishek Tejpaul:
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Grails GSP tag: grep
31 Oct 2012 in Grails -
Sending email through command line with Mutt
In one of the shell scripts I was reading I saw a usage of mutt text-based command line email client. Mutt can be used to send out the emails from the production machines or any other servers where you do not have access to the browsers or UI based email clients. Sometimes it is essential to send out a particular log file to certain people for bug fixing while you are checking on the prod machine.
Here is how you can send out the emails from the Linux (at-least for most of the Debian based distributions such as Ubuntu)
sudo apt-get install mutt
Now you can type in the following command:
mutt -s <emailSubject> <recipientEmail> -c <ccRecipientEmail> -b <bccRecipientEmail> -a </path/to/the/attachmentFile> < </path/to/the/fileContainingTheMessage>
We can see an example here:
mutt -s "Demo Subject Line" xyz@xyz.com -c abc@xyz.com -b blind-recipient@xyz.com -a /tmp/meetingDetails.xls < /tmp/emailBody.txt
As you can see, the -s flag is used to specify the subject line of the email and the optional flags -c and -b are used to specify the “cc” recipient and “bcc” recipient respectively. Also, -a flag is used to specify the location of the attachment file. There is one caveat regarding the -a flag i.e. -a option must be placed at the end of command line options. You can read the mutt documentation for more information on this.
This utility is a nice to know tool if you are a developer working in Linux and can use it in shell scripts when ever there is an apt use-case. You can read more about mutt and other cool things it can do by clicking here or by typing the ‘man mutt‘ command in your terminal.
Hope this helps !! -
Using git diff feature on Github
07 Jun 2011 in SystemHi Folks,
Recently I came across a cool way to compare differences between two branches, two tags, or between two commits on Github. Many a times in our project we have to thoroughly see what has been a specific change in the code base before we push it on our production branch.
Here is how you can view the differences in commits:
On the Github, go to the Source view of your project. You will see a link named ‘Branch List’. Once the page opens you can see a list of all the remote branches. Hit on the Compare button in front of any of the available branches to see the difference between two branches.
Now note the URL in the address bar. It should end with something like ‘…/compare/<x>…<y>’ where x and y are separated by three dots(…) and their values could be project’s branch names
Isn’t it good?
Well, Git’s (read about git diff) and Github’s goodness does not stop just here. Instead of branch names as the values of x and y, you can also put two different commit hashes or tag names to view the differences in the code-base. More so, the commit hashes do not have to belong to the same branch. So you can pretty much compare your code’s current snap-shot with any of its past snap-shot irrespective of branch or a tag or a commit hash.
Hope this helps someone.
Abhishek Tejpaul
abhishek@intelligrape.com
[IntelliGrape Software Pvt. Ltd.] -
Fetching an old deleted file of a project using Git
15 Nov 2010 in SystemRecently in one of my projects, I had to bring the content of old-forgotten-deleted file back to the application code base using Git. The problem took a ‘fancy’ turn as the path to the file was no more a valid path in Git as there had been a major refactorings in our project esp. renaming of the packages as well as the folder structure since the file in question was deleted.
This is how I could see the deleted file (& its content) using the Git commands:1. First of all, we will try to get the revision number of the git commit which committed our deleted file.
Type in, git log
on your console at the location where you have cloned/forked your Git project. This command will show a all the commits that has been done since the start of the project.
If you know at what period of time the deleted file was committed, you can use another flavor of git log command i.e.
git log –before=”Oct 01 2010″, or,
git log –after=”Oct 01 2010″
If you have an idea about who all could have or have committed the deleted file, then you can also use another option provided by git log command i.e
git log –author=”Author Name” or
git log –author=authorId
The commands above are self-explanatory.
NOTE: The step above would be helpful only if your team has followed good practices such as putting appropriate and self-explanatory comments while committing. So like if you have deleted a file & the comment while committing specifies that in some way or other it would be really easy for you to find out the probable revision number of that old commit by seeing the logs.2. Now once we have the revision number, we can try using the
git show
command to make sure that the deleted file was actually present in the commit. Type the following:
git show --name-only <revisionNumber>
Once you see your sought-after file there, then you can see the contents of your file by typing
git show <revisionNumber>
This command will show you the change-set and the content of the file. Now you can fetch or copy the code out of the deleted file and place it in your new code or create an entirely new file out of it.
Hope this helps !!!
Reference: http://cheat.errtheblog.com/s/git
– Abhishek Tejpaul
abhishek@intelligrape.com
[IntelliGrape Software Pvt. Ltd.] -
Passing parameters to sub-reports in Jasper
15 Sep 2010 in Java toolsIn one of our recent project, we made quite a good use of Grails Jasper plugin (v-0.9.7) to generate reports in the form of PDFs and Excel sheets. The reports contain sub-reports, which in many cases, contain sub-reports again. The way to pass parameters to sub-reports at the deeper level is to use the following tag in the .jrxml file: <subreportParameter>
In the Grails Jasper plugin, the Jasper controller searches for the “SUBREPORT_DIR” in the params when the request is forwarded to it. If it does not find any key named “SUBREPORT_DIR” in the params map, it initializes the variable SUBREPORT_DIR with the value of “jasperFilePath” i.e. the location where the Jasper’s .jasper and .jrxml files can be found. Now suppose you want to give seperate file location for your sub-reports, you can easily do this using the following code in the .jrxml file inside the tag:
... ...Note the value of “![CDATA[$P{SUBREPORT_DIR}]]” parameter can be passed as a parameter from the controller to the master report but afterwards you can make use of the tag to pass on the information to the subsequent sub-reports to any level.
For more information on the jasper .jrxml tags, please visit the following link here: http://jasperforge.org/uploads/publish/jasperreportswebsite/trunk/schema.reference.html
Hope this helps !!
- Abhishek Tejpaul
abhishek@intelligrape.com
[IntelliGrape Software Pvt. Ltd] -
Grails Transactions using @Transactional annotations and Propagation.REQUIRES_NEW
Hi All,
Here is how you can implement a new transaction in an already executing transaction in Grails which uses nothing but Spring framework’s Transaction mechanism as an underlying implementation. Spring provides @Transactional annotations to provide declarative transactions. We can use the same in our Grails project to achieve the transactional behavior.
Here is the scenario: You have two domain classes named SecuredAccount and AccountCreationAttempt. You try to transactionally save the SecuredAccount object which in turn creates a AccountCreationAttempt object which writes to the database stating: “There is an attempt to create a new SecuredAccount at this time: <current date and time>”. Point to note here is that even if the creation of the new SecuredAccount object fails, the record must still be written to the database so that the Administrator can validate whether the attempt at the specific time was by a legitimate user or an attacker.
Here is the code:
import org.springframework.transaction.annotation.* Class MyService { static transactional = false def anotherService @Transactional def createSecuredAccount() { def securedAccount = new SecuredAccount(userId:"John") securedAccount.save(flush:true) anotherService.createAccountCreationAttempt() throw new RuntimeException("Error thrown in createSecuredAccount()") } }
import org.springframework.transaction.annotation.* class AnotherService { static transactional = false @Transactional(propagation = Propagation.REQUIRES_NEW) def createAccountCreationAttempt() { def accountCreationAttempt = new AccountCreationAttempt(logRemarks: "There is an attempt to create a new SecuredAccount at this time: {new Date()}") accountCreationAttempt.save(flush:true) } }
Now in this scenario, AccountCreationAttempt object always gets persisted whether or not the transaction for creating SecuredAccount object fails.
Here are few gotchas regarding the above transactions:
1.) First of all, for Propagation.REQUIRES_NEW to work as intended, it has to be inside a new object i.e. a new service in our example. If we had put the createAccountCreationAttempt() method in the MyService there would be no new transaction spawned and even no log entry would be made. This is Spring’s proxy object implementation of transactions and you can read more about it here:
Please pay special attention to the “NOTE” sub-section.This is what it states:
“In proxy mode (which is the default), only external method calls coming in through the proxy are intercepted. This means that self-invocation, in effect, a method within the target object calling another method of the target object, will not lead to an actual transaction at runtime even if the invoked method is marked with @Transactional.”
2.) Secondly, all the @Transactional methods should have a public visibility i.e. createSecuredAccount() and createAccountCreationAttempt() methods should be public methods, and not private or protected. This again is Spring’s @Transactional annotations implementation and you can read about it at the same link as provided above. Note the right side-bar titled “Method visibility and @Transactional“.
Well, once you keep note of these gotchas I guess you are all set to make good use of @Transactional annotations and its full power.
Cheers !!!
- Abhishek Tejpaul
abhishek@intelligrape.com
[IntelliGrape Software Pvt. Ltd.] -
Tomcat 6 in-memory session replication
22 Jul 2010 in SystemHi All,
Here are the few basic steps that you need to follow in order to achieve the in-memory session replication between two or more Tomcat 6 instances.
This blog refers the Apache Tomcat documentation as found here: http://tomcat.apache.org/tomcat-6.0-doc/cluster-howto.html.
The Tomcat documentation provides more detailed explanation of the Clustering concepts as well as the definition of tags, attributes etc. used in server.xml file.Step 1: Include the <distributable> tag in web.xml file i.e. you can simple write the following line in your deployment descriptor(i.e. web.xml):
<distributable />
Please read the following link to know more about this tag: http://wiki.metawerx.net/wiki/Web.xml.Distributable
Step 2: Add the following lines of XML in the server.xml file inside the <Engine> element/tag:
<Engine name="Catalina" defaultHost="localhost"> <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster" channelSendOptions="8"> <Manager className="org.apache.catalina.ha.session.DeltaManager" expireSessionsOnShutdown="false" notifyListenersOnReplication="true"/> <Channel className="org.apache.catalina.tribes.group.GroupChannel"> <Membership className="org.apache.catalina.tribes.membership.McastService" address="228.0.0.4" port="45564" frequency="500" dropTime="3000"/> <Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver" address="auto" port="4000" autoBind="100" selectorTimeout="5000" maxThreads="6"/> <Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter"> <Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/> </Sender> <Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/> <Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/> </Channel> <Valve className="org.apache.catalina.ha.tcp.ReplicationValve" filter=""/> <Valve className="org.apache.catalina.ha.session.JvmRouteBinderValve"/> <Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer" tempDir="/tmp/war-temp/" deployDir="/tmp/war-deploy/" watchDir="/tmp/war-listen/" watchEnabled="false"/> <ClusterListener className="org.apache.catalina.ha.session.JvmRouteSessionIDBinderListener"/> <ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/> </Cluster> . . . . </Engine>
Please note that there might be some other elements such as <Host>, <Realm> etc. inside the <Engine> element. Also, for each tomcat instance the value defined for the ‘port’ attribute of the <Receiver> tag must be unique.
And, that’s all. You now have a basic session-replication in place. Your sessions are replicated amongst all the tomcat instances which are part of your cluster.
NOTE: Please make sure if you make any changes to the “context.xml” file, you have to delete the following xml file located in the <TOMCAT_HOME>/conf/Catalina/localhost/{yourAppName}.xml. If you don’t delete this file, then your changes will be ignored and the settings defined in this file will take effect.
Cheers!!!
Abhishek Tejpaul
abhishek@intelligrape.com
[Intelligrape Software Pvt. Ltd.] -
Tomcat 6 Session Persistence through JDBCStore
In one of our recent projects, we needed to save the HTTP session in the database.
This blog refers the Apache documentation as found here: http://tomcat.apache.org/tomcat-6.0-doc/config/manager.htmlThese are the following steps that need to be followed:
Step 1: Create a database named tomcat (as shown in our example in Step 3 below) or any other name as specified in the ‘connectionURL’ attribute of the <Store> element.
Step 2: Create the following table in the newly created database:
create table sessions ( session_id varchar(100) not null primary key, valid_session char(1) not null, max_inactive int not null, last_access bigint not null, app_name varchar(255), session_data mediumblob, KEY kapp_name(app_name) );
Step 3: Copy the context.xml file available at the global level at this location: <TOMCAT_HOME>/conf to your application’s META-INF folder and replace the <Manager> element with the following:
<Manager className='org.apache.catalina.session.PersistentManager' saveOnRestart='false' minIdelSwap='0' maxIdleSwap='0' maxIdleBackup='1'> <Store className="org.apache.catalina.session.JDBCStore" driverName="com.mysql.jdbc.Driver" connectionURL="jdbc:mysql://localhost/tomcat?user=username&password=password" sessionTable="sessions" sessionIdCol="session_id" sessionDataCol="session_data" sessionValidCol="valid_session" sessionMaxInactiveCol="max_inactive" sessionLastAccessedCol="last_access" sessionAppCol='app_name' /> </Manager>
If these settings are placed in the ‘<TOMCAT_HOME>/conf/context.xml‘, it will have a global effect on all the applications running on the server. For an application specific setting, you can place a newly created “context.xml” file in the <APP_HOME>/META-INF sub-folder inside the webapps folder.
Please note that attributes used in the <Store> element make use of the table columns as created in the database in Step 2.Step 4: Set the following system properties named – ‘org.apache.catalina.session.StandardSession.ACTIVITY_CHECK‘ to ‘true‘.
For further information on this, read the ‘Persistent Manager Implementation’ section at this link : http://tomcat.apache.org/tomcat-6.0-doc/config/manager.htmlStep 5: Make sure you have placed the MySql jar in the <TOMCAT_HOME>/lib folder.
Now you can try to hit the application’s URL and check the database table to see the newly persisted session. Please note that it takes around 60 seconds to see the stored session in the database so you might have to wait a bit. You can have multiple instances of tomcat running your application pointing to the same database and can share this persisted session in case any of the tomcat instance crashes.
Cheers!!!
Abhishek & Imran
abhishek@intelligrape.com | imran@intelligrape.com
[Intelligrape Software Pvt. Ltd.]
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Finding User’s Session Locale in GSP
14 Jul 2010 in Grails&HTML-UI-CSSThere are situations where you might want to know the Locale of the user as set in the session in your GSP so that you can show text in a particular language/manner. Here the Spring framework’s SessionLocaleResolver can come to the rescue. In one of our recent Grails projects, we did the following in one of our GSPs to display the content accordingly.
Please note that our use-case was like that in which we have fields for different languages in our domain classes. For example:
class MyDomain { String greetingEnglish String greetingFrench }
And now you have to conditionally display the object’s fields based on the user’s locale. If this were a static text, then the Grails in-built i18n support in the form of message bundles would be the ideal solution. But this is a different scenario.
So let’s start by writing the following in your GSP :
<g:set var="lang" value="${session.'org.springframework.web.servlet.i18n.SessionLocaleResolver.LOCALE'}"/>The above statement returns the ISO Language Code from the session object which is a map. In Grails, the map values can be retrieved by the ‘.’ operator too by passing the keys. These codes are the lower-case, two-letter codes as defined by ISO-639. For example: ‘en’ for English, ‘fr’ for French and so on.
Once you know the locale, you can show the conditional content on your page. It could be something like this:
<g:if test="${lang.startsWith('en')}"> <h1> ${myDomainobject.greetingEnglish} </h1> </g:if> <g:elseif test="${lang.startsWith('fr')}"> <h1> ${myDomainobject.greetingFrench}</h1> </g:elseif> <g:else> <h1> Default Greeting </h1> </g:else>Hope this will help !!!
- Abhishek Tejpaul
[Intelligrape Software Pvt. Ltd.]
Configuring multiple senders in Grails Mail plugin
In one of the recent projects, we had to set up a system where emails for different purposes (i.e registration, error reporting etc.) were to be sent out from different email accounts. We were using the Grails Mail plugin which does not provide a clear way to set-up or configure multiple email addresses. With the help of my colleague, Chandan Luthra, I tried to dig into the workings of plugin and found out that Mail plugin injects the properties such as username, password etc. through a bean named mailSender. So we just did the following to customize/configure these properties on the fly from the controller or a service:
CH.config.grails.mail.default.from = "username@domain.com" // Mail plugin uses this to connect to the account in e-mail provider's server. So it should be set. mailSender.username = "username@domain.com" mailSender.password = "sample123!"
And yes, once you are done with your email sending functionality you can set mailSender properties back to the default settings set originally in Config.groovy. Your service/controller code could look something like this:
// Store the default mail settings in variables def defaultFrom = CH.config.grails.mail.default.from String oldUsername = mailSender.username String oldPassword = mailSender.password // Change the properties here; send the email try { CH.config.grails.mail.default.from = "username@domain.com" mailSender.username = "username@domain.com" mailSender.password = "sample123!" mailService.sendMail { to(['exampple1@domain1.com', 'example2@domain2.com'].toArray()) subject("Example Subject") body("Sample body") } } catch (Exception e) { // catch block code } // Set the original settings back finally { CH.config.grails.mail.default.from = defaultFrom mailSender.username = oldUsername mailSender.password = oldPassword }I guess this scenario can be quite prevalent in many projects. Hope you find the blog useful.
- Abhishek Tejpaul
Intelligrape Software Pvt. Ltd.