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Showing posts from 2012

Custom Database function using JPA JPQL and eclipselink

Sometimes it's useful to exploit some of the native functions provided by database vendors while using JPA and JPQL.And although it's possible to execute native queries, you'll loose some of the JPA magic in the process.Thankfully Eclipselink (and probably Hibernate) allow some degree of flexibility through special operators.According to the official documentation the FUNC operator allows for a database function to be call from JPQL. It allows calling any database functions not supported directly in JPQL, and calling user or library specific functions. @Entity public class Employee implements Serializable{ @Id private Long id; private String firstname; private String lastname; // Setters - Getters } Below is a quick example on how to use FUNC to call Oracle's TO_CHAR function and convert all the ids in the query to a String. Let's say that for an obscure reason we would like to get a list of all the employees id's but not as a list o…

JPA Eclipselink optimizing query performance on a large resultset

Recently I had to look for ways of optimizing a very slow database request on a very large ResultSet (more than 5000 results with several @OneToMany associations).While reading EclipseLink's documentation I stumbled upon an interesting piece of information regarding handling large ResultSets. EclipseLink recommends using Streamed Cursors when handling large resultsets According to the documentation : Cursored streams provide the ability to read back a query result set from the database in manageable subsets, and to scroll through the result set stream.The initial query would load practically the entire database into memory in one request (all the associations are marked with QueryHint.LEFT_FETCH in order to eagerly fetch the associations upon the initial request). I will not go into details regarding the process since it's irrelevant to this post not the goal of this post, but basically the program would query an Oracle Database transform every JPA Entity and store it in a so…

MongoDB $where clause to query array length

There is no direct way in mongodb to return all the documents in where a sub collection has at least X number of entries { "name" : "Terry Brooks", "books" : [ "The Sword of Shannara", "The Elfstones of Shannara", "The Wishsong of Shannara" ] }, { "name" : "Oscar Wilde", "books" : [ "The Picture of Dorian Gray" ] } Let's say that I want all the authors that have written more than 1 book. There is no direct way in mongodb to do this. it needs to be done either by map reduce or perhaps with the new aggregation framework but you cannot combine $gt and $size operators like so : db.AUTHORS.find({"books" : {$size : {$gt : 1}}}); It doesn't work, you wont get any error messages but an empty result. MongoDb allows Javascript evaluation…

EclipseLink (JPA) Spring - No persistence exception translators exception

I recently stumbled upon a Spring Exception while working with Spring Data JPA and attempting to deploy a WAR file on glassfish. I have a JTA datasource defined in my glassfish which is used by Spring (through JNDI) to instantiate my entity manager. The JPA configuration was pretty straight forward but every time I tried to deploy the app on Glassfish I stumbled upon the error : org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor#0': Initialization of bean failed; nested exception is java.lang.IllegalStateException: No persistence exception translators found in bean factory. Cannot perform exception translation. Apparently when using Hibernate there is an easy fix for this : Declaring a Hibernate Exception Translator in the spring beans config file : When using EclipseLink though this doesn't work... So I looked for implementations of the sp…

REST load testing with siege

Siege is a pretty nifty linux utility for HTTP/HTTPS load testing it can be downloaded here : siege homepage
Siege allows you to write a list of URLs in a file (one URL per line) the utility will then parse the file and execute the load tests according to your configuration file .siegerc or the command's arguments. The only thing I do not like about siege is the fact that I would like to have stats per URL; when siege parses the url file it will write a log file with global statics for the siege session Something like so : 2012-09-21 12:03:52, 1813, 121.43, 19, 0.49, 14.93, 0.16, 7.38, 1813, 0 which to my taste is not very clear While executing the tests siege prints out statistics that are not found in the siege log file (which in my case are more useful) but if you have multiple URLs you don't know for which URL the printed stats are. So I decided to handle the situation differently using awk and a bash script Note that…

MongoDB & Spring Data remove Elements from Arrays

In this blogpost I will be showing how to execute a Mongo Query using Spring Data in order to "pull" multiple items in a object's collection. The model : @Document(collection="Book") public class Book{ private String ISBN; private String bookTitle; private List< String > categories; //SETTERS - GETTERS } public class Category{ private String categoryId; private String categoryName; } A typical Mongo Document would be something like : { "isbn" : "11111111", "bookTitle" : "My Book Title", "categories" :[ { "categoryId" : "cat1", "categoryName": "My Category 1" }, { "categoryId" : "cat2", "categoryName": "My Category 2" } ] } { "isbn…

Resolving Spring's data error message "Cannot resolve the name to a(n) 'element declaration"

While working on a project combining Spring Data JPA and Spring Data MongoDB I run into a lot of trouble when I introduced the The error message : Cannot resolve the name to a(n) 'element declaration I spent some time looking for the cause, and then I realised that the version of SpringDataMongoDB and SpringDataJPA had dependencies on different versions of SpringDataCore. and it was causing trouble So if you get the same error just check your maven dependency hierarchy or go check the POM files for the libraries. You can check out the POM files in Spring's repo : Spring Maven repository

Untar a multi-volume tar splitted file

One way to untar multi-volume tar files (splitted) is to use a combination of CAT and TAR command. cat splitFile1 splitFile2 | tar -xf - You can also add a v to your tar command to make verbose.

Bash - create subfolder in each folder

I recently had to add a subfolder to each folder inside a directory in a directory that had more than 100 folders it was a bit tedious to copy paste the folder inside each one of them.Thankfully this can easily done with a bash command :


cd /opt/myfolder for f in *; do mkdir "$f/myFolder"; done

Find out linux distribution

This post is mostly for me as I keep forgeting how to do it.Type the following command in the terminal in order to find the Linux distribution : cat /etc/issue

MyFaces Trinidad invalid_lhs_in_assignment using Partial Page Rendering (PPR)

I stumbled upon a weird error last week. I was having the following error in some of my pages after a PPR request : ["Error ", ReferenceError arguments: Array[0] get message: function getter() { [native code] } get stack: function getter() { [native code] } set message: function setter() { [native code] } set stack: function setter() { [native code] } type: "invalid_lhs_in_assignment" __proto__: Error , " delivering XML request status changed to ", function (requestEvent) { if (requestEvent.getStatus() == TrXMLRequestEvent.STATUS_COMPLETE) { var statusCode = requestEvent.getResponseStatusCode(); // The server might not return successfully, for example if an // exception is thrown. When that happens, a non-200 (OK) status // code is returned as part of the HTTP prototcol. if (statusCode == 200) { _pprStopBlocking(window); if (requestEvent.isPprResponse()) { var responseDocument = requestEvent.get…