inverse attribute in Hibernate - What does it mean?

This is the best explanation, that i have seen till date, about Hibernate's "inverse" attribute:
Meaning of "inverse" in Hibernate

Evict collection from Hibernate second level cache

Hibernate allows persistent objects to be cached in its second level cache(The first level cache in Hibernate is the Session object which is ON by default). Applications can switch on the second level cache. When a object is being retrieved by the application through Hibernate, Hibernate first checks in its Session cache and then the Second level cache to see if the object has be retrieved already. If it finds it either the Session cache or the Second level cache, it will NOT fire a query to the database.
While configuring second level cache, the object can be cached and also the collections contained in the object can be cached. Have a look at the following example:

<hibernate-mapping default-lazy="false" > 
 <class name="org.myapp.ho.Parent" table="Parent"> 
    <b><cache usage="read-only" /> </b>
    <id name="id" type="Integer" column="ID" /> 
  <set name="myChildren"> 
     <b> <cache usage="read-only"/> </b>
     <one-to-many class="org.myapp.ho.Child"/> 
  </set> 
 </class> 
</hibernate-mapping>

<hibernate-mapping default-lazy="false" > 
   <class name="org.myapp.ho.Child" table="Child">
      <b><cache usage="read-only" /></b>
      <id name="id" type="Integer" column="ID" /> 
   </class> 
</hibernate-mapping>

Note that we have used the cache setting at 3 places:
1) The org.myapp.ho.Parent object
2) The "myChildren" collection in the org.myapp.ho.Parent object
3) The org.myapp.ho.Child object
When you configure a collection to be second level cached in Hibernate, it internally maintains a SEPERATE cache for these collection than the one which it uses to cache the parent objects. So in the example above, the “myChildren” will be cached separately than the org.myapp.ho.Parent object.
There might be cases where applications would want to evict objects from the cache. If its the Session cache from which the application has to evict the object then the call to Session.evict will cascade even to collections and will evict the collection from the *Session cache*. However, if the object(and the collections contained in it) have to be evicted from the second level cache, then the application has to *explicitly* call the evictCollection method on the SessionFactory to remove the *collection* contained in the Parent object. The reason behind this is, as already mentioned, the collections are cached separately, than the parent objects, in the second level cache.
So, in our example above, if we have to evict the Parent with id 500 and its collection from the second level cache, then here’s what has to be done:

SessionFactory sf = MyUtil.getSessionFactory();
//this will evict the Parent Object from the second level cache
sf.evict(org.myapp.ho.Parent.class,new Integer(500)); 
//this will evict the collection from the second level cache for the Parent with id=500
sf.evictCollection(org.myapp.ho.Parent.class.getName() +  ".myChildren", new Integer(500)); 

The first parameter to the evictCollection method is the ‘roleName’ of the collection. The roleName is formed as follows:

roleName = NameOfTheParentClass + "." + NameOfTheCollectionInsideTheParent

The second parameter the evictCollection method is the id of the parent object, to which this collection belongs.

Custom reverse engineering strategy in Hibernate

Hibernate has tools to create mapping files(hbm files) and domain model classes from database schemas(reverse engineering). can be used as part of ant task to do this. Hibernate creates the property names using its default reverse engineering strategy. Hibernate also, provides a way through which the user can specify his own custom reverse engineering strategy through which he can follow his own naming conventions etc…

There is a reversestrategy attribute which can be set to some custom class, which implements org.hibernate.cfg.reveng.ReverseEngineeringStrategy, in the . Here’s an example:

<jdbcconfiguration configurationfile="hibernate.cfg.xml"  

    packagename="${package.name}"  

    revengfile="hibernate.reveng.xml"  

    reversestrategy="org.myapp.hibernate.tool.SampleReverseEngineeringStrategy"/>

The org.myapp.hibernate.tool.SampleReverseEngineeringStrategy is our own custom class which implements org.hibernate.cfg.reveng.ReverseEngineeringStrategy. In this example, our SampleReverseEngineeringStrategy, overrides the columnToPropertyName(TableIdentifier table, String column) method to provide a custom implementation for generating property names out of a column name. Here’s the SampleReverseEngineeringStrategy code:

package org.myapp.hibernate.tool;  

import java.util.regex.Matcher;  
import java.util.regex.Pattern;  
import org.hibernate.cfg.reveng.DelegatingReverseEngineeringStrategy;  
import org.hibernate.cfg.reveng.ReverseEngineeringStrategy;  
import org.hibernate.cfg.reveng.TableIdentifier;  
/**  
 *  
 * @author Jaikiran Pai  
 *  
 */  
public class SampleReverseEngineeringStrategy extends DelegatingReverseEngineeringStrategy {  
/**  
  * Constructor  
  *  
  * @param delegate {@link org.hibernate.cfg.reveng.ReverseEngineeringStrategy}  
  */  
 public SampleReverseEngineeringStrategy(ReverseEngineeringStrategy delegate) {  
  super(delegate);  
 }  

/**  
  * Changes the default behaviour of naming the property. 
  
  * Does the following replacements(not neccessarily in the order) and returns the resulting  
  * {@link String} as property name:
  *  - Converts the first letter of the column to uppercase
  *  - Converts the letters following a ‘_’ character to uppercase in the column  
  *  - Removes any underscores present from column  
  *  
  * @see org.hibernate.cfg.reveng.DelegatingReverseEngineeringStrategy  
  * @see org.hibernate.cfg.reveng.ReverseEngineeringStrategy  
  *  
  * @param table {@link TableIdentifier}  
  * @param column  
  * @return Returns the propert name after converting it appropriately  
  */  
public String columnToPropertyName(TableIdentifier table, String column) {  

  String replacedColumn = replaceFirstLetterToUpperCase(column);  
   
   replacedColumn = removeUnderScoresAndConvertNextLetterToUpperCase(replacedColumn);  

   if (anyReplacementsMadeToOriginalColumnName(column,replacedColumn)) {  
  
  return replacedColumn;  

   }  

   /*  
    * Let DelegatingReverseEngineeringStrategy handle this  
    */  
   return super.columnToPropertyName(table, column);  
 }  

 /**  
  *  
  * Returns true if the originalString and replacedString are NOT equal  
  * (meaning there was some replacement done to the original column name). Else returns false.
  *  
  * @param originalString The original column name  
  * @param replacedString The column name after doing necessary replacements  
  * @return Returns true if the originalString and replacedString are NOT equal  
  * (meaning there was some replacement done to the original column name). Else returns false.  
  *   
  * @throws {@link NullPointerException} if originalString is null.  
  */  
 protected boolean anyReplacementsMadeToOriginalColumnName(String originalString, String replacedString) {  
  if (originalString.equals(replacedString)) {  
    return false;  

   }  

   return true;  
 }  

 /**  
  * Converts the first letter of the input to uppercase and  
  * returns the resultant {@link String}.  
  *   
  * Ex: If the input is startDate then the resulting {@link String}  
  * after replacement will be StartDate  
  *  
  * @param input The {@link String} whose contents have to be replaced  
  * @return Returns a {@link String} after doing the appropriate replacements  
  */  
 protected String replaceFirstLetterToUpperCase(String input) {  

   /*  
    * The pattern to match a String starting with lower case  
    */  
   final String startsWithLowerCasePattern = "^[a-z]";  
   
   Pattern patternForReplacingLowerCase = Pattern.compile(startsWithLowerCasePattern);  
   Matcher regexMatcher = patternForReplacingLowerCase.matcher(input);  

 /*  
  * This will hold the replaced contents  
  */  
 StringBuffer replacedContents = new StringBuffer();  

 /*  
  * Check whether the first letter starts with lowercase.  
  * If yes, change it to uppercase, else pass on the control to  
  * DelegatingReverseEngineeringStrategy  
  *  
  */  
 if (regexMatcher.find()) {  

  String firstCharacter = regexMatcher.group();  
  /*  
   * Convert it to uppercase  
   */  
  regexMatcher.appendReplacement(replacedContents,firstCharacter.toUpperCase());  
  regexMatcher.appendTail(replacedContents);  
  regexMatcher.reset();  
  
  /*  
   * Return the replaced contents  
   */  
    return replacedContents.toString();  

   }  

        //no replacements to do, just return the original input  
        return input;  

 }  

 /**  
  * Converts the letters following a ‘_’ character to uppercase and also removes  
  * the ‘_’ character from the input and returns the resulting {@link String}.   
  * Carries out a 2 pass strategy to do the replacements. During the first pass,  
  * replaces all the letters that immidiately follow a ‘_’ to uppercase.  
  * Ex: If the input is _start_Date__today_ then after the first pass of replacement, the  
  * resultant string will be _Start_Date__Today_  
  *   
  * This replaced {@link String} is then passed ahead for second pass (if no replacements were  
  * done during first pass, then the original {@link String} is passed). During the second pass  
  * the underscores are removed.  
  * Ex: If the input is _start_Date__today_ then after BOTH the passes the  
  * resultant string will be StartDateToday  
  *  
  * @param input The {@link String} whose contents have to be replaced  
  * @return Returns a {@link String} after doing the appropriate replacements  
  */  
 protected String removeUnderScoresAndConvertNextLetterToUpperCase(String input) {  

   /*  
    * The pattern which matches a String that starts with a letter immidiately after  
    * a ‘_’ character  
    */  
   final String stringFollowingUnderScore = "[.]*_[a-zA-Z]+";  

   Pattern patternForReplacingLowerCase = Pattern.compile(stringFollowingUnderScore);  
          Matcher regexMatcher = patternForReplacingLowerCase.matcher(input);  
          /*  
           * This will hold the replaced contents  
           */  
          StringBuffer replacedContents = new StringBuffer();  

          boolean foundAnyMatch = false;  

          while (regexMatcher.find()) {  
   foundAnyMatch = true;  
   String matchedString = regexMatcher.group();  
       /*  
          * The character immidiately following the underscore  
   * Example:  
          * If matchedString is _tMn then originalCharAfterUnderScore will be the  
          * character t  
          */  

        char originalCharAfterUnderScore = matchedString.charAt(1);  
         /*  
          * Convert the character to uppercase  
          */  

        String replacedCharAfterUnderScore = String.valueOf(originalCharAfterUnderScore).toUpperCase();  

        /*  
          * Now place this replaced character back into the matchedString  
          */  

        String replacement = matchedString.replace(originalCharAfterUnderScore,replacedCharAfterUnderScore.charAt(0));  

        /*  
   * Append this to the replacedColumn, which will be returned back to the user  
          */  
  regexMatcher.appendReplacement(replacedContents,replacement);  

     } //end of while  

        regexMatcher.appendTail(replacedContents);  
 regexMatcher.reset();  
 
 /*  
         * Now the input string has been replaced to contain uppercase letters after the underscore.  
         * Ex: If input string was "_start_Date_today" then at this point after the above processing,  
        * the replaced string will be "_Start_Date_Today"  
  * The only thing that remains now is to remove the underscores from the input string.  
        * The following statements do this part.  
        *  
        */  

      if (foundAnyMatch) {  
       return removeUnderScores(replacedContents.toString());  
       
   } else {  
     return removeUnderScores(input);  
     
   }  

 }  

 /**  
  * Removes any underscores present from input and returns the  
  * resulting {@link String}  
  * Ex: If the input is _start_Date__today_ then the resulting {@link String}  
  * after replacement will be startDatetoday  
  *
  * @param input The {@link String} whose contents have to be replaced  
  * @return Returns a {@link String} after doing the appropriate replacements  
  */  
 protected String removeUnderScores(String input) {  

   /*  
  * Pattern for matching underscores  
     */  
  Pattern patternForUnderScore = Pattern.compile("[.]*_[.]*");  

    Matcher regexMatcher = patternForUnderScore.matcher(input);  
  /*  
      * This will hold the return value  
   */  
    StringBuffer returnVal = new StringBuffer();  
       boolean foundAnyMatch = false;  
   
     while (regexMatcher.find()) {  
        foundAnyMatch = true;  

     String matchedString = regexMatcher.group();  

     /*  
       * Remove the underscore  
   */  
     regexMatcher.appendReplacement(returnVal,"");  

      }  

      regexMatcher.appendTail(returnVal);  
 regexMatcher.reset();  

      /*  
        * If any match was found(and replaced) then return the replaced string.  
        * Else return the original input.  
  */  
      if (foundAnyMatch) {  
        return returnVal.toString();  

      }  
 return input;  

 }  

}

In the example above, the columnToPropertyName method is overridden to do the following:

- Creates property names that start with a Capital case(By default, Hibernate creates property names in camel-case)
- Converts the letter, that follows a ‘_’ (underscore character) to uppercase in the property name
- Removes any underscores in the property name

Ex: If the column name is start_Date_today, then the resulting property name after using the SampleReverseEngineeringStrategy would be StartDateToday.

Here’s a documentation from Hibernate about Controlling Reverse Engineering

Convert exception stacktrace to String object

Usually whenever a exception occurs, we use the method printStackTrace() on the exception object to display the stacktrace. However, if the stacktrace has to be stored into a String object then the following piece of code will prove handy:

  /**
     * Creates and returns a {@link java.lang.String} from t’s stacktrace 
     * @param t Throwable whose stack trace is required
     * @return String representing the stack trace of the exception
     */
    public String getStackTrace(Throwable t) {
        StringWriter stringWritter = new StringWriter();
        PrintWriter printWritter = new PrintWriter(stringWritter, true);
        t.printStackTrace(printWritter);
        printWritter.flush();
        stringWritter.flush(); 

       return stringWritter.toString();
    } 

Lookup an EJB from a different application, deployed on the same server, in JBoss

Question:
I have a web application through which i want to access an EJB which is deployed as a separate appliaction on the same server. How do i do it?
Answer:
In the web.xml of your war(the web application through which you want to access the EJB), have the following entry:

<ejb-ref>

<ejb-ref-name>GiveAnNameByWhichYouWouldLikeToReferTheBeanInYourWebApp</ejb-ref-name>
    <ejb-ref-type>session</ejb-ref-type>
    <home>packageName.ClassNameOfTheHomeObjectOfTheBeanYouWantToRefer</home>
    <remote>packageName.ClassNameOfTheRemoteObjectOfTheBeanYouWantToRefer</remote>
  </ejb-ref>
  

In the jboss-web.xml of your war, have the following entry:

  <ejb-ref>
      <!--The ejb-ref-name should be same as the one given in the web.xml above -->
      <ejb-ref-name>GiveANameByWhichYouWouldLikeToReferTheBeanInYourWebApp</ejb-ref-name>
      <!--Example:somecontext/somejndiName.YouWillFindThisJndiNameInTheJboss.xmlOfTheEJB-->
      <jndi-name>TheJndiNametoWhichTheBeanIsBound</jndi-name>
   </ejb-ref>
   
  

For more info, have a look at the dtds of web.xml (http://java.sun.com/dtd/web-app_2_3.dtd) and jboss-web.xml(http://www.jboss.org/j2ee/dtd)
In your code, do the lookup as:

 
Context ic = new InitialContext();
Object home=ic.lookup("java:comp/env/TheNameThatYouHadGivenInEjb-ref-nameTagOfJbossWeb.xml");

Here’s an example:

web.xml:

<ejb-ref>
    <ejb-ref-name>MyTestBean</ejb-ref-name>
    <ejb-ref-type>session</ejb-ref-type>
    <home>com.test.ejb.MyBeanHome</home>
    <remote>com.test.ejb.MyBeanRemote</remote>
  </ejb-ref>
  

jboss-web.xml:

 <ejb-ref>
   <ejb-ref-name>MyTestBean</ejb-ref-name>
   <jndi-name>myejb/test/MyTestBean</jndi-name>
  </ejb-ref>
 
 

Lookup code:

Context ic = new InitialContext();
  Object ejbHome = ic.lookup("java:comp/env/MyTestBean");

How to create a Queue/Topic in JBoss?

Queues/Topics are known as administered objects and in jboss they can be deployed through ServiceDotXml files. Let’s name our file, myapp-destination-service.xml. The contents of the file should look like:

<?xml version="1.0" encoding="UTF-8"?>

<server>
   <!--My queue, named: myAppQueue -->
  <mbean code="org.jboss.mq.server.jmx.Queue"
  name="jboss.mq.destination:service=Queue,name=myAppQueue">
    <depends optional-attribute-name="DestinationManager">jboss.mq:service=DestinationManager</depends>
  </mbean>
   <!-- My topic, named: myAppTopic -->
  <mbean code="org.jboss.mq.server.jmx.Topic"
  name="jboss.mq.destination:service=Topic,name=myAppTopic">
    <depends optional-attribute-name="DestinationManager">jboss.mq:service=DestinationManager</depends>
  </mbean>
    
  </server>

Place this file in your %JBOSS_HOME%/server/default/deploy directory and start the server.
(Note: If you are using the ‘all’ configuration or the ‘minimal’ configuration then you will have to place this file in the %JBOSS_HOME%/server/all/deploy or %JBOSS_HOME%/server/minimal/deploy directory, as appropriate).

The above queue/topic will be bound to the jndi name: queue/myAppQueue and topic/myAppTopic respectively.

Note:

1) The above file is meant for JBoss-3.2.3. Though there might be slight changes to later versions of JBoss, the basic configuration mechanism remains the same.

2) JBoss also comes with preconfigured queues/topics. These configurations can be found in the jbossmq-destinations-service.xml file which is present in %JBOSS_HOME%/server/default/deploy/jms directory


Update:

For JBoss AS 6.x see this FAQ for more details on how to configure JMS queue/topics

How to enable Garbage Collection (GC) logs

To enable GC logs, the -Xloggc:logFileName option will have to be passed when java command is being executed. Additionally if the detailed log of the GC is required, then an additional -XX:+PrintGCDetails option will have to be passed.

Example: java -Xloggc:D:/log/myLogFile.log -XX:+PrintGCDetails myProg