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"> 

    <cache usage="read-only" /> 

    <id name="id" type="Integer" column="ID" /> 

  <set name="myChildren"> 

     <cache usage="read-only"> 

     <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">
      <cache usage="read-only" />
      <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();

sf.evict(org.myapp.ho.Parent.class,new Integer(500)); //this will evict the Parent Object from the second level cache

sf.evictCollection(org.myapp.ho.Parent.class.getName() +  “.myChildren”, new Integer(500)); //this will evict the collection from the second level cache for the Parent with id=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). <jdbcconfguration> 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 <jdbcconfiguration>. 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. <br>  

  * Does the following replacements(not neccessarily in the order) and returns the resulting  

  * {@link String} as property name  

  *  

  * <ul>  

  *  <li>Converts the first letter of the <code>column</code> to uppercase</li>  

  *  <li>Converts the letters following a '_' character to uppercase in the <code>column</code></li>  

  *  <li>Removes any underscores present from <code>column</code></li>  

  * </ul>  

  *   

  *  

  * @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 <code>originalString</code> and <code>replacedString</code> are NOT equal  

  * (meaning there was some replacement done to the original column name). Else returns false. <br/>  

  *  

  * @param originalString The original column name  

  * @param replacedString The column name after doing necessary replacements  

  * @return Returns true if the <code>originalString</code> and <code>replacedString</code> are NOT equal  

  * (meaning there was some replacement done to the original column name). Else returns false.  

  *   

  * @throws {@link NullPointerException} if <code>originalString</code> is null.  

  */  

 protected boolean anyReplacementsMadeToOriginalColumnName(String originalString, String replacedString) {  

  if (originalString.equals(replacedString)) {  

   return false;  

  }  

  return true;  

 }  

   

 /**  

  * Converts the first letter of the <code>input</code> to uppercase and  

  * returns the resultant {@link String}.<br/>  

  * <p>  

  * Ex: If the <code>input</code> is startDate then the resulting {@link String}  

  * after replacement will be StartDate  

  * </p>  

  *  

  * @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 <code>input</code> and returns the resulting {@link String}. <br/>  

  * Carries out a 2 pass strategy to do the replacements. During the first pass,  

  * replaces all the letters that immidiately follow a '_' to uppercase.  

  * <p>  

  * Ex: If the <code>input</code> is _start_Date__today_ then after the first pass of replacement, the  

  * resultant string will be _Start_Date__Today_  

  * </p>  

  * <p>  

  * 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.  

  * </p>  

  * <p>  

  * Ex: If the <code>input</code> is _start_Date__today_ then after BOTH the passes the  

  * resultant string will be StartDateToday  

  * </p>  

  *  

  * @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);  

    }  

        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 <code>input</code> and returns the  

  * resulting {@link String}  

  * <p>  

  * Ex: If the <code>input</code> is _start_Date__today_ then the resulting {@link String}  

  * after replacement will be startDatetoday  

  * </p>  

  *  

  * @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