Articles

Whilst I am very grateful to the folks who developed SimpleJPA, I found it a bit problematic building (with Maven) a project having SimpleJPA as of its dependencies. In this article, I attempt to make it easier for others who intend to use Maven for building their project which uses SimpleJPA.

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Having @ManyToMany(fetch=FetchType.EAGER) attributes can slow down retrieval quite significantly (up to 40 times slower).

I recall reading somewhere that FetchType.EAGER is the default for @ManyToMany associations. Also, from experience I noticed that setting FetchType.LAZY caused a org.hibernate.LazyInitializationException thrown with exception message similar to, failed to lazily initialize a collection of role: com.xyz.domain.EntityOne.images, no session or session was closed.

So, it seemed that the only other way to quickly reduce the time it took (for the retrieval) was to look into the caching options such as query caching, second-level caching, both of which are supported by Hibernate (the webapp’s JPA provider). For more information on second-level caching, please refer to this article.

Integrating Spring + JPA + Hibernate + Tomcat + EHCache took me a few hours this afternoon, but the effort paid off. The retrieval is now 40 times faster!

This post is summarizing the setup involved in getting them all to work together.

Relevant Section Of persistence.xml:

<provider>org.hibernate.ejb.HibernatePersistence</provider>
<shared-cache-mode>ENABLE_SELECTIVE</shared-cache-mode>
<properties>
  <property name="hibernate.cache.provider_class" 
    value="org.hibernate.cache.SingletonEhCacheProvider" />
  <property name="hibernate.cache.provider_configuration" value="/ehcache.xml" />
  <property name="hibernate.cache.use_second_level_cache" value="true" />
  <property name="hibernate.cache.use_query_cache" value="true" />
</properties>

Notes:

• The persistence.xml is located at {tomcat}/webapps/{your-webapp}/META-INF.
• I used org.hibernate.cache.SingletonEhCacheProvider instead of org.hibernate.cache.EhCacheProvider. I was getting a WARN message if I used org.hibernate.cache.EhCacheProvider. I referred to this post for a fix.

Relevant Section Of ehcache.xml:

  <diskStore path="user.dir/mywebapp-special-cache-folder"/>
 
  <defaultCache eternal="false" overflowToDisk="false"
    maxElementsInMemory="1000" timeToIdleSeconds="30" timeToLiveSeconds="60"/>
 
  <cache name="com.xyz.domain.EntityTwo" eternal="false" overflowToDisk="true" 
    maxElementsInMemory="1000" timeToIdleSeconds="300" timeToLiveSeconds="600"
    diskPersistent="true" diskExpiryThreadIntervalSeconds="300"/>
 
  <cache name="com.xyz.domain.EntityOne" eternal="false" overflowToDisk="true" 
    maxElementsInMemory="1000" timeToIdleSeconds="300" timeToLiveSeconds="600" 
    diskPersistent="true" diskExpiryThreadIntervalSeconds="300"/>
 
  <cache name="com.xyz.domain.EntityOne.images" eternal="false" 
    overflowToDisk="true" 
    maxElementsInMemory="1000" timeToIdleSeconds="300" timeToLiveSeconds="600" 
    diskPersistent="true" diskExpiryThreadIntervalSeconds="300"/>

Notes:

• The ehcache.xml is located at {tomcat}/webapps/{your-webapp}/WEB-INF/classes.
• The diskStore element indicates where the files to be used for caching will be stored for the entities I wish to be made persistent to disk. In my web application, the files are stored under {tomcat}/bin/{mywebapp-special-cache-folder}
• diskPersistent="true" indicates that the disk store (for the specific entity) is persistent between cache and VM restarts. Please refer to the EHCache documentation on disk storage for more information.

Relevant JARs: under {tomcat}/webapps/{your-webapp}/WEB-INF/lib

• ehcache-core-2.0.1.jar
• hibernate3.jar
• hibernate-jpa-2.0-api-1.0.0.Final.jar
• slf4j-api-1.5.8.jar
• slf4j-log4j12-1.5.6.jar
• Spring 3.0.2 JARs..

Relevant Section Of Spring Configuration:

<!-- there should be a way out of hardcoding the location of the properties file -->
<bean 
  class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
  <property name="location" value="/WEB-INF/spring-hes-db.properties" />
</bean>
 
<!-- ENTITY MANAGER FACTORY -->
<!-- LocalEntityManagerFactoryBean did not work for me -->
<bean id="emf-p" 
  class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
  <property name="persistenceUnitManager" ref="pum"/>
  <property name="persistenceUnitName" value="pu1"/>
  <property name="dataSource" ref="dataSource-p" />
  <property name="jpaVendorAdapter">
    <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
      <property name="database" value="MYSQL" />
      <property name="showSql" value="true" />
      <property name="generateDdl" value="true" />
      <property name="databasePlatform" value="org.hibernate.dialect.MySQLDialect"/>
    </bean>
  </property>
</bean>
 
<!-- TRANSACTION MANAGER -->
<bean id="transactionManager" 
  class="org.springframework.orm.jpa.JpaTransactionManager">
  <property name="entityManagerFactory" ref="emf-p"/>
  <property name="dataSource" ref="dataSource-p"/>
</bean>
 
<!-- DATA SOURCES -->
<bean id="dataSource-p" 
    class="org.springframework.jdbc.datasource.DriverManagerDataSource">
  <property name="driverClassName" value="com.mysql.jdbc.Driver" />
  <property name="url" 
    value="jdbc:mysql://${db.host}:${db.port}/${db.name}"/>
  <property name="username" value="${db.username}" />
  <property name="password" value="${db.password}" />
</bean>
 
<!-- JPA TEMPLATE -->
<bean id="jpaTemplate" class="org.springframework.orm.jpa.JpaTemplate">
 <property name="entityManagerFactory" ref="emf-p" />
</bean>
 
<!-- DAOs -->
<bean id="entityOneDao" class="hes.db.impl.EntityOneDAOImpl">
  <property name="jpaTemplate" ref="jpaTemplate"/>
</bean>
 
<!-- PERSISTENCE UNIT -->
<bean id="pum" 
  class="org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager">
  <property name="persistenceXmlLocations">
    <list>
      <value>META-INF/persistence.xml</value>
    </list>
  </property>
  <property name="dataSources">
    <map>
      <entry key="remoteDataSource" value-ref="dataSource-p" />
    </map>
  </property>
  <property name="defaultDataSource" ref="dataSource-p"/>
</bean>

Some of this may be redundant and will be cleaned up later.

If you have some better ideas, please do share. Thanks.

Problem: I was getting an org.hibernate.AnnotationException when creating a bidirectional one-to-one relationship between 2 entities (EntityOne and EntityTwo).

Relevant Section Of StackTrace:
Caused by: org.hibernate.AnnotationException: @OneToOne or @ManyToOne on com.xyz.domain.EntityOne.anImage references an unknown entity: com.xyz.domain.EntityTwo
at org.hibernate.cfg.ToOneFkSecondPass.doSecondPass(ToOneFkSecondPass.java:103)
at org.hibernate.cfg.AnnotationConfiguration.processEndOfQueue(AnnotationConfiguration.java:541)
at org.hibernate.cfg.AnnotationConfiguration.processFkSecondPassInOrder(AnnotationConfiguration.java:523)
at org.hibernate.cfg.AnnotationConfiguration.secondPassCompile(AnnotationConfiguration.java:380)
at org.hibernate.cfg.Configuration.buildMappings(Configuration.java:1206)
at org.hibernate.ejb.Ejb3Configuration.buildMappings(Ejb3Configuration.java:1449)

Relevant Section Of EntityOne’s Code:

@Entity
@Table(name = "entity1")
public class EntityOne implements Serializable {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  @Column(name = "e1_id")
  private Long id;
 
  @OneToOne(fetch = FetchType.LAZY, optional = true)
  @JoinTable(name = "entity1_entity2_map",
    joinColumns = @JoinColumn(name = "e1_id"),
    inverseJoinColumns = @JoinColumn(name = "e2_id"))
  private EntityTwo anImage;
 
  //Other code
}

Relevant Section Of EntityTwo’s Code:

@Entity
@Table(name = "entity2")
public class EntityTwo implements Serializable {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  @Column(name = "e2_id")
  private Long id;
 
  @OneToOne(mappedBy = "anImage", optional = true)
  private EntityOne e1;
 
  //Other code
}

Solution That Worked For Me:
Check the persistence.xml to ensure that the 2 entities (EntityOne and EntityTwo) are listed. Previously I had not included EntityTwo, which is why I kept getting the message com.xyz.domain.EntityOne.anImage references an unknown entity: com.xyz.domain.EntityTwo

    com.xyz.domain.EntityOne
    com.xyz.domain.EntityTwo
    <!-- other configurations -->

3120 bytes to be precise.

I had access to code which instantiated 3 java.text.SimpleDateFormat objects per method call.

I recently came across simple yet effective instrumentation to query the size of a Java object, javamex. Using it, I found that each instance of java.text.SimpleDateFormat has a deep memory usage of 3120 bytes! In comparison, a 3 character String, say, “ABC” has a deep memory usage of 48 bytes.

This article suggests using an instance of the java.text.SimpleDateFormat per thread by making use of the java.lang.ThreadLoca class.

After referring to two articles online (which you can read here & here), I made some modifications to the code and sure enough, I no longer face the earlier problems related to memory usage.

Some excerpts. I created a per-thread Singleton, DateUtil.

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public class DateUtil
{
  private static SDF1 sdf1 = new SDF1();
  private static SDF2 sdf2 = new SDF2();
 
  // Returns the SimpleDateFormat object
  // for the date pattern "ddMMMyyyy".
  public static SimpleDateFormat getSimpleDateFormat_ddMMMyyyy() {
    return (SimpleDateFormat) sdf1.get();
  }
 
  // Returns the SimpleDateFormat object
  // for the date pattern "EEE dd MMM yyyy".
  public static SimpleDateFormat getSimpleDateFormat_EEE_dd_MMM_yyyy() {
    return (SimpleDateFormat) sdf2.get();
  }
 
  // A ThreadLocal subclass for the SimpleDateFormat
  // for the date pattern "ddMMMyyyy"
  private static class SDF1 extends ThreadLocal<SimpleDateFormat> {
    public SimpleDateFormat initialValue() {
      return new SimpleDateFormat("ddMMMyyyy");
    }
  }
 
  // A ThreadLocal subclass for the SimpleDateFormat 
  // for the date pattern "EEE dd MMM yyyy"
  private static class SDF2 extends ThreadLocal<SimpleDateFormat> {
    public SimpleDateFormat initialValue() {
      return new SimpleDateFormat("EEE dd MMM yyyy");
    }
  }
}

In the older code which previously instantiated a java.text.SimpleDateFormat object, I replaced it with the following,

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  SimpleDateFormat travelDateFormat = DateUtil.getSimpleDateFormat_ddMMMyyyy();
  Date date = travelDateFormat.parse(strToParse);
  //where, strToParse is a String similar to "31Jan2010"

Here’s something that worked for me after having spent several hours on it. First the relevant part of the build.xml file.

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    <copy todir="${build.dir}${file.separator}schemaSrc">
      <fileset dir="${schemas.dir}">
        <include name="*.xsd"/>
        <include name="*.xsdconfig"/>
      </fileset>
    </copy>
    <path id="xmlbeans.path">
      <fileset dir="${lib.dir}/xmlbeans">
        <include name="xbean.jar"/>
        <include name="jsr173_1.0_api.jar"/>
      </fileset>
    </path>
    <property name="xmlbeans.compiler" value="javac1.5"/>
    <taskdef name="xmlbean" classname="org.apache.xmlbeans.impl.tool.XMLBean" 
             classpath="${toString:xmlbeans.path}" />
    <xmlbean schema="${build.dir}${file.separator}schemaSrc"
             destfile="schemas.jar"
             srcgendir="${build.dir}"
             debug="on"
             classpath="${toString:xmlbeans.path}"
             javasource="1.5"
             />

A sample .xsd file,

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<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:element name="Employee">
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element name="SSN" type="xsd:string"/>
        <xsd:element name="Name" type="xsd:string"/>
        <xsd:element name="DateOfBirth" type="xsd:date"/>
        <xsd:element name="EmployeeType" type="xsd:string"/>
        <xsd:element name="Salary" type="xsd:long"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>
</xsd:schema>

A sample .xsdconfig file,

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<xb:config xmlns:xb="http://xml.apache.org/xmlbeans/2004/02/xbean/config">
  <xb:namespace uri="##any">
    <xb:package>com.acompany.xsd</xb:package>
  </xb:namespace>
</xb:config>

Very trivial Java code to get the dates in a window around a specified date (i.e., X days before and Y days after a specified date). The dates are returned as strings of a configurable format.

For example, to get the dates 2 days before and 1 day after 25Dec2009, the returned String[] contains the Strings “23Dec2009“, “24Dec2009“, “25Dec2009“, “26Dec2009“. In this example, I chose to use “ddMMMsyyyy” format, so the date strings look similar to 25Dec2009.

Calling The getDatesAround Method

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SimpleDateUtil util = new SimpleDateUtil();
final String pattern = "ddMMMsyyyy";
try {
  util.getDatesAround(util.constructDate( pattern, "25Dec2009"), 2, 1, pattern);
  util.getDatesAround(util.constructDate( pattern, "14Feb2010"), 4, 1, pattern);
} catch (ParseException e) {
  e.printStackTrace();
}

Calling the getDatesAround method to get the dates 2 days before and 1 day after 25Dec2009, the returned String[] contains the Strings “23Dec2009″, “24Dec2009″, “25Dec2009″, “26Dec2009″.

And now, the source code

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public Date constructDate(String pattern, String dateString) throws ParseException {
  dateFormatter.applyPattern(pattern);
  return dateFormatter.parse(dateString);
}
 
public String[] getDatesAround(Date centredDate, int numDaysBefore, int numDaysAfter, String pattern) {
  int totalNumDays = ((numDaysBefore > 0) ? numDaysBefore : 0) + 1 + ((numDaysAfter > 0) ? numDaysAfter : 0), pos = 0;
  String[] dateStrings = new String[totalNumDays];
  SimpleDateFormat sdf = new SimpleDateFormat(pattern);
  // Get a Date object, which we will be using for manipulations
  Calendar cal = GregorianCalendar.getInstance();
  cal.setTime(centredDate);
  Date dateObj = cal.getTime();
  long time = dateObj.getTime();
  // Constructing the date Strings before the centredDate
  if (numDaysBefore > 0) {
    time = dateObj.getTime();
  for (int i = numDaysBefore; i > 0; i--) {
    dateObj.setTime(time - oneDayInMillis * i);
    dateStrings[pos] = sdf.format(dateObj);
    pos++;
    }
  }
  // centredDate
  dateStrings[pos] = sdf.format(centredDate);
  dateObj.setTime(centredDate.getTime());
  pos++;
 
  // Constructing the date Strings after the centredDate
  if (numDaysAfter > 0) {
    for (int i = pos; i < totalNumDays; i++) {
      time = dateObj.getTime();
      dateObj.setTime(time + oneDayInMillis);
      dateStrings[i] = sdf.format(dateObj);
    }
  }
  return dateStrings;
}

Java code to split up a cAmelCaSe string by using the uppercase characters as the separators. For example, aBcDe into a_b_c_d_e.

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private String convertCamelCaseString(String camelCaseString, String optionalSeparator) {
  int n = camelCaseString.trim().length();
  StringBuilder sb = new StringBuilder(n * 2);
  for (int i = 0; i < n; i++) {
    char c = camelCaseString.charAt(i);
    int x = (int) c;
    // A=65, N=78, Z=90, a=97
    // See http://blossomassociates.net/ascii.html
    if(optionalSeparator==null) optionalSeparator="";
    if (x >= 65 && x <= 90) { // Converting to lower case
      c = (char) (x + 32);
      sb.append(optionalSeparator).append(c);
    } else
      sb.append(c);
  }
 
  // Final string converted from camelCaseString
  String convertedString = sb.toString();
  sb = null;
  return convertedString;
}

In my code, there is a Struts 2 ActionSupport subclass passing a List of Item objects to a form, to be used to populate a Struts 2 Select tag.

The relevant part of the form, is

<s:form action="someAction" method="POST">
<s:select label="Associate This With Item" 
  name="itemId" 
  list="items" 
  listKey="id" 
  listValue="title" 
  value="%{items.{title}}"/>
<s:submit type="button" value="Associate" />
</s:form>

Here, the Item object’s title attribute is being displayed and upon form submit, the id attribute of the selected Item object is passed to the backend. This is specified by using the listValue and listKey parameters respectively.

Form used to update an entity, involving a radio button.

<s:form action="editSomethingAction" method="POST">
<s:radio name="moduleStateSelected" key="label.state" 
    list="#{1:'Draft', 2:'Published'}" 
    value="%{module.state}" />
<s:submit value="Update" align="right"/>
</s:form>

The form parameter moduleStateSelected takes in the value of the state attribute of an object of the Module class, passed by the Struts 2 action.

The related code in the Struts 2 ActionSupport subclass,

public class SomeWebAction extends ActionSupport {
private Module module;
public Module getModule() {
  return module;
}
public void setModule(Module m) {
  this.module = m;
}
//.. other methods not listed here
}

And, relevant code of the Module class,

public class Module {
private short state;
public short getState() {
  return module;
}
public void setState(short newState) {
  this.state = newState;
}
//.. other methods not listed here
}

I have a Struts 2 action with getter/setter methods for 3 attribute, say, name, id and an object of a class Question (which has a string attribute, questionText, which I want to access in the form).

I somehow forgot how to access the attributes to be used as the default values in a form, so after a bit of searching old code, here’s what it should look like:

<s:form action="editSomethingAction" method="POST">
<s:hidden name="id" value="%{#parameters['id']}" />
<s:hidden name="name" value="%{#parameters['name']}" />
<s:textarea name="questionText" label="" rows="3" cols="80" value="%{question.questionText}"/>
<s:submit value="Update" align="right"/>
</s:form>