Develop java persistence example with Maven2
Getting Started with Maven2 : Download Maven2
from maven.apache.org, unzip the archive
into your local directory. Here we are assuming that the local repository already
exists.
Set the JAVA_HOME variable to point to the JDK installation directory
and MAVEN_HOME to point to the Maven directory and add
the MAVEN_HOME/bin to the PATH environment
variable.
To
test, whether the path has been set properly. Type mvn
-version on
the command prompt.
|
C:\>mvn -versionMaven
version: 2.0.7Java version:1.5.0OS name: “windows 2000” version: “5.0”
arch:“x86” |
Developing java presistence example with Maven2
Here I will show, how to build a very simple example that uses java persistence
API. Maven2 highly simplifies the build process, all the required libraries
are downloaded automatically only when it really requires.
Directory Structure of the example: Our
application must maintain a directory structure understandable by the maven
project build tool. It is shown below:
Our Application includes: Our application
includes the source folder "src" and an pom.xml file. Source
folder contains java files, here HelloWorld.java and Message.java files are
contained in the hello folder. Resource folder exists parallel to the
java folder inside the main folder and contains log4j.properties
file and the META-INF folder. META-INF folder contains persistence.xml
file. "pom.xml" is the main file for the maven build system
that includes information about the tools required to build the
application.
1. Create a java class (lets take it Message.java ), where you map java class/properties to database table/fields. Java persistence
annotations will do this job for us:
package hello;
import javax.persistence.*;
@Entity
@Table(name = "MESSAGES")
public class Message {
@Id @GeneratedValue @Column(name = "MESSAGE_ID")
private Long id;
@Column(name = "MESSAGE_TEXT")
private String text;
@ManyToOne(cascade = CascadeType.ALL) @JoinColumn(name = "NEXT_MESSAGE_ID")
private Message nextMessage;
public Message() {}
public Message(String text) {
this.text = text;
}
public Long getId() {
return id;
}
private void setId(Long id) {
this.id = id;
}
public String getText() {
return text;
}
public void setText(String text) {
this.text = text;
}
public Message getNextMessage() {
return nextMessage;
}
public void setNextMessage(Message nextMessage) {
this.nextMessage = nextMessage;
}
}
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Here "Message" class is mapped to "MESSAGES" table,
"id", "text" and "nextMessage" properties - to
"MESSAGE_ID", "MESSAGE_TEXT" and "NEXT_MESSAGE_ID"
fields.
2. Now create a simple program ( HelloWorld.java ) that uses persistent "Message" object:
HelloWorld.java
package hello;
import java.util.*;
import javax.persistence.*;
public class HelloWorld {
public static void main(String[] args) {
// Start EntityManagerFactory
EntityManagerFactory emf =
Persistence.createEntityManagerFactory("helloworld");
// First unit of work
EntityManager em = emf.createEntityManager();
EntityTransaction tx = em.getTransaction();
tx.begin();
Message message = new Message("Hello World with JPA");
em.persist(message);
message = new Message("This is message 2");
em.persist(message);
message = new Message("This is message 3");
em.persist(message);
tx.commit();
em.close();
// Second unit of work
EntityManager newEm = emf.createEntityManager();
EntityTransaction newTx = newEm.getTransaction();
newTx.begin();
List messages =
newEm.createQuery("select m from Message m order by m.text asc").getResultList();
System.out.println( messages.size() + " message(s) found:" );
for (Object m : messages) {
Message loadedMsg = (Message) m;
System.out.println(loadedMsg.getText());
}
newTx.commit();
newEm.close();
// Shutting down the application
emf.close();
}
}
|
This example does not refer to any persistent framework directly. Instead, it
uses symbolic names to access the framework in indirect way. In the above
mentioned example we have used "helloworld" name to refer to the ORM
framework.
3. In this example we used Hibernate (http://hibernate.org) as persistence
framework and hsqldb (http://hsqldb.org) as database. Let's take a look at the
hibernate configuration file (persistence.xml) where we describe "helloworld"
factory:
persistence.xml
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
version="1.0">
<!-- persistence.xml -->
<persistence-unit name="helloworld">
<!-- The provider only needs to be set if you use several JPA providers -->
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<properties>
<!-- Scan for annotated classes and Hibernate mapping XML files -->
<property name="hibernate.archive.autodetection" value="class, hbm"/>
<!-- SQL stdout logging -->
<property name="hibernate.show_sql" value="true"/>
<property name="hibernate.format_sql" value="true"/>
<property name="use_sql_comments" value="true"/>
<property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/>
<property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver"/>
<property name="hibernate.connection.url" value="jdbc:hsqldb:file:persistence-db/test"/>
<property name="hibernate.connection.username" value="sa"/>
<property name="hibernate.hbm2ddl.auto" value="create"/>
<property name="hibernate.c3p0.min_size" value="5"/>
<property name="hibernate.c3p0.max_size" value="20"/>
<property name="hibernate.c3p0.timeout" value="300"/>
<property name="hibernate.c3p0.max_statements" value="50"/>
<property name="hibernate.c3p0.idle_test_period" value="3000"/>
</properties>
</persistence-unit>
</persistence>
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persistence.xml should be located on your CLASSPATH within META-INF
directory. "hibernate.hbm2ddl.auto" property will take care of
creating database table automatically.
4. Create a Maven2 file i.e. pom.xml file, responsible for downloading all dependent libraries, building
correct CLASSPATH for the project and running the example. POM is the fundamental unit of work in Maven. It is an xml file that contains
information about the project and configuration details used by Maven to build
the project. It contains default values for most projects. For example, the build directory, which is “target”,
the source directory, which is “src/main/java” the test source
directory, which is
“src/main/test” and so on. POM also contains the goals and plugins. So, while executing a task or goal,
Maven looks for the POM in the current directory. It reads the POM, gets the
needed configuration information, and then executes the goal. Some of the
configuration that can be specified in the POM are the project dependencies, the
plugins or goals that can be executed, the build profiles, and so on. Other
information such as the project version, description, developers, mailing lists
and such can also be specified.
pom.xml
<?xml version="1.0" encoding="UTF-8"?>
<project>
<modelVersion>4.0.0</modelVersion>
<groupId>persistence-deps</groupId>
<artifactId>persistence-deps</artifactId>
<version>1.0</version>
<dependencies>
<dependency>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
<version>1.0.4</version>
</dependency>
<dependency>
<groupId>hsqldb</groupId>
<artifactId>hsqldb</artifactId>
<version>1.8.0.7</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate</artifactId>
<version>3.2.2.ga</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-annotations</artifactId>
<version>3.2.1.ga</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-entitymanager</artifactId>
<version>3.2.1.ga</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-tools</artifactId>
<version>3.2.0.beta9a</version>
</dependency>
<dependency>
<groupId>c3p0</groupId>
<artifactId>c3p0</artifactId>
<version>0.9.1</version>
</dependency>
</dependencies>
<build>
<defaultGoal>compile</defaultGoal>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.5</source>
<target>1.5</target>
</configuration>
</plugin>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<executions>
<execution>
<goals>
<goal>java</goal>
</goals>
</execution>
</executions>
<configuration>
<mainClass>hello.HelloWorld</mainClass>
</configuration>
</plugin>
</plugins>
</build>
<!--
<repositories>
<repository>
<id>scriptlandia-repo</id>
<name>Scriptlandia Maven2 repository</name>
<url>http://scriptlandia-repository.googlecode.com/svn/trunk/tools</url>
</repository>
</repositories>
-->
</project> |
4. Open the command prompt. Go inside the project directory of your application
(MyApplication in this case) and run the "mvn compile exec:java"
command.
| C:\MyApplication >mvn compile exec:java |
After successfully running the command, your command prompt will
display the output on the console shown below:
Hibernate:
insert
into
MESSAGES
(MESSAGE_ID, MESSAGE_TEXT, NEXT_MESSAGE_ID)
values
(null, ?, ?)
Hibernate:
call identity()
Hibernate:
insert
into
MESSAGES
(MESSAGE_ID, MESSAGE_TEXT, NEXT_MESSAGE_ID)
values
(null, ?, ?)
Hibernate:
call identity()
Hibernate:
insert
into
MESSAGES
(MESSAGE_ID, MESSAGE_TEXT, NEXT_MESSAGE_ID)
values
(null, ?, ?)
Hibernate:
call identity()
Hibernate:
select
message0_.MESSAGE_ID as MESSAGE1_0_,
message0_.MESSAGE_TEXT as MESSAGE2_0_,
message0_.NEXT_MESSAGE_ID as NEXT3_0_
from
MESSAGES message0_
order by
message0_.MESSAGE_TEXT asc
3 message(s) found:
Hello World with JPA
This is message 2
This is message 3
Aug 21, 2007 3:18:06 AM org.hibernate.impl.SessionFactoryImpl close
INFO: closing
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 34 seconds
[INFO] Finished at: Tue Aug 21 03:18:06 GMT+05:30 2007
[INFO] Final Memory: 6M/11M
[INFO] ------------------------------------------------------------------------ |
On building successfully, our application displays the
messages given below on the console:
Hello World with JPA
This is message 2
This is message 3
|
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