Chapter 4. Introduction to Blob and Clob support in EJB3

The EJB 3.0 specification has support for Blob and Clob types. The specification allows you to map the following types to an entity property:

java.sql.Blob
java.sql.Clob
Any Serializable Object
byte[], Byte[]
char[], String, Character[]

To use this feature just need to use the @javax.persistence.Lob annotation. The Lob annotation is an encapsulation of what type of lob you want. Below is an example of defining fields in an entity that are blobs or clobs.

@Entity
public class BlobEntity implements Serializable
{
   private long id;
   private Blob blobby;
   private Clob clobby;

   @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
   public long getId()
   {
      return id;
   }

   public void setId(long id)
   {
      this.id = id;
   }

   @Lob @Basic(fetch = FetchType.EAGER)
   public Blob getBlobby()
   {
      return blobby;
   }

   public void setBlobby(Blob blobby)
   {
      this.blobby = blobby;
   }

   @Lob @Basic(fetch = FetchType.EAGER)
   public Clob getClobby()
   {
      return clobby;
   }

   public void setClobby(Clob clobby)
   {
      this.clobby = clobby;
   }


}

Working with Blobs and Clobs:

Open up org.jboss.tutorial.blob.bean.LobTesterBean and look for the create() method. JBoss EJB3 is built on top of the Hibernate persistence engine. Hibernate has some helper methods for creating blobs and clobs that {{LobTesterBean}} uses.

Blob creation:

org.hibernate.Hibernate.createBlob(byte[] bytes) org.hibernate.Hibernate.createBlob(InputStream stream, int length) org.hibernate.Hibernate.createBlob(InputStream stream)

Clob creation:

org.hibernate.Hibernate.createClob(String string) org.hibernate.Hibernate.createClob(Reader reader, int length)

Blobs and clobs must only be accessed within a transaction. Blobs and clobs are also not serializable or detachable.

Mapping Strings/byte[] to Clob/Blob:

This is pretty easy, just look at BlobEntity2.java

@Entity
public class BlobEntity2 implements Serializable
{
   private long id;
   private byte[] blobby;
   private String clobby;

   @Id @GeneratedValue(strategy=GenerationType.AUTO)
   public long getId()
   {
      return id;
   }

   public void setId(long id)
   {
      this.id = id;
   }

   @Lob @Basic(fetch = FetchType.EAGER)
   public byte[] getBlobby()
   {
      return blobby;
   }

   public void setBlobby(byte[] blobby)
   {
      this.blobby = blobby;
   }

   @Lob @Basic(fetch = FetchType.EAGER)
   public String getClobby()
   {
      return clobby;
   }

   public void setClobby(String clobby)
   {
      this.clobby = clobby;
   }


}

Building and Running:

Note

To build and run the example, make sure you have installed JBoss 5.x. See the Section 1.1, “JBoss Application Server 5.x” for details.

From the command prompt, move to the "blob" folder under the Section 1.3, “Set the EJB3_TUTORIAL_HOME”

Ant Users:

Make sure your JBossAS-5.x is running

$ ant
$ ant run

Maven Users: Make sure the AS is not running.

$ mvn clean install -PRunSingleTutorial

View the tables and rows:

You can view the tables created by JBoss by going to the Hypersonic Service, scrolling down to the startDatabaseManager button and clicking it. A Hypersonic SQL window will be minimized, but you can open it up to look at the tables and do queries.