Help

Inactive Bloggers
06. Jul 2004, 01:51 CET, by Gavin King

Now that the EJB 3.0 early draft is out there, it's definitely time to move on from all the recent politics, and start thinking about what we can actually /do/ with this stuff. EJB is the only Java standard which addresses server-side /business logic/, and is thus central to J2EE. The EJB spec committee recognizes that EJB has fallen short of achieving some of its ambitious goals, and is in need of an overhaul. This overhaul has focused very much upon ease of use, primarily via simplification of the requirements upon bean implementors. However, the spec committe also identified a number of critical functional enhancements that facilitate ease of use and the elimination of certain J2EE anti-patterns.

Fortunately, the Java community now knows much more about the problems involved in building web and enterprise applications than it did five years ago. We have been learning by doing. A powerful, easy to use, standard approach to building server-side business objects is now well within our grasp!

The spec committee thought very hard about how to simplify certain extremely common use cases, expecially two cases that I'll canonicalize as follows:

/Updating data/

1. retrieve an item of data

2. display on screen

3. accept user input

4. update the data

5. report success to the user

/Entering new data/

1. retrieve an item of data

2. display on screen

3. accept user input

4. create a new item of data, with an association to the first item

5. report success to the user

Each of these cases involves two full application request/response cycles. Both demonstrate that optimistic locking is essential in an application that requires high concurrency (both cases would need to be implemented as a single conversation spanning two distinct ACID database/JTA transactions).

A further common use case, that is especially difficult to do in EJB 2.1, given the limitations of the query language is the following:

/Listing and reporting data/

1. retrieve an aggregated or summarized list of data

2. display it to the user

Finally, we considered two canonical physical architectures. The /colocated/ case - which we considered most important, at least for typical web apps - has a presentation layer acting as a local client of the business layer. The /remote case/ has a remote client (for example, a servlet engine or swing client running in a different physical tier) accessing the business layer.

First we need data. Interaction with relational data is central to almost every web or enterprise application. Applications that implement nontrivial business logic benefit from an object-oriented representation of the data (a /domain model/) that takes full advantage of object oriented techniques like inheritance and polymorphism. For various reasons (I won't repeat the arguments we made at length in Hibernate in Action), applications which use fully object oriented domain models need an automated solution to the OR mismatch problem. EJB3 incorporates a very sophisticated ORM specification that draws heavily upon the experience of CMP 2.1, Hibernate and Oracle's TopLink.

Our auction application has Users, Items and Bids. Let's implement these as 3.0-style entity beans. We'll start with Item:

@Entity 
public class Item {
    private Long id;
    private User seller;
    private Collection<Bid> bids = new ArrayList<Bid>();
    private String description;
    private String shortDescription;
    private Date auctionEnd;
    private int version;
    
    public Item(User seller, String desc, String shortDesc, Date end) {
        this.seller = seller;
        this.description = desc;
        this.shortDescription = shortDesc;
        this.auctionEnd = end;
    }
    
    protected Item() {}
    
    @Id(generate=AUTO)
    public Long getId() {
        return id;
    }
    protected void setId(Long id) {
        this.id = id;
    }
    
    @Column(length=500)
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    
    public Date getAuctionEnd() {
        return auctionEnd;
    }
    protected void setAuctionEnd(Date end) {
        this.auctionEnd = end;
    }
    
    @Column(length=100)
    public String getShortDescription() {
        return shortDescription;
    }
    public void setShortDescription(String shortDescription) {
        this.shortDescription = shortDescription;
    }
    
    @JoinColumn(nullable=false, updatable=false)
    public User getSeller() {
        return seller;
    }
    protected void setSeller(User seller) {
        this.seller = seller;
    }
    
    @OneToMany(cascade=ALL)
    protected Collection<Bid> getBids() {
        return bids;
    }
    protected void setBids(Collection<Bid> bids) {
        this.bids = bids;
    }
    
    @Version
    public int getVersion() {
        return version;
    }
    protected void setVersion(int version) {
        this.version = version;
    }
    
    public Bid bid(BigDecimal amount, User bidder) {
        Bid newBid = new Bid(this, amount, bidder);
        bids.add(newBid);
        return bid;
    }
    
}

The most striking thing, initially, is the annotations. No deployment descriptor is required! EJB 3.0 will let you choose between the use of annotations and XML deployment descriptors, but we expect that annotations will be the most common case.

The @Entity annotation tells the container that this class is a CMP entity bean. The optional @Column annotations define the column mappings for persistent properties (if we were mapping legacy data, we would define column names in these annotations). The @Id annotations selects the primary key property of the entity bean. In this case, the entity bean has a generated primary key. The optional @OneToMany annotations defines a one to many association, and specifies a /cascade style/ of ALL. The optional @JoinColumn annotation defines a column mapping for an association. In this case, it constrains the association to be required and immutable. The @Version annotation defines the property used by the container for optimistic locking.

Perhaps the most important thing to notice here is that almost all of the annotations are optional. EJB3 uses /configuration by exception/, meaning that the specification defines natural, useful defaults so that you do /not/ need to specify an annotation in the common case. For example, we have not included any annotation at all on one of the persistent properties.

Also striking is that, apart from the annotations, Item has no dependency to classes or interfaces in javax.ejb. This was a major goal of EJB3, and helps to significantly reduce noise.

Entity beans do not require a local or remote interface, which also reduces noise.

It's time to implement Bid:

@Entity
public class Bid {
    private Long id;
    private Item item;
    private BigDecimal amount;
    private Date datetime;
    private User bidder;
    private boolean approved;
    
    protected Bid() {}
    
    public Bid(Item item, BigDecimal amount, User bidder) {
        this.item = item;
        this.amount = amount;
        this.bidder = bidder;
        this.datetime = new Date();
    }
    
    @Id(generate=AUTO)
    public Long getId() {
        return id;
    }
    protected void setId(Long id) {
        this.id = id;
    }
    
    @Column(nullable=false) 
    public BigDecimal getAmount() {
        return amount;
    }
    protected void setAmount(BigDecimal amount) {
        this.amount = amount;
    }
    
    @Column(nullable=false) 
    public Date getDatetime() {
        return datetime;
    }
    protected void setAmount(Date datetime) {
        this.datetime = datetime;
    }
    
    @JoinColumn(nullable=false)
    public User getBidder() {
        return bidder;
    }
    protected void setBidder(User bidder) {
        this.bidder = bidder;
    }
    
    @JoinColumn(nullable=false)
    public Item getItem() {
        return item;
    }
    protected void setItem(Item item) {
        this.item = item;
    }
    
    public boolean isApproved() {
        return approved;
    }
    public void setApproved(boolean approved) {
        this.approved = approved;
    }
    
    
}

I'll leave it to you to write the User entity bean!

Notice that our domain model classes are now simple JavaBeans. This means that they are testable outside the EJB container.

What about the home interfaces? Well, the requirement for home interfaces has been eliminated by EJB3, so we won't be needing them.

Now, let's look at our first use case, updating an Item. We'll assume a colocated architecture, and implement our business logic in a stateless session bean with a local business interface. A business interface defines operations visible to the client (your session beans may have as many business interfaces as you like). We'll define the Auction local interface as follows:

public interface Auction {
    public Item getItemById(Long itemId);
    public void updateItem(Item item);
}

Business interfaces are local by default, so no @Local annotation is required.

Since our Item entity bean is also a JavaBean, we can pass it directly to a JSP (for example). We do not need any DTOs, or complex method signatures.

The AuctionImpl bean class implements this interface:

@Stateless
public class AuctionImpl implements Auction {
    @Inject public EntityManager em;
    
    public Item getItemById(Long itemId) {
        return em.find(Item.class, itemId);
    }
    
    public void updateItem(Item item) {
        em.merge(item);
    }
}

Wow. That was easy!

The @Inject annotation is used to indicate that a field of a session bean is set by the container. In EJB 3.0, session beans do not need to use JNDI to obtain references to resources or other EJBs. In this case, the container /injects/ a reference to the EntityManager, the interface for persistence-related operations on entity beans.

Notice that the container handles optimistic locking transparently.

Suppose our client was a servlet. The code to display an Item might look something like this:

Long itemId = new Long( request.getParameter("itemId") );

Auction auction = (Auction) new InitialContext().lookup("Auction");

Item item = auction.getItemById(itemId);

session.setAttribute("item", item);

Since the Item entity bean is also just a simple JavaBean, the JSP can use it directly. The second request, which updates the Item, might look like this:

Item item = (Item) session.getAttribute("item");

item.setDescription( request.getParameter("description") );
item.setShortDescription( request.getParameter("shortDescription") );

Auction auction = (Auction) new InitialContext().lookup("Auction");
    
auction.updateItem(item);

Note that other expert groups are exploring ways to eliminate JNDI lookups, such as these lookups for the session bean.

Now let's turn to the second use case, creating a new Bid. We'll add a new method to Auction:

public interface Auction {
    public Item getItemById(Long itemId);
    public void updateItem(Item item);
    public Bid bidForItem(Item item, BigDecimal amount, User bidder)
        throws InvalidBidException;
    public void approveBid(Long bidId);
}

We must implement these new methods in AuctionImpl, of course:

@Stateless
public class AuctionImpl implements Auction {
    @Inject public EntityManager em;
    @Inject QueueConnectionFactory bidQueue;
    
    ...
    
    public void approveBid(Long bidId) {
        Bid bid = em.find(Bid.class, bidId);
        bid.setApproved(approved);
    }
    
    public Bid bidForItem(Item item, BigDecimal amount, User bidder)
        throws InvalidBidException {
        
        String query = "select max(bid.amount) from Bid bid where "
            + "bid.item.id = :itemId and bid.approved = true";
        
        BigDecimal maxApprovedBid = 
            (BigDecimal) em.createNamedQuery(query)
                .setParameter( "itemId", item.getId() )
                .getUniqueResult();
        
        if ( amount.lessThan(maxApprovedBid) ) {
            throw new InvalidBidException();
        }
        
        Bid bid = item.createBid(amount, bidder);
        
        em.create(bid);
        
        requestApproval(bid, bidQueue);
        
        return bid;            
        
    }
    
    
    private static void requestApproval(Bid bid, Queue queue) {
        ...
    }
}

The bidForItem() method executes an EJB QL query that determines the current maximum approved Bid. If the amount of the new bid is larger, we instantiate a new Bid and make it persistent by calling EntityManager.create().

The requestApproval() method sends a message to the queue, asking a backend system to verify the bidder's ability to pay. The session bean gets a reference to a JMS QueueConnectionFactory by dependency injection.

We need a message driven bean, BidApprovalListener, to receive the reply from the backend system. When the reply is received, we'll send an email to the bidder, and mark the Bid approved.

@MessageDriven 
public class BidApprovalListener implements MessageListener {
    @Inject javax.mail.Session session;
    @Inject Auction auction;
    
    public void onMessage(Message msg) {
    
        MapMessage mapMessage = (MapMessage) msg;
        boolean approved = mapMessage.getBoolean("approved");
        long bidId = mapMessage.getLong("bidId");
        
        auction.approveBid(bidId);
                    
        notifyApproval(bid, session);
        
    }
    
    private static void notifyApproval(Bid bid, Session session) {
        ...
    }
}

Notice that the message driven bean gets its JavaMail Session and a reference to the Auction session bean by dependency injection.

Our final use case is easy. We add one more method to Auction:

public interface Auction {
    public Item getItemById(Long itemId);
    public void updateItem(Item item);
    public Bid bidForItem(Item item, BigDecimal amount, User bidder)
        throws InvalidBidException;
    public void approveBid(Long bidId);
    public List<ItemSummary> getItemSummaries();
}

And implement it in AuctionImpl:

@Stateless
public class AuctionImpl implements Auction {
    ...
    
    public List<ItemSummary> getItemSummaries() {
        String query = "select new "
        + "ItemSummary(item.shortDescription, item.id, max(bid.amount)) "
        + "from Item item left outer join item.bids bid";
        
        return (List<ItemSummary>) em.createQuery(query)
            .listResults();
    }
}

ItemSummary is just a simple JavaBean, with an appropriate constructor, so we don't need to show it here.

The EJB QL query demonstrates how easy it is to do outer joins, projection and aggregation in EJB3. These (and other) new features make the Fast Lane Reader antipattern pretty much obsolete.

Well, that's the end of part one. We've seen the following simplifications:

  • Annotations replace complex XML deployment descriptors
  • Elimination of home interfaces
  • Elimination of dependencies to classes and interfaces in javax.ejb
  • Entity beans are simple JavaBeans, with no local or remote interfaces
  • Dependency injection replaces JNDI lookups
  • Elimination of DTOs
  • Elimination of Fast Lane Reader antipattern

In the next installment, we'll consider remote clients...