Difference between revisions of "Hibernate-Guidelines"

Latest revision as of 14:32, 2 February 2016

Status

In progress

Important

SQL Injection

It is commonly thought that ORM layers, like Hibernate are immune to SQL injection. This is not the case as Hibernate includes a subset of SQL called HQL, and allows "native" SQL queries. Often the ORM layer only minimally manipulates the inbound query before handing it off to the database for processing.

Transaction Scope

All database communication should occur within the scope of a Transaction.

Persisting Tainted Data

If an application sets tainted properties in an object, then makes that object persistent, it is highly likely that data will be accessed at some point and displayed. If database values are never displayed to the user then obviously this is nullified. Better yet if we could figure out which values from the db are tainted but I have a feeling it's not a feasible scan.

Rollback

When an exception occurs, roll back the Transaction and close the Session. If you don't, Hibernate can't guarantee that in-memory state accurately represents persistent state. If your Session is bound to the application, you have to stop the application. Rolling back the database transaction doesn't put your business objects back into the state they were at the start of the transaction, so the database state and the business objects do get out of sync. Objects that are manipulated after a rollback as if they were synchronized.

Simple

Don't use load() to determine existence

Do not use Session.load() to determine if an instance with the given identifier exists on the database; use Session.get() or a query instead.

Constructing Query Strings

Hibernate has a set of functions that are used internally to construct their query strings, but there is no limit to using them in application code - what if they were used with tainted data? This is pretty self explanatory.

Place each class mapping in its own file

Don't use a single monolithic mapping document. Map com.eg.Foo in the file com/eg/Foo.hbm.xml. This makes particularly good sense in a team environment. Nothing more than a app dev good practice.

Use bind variables

As in JDBC, always replace non-constant values by "?". Never use string manipulation to bind a nonconstant value in a query! Even better, consider using named parameters in queries.

Load mappings as resources

Deploy the mappings along with the classes they map.

Passwords in the hibernate.cfg.xml in plain text

Sensitive information such as passwords should be encrypted to prevent attacks like connecting directly to the database, you can extend Hibernate functionality to implement password encryption.

Detailed

Transaction Timeout

Transaction timeouts ensure that no misbehaving transaction can indefinitely tie up resources while returning no response to the user.
Outside a managed (JTA) environment, Hibernate cannot fully provide this functionality. However, Hibernate can at least control data access operations, ensuring that database level deadlocks and queries with huge result sets are limited by a defined timeout.
In a managed environment, Hibernate can delegate transaction timeout to JTA.

Session sess = factory.openSession();
try {
     //set transaction timeout to 3 seconds
     sess.getTransaction().setTimeout(3);
     sess.getTransaction().begin();
     // do some work
          ...
     sess.getTransaction().commit()
}
catch (RuntimeException e) {
     sess.getTransaction().rollback();
     throw e; // or display error message
}
finally {
     sess.close();
}

Identify natural keys

Even though we recommend the use of surrogate keys as primary keys, you should still try to identify natural keys for all entities.
A natural key is a property or combination of properties that is unique and non-null. If it is also immutable, even better.
Map the properties of the natural key inside the <natural-id> element. Hibernate will generate the necessary unique key and nullability constraints, and your mapping will be more selfdocumenting.
We strongly recommend that you implement equals() and hashCode() to compare the natural key properties of the entity.
This mapping is not intended for use with entities with natural primary keys.

Example in hbm.xml:

 <natural-id mutable="true|false"/>
     <property ... />
     <many-to-one ... />
     ......
 </natural-id>

Parameter Binding

Hibernate parameter binding works like prepared statements. An edge case it is, but say you had an already tainted string, and then used Query setters to bind more parameters to the "?" placeholders.

Example of proper use:

     Query q = sess.createQuery("from DomesticCat cat where cat.name = ?");
     q.setString(0, "Izi");
     Iterator cats = q.iterate();

Improper use:

     Insert insert = new Insert(dialect);
     insert.setComment(taint);
     insert.addSelectColumn(columnName, alias);
     String sql = insert.toQueryString();
           ....
     Query query = sess.createQuery(sql); /*B00*///creating a Query with an already tainted string
     query.setString(position, val); //this doesn't cleanse the damage that's been done.
           ....
     transaction.commit(); //sink the ship

Declare identifier properties on persistent classes

Hibernate makes identifier properties optional. There are all sorts of reasons why you should use them. Mapped classes must declare the primary key column of the database table. Most classes will also have a Java-Beans-style property holding the unique identifier of an instance. This is the easiest and most recommended route. any

Don't use session.find()

This pretty much looks like a duplicate of sql injection but using the deprecated method find() instead. This rule was pulled straight from the best practice list from the reference manual.

If using Hibernate, do not use the depreciated session.find() method without using one of the query binding overloads. Using session.find() with direct user input allows the user input to be passed directly to the underlying SQL engine and will result in SQL injections on all supported RDBMS.


  Payment payment = (Payment) session.
       find("from com.example.Payment as payment where payment.id = " + paymentIds.get(i));

The above Hibernate HQL will allow SQL injection from paymentIds, which are obtained from the user. A safer way to express this is:


  int pId = paymentIds.get(i);

  TsPayment payment = (TsPayment) session.
       find("from com.example.Payment as payment where payment.id = ?", pId, StringType);

Cache Management and !OutOfMemoryException

The Session caches every object that is in persistent state (watched and checked for dirty state by Hibernate).
Whenever you pass an object to save(), update() or saveOrUpdate() and whenever you retrieve an object using load(), get(), list(), iterate() or scroll(), that object is added to the internal cache of the Session.
When flush() or commit() is subsequently called, the state of that object will be synchronized with the database.
Otherwise it grows endlessly until you get an OutOfMemoryException, if you keep it open for a long time or simply load too much data.
One solution for this is to call clear() and evict() to manage the Session cache, but you most likely should consider a Stored Procedure if you need mass data operations.
Some solutions are shown in Chapter 13, Batch processing. Keeping a Session open for the duration of a user session also means a high probability of stale data.

In this example, it would fall over with an OutOfMemoryException somewhere around the 50 000th row. That's because Hibernate caches all the newly inserted Customer instances in the session-level cache.

     Session session = sessionFactory.openSession();
     Transaction tx = session.beginTransaction();
     for ( int i=0; i<100000; i++ ) {
     Customer customer = new Customer(.....);
     session.save(customer);
     }
     tx.commit();
     session.close();

When making new objects persistent, you must flush() and then clear() the session regularly, to control the

     size of the first-level cache.
     Session session = sessionFactory.openSession();
     Transaction tx = session.beginTransaction();
     for ( int i=0; i<100000; i++ ) {
          Customer customer = new Customer(.....);
          session.save(customer);
          if ( i % 20 == 0 ) { //20, same as the JDBC batch size
               //flush a batch of inserts and release memory:
               session.flush();
               session.clear();
          }
     }
     tx.commit();
     session.close();

About StatelessSession

A StatelessSession has no persistence context associated with it and does not provide many of the higher-level life cycle semantics. In particular, a stateless session does not implement a first-level cache nor interact with any second-level or query cache.
This may be used as a solution for the cache out of memory problem.
Operations performed using a stateless session do not ever cascade to associated instances. Collections are ignored by a stateless session.
The insert(), update() and delete() operations defined by the StatelessSession interface are considered

to be direct database row-level operations, which result in immediate execution of a SQL INSERT, UPDATE or DELETE respectively.

Example of proper use:

     StatelessSession session = sessionFactory.openStatelessSession();
     Transaction tx = session.beginTransaction();

     ScrollableResults customers = session.getNamedQuery("GetCustomers")
          .scroll(ScrollMode.FORWARD_ONLY);
     while ( customers.next() ) {
          Customer customer = (Customer) customers.get(0);
          customer.updateStuff(...);
          session.update(customer);
     }
     tx.commit();
     session.close();

Aboot POJOs

You can tell what objects are the persistent ones by looking at the xxx.hbm.xml files.
Persistent objects are usually POJO's with a default no-arg constructor so that Hibernate can instantiate them using Constructor.newInstance().
It is recommended that these POJO's have a property that is explicitly mapped as a primary key.

<hibernate-mapping>
   <class entity-name="Customer">
      <id name="id"
         type="long"
         column="ID">
         <generator class="sequence"/>
      </id>
         ......

Accessors and mutators should be declared for persistent fields - it is optional but is good practice because Hibernate persists JavaBeans style properties.

Session and Concurrency issues

A Session is not thread-safe.
Things which are supposed to work concurrently, like HTTP requests, session beans, or Swing workers, will cause race conditions if a Session instance would be shared.
If you keep your Hibernate Session in your HttpSession, you should consider synchronizing access to your Http session.
Otherwise, a user that clicks reload fast enough may use the same Session in two concurrently running threads.

The equals() and hashCode() problem:

Most Java objects provide a built-in equals() and hashCode() based on the object's identity; so each new() object will be different from all others.
If all your objects are in memory, this is a fine model, but Hibernate's whole job, of course, is to move your objects out of memory.
Hibernate uses the Hibernate session to manage this uniqueness. When you create an object with new(), and then save it into a session, Hibernate now knows that whenever you query for an object and find that particular object, Hibernate should return you that instance of the object.
However, once you close the Hibernate session, all bets are off. If you keep holding onto an object that you either created or loaded in a Hibernate session that you have now closed, Hibernate has no way to know about those objects.
So if you open another session and query for "the same" object, Hibernate will return you a new instance. Hence, if you keep collections of objects around between sessions, you will start to experience odd behavior (duplicate objects in collections, mainly).
Long story short equals() and hashCode() should be implemented to compare object properties and not just compare on the basis of identifier (the property mapped as the primary key see the above example). The problem is discussed in detail here.

public class Cat {
...
 public boolean equals(Object other) {
   if (this == other) return true;
   if ( !(other instanceof Cat) ) return false;
   final Cat cat = (Cat) other;
   if ( !cat.getLitterId().equals( getLitterId() ) ) return false;
   if ( !cat.getMother().equals( getMother() ) ) return false;
   return true;
 }
 public int hashCode() {
   int result;
   result = getMother().hashCode();
   result = 29 * result + getLitterId();
   return result;
 }
}

Using Detached Objects

In a three tiered architecture, consider using detached objects.
When using a servlet / session bean architecture, you could pass persistent objects loaded in the session

bean to and from the servlet / JSP layer.

Use a new session to service each request.
Use Session.merge() or Session.saveOrUpdate() to synchronize objects with the database.

Usually update() or saveOrUpdate() are used in the following scenario:

the application loads an object in the first session
the object is passed up to the UI tier
some modifications are made to the object
the object is passed back down to the business logic tier
the application persists these modifications by calling update() in a second session

// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catID);
// in a higher tier of the application
Cat mate = new Cat();
cat.setMate(mate);
// later, in a new session
secondSession.saveOrUpdate(cat); // update existing state (cat has a non-null id)
secondSession.saveOrUpdate(mate); // save the new instance (mate has a null id)

Consider externalising query strings

This is a good practice if your queries call non-ANSI-standard SQL functions. Externalising the query strings to mapping files will make the application more portable. It also decreases the likelihood of injection because the Query objects let you set individual parameters but not the entire query string - you can only do that when you pass the string into the Query implementation constructor or call Session.createQuery.

Edge cases are still possibilities!

Basically the application would have to be messy enough to be binding parameters here, and concatenating variables there to the same query string.
You can extract the query string using Query.getQueryString(), and although you can't manipulate the string and 'add' it back to the Query object, you could theoretically taint it and use it in another instance of Query. Dumb? yes. Possible? Yes.

So how to externalize?

You may define named queries in the mapping document.

  <query name="ByNameAndMaximumWeight"><![CDATA[
     from eg.DomesticCat as cat
     where cat.name = ?
     and cat.weight > ?
     ] ]></query>

Parameter binding and executing is done programatically:

     Query q = sess.getNamedQuery("ByNameAndMaximumWeight");
     q.setString(0, name);
     q.setInt(1, minWeight);
     List cats = q.list();

Note that the actual program code is independent of the query language that is used, you may also define native

SQL queries in metadata, or migrate existing queries to Hibernate by placing them in mapping files.

Also note that a query declaration inside a <hibernate-mapping> element requires a global unique name for the

query, while a query declaration inside a <class> element is made unique automatically by prepending the fully qualified name of the class, for example eg.Cat.ByNameAndMaximumWeight.

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