Like we have mentioned, traditional cache system does not support object relationship management during serialization (be it to the persistent data store or replicated to the other in-memory nodes.) Examples of object relationship are like an address object is shared by members of the household, and a child-parent relationship. All these relationship will be lost once the objects are replicated or persisted. As a result, explicit mapping will be needed outside of the cache system to express the object relationship. PojoCache, in contrast, can manage object relationship transparently for users.

During the mapping process, we will check whether any of its associated object is multiple or circular referenced. A reference counting mechanism has been implemented associating with the CacheInterceptor. If a new object created in the cache referenced to another POJO, a referenced fqn will be stored there to redirect any query and update to the original node.

To look at one example, let's say that multiple Persons ("joe" and "mary") objects can own the same Address (e.g., a household). Graphically, here is what it will look like in the tree nodes. Like we have covered in the previous section on the mapping by reachability, the POJO will map recursively into the cache. However, when we detect a multiple reference (in this case, the Address), we will keep track of the reference counting for the sub-object addr.

In the following code snippet, we show programmatically the object sharing example.

import org.jboss.cache.PropertyConfigurator;
import org.jboss.cache.aop.PojoCache;
import org.jboss.test.cache.test.standAloneAop.Person;
import org.jboss.test.cache.test.standAloneAop.Address;

PojoCache tree = new PojoCache();
PropertyConfigurator config = new PropertyConfigurator(); // configure tree cache.
config.configure(tree, "META-INF/replSync-service.xml");

Person joe = new Person(); // instantiate a Person object named joe
joe.setName("Joe Black");
joe.setAge(31);

Person mary = new Person(); // instantiate a Person object named mary
mary.setName("Mary White");
mary.setAge(30);

Address addr = new Address(); // instantiate a Address object named addr
addr.setCity("Sunnyvale");
addr.setStreet("123 Albert Ave");
addr.setZip(94086);

joe.setAddress(addr); // set the address reference
mary.setAddress(addr); // set the address reference

tree.startService(); // kick start tree
tree.putObject("/aop/joe", joe); // add aop sanctioned object (and sub-objects) into cache.
tree.putObject("/aop/mary", mary); // add aop sanctioned object (and sub-objects) into cache.

Address joeAddr = joe.getAddress();
Address maryAddr = mary.getAddress(); // joeAddr and maryAddr should be the same instance

tree.removeObject("/aop/joe");
maryAddr = mary.getAddress(); // Should still have the address.

Notice that after we remove joe instance from the cache, mary should still have reference the same Address object in the cache store.

To further illustrate this relationship management, let's examine the Java code under a replicated environment. Imagine that we have two separate cache instances in the cluster now (cache1 and cache2). Let's say, on the first cache instance, we put both joe and mary under cache management as above. Then, we failover to cache2. Here is the code snippet:

/**
 * Code snippet on cache2 during fail-over
 */
import org.jboss.cache.PropertyConfigurator;
import org.jboss.cache.aop.PojoCache;
import org.jboss.test.cache.test.standAloneAop.Person;
import org.jboss.test.cache.test.standAloneAop.Address;

PojoCache tree = new PojoCache();
PropertyConfigurator config = new PropertyConfigurator(); // configure tree cache.
config.configure(tree, "META-INF/replSync-service.xml");

tree.startService(); // kick start tree
Person joe = tree.getObject("/aop/joe"); // retrieve the POJO reference.
Person mary = tree.getObject("/aop/mary"); // retrieve the POJO reference.

Address joeAddr = joe.getAddress();
Address maryAddr = mary.getAddress(); // joeAddr and maryAddr should be the same instance!!!

maryAddr = mary.getAddress().setZip(95123);
int zip = joeAddr.getAddress().getZip(); // Should be 95123 as well instead of 94086!