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Don't call us, Hazelcast will call you!

29 Oct 2014 java dependency injection hazelcast cdi jsr330 spring guice

Sending tasks or application logic to the data is a good step to achieve greater scalability, generally you need to inject some local dependencies into remote tasks before they are executed. In Hazelcast, there are a few ways of injecting dependencies into user objects.

Spring

Who don’t use Spring nowadays… If you are a Spring user, you can also configure Hazelcast with Spring. By doing this you also get ability of injecting Spring dependencies into your remote tasks (Callables, Runnables, EntryProcessors etc). Only thing you need to do is mark those classes with @SpringAware annotation and Hazelcast will let Spring to inject dependencies just after a task arrives to the remote node.

<beans>
    <context:annotation-config />
    <hz:hazelcast id="hazelcast">
        <hz:config>
            ...
        </hz:config>
    </hz:hazelcast>

    <bean id="someBean" class="com.hazelcast.examples.spring.SomeBean" scope="singleton" />
    ...
</beans>

@SpringAware
public class SomeTask implements Callable, ApplicationContextAware,
    java.io.Serializable {

    private transient ApplicationContext context;

    private transient SomeBean someBean;

    public Object call() throws Exception {
        assert context != null;
        assert someBean != null;

        // do something meaningful with someBean
        // ...
        return result;
    }

    public void setApplicationContext(ApplicationContext applicationContext)
        throws BeansException {
        context = applicationContext;
    }

    @Autowired
    public void setSomeBean(final SomeBean someBean) {
        this.someBean = someBean;
    }
}

ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
HazelcastInstance hazelcast = (HazelcastInstance) context.getBean("hazelcast");
hazelcast.getExecutorService().submit(new SomeTask());

ManagedContext

ManagedContext is used to initialize an object upon it’s created. By default Hazelcast has two out-of-the-box implementation of it, one is used to inject HazelcastInstance into HazelcastInstanceAware objects and the other one is to inject Spring dependencies. Apart from these one can implement its own ManagedContext to inject its dependencies using its own way or using any other DI framework, guice etc.

ManagedContext is a single-abstract-method interface, it’s very trivial to implement.

/**
 * Container managed context, such as Spring, Guice and etc.
 */
public interface ManagedContext {
    /**
     * Initialize the given object instance.
     * This is intended for repopulating select fields and methods for deserialized instances.
     * It is also possible to proxy the object, e.g. with AOP proxies.
     *
     * @param obj Object to initialize
     * @return the initialized object to use
     */
    Object initialize(Object obj);
}

A sample implementation that injects JPA EntityManager into data access objects:

public interface DAO {
    void setEntityManager(EntityManager entityManager);
    ...
}

public class JPAManagedContext implements ManagedContext {
    final EntityManagerFactory factory;

    JPAManagedContext(EntityManagerFactory factory) {
        this.factory = factory;
    }

    public Object initialize(Object obj) {
        if (obj instanceof DAO) {
            EntityManager entityManager factory.createEntityManager();
            ((DAO) obj).setEntityManager(entityManager);
        }
        return obj;
    }
}

You should enable ManagedContext implementation in Hazelcast configuration:

Config config = new Config();
config.setManagedContext(new JPAManagedContext());

Manual way: User context map

Each Hazelcast instance has its own user context map, which can be initialized in configuration and can be accessed directly from HazelcastInstance interface. It’s just a plain ConcurrentMap<String, Object>.

ConcurrentMap<String, Object> context = new ConcurrentHashMap<String, Object>();
context.put("key1", value1);
context.put("key2", value2);

Config config = new Config();
config.setUserContext(context);

HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance(config);

You can access that context map inside a distributed task by implementing HazelcastInstanceAware interface.

public class SomeTask implements Callable, HazelcastInstanceAware,
    java.io.Serializable {

    private transient HazelcastInstance hazelcastInstance;

    public Object call() throws Exception {
        assert hazelcastInstance != null;

        ConcurrentMap<String, Object> context = hazelcastInstance.getUserContext();
        Object value1 = context.get("key1");
        Object value2 = context.get("key2");
        // do something meaningful
        // ...
        return result;
    }

    public void void setHazelcastInstance(HazelcastInstance hazelcastInstance) {
        this.hazelcastInstance = hazelcastInstance;
    }
}

Now submit all your remote tasks and let Hazelcast execute them in place…

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