聊聊dubbo的ConcurrentHashSet

序

本文主要研究一下dubbo的ConcurrentHashSet

ConcurrentHashSet

dubbo-2.7.2/dubbo-common/src/main/java/org/apache/dubbo/common/utils/ConcurrentHashSet.java

public class ConcurrentHashSet<E> extends AbstractSet<E> implements Set<E>, java.io.Serializable {

    private static final long serialVersionUID = -8672117787651310382L;

    private static final Object PRESENT = new Object();

    private final ConcurrentMap<E, Object> map;

    public ConcurrentHashSet() {
        map = new ConcurrentHashMap<E, Object>();
    }

    public ConcurrentHashSet(int initialCapacity) {
        map = new ConcurrentHashMap<E, Object>(initialCapacity);
    }

    /**
     * Returns an iterator over the elements in this set. The elements are
     * returned in no particular order.
     *
     * @return an Iterator over the elements in this set
     * @see ConcurrentModificationException
     */
    @Override
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    @Override
    public int size() {
        return map.size();
    }

    /**
     * Returns <tt>true</tt> if this set contains no elements.
     *
     * @return <tt>true</tt> if this set contains no elements
     */
    @Override
    public boolean isEmpty() {
        return map.isEmpty();
    }

    /**
     * Returns <tt>true</tt> if this set contains the specified element. More
     * formally, returns <tt>true</tt> if and only if this set contains an
     * element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this set is to be tested
     * @return <tt>true</tt> if this set contains the specified element
     */
    @Override
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    /**
     * Adds the specified element to this set if it is not already present. More
     * formally, adds the specified element <tt>e</tt> to this set if this set
     * contains no element <tt>e2</tt> such that
     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>. If this
     * set already contains the element, the call leaves the set unchanged and
     * returns <tt>false</tt>.
     *
     * @param e element to be added to this set
     * @return <tt>true</tt> if this set did not already contain the specified
     * element
     */
    @Override
    public boolean add(E e) {
        return map.put(e, PRESENT) == null;
    }

    /**
     * Removes the specified element from this set if it is present. More
     * formally, removes an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if this
     * set contains such an element. Returns <tt>true</tt> if this set contained
     * the element (or equivalently, if this set changed as a result of the
     * call). (This set will not contain the element once the call returns.)
     *
     * @param o object to be removed from this set, if present
     * @return <tt>true</tt> if the set contained the specified element
     */
    @Override
    public boolean remove(Object o) {
        return map.remove(o) == PRESENT;
    }

    /**
     * Removes all of the elements from this set. The set will be empty after
     * this call returns.
     */
    @Override
    public void clear() {
        map.clear();
    }

}

• ConcurrentHashSet继承了AbstractSet，实现了Set、Serializable接口；它底层使用ConcurrentHashMap来实现，其value固定为PRESENT

实例

dubbo-2.7.2/dubbo-config/dubbo-config-spring/src/main/java/org/apache/dubbo/config/spring/extension/SpringExtensionFactory.java

public class SpringExtensionFactory implements ExtensionFactory {
    private static final Logger logger = LoggerFactory.getLogger(SpringExtensionFactory.class);

    private static final Set<ApplicationContext> CONTEXTS = new ConcurrentHashSet<ApplicationContext>();
    private static final ApplicationListener SHUTDOWN_HOOK_LISTENER = new ShutdownHookListener();

    public static void addApplicationContext(ApplicationContext context) {
        CONTEXTS.add(context);
        if (context instanceof ConfigurableApplicationContext) {
            ((ConfigurableApplicationContext) context).registerShutdownHook();
            DubboShutdownHook.getDubboShutdownHook().unregister();
        }
        BeanFactoryUtils.addApplicationListener(context, SHUTDOWN_HOOK_LISTENER);
    }

    public static void removeApplicationContext(ApplicationContext context) {
        CONTEXTS.remove(context);
    }

    public static Set<ApplicationContext> getContexts() {
        return CONTEXTS;
    }

    // currently for test purpose
    public static void clearContexts() {
        CONTEXTS.clear();
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T> T getExtension(Class<T> type, String name) {

        //SPI should be get from SpiExtensionFactory
        if (type.isInterface() && type.isAnnotationPresent(SPI.class)) {
            return null;
        }

        for (ApplicationContext context : CONTEXTS) {
            if (context.containsBean(name)) {
                Object bean = context.getBean(name);
                if (type.isInstance(bean)) {
                    return (T) bean;
                }
            }
        }

        logger.warn("No spring extension (bean) named:" + name + ", try to find an extension (bean) of type " + type.getName());

        if (Object.class == type) {
            return null;
        }

        for (ApplicationContext context : CONTEXTS) {
            try {
                return context.getBean(type);
            } catch (NoUniqueBeanDefinitionException multiBeanExe) {
                logger.warn("Find more than 1 spring extensions (beans) of type " + type.getName() + ", will stop auto injection. Please make sure you have specified the concrete parameter type and there's only one extension of that type.");
            } catch (NoSuchBeanDefinitionException noBeanExe) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Error when get spring extension(bean) for type:" + type.getName(), noBeanExe);
                }
            }
        }

        logger.warn("No spring extension (bean) named:" + name + ", type:" + type.getName() + " found, stop get bean.");

        return null;
    }

    private static class ShutdownHookListener implements ApplicationListener {
        @Override
        public void onApplicationEvent(ApplicationEvent event) {
            if (event instanceof ContextClosedEvent) {
                DubboShutdownHook shutdownHook = DubboShutdownHook.getDubboShutdownHook();
                shutdownHook.doDestroy();
            }
        }
    }
}

• SpringExtensionFactory实现了ExtensionFactory接口，它使用ConcurrentHashSet来注册spring的ApplicationContext

小结

ConcurrentHashSet继承了AbstractSet，实现了Set、Serializable接口；它底层使用ConcurrentHashMap来实现，其value固定为PRESENT

doc

• ConcurrentHashSet