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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 ? e==null : 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 ? e2==null : 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 ? e==null : 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