/**
 * 共享bean實例的通用注冊表，實現了SingletonBeanRegistry. 允許注冊表中注冊的單例應該被所有調用者共享，通過bean名稱獲得。
 * 
 * 還支持登記的DisposableBean實例，（這可能會或不能正確的注冊單例），關閉注冊表時destroyed.
 * 可以注冊bean之間的依賴關系，執行適當的關閉順序。
 * 
 * 
 * 這個類主要用作基類的BeanFactory實現， 提供基本的管理
 * singleton bean 實例功能。
 * 
 */
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements
		SingletonBeanRegistry {

//內部標記為一個空的單例對象： 并發 Maps( 不支持空值 )作為標志值。
	protected static final Object NULL_OBJECT = new Object();

// 日記用來記錄子類
	protected final Log logger = LogFactory.getLog(getClass());

//是存放singleton對象的緩存
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>();

// 是存放制造singleton的工廠對象的緩存
	private final Map<String, ObjectFactory> singletonFactories = new HashMap<String, ObjectFactory>();

//是存放singletonFactory 制造出來的 singleton 的緩存
      
      
        
	private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>();
//以上三個緩存是這個類存放單例bean的主要Map
//就是單例注冊表
      
      
        
	private final Set<String> registeredSingletons = new LinkedHashSet<String>(
			16);

//目前正在創建中的單例bean的名稱的集合
      
      
        
	private final Set<String> singletonsCurrentlyInCreation = Collections
			.synchronizedSet(new HashSet<String>());

//存放異常出現的相關的原因的集合
      
      
        
	private Set<Exception> suppressedExceptions;

//標志，指示我們目前是否在銷毀單例中
      
      
        
	private boolean singletonsCurrentlyInDestruction = false;

//存放一次性bean的緩存
      
      
        
	private final Map<String, Object> disposableBeans = new LinkedHashMap<String, Object>();

//外部bean與被包含在外部bean的所有內部bean集合包含關系的緩存
	private final Map<String, Set<String>> containedBeanMap = new ConcurrentHashMap<String, Set<String>>();

//指定bean與依賴指定bean的所有bean的依賴關系的緩存
	private final Map<String, Set<String>> dependentBeanMap = new ConcurrentHashMap<String, Set<String>>();

//指定bean與創建這個bean所需要依賴的所有bean的依賴關系的緩存
      
      
        
	private final Map<String, Set<String>> dependenciesForBeanMap = new ConcurrentHashMap<String, Set<String>>();

	// SingletonBeanRegistry接口的registerSingleton方法的實現
	public void registerSingleton(String beanName, Object singletonObject)
			throws IllegalStateException {
		Assert.notNull(beanName, "'beanName' must not be null");
		synchronized (this.singletonObjects) {
			Object oldObject = this.singletonObjects.get(beanName);

      
      
        //如果singletonObjects緩存找到有指定名稱為beanName的對象，則表示該名稱已被占用
			if (oldObject != null) {
				throw new IllegalStateException("Could not register object ["
						+ singletonObject + "] under bean name '" + beanName
						+ "': there is already object [" + oldObject
						+ "] bound");
			}

      
      
        //若該名稱沒被占用，真正的注冊操作在這里實現
			addSingleton(beanName, singletonObject);
		}
	}

	
	protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
			// 因為singletonObjects類型是ConcurrentHashMap,并發Map不支持空值作為標志值，所以用NULL_OBJECT來代替
			this.singletonObjects.put(beanName,
					(singletonObject != null ? singletonObject : NULL_OBJECT));
			// beanName已被注冊存放在singletonObjects緩存，那么singletonFactories不應該再持有名稱為beanName的工廠
			this.singletonFactories.remove(beanName);
			// beanName已被注冊存放在singletonObjects緩存，那么earlySingletonObjects不應該再持有名稱為beanName的bean。
			this.earlySingletonObjects.remove(beanName);
			// beanName放進單例注冊表中
			this.registeredSingletons.add(beanName);
		}
	}

	/**
	 * 添加 名稱為beanName的singletonFactory對象
	 * 
	 */
	protected void addSingletonFactory(String beanName,
			ObjectFactory singletonFactory) {
		Assert.notNull(singletonFactory, "Singleton factory must not be null");
		synchronized (this.singletonObjects) {
			// 判斷singletonObjects內名字為beanName是否被占用，若沒有，進行注冊操作
			if (!this.singletonObjects.containsKey(beanName)) {
				this.singletonFactories.put(beanName, singletonFactory);
				this.earlySingletonObjects.remove(beanName);
				this.registeredSingletons.add(beanName);
			}
		}
	}

	// SingletonBeanRegistry接口的getSingleton方法的實現
	public Object getSingleton(String beanName) {
		return getSingleton(beanName, true);
	}


	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		// 如果singletonObjects指定beanName的對象是不存在的
		if (singletonObject == null) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				// 如果earlySingletonObjects指定的beanName的對象是不存在的且allowEarlyReference是允許的
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory singletonFactory = this.singletonFactories
							.get(beanName);
					// 如果存在指定beanName的singletonFactory對象
					if (singletonFactory != null) {
						// singletonFactory創建指定的單例對象
						singletonObject = singletonFactory.getObject();
						// 這里可以看出earlySingletonObjects緩存應該是存放singletonFactory產生的singleton
						this.earlySingletonObjects.put(beanName,
								singletonObject);
						// 這里表示指定的beanName已被占用，所以要在singletonFactories移除該名稱
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return (singletonObject != NULL_OBJECT ? singletonObject : null);
	}


	public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
		Assert.notNull(beanName, "'beanName' must not be null");
		synchronized (this.singletonObjects) {
			Object singletonObject = this.singletonObjects.get(beanName);
			// 如果singetonObjects緩存不存在名稱為beanName的對象
			if (singletonObject == null) {
				// 如果目前在銷毀singellton
				if (this.singletonsCurrentlyInDestruction) {
					throw new BeanCreationNotAllowedException(
							beanName,
							"Singleton bean creation not allowed while the singletons of this factory are in destruction "
									+ "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Creating shared instance of singleton bean '"
							+ beanName + "'");
				}
				// 單例對象創建前的回調,默認實現注冊正在創建的單例
				beforeSingletonCreation(beanName);
				// 判斷存儲異常相關原因的集合是否已存在
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				// 若沒有，剛創建異常集合的實例
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<Exception>();
				}
				try {
					// 由參數給定的singletonFactory創建singleton對象,getObject方法的具體實現由ObjectFactory的子類決定
					singletonObject = singletonFactory.getObject();
				} catch (BeanCreationException ex) {
					// 如果異常被抓取，在這里將出現異常的原因拋出
					if (recordSuppressedExceptions) {
						for (Exception suppressedException : this.suppressedExceptions) {
							ex.addRelatedCause(suppressedException);
						}
					}
					throw ex;
				} finally {
					// 結束前，將異常集合銷毀掉
					if (recordSuppressedExceptions) {
						this.suppressedExceptions = null;
					}
					// 單例創建之后的回調,默認的實現標志單例不要在創建了。
					afterSingletonCreation(beanName);
				}
				// 注冊創建后的單例
				addSingleton(beanName, singletonObject);
			}
			return (singletonObject != NULL_OBJECT ? singletonObject : null);
		}
	}

	/**
	 * 注冊 發生在singeton bean 實例創建之間發生的異常 
	 */
	protected void onSuppressedException(Exception ex) {
		synchronized (this.singletonObjects) {
			if (this.suppressedExceptions != null) {
				this.suppressedExceptions.add(ex);
			}
		}
	}

	/**
	 * 移除名稱為beanName的單例,主要在四個集合中移除，
	 * 如singletonObjects,singletonFactories,earlySingletonObjects
	 * ,registeredSingletons
	 * 
	 */
	protected void removeSingleton(String beanName) {
		synchronized (this.singletonObjects) {
			this.singletonObjects.remove(beanName);
			this.singletonFactories.remove(beanName);
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.remove(beanName);
		}
	}

	// singletonBeanRegistry接口的containsSingleton方法實現
	public boolean containsSingleton(String beanName) {
		return (this.singletonObjects.containsKey(beanName));
	}

	// singletonBeanRegistry接口的getSingletonNames方法實現
	public String[] getSingletonNames() {
		// 對singletonObjects加鎖，可能是為了防止registeredSingletons和singletonObjects出現不一致的問題
		synchronized (this.singletonObjects) {
			return StringUtils.toStringArray(this.registeredSingletons);
		}
	}

	// singletonBeanRegistry接口的getSingletonCount方法實現
	public int getSingletonCount() {
		synchronized (this.singletonObjects) {
			return this.registeredSingletons.size();
		}
	}

	/**
	 * 單例對象創建前的回調,默認實現singletonsCurrentlyInCreation集合注冊正在創建的單例.
	 * 
	 */
	protected void beforeSingletonCreation(String beanName) {
		if (!this.singletonsCurrentlyInCreation.add(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}
	}

	/**
	 * 單例創建之后的回調,默認實現singletonCurrentlyInCreation集合移除正在創建的單例
      
      
        
	 * 
	 */
	protected void afterSingletonCreation(String beanName) {
		if (!this.singletonsCurrentlyInCreation.remove(beanName)) {
			throw new IllegalStateException("Singleton '" + beanName
					+ "' isn't currently in creation");
		}
	}

	/**
	 * 返回 存放正在創建單例的集合是否包含指定名稱為beanName的單例存在
	 * 
	 */
	public final boolean isSingletonCurrentlyInCreation(String beanName) {
		return this.singletonsCurrentlyInCreation.contains(beanName);
	}

	/**
	 * 一次性bean注冊，存放在disponsableBeans集合中
	 * 
	 */
	public void registerDisposableBean(String beanName, DisposableBean bean) {
		synchronized (this.disposableBeans) {
			this.disposableBeans.put(beanName, bean);
		}
	}

	/**
	 * 注冊兩個bean之間的控制關系,例如內部bean和包含其的外部bean之間
	 * 
	 */
	public void registerContainedBean(String containedBeanName,
			String containingBeanName) {
		synchronized (this.containedBeanMap) {
			// 從containedBeanMap緩存中查找外部bean名為containingBeanName的內部bean集合
			Set<String> containedBeans = this.containedBeanMap
					.get(containingBeanName);
			// 如果沒有，剛新建一個存放內部bean的集合，并且存放在containedBeanMap緩存中
			if (containedBeans == null) {
				containedBeans = new LinkedHashSet<String>(8);
				this.containedBeanMap.put(containingBeanName, containedBeans);
			}
			// 將名為containedBeanName的內部bean存放到內部bean集合
			containedBeans.add(containedBeanName);
		}
		// 緊接著調用注冊內部bean和外部bean的依賴關系的方法
		registerDependentBean(containedBeanName, containingBeanName);
	}

	/**
	 * 注冊給定bean的一個依賴bean，給定的bean銷毀之前被銷毀。
	 * 
	 */
	public void registerDependentBean(String beanName, String dependentBeanName) {
		// 調用SimpleAliasRegistry的canonicalName方法，這方法是將參數beanName當做別名尋找到注冊名，并依此遞歸
		String canonicalName = canonicalName(beanName);
		synchronized (this.dependentBeanMap) {
			// 從dependentBeanMap緩存中找到依賴名為canonicalName這個bean的 依賴bean集合
			Set<String> dependentBeans = this.dependentBeanMap
					.get(canonicalName);
			// 如果為空，則新建一個依賴bean集合，并且存放到dependentBeanMap緩存中
			if (dependentBeans == null) {
				dependentBeans = new LinkedHashSet<String>(8);
				this.dependentBeanMap.put(canonicalName, dependentBeans);
			}
			// 依賴bean集合添加參數2指定的dependentBeanName
			dependentBeans.add(dependentBeanName);
		}
		synchronized (this.dependenciesForBeanMap) {
			// 從dependenciesForBeanMap緩存中找到dependentBeanName要依賴的所有bean集合
			Set<String> dependenciesForBean = this.dependenciesForBeanMap
					.get(dependentBeanName);
			if (dependenciesForBean == null) {
				dependenciesForBean = new LinkedHashSet<String>(8);
				this.dependenciesForBeanMap.put(dependentBeanName,
						dependenciesForBean);
			}
			dependenciesForBean.add(canonicalName);
		}
	}

	/**
	 *  確定是否還存在名為beanName的被依賴關系
	 */
	protected boolean hasDependentBean(String beanName) {
		return this.dependentBeanMap.containsKey(beanName);
	}

	/**
	 * 返回依賴于指定的bean的所有bean的名稱，如果有的話。
	 * 
	 */
	public String[] getDependentBeans(String beanName) {
		Set<String> dependentBeans = this.dependentBeanMap.get(beanName);
		if (dependentBeans == null) {
			return new String[0];
		}
		return StringUtils.toStringArray(dependentBeans);
	}

	/**
	 * 返回指定的bean依賴于所有的bean的名稱，如果有的話。
	 * 
	 */
	public String[] getDependenciesForBean(String beanName) {
		Set<String> dependenciesForBean = this.dependenciesForBeanMap
				.get(beanName);
		// 如果沒有的話返回new String[0]而不是null
		if (dependenciesForBean == null) {
			return new String[0];
		}
		return dependenciesForBean.toArray(new String[dependenciesForBean
				.size()]);
	}

	// 銷毀單例
	public void destroySingletons() {
		if (logger.isInfoEnabled()) {
			logger.info("Destroying singletons in " + this);
		}
		// 單例目前銷毀標志開始
		synchronized (this.singletonObjects) {
			this.singletonsCurrentlyInDestruction = true;
		}

		// 銷毀disponsableBeans緩存中所有單例bean
		synchronized (this.disposableBeans) {
			String[] disposableBeanNames = StringUtils
					.toStringArray(this.disposableBeans.keySet());
			for (int i = disposableBeanNames.length - 1; i >= 0; i--) {
				destroySingleton(disposableBeanNames[i]);
			}
		}

		// containedBeanMap緩存清空,dependentBeanMap緩存清空，dependenciesForBeanMap緩存清空
		this.containedBeanMap.clear();
		this.dependentBeanMap.clear();
		this.dependenciesForBeanMap.clear();

		// singeltonObjects緩存清空，singletonFactories緩存清空，earlySingletonObjects緩存清空，registeredSingletons緩存清空
		synchronized (this.singletonObjects) {
			this.singletonObjects.clear();
			this.singletonFactories.clear();
			this.earlySingletonObjects.clear();
			this.registeredSingletons.clear();
			// 單例目前正在銷毀標志為結束
			this.singletonsCurrentlyInDestruction = false;
		}
	}


	public void destroySingleton(String beanName) {
		// Remove a registered singleton of the given name, if any.
		removeSingleton(beanName);

		// Destroy the corresponding DisposableBean instance.
		DisposableBean disposableBean;
		synchronized (this.disposableBeans) {
			disposableBean = (DisposableBean) this.disposableBeans
					.remove(beanName);
		}
		destroyBean(beanName, disposableBean);
	}


	protected void destroyBean(String beanName, DisposableBean bean) {
		// Trigger destruction of dependent beans first...
		// 這段代碼告訴我們先移除要銷毀依賴bean
		Set<String> dependencies = this.dependentBeanMap.remove(beanName);
		if (dependencies != null) {
			if (logger.isDebugEnabled()) {
				logger.debug("Retrieved dependent beans for bean '" + beanName
						+ "': " + dependencies);
			}
			for (String dependentBeanName : dependencies) {
				destroySingleton(dependentBeanName);
			}
		}

		// Actually destroy the bean now...
		// 銷毀bean實例
		if (bean != null) {
			try {
				bean.destroy();
			} catch (Throwable ex) {
				logger.error("Destroy method on bean with name '" + beanName
						+ "' threw an exception", ex);
			}
		}

		// Trigger destruction of contained beans...
		// 從containedBeanMap緩存中移除要銷毀的bean，遞歸移除它的包含內部bean集合
		Set<String> containedBeans = this.containedBeanMap.remove(beanName);
		if (containedBeans != null) {
			for (String containedBeanName : containedBeans) {
				destroySingleton(containedBeanName);
			}
		}

		// Remove destroyed bean from other beans' dependencies.
		// 從其它bean的依賴bean集合中移除要銷毀的bean
		synchronized (this.dependentBeanMap) {
			for (Iterator<Map.Entry<String, Set<String>>> it = this.dependentBeanMap
					.entrySet().iterator(); it.hasNext();) {
				Map.Entry<String, Set<String>> entry = it.next();
				Set<String> dependenciesToClean = entry.getValue();
				dependenciesToClean.remove(beanName);
				if (dependenciesToClean.isEmpty()) {
					it.remove();
				}
			}
		}

		// Remove destroyed bean's prepared dependency information.
		// 最后 從dependenciesForBeanMap緩存中移除要銷毀的bean
		this.dependenciesForBeanMap.remove(beanName);
	}

	/**
	 * Expose the singleton mutex to subclasses.
	 * <p>
	 * Subclasses should synchronize on the given Object if they perform any
	 * sort of extended singleton creation phase. In particular, subclasses
	 * should <i>not</i> have their own mutexes involved in singleton creation,
	 * to avoid the potential for deadlocks in lazy-init situations.
	 */
	protected final Object getSingletonMutex() {
		return this.singletonObjects;
	}

}