spring-ioc

Spring IOC

在Spring Framework简介中介绍了spring的整体框架，这篇文章主要介绍Spring IOC。那么，什么是IOC呢？IOC是Inversion of Control的缩写，是一个非常重要的面向对象法则，主要目的是用来消减计算机程序的耦合问题，同样也是Spring框架的核心。IOC一般分为两种类型，依赖注入（Dependency
Injection）和依赖查找（Dependency Lookup），在Spring Framework框架中主要是依赖注入。作为spring的核心，那IOC到底是什么呢？
在揭开谜底之前，下面先看看传统模式下的编程风格，如下代码所示

public class User{
    private String id;
    private String name;
    private String password;
    private Date birthday;
    //ignore other properties...
    //ignore setter getter method	
}

public interface IUserDao{
    public void add(User user);
    public void delete(User user);
    public List<User> findAll();
}

public class UserDaoImpl{
    public void add(User user){
        //ignore the implementation
    }
        
    public void delete(User user){
        //ignore the implementation
    }

    public List<User> findAll(){
        //ignore the implementation
    } 
}

public class UserController{
    private IUserDao userDao = new UserDaoImpl();

    @RequestMapping("/user/add")
    public void add(User user){
        userDao.add(user);
    }

    @RequestMapping("/user/delete")
    public void delete(User user){
        userDao.add(user);
    }
    
    @RequestMapping("/user/findAll")
    public void findAll(User user){
        List<User> allUser = userDao.findAll();
    }
}

在这里面，UserController调用private IUserDao userDao = new UserDaoImpl()来初始化userDao。由于java是一门编译语言，也就是将.java变成.class的过程。在这个过程中捆绑了IUserDao的实现，也就是UserDaoImpl。虽然开发者使用了面向接口编程的风格来书写代码，但是还是和UserDaoImpl捆绑在了一起。如果后期开发中，为IUserDao提供另外一种实现，比如：将orm框架由mybatis换成了hibernate，那么就需要修改源码UserController的实现。传统模式中，一个类依赖的其他类都由自己维护，需要什么类，就new一个。这种开发方式的耦合性比较高，不利于后期的维护。那么既然这种方式不好，那么有没有一种其他办法来解决这个问题呢？没错，IOC可以非常好的解决这个问题。

IOC组成

IOC是一个容器，将所有的java bean对象交给IOC来管理，其他类在初始化时，依赖的类不是自己主动new一个，而是由IOC主动注入，这个过程恰好与传统模式相反，所以我们称之为控制反转。由于控制反转这个词语非常的抽象，所以IOC也叫做依赖注入，按字面意思理解就是：这个类依赖其他什么类，那么IOC就帮你注入这个类。下面看如下代码

public class User{
    private String id;
    private String name;
    private String password;
    private Date birthday;
    //ignore other properties...
    //ignore setter getter method<span style="white-space:pre">	</span>
}

public interface IUserDao{
    public void add(User user);
    public void delete(User user);
    public List<User> findAll();
}

@Repository
public class UserDaoImpl{
    public void add(User user){
        //ignore the implementation
    }
        
    public void delete(User user){
        //ignore the implementation
    }

    public List<User> findAll(){
        //ignore the implementation
    } 
}

@Controller
public class UserController{
    //主动注入这个类
      @Autowired
    private IUserDao userDao userDao;

    @RequestMapping("/user/add")
    public void add(User user){
        userDao.add(user);
    }

    @RequestMapping("/user/delete")
    public void delete(User user){
        userDao.add(user);
    }
    
    @RequestMapping("/user/findAll")
    public void findAll(User user){
        List<User> allUser = userDao.findAll();
    }
}

从代码中我们可以看到，这两份代码几乎是一样的，就只是去掉了userDao的实现类，在userDao变量中加入了@Autowired注解而已。没错，这就是spring的厉害之处，因为spring是一种无侵入式的轻量级框架。那么spring是如何实现控制反转的呢？下面先给出一个spring
ioc图




从图中可以看到，图中一种由三个元素构成：configuration metadata（源信息），business object，spring container。由三者可以产生一个fully configured system（可运行的系统）。其中business
object为业务逻辑代码，也就是前面例子中的UserDaoImpl，UserController。configuration metadata为源信息，也就是告诉spring container如何去实例化bean，注入bean依赖的类，在前面的例子中，@Controller，@Repository，@Autowired就是告诉spring container UserController与UserDaoImpl为java bean对象，然后UserController依赖于IUserDao。spring
container拿到这些源信息后，就会尝试去完成任务，如果成功了，那么系统就准备就绪了。

IOC的实现

有了前面的内容，想必读者对IOC有了一定的了解。IOC主要就是做如下的几件事：
1.拿到源信息
2.初始化容器，初始化时，自动注入java bean之间的依赖对象

那么spring是如何初始化容器，并且自动注入java bean之间的依赖对象呢？作为一个开发者，相比读者也可以猜测到，spring做的事情应该是先实例化一个bean，然后查看这个bean的依赖关系，如果这个bean没有依赖，那么实例化成功，加入一个集合中（比如使用Map<String,Object>）；如果这个bean有依赖，那么查看容器中是否已经有相应的bean，如果有，直接注入，bean实例化成功；没有就先将bean缓存起来，继续实例化其他bean；当所有bean都初始化完毕后，在解决先前缓存起来bean之间的依赖关系。下面是根据这个思路画的一个简单流程图，图有点丑，大家莫见怪



有了该图后，相比你对IOC的神秘之处，有了些许的了解。当然，这个图仅仅是一个思维导图，具体代码的细节，肯定得复杂的多。下面给出一个spring默认的实现类，不必要的信息做了删除

/**
 * 
 * 一个BeanFactory的实现类，不必要的信息作了删除，有兴趣的读者可以查看源码
 * */
@SuppressWarnings("serial")
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
		implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {

	private static Class<?> javaxInjectProviderClass = null;

	static {
		ClassLoader cl = DefaultListableBeanFactory.class.getClassLoader();
		try {
			javaxInjectProviderClass = cl.loadClass("javax.inject.Provider");
		}
		catch (ClassNotFoundException ex) {
			// JSR-330 API not available - Provider interface simply not supported then.
		}
	}

	/** Map from serialized id to factory instance */
	private static final Map<String, Reference<DefaultListableBeanFactory>> serializableFactories =
			new ConcurrentHashMap<String, Reference<DefaultListableBeanFactory>>(8);

	/** Optional id for this factory, for serialization purposes */
	private String serializationId;

	/** Whether to allow re-registration of a different definition with the same name */
	private boolean allowBeanDefinitionOverriding = true;

	/** Whether to allow eager class loading even for lazy-init beans */
	private boolean allowEagerClassLoading = true;

	/** Resolver to use for checking if a bean definition is an autowire candidate */
	private AutowireCandidateResolver autowireCandidateResolver = new SimpleAutowireCandidateResolver();

	/** 用于缓存解决好依赖关系的bean */
	private final Map<Class<?>, Object> resolvableDependencies = new HashMap<Class<?>, Object>(16);

	/** 保存源信息 */
	private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(64);

	/** Map of singleton and non-singleton bean names keyed by dependency type */
	private final Map<Class<?>, String[]> allBeanNamesByType = new ConcurrentHashMap<Class<?>, String[]>(64);

	/** Map of singleton-only bean names keyed by dependency type */
	private final Map<Class<?>, String[]> singletonBeanNamesByType = new ConcurrentHashMap<Class<?>, String[]>(64);

	/***/
	private final List<String> beanDefinitionNames = new ArrayList<String>();

	/** Whether bean definition metadata may be cached for all beans */
	private boolean configurationFrozen = false;

	/** Cached array of bean definition names in case of frozen configuration */
	private String[] frozenBeanDefinitionNames;

	/**
	 * Create a new DefaultListableBeanFactory.
	 */
	public DefaultListableBeanFactory() {
		super();
	}

	/**
	 * Create a new DefaultListableBeanFactory with the given parent.
	 * @param parentBeanFactory the parent BeanFactory
	 */
	public DefaultListableBeanFactory(BeanFactory parentBeanFactory) {
		super(parentBeanFactory);
	}

	/**
	 * Specify an id for serialization purposes, allowing this BeanFactory to be
	 * deserialized from this id back into the BeanFactory object, if needed.
	 */
	public void setSerializationId(String serializationId) {
		if (serializationId != null) {
			serializableFactories.put(serializationId, new WeakReference<DefaultListableBeanFactory>(this));
		}
		else if (this.serializationId != null) {
			serializableFactories.remove(this.serializationId);
		}
		this.serializationId = serializationId;
	}

	/**
	 * Set whether it should be allowed to override bean definitions by registering
	 * a different definition with the same name, automatically replacing the former.
	 * If not, an exception will be thrown. This also applies to overriding aliases.
	 * <p>Default is "true".
	 * @see #registerBeanDefinition
	 */
	public void setAllowBeanDefinitionOverriding(boolean allowBeanDefinitionOverriding) {
		this.allowBeanDefinitionOverriding = allowBeanDefinitionOverriding;
	}

	/**
	 * Set whether the factory is allowed to eagerly load bean classes
	 * even for bean definitions that are marked as "lazy-init".
	 * <p>Default is "true". Turn this flag off to suppress class loading
	 * for lazy-init beans unless such a bean is explicitly requested.
	 * In particular, by-type lookups will then simply ignore bean definitions
	 * without resolved class name, instead of loading the bean classes on
	 * demand just to perform a type check.
	 * @see AbstractBeanDefinition#setLazyInit
	 */
	public void setAllowEagerClassLoading(boolean allowEagerClassLoading) {
		this.allowEagerClassLoading = allowEagerClassLoading;
	}

	/**
	 * Set a custom autowire candidate resolver for this BeanFactory to use
	 * when deciding whether a bean definition should be considered as a
	 * candidate for autowiring.
	 */
	public void setAutowireCandidateResolver(final AutowireCandidateResolver autowireCandidateResolver) {
		Assert.notNull(autowireCandidateResolver, "AutowireCandidateResolver must not be null");
		if (autowireCandidateResolver instanceof BeanFactoryAware) {
			if (System.getSecurityManager() != null) {
				final BeanFactory target = this;
				AccessController.doPrivileged(new PrivilegedAction<Object>() {
					public Object run() {
						((BeanFactoryAware) autowireCandidateResolver).setBeanFactory(target);
						return null;
					}
				}, getAccessControlContext());
			}
			else {
				((BeanFactoryAware) autowireCandidateResolver).setBeanFactory(this);
			}
		}
		this.autowireCandidateResolver = autowireCandidateResolver;
	}

	/**
	 * Return the autowire candidate resolver for this BeanFactory (never {@code null}).
	 */
	public AutowireCandidateResolver getAutowireCandidateResolver() {
		return this.autowireCandidateResolver;
	}

	@Override
	public void copyConfigurationFrom(ConfigurableBeanFactory otherFactory) {
		super.copyConfigurationFrom(otherFactory);
		if (otherFactory instanceof DefaultListableBeanFactory) {
			DefaultListableBeanFactory otherListableFactory = (DefaultListableBeanFactory) otherFactory;
			this.allowBeanDefinitionOverriding = otherListableFactory.allowBeanDefinitionOverriding;
			this.allowEagerClassLoading = otherListableFactory.allowEagerClassLoading;
			this.autowireCandidateResolver = otherListableFactory.autowireCandidateResolver;
			this.resolvableDependencies.putAll(otherListableFactory.resolvableDependencies);
		}
	}

	//---------------------------------------------------------------------
	// Implementation of ListableBeanFactory interface
	//---------------------------------------------------------------------
    
	public <T> T getBean(Class<T> requiredType) throws BeansException {
		Assert.notNull(requiredType, "Required type must not be null");
		String[] beanNames = getBeanNamesForType(requiredType);  
		if (beanNames.length > 1) {  
			ArrayList<String> autowireCandidates = new ArrayList<String>();
			for (String beanName : beanNames) {
				if (getBeanDefinition(beanName).isAutowireCandidate()) {
					autowireCandidates.add(beanName);
				}
			}
			if (autowireCandidates.size() > 0) {
				beanNames = autowireCandidates.toArray(new String[autowireCandidates.size()]);
			}
		}
		if (beanNames.length == 1) {
			return getBean(beanNames[0], requiredType);
		}
		else if (beanNames.length > 1) {
			T primaryBean = null;
			for (String beanName : beanNames) {
				T beanInstance = getBean(beanName, requiredType);
				if (isPrimary(beanName, beanInstance)) {
					if (primaryBean != null) {
						throw new NoUniqueBeanDefinitionException(requiredType, beanNames.length,
								"more than one 'primary' bean found of required type: " + Arrays.asList(beanNames));
					}
					primaryBean = beanInstance;
				}
			}
			if (primaryBean != null) {
				return primaryBean;
			}
			throw new NoUniqueBeanDefinitionException(requiredType, beanNames);
		}
		else if (getParentBeanFactory() != null) {
			return getParentBeanFactory().getBean(requiredType);
		}
		else {
			throw new NoSuchBeanDefinitionException(requiredType);
		}
	}

	@Override
	public boolean containsBeanDefinition(String beanName) {
		Assert.notNull(beanName, "Bean name must not be null");
		return this.beanDefinitionMap.containsKey(beanName);
	}

	public int getBeanDefinitionCount() {
		return this.beanDefinitionMap.size();
	}

	public String[] getBeanDefinitionNames() {
		synchronized (this.beanDefinitionMap) {
			if (this.frozenBeanDefinitionNames != null) {
				return this.frozenBeanDefinitionNames;
			}
			else {
				return StringUtils.toStringArray(this.beanDefinitionNames);
			}
		}
	}

	public String[] getBeanNamesForType(Class<?> type) {
		return getBeanNamesForType(type, true, true);
	}

	public String[] getBeanNamesForType(Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
		if (!isConfigurationFrozen()  || type == null || !allowEagerInit) {
			return doGetBeanNamesForType(type, includeNonSingletons, allowEagerInit);
		}
		Map<Class<?>, String[]> cache =
				(includeNonSingletons ? this.allBeanNamesByType : this.singletonBeanNamesByType);
		String[] resolvedBeanNames = cache.get(type);
		if (resolvedBeanNames != null) {
			return resolvedBeanNames;
		}
		resolvedBeanNames = doGetBeanNamesForType(type, includeNonSingletons, allowEagerInit);
		cache.put(type, resolvedBeanNames);
		return resolvedBeanNames;
	}

	private String[] doGetBeanNamesForType(Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
		List<String> result = new ArrayList<String>();

		// Check all bean definitions.
		String[] beanDefinitionNames = getBeanDefinitionNames();
		for (String beanName : beanDefinitionNames) {
			// Only consider bean as eligible if the bean name
			// is not defined as alias for some other bean.
			if (!isAlias(beanName)) {
				try {
					RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
					// Only check bean definition if it is complete.
					if (!mbd.isAbstract() && (allowEagerInit ||
							((mbd.hasBeanClass() || !mbd.isLazyInit() || this.allowEagerClassLoading)) &&
									!requiresEagerInitForType(mbd.getFactoryBeanName()))) {
						// In case of FactoryBean, match object created by FactoryBean.
						boolean isFactoryBean = isFactoryBean(beanName, mbd);
						boolean matchFound = (allowEagerInit || !isFactoryBean || containsSingleton(beanName)) &&
								(includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type);
						if (!matchFound && isFactoryBean) {
							// In case of FactoryBean, try to match FactoryBean instance itself next.
							beanName = FACTORY_BEAN_PREFIX + beanName;
							matchFound = (includeNonSingletons || mbd.isSingleton()) && isTypeMatch(beanName, type);
						}
						if (matchFound) {
							result.add(beanName);
						}
					}
				}
				catch (CannotLoadBeanClassException ex) {
					if (allowEagerInit) {
						throw ex;
					}
					// Probably contains a placeholder: let's ignore it for type matching purposes.
					if (this.logger.isDebugEnabled()) {
						this.logger.debug("Ignoring bean class loading failure for bean '" + beanName + "'", ex);
					}
					onSuppressedException(ex);
				}
				catch (BeanDefinitionStoreException ex) {
					if (allowEagerInit) {
						throw ex;
					}
					// Probably contains a placeholder: let's ignore it for type matching purposes.
					if (this.logger.isDebugEnabled()) {
						this.logger.debug("Ignoring unresolvable metadata in bean definition '" + beanName + "'", ex);
					}
					onSuppressedException(ex);
				}
			}
		}

		// Check singletons too, to catch manually registered singletons.
		String[] singletonNames = getSingletonNames();
		for (String beanName : singletonNames) {
			// Only check if manually registered.
			if (!containsBeanDefinition(beanName)) {
				// In case of FactoryBean, match object created by FactoryBean.
				if (isFactoryBean(beanName)) {
					if ((includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type)) {
						result.add(beanName);
						// Match found for this bean: do not match FactoryBean itself anymore.
						continue;
					}
					// In case of FactoryBean, try to match FactoryBean itself next.
					beanName = FACTORY_BEAN_PREFIX + beanName;
				}
				// Match raw bean instance (might be raw FactoryBean).
				if (isTypeMatch(beanName, type)) {
					result.add(beanName);
				}
			}
		}

		return StringUtils.toStringArray(result);
	}

	/**
	 * Check whether the specified bean would need to be eagerly initialized
	 * in order to determine its type.
	 * @param factoryBeanName a factory-bean reference that the bean definition
	 * defines a factory method for
	 * @return whether eager initialization is necessary
	 */
	private boolean requiresEagerInitForType(String factoryBeanName) {
		return (factoryBeanName != null && isFactoryBean(factoryBeanName) && !containsSingleton(factoryBeanName));
	}

	public <T> Map<String, T> getBeansOfType(Class<T> type) throws BeansException {
		return getBeansOfType(type, true, true);
	}

	public <T> Map<String, T> getBeansOfType(Class<T> type, boolean includeNonSingletons, boolean allowEagerInit)
			throws BeansException {

		String[] beanNames = getBeanNamesForType(type, includeNonSingletons, allowEagerInit);
		Map<String, T> result = new LinkedHashMap<String, T>(beanNames.length);
		for (String beanName : beanNames) {
			try {
				result.put(beanName, getBean(beanName, type));
			}
			catch (BeanCreationException ex) {
				Throwable rootCause = ex.getMostSpecificCause();
				if (rootCause instanceof BeanCurrentlyInCreationException) {
					BeanCreationException bce = (BeanCreationException) rootCause;
					if (isCurrentlyInCreation(bce.getBeanName())) {
						if (this.logger.isDebugEnabled()) {
							this.logger.debug("Ignoring match to currently created bean '" + beanName + "': " +
									ex.getMessage());
						}
						onSuppressedException(ex);
						// Ignore: indicates a circular reference when autowiring constructors.
						// We want to find matches other than the currently created bean itself.
						continue;
					}
				}
				throw ex;
			}
		}
		return result;
	}

	public Map<String, Object> getBeansWithAnnotation(Class<? extends Annotation> annotationType) {
		Set<String> beanNames = new LinkedHashSet<String>(getBeanDefinitionCount());
		beanNames.addAll(Arrays.asList(getBeanDefinitionNames()));
		beanNames.addAll(Arrays.asList(getSingletonNames()));
		Map<String, Object> results = new LinkedHashMap<String, Object>();
		for (String beanName : beanNames) {
			if (findAnnotationOnBean(beanName, annotationType) != null) {
				results.put(beanName, getBean(beanName));
			}
		}
		return results;
	}

	/**
	 * Find a {@link Annotation} of {@code annotationType} on the specified
	 * bean, traversing its interfaces and super classes if no annotation can be
	 * found on the given class itself, as well as checking its raw bean class
	 * if not found on the exposed bean reference (e.g. in case of a proxy).
	 */
	public <A extends Annotation> A findAnnotationOnBean(String beanName, Class<A> annotationType) {
		A ann = null;
		Class<?> beanType = getType(beanName);
		if (beanType != null) {
			ann = AnnotationUtils.findAnnotation(beanType, annotationType);
		}
		if (ann == null && containsBeanDefinition(beanName)) {
			BeanDefinition bd = getMergedBeanDefinition(beanName);
			if (bd instanceof AbstractBeanDefinition) {
				AbstractBeanDefinition abd = (AbstractBeanDefinition) bd;
				if (abd.hasBeanClass()) {
					ann = AnnotationUtils.findAnnotation(abd.getBeanClass(), annotationType);
				}
			}
		}
		return ann;
	}

	//---------------------------------------------------------------------
	// Implementation of ConfigurableListableBeanFactory interface
	//---------------------------------------------------------------------

	public void registerResolvableDependency(Class<?> dependencyType, Object autowiredValue) {
		Assert.notNull(dependencyType, "Type must not be null");
		if (autowiredValue != null) {
			Assert.isTrue((autowiredValue instanceof ObjectFactory || dependencyType.isInstance(autowiredValue)),
					"Value [" + autowiredValue + "] does not implement specified type [" + dependencyType.getName() + "]");
			this.resolvableDependencies.put(dependencyType, autowiredValue);
		}
	}

	public boolean isAutowireCandidate(String beanName, DependencyDescriptor descriptor)
			throws NoSuchBeanDefinitionException {

		// Consider FactoryBeans as autowiring candidates.
		boolean isFactoryBean = (descriptor != null && descriptor.getDependencyType() != null &&
				FactoryBean.class.isAssignableFrom(descriptor.getDependencyType()));
		if (isFactoryBean) {
			beanName = BeanFactoryUtils.transformedBeanName(beanName);
		}

		if (containsBeanDefinition(beanName)) {
			return isAutowireCandidate(beanName, getMergedLocalBeanDefinition(beanName), descriptor);
		}
		else if (containsSingleton(beanName)) {
			return isAutowireCandidate(beanName, new RootBeanDefinition(getType(beanName)), descriptor);
		}
		else if (getParentBeanFactory() instanceof ConfigurableListableBeanFactory) {
			// No bean definition found in this factory -> delegate to parent.
			return ((ConfigurableListableBeanFactory) getParentBeanFactory()).isAutowireCandidate(beanName, descriptor);
		}
		else {
			return true;
		}
	}

	/**
	 * Determine whether the specified bean definition qualifies as an autowire candidate,
	 * to be injected into other beans which declare a dependency of matching type.
	 * @param beanName the name of the bean definition to check
	 * @param mbd the merged bean definition to check
	 * @param descriptor the descriptor of the dependency to resolve
	 * @return whether the bean should be considered as autowire candidate
	 */
	protected boolean isAutowireCandidate(String beanName, RootBeanDefinition mbd, DependencyDescriptor descriptor) {
		resolveBeanClass(mbd, beanName);
		if (mbd.isFactoryMethodUnique) {
			boolean resolve;
			synchronized (mbd.constructorArgumentLock) {
				resolve = (mbd.resolvedConstructorOrFactoryMethod == null);
			}
			if (resolve) {
				new ConstructorResolver(this).resolveFactoryMethodIfPossible(mbd);
			}
		}
		return getAutowireCandidateResolver().isAutowireCandidate(
				new BeanDefinitionHolder(mbd, beanName, getAliases(beanName)), descriptor);
	}

	@Override
	public BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException {
		BeanDefinition bd = this.beanDefinitionMap.get(beanName);
		if (bd == null) {
			if (this.logger.isTraceEnabled()) {
				this.logger.trace("No bean named '" + beanName + "' found in " + this);
			}
			throw new NoSuchBeanDefinitionException(beanName);
		}
		return bd;
	}

	public void freezeConfiguration() {
		this.configurationFrozen = true;
		synchronized (this.beanDefinitionMap) {
			this.frozenBeanDefinitionNames = StringUtils.toStringArray(this.beanDefinitionNames);
		}
	}

	public boolean isConfigurationFrozen() {
		return this.configurationFrozen;
	}

	/**
	 * Considers all beans as eligible for metadata caching
	 * if the factory's configuration has been marked as frozen.
	 * @see #freezeConfiguration()
	 */
	@Override
	protected boolean isBeanEligibleForMetadataCaching(String beanName) {
		return (this.configurationFrozen || super.isBeanEligibleForMetadataCaching(beanName));
	}

	public void preInstantiateSingletons() throws BeansException {
		if (this.logger.isInfoEnabled()) {
			this.logger.info("Pre-instantiating singletons in " + this);
		}
		List<String> beanNames;
		synchronized (this.beanDefinitionMap) {
			// Iterate over a copy to allow for init methods which in turn register new bean definitions.
			// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
			beanNames = new ArrayList<String>(this.beanDefinitionNames);
		}
		for (String beanName : beanNames) {
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				if (isFactoryBean(beanName)) {
					final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
					boolean isEagerInit;
					if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
						isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
							public Boolean run() {
								return ((SmartFactoryBean<?>) factory).isEagerInit();
							}
						}, getAccessControlContext());
					}
					else {
						isEagerInit = (factory instanceof SmartFactoryBean &&
								((SmartFactoryBean<?>) factory).isEagerInit());
					}
					if (isEagerInit) {
						getBean(beanName);
					}
				}
				else {
					getBean(beanName);
				}
			}
		}
	}

	//---------------------------------------------------------------------
	// Implementation of BeanDefinitionRegistry interface
	//---------------------------------------------------------------------

	public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException {

		Assert.hasText(beanName, "Bean name must not be empty");
		Assert.notNull(beanDefinition, "BeanDefinition must not be null");

		if (beanDefinition instanceof AbstractBeanDefinition) {
			try {
				((AbstractBeanDefinition) beanDefinition).validate();
			}
			catch (BeanDefinitionValidationException ex) {
				throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
						"Validation of bean definition failed", ex);
			}
		}

		synchronized (this.beanDefinitionMap) {
			Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
			if (oldBeanDefinition != null) {
				if (!this.allowBeanDefinitionOverriding) {
					throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
							"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
							"': There is already [" + oldBeanDefinition + "] bound.");
				}
				else {
					if (this.logger.isInfoEnabled()) {
						this.logger.info("Overriding bean definition for bean '" + beanName +
								"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
					}
				}
			}
			else {
				this.beanDefinitionNames.add(beanName);
				this.frozenBeanDefinitionNames = null;
			}
			this.beanDefinitionMap.put(beanName, beanDefinition);
		}

		resetBeanDefinition(beanName);
	}

	public void removeBeanDefinition(String beanName) throws NoSuchBeanDefinitionException {
		Assert.hasText(beanName, "'beanName' must not be empty");

		synchronized (this.beanDefinitionMap) {
			BeanDefinition bd = this.beanDefinitionMap.remove(beanName);
			if (bd == null) {
				if (this.logger.isTraceEnabled()) {
					this.logger.trace("No bean named '" + beanName + "' found in " + this);
				}
				throw new NoSuchBeanDefinitionException(beanName);
			}
			this.beanDefinitionNames.remove(beanName);
			this.frozenBeanDefinitionNames = null;
		}

		resetBeanDefinition(beanName);
	}

	/**
	 * Reset all bean definition caches for the given bean,
	 * including the caches of beans that are derived from it.
	 * @param beanName the name of the bean to reset
	 */
	protected void resetBeanDefinition(String beanName) {
		// Remove the merged bean definition for the given bean, if already created.
		clearMergedBeanDefinition(beanName);

		// Remove corresponding bean from singleton cache, if any. Shouldn't usually
		// be necessary, rather just meant for overriding a context's default beans
		// (e.g. the default StaticMessageSource in a StaticApplicationContext).
		destroySingleton(beanName);

		// Remove any assumptions about by-type mappings.
		clearByTypeCache();

		// Reset all bean definitions that have the given bean as parent (recursively).
		for (String bdName : this.beanDefinitionNames) {
			if (!beanName.equals(bdName)) {
				BeanDefinition bd = this.beanDefinitionMap.get(bdName);
				if (beanName.equals(bd.getParentName())) {
					resetBeanDefinition(bdName);
				}
			}
		}
	}

	/**
	 * Only allows alias overriding if bean definition overriding is allowed.
	 */
	@Override
	protected boolean allowAliasOverriding() {
		return this.allowBeanDefinitionOverriding;
	}

	@Override
	public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
		super.registerSingleton(beanName, singletonObject);
		clearByTypeCache();
	}

	@Override
	public void destroySingleton(String beanName) {
		super.destroySingleton(beanName);
		clearByTypeCache();
	}

	/**
	 * Remove any assumptions about by-type mappings.
	 */
	private void clearByTypeCache() {
		this.allBeanNamesByType.clear();
		this.singletonBeanNamesByType.clear();
	}

	//---------------------------------------------------------------------
	// Dependency resolution functionality
	//---------------------------------------------------------------------

	public Object resolveDependency(DependencyDescriptor descriptor, String beanName,
			Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {

		descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
		if (descriptor.getDependencyType().equals(ObjectFactory.class)) {
			return new DependencyObjectFactory(descriptor, beanName);
		}
		else if (descriptor.getDependencyType().equals(javaxInjectProviderClass)) {
			return new DependencyProviderFactory().createDependencyProvider(descriptor, beanName);
		}
		else {
			return doResolveDependency(descriptor, descriptor.getDependencyType(), beanName, autowiredBeanNames, typeConverter);
		}
	}

	protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,
			Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {

		Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
		if (value != null) {
			if (value instanceof String) {
				String strVal = resolveEmbeddedValue((String) value);
				BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
				value = evaluateBeanDefinitionString(strVal, bd);
			}
			TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
			return (descriptor.getField() != null ?
					converter.convertIfNecessary(value, type, descriptor.getField()) :
					converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
		}

		if (type.isArray()) {
			Class<?> componentType = type.getComponentType();
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType, descriptor);
			if (matchingBeans.isEmpty()) {
				if (descriptor.isRequired()) {
					raiseNoSuchBeanDefinitionException(componentType, "array of " + componentType.getName(), descriptor);
				}
				return null;
			}
			if (autowiredBeanNames != null) {
				autowiredBeanNames.addAll(matchingBeans.keySet());
			}
			TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
			return converter.convertIfNecessary(matchingBeans.values(), type);
		}
		else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
			Class<?> elementType = descriptor.getCollectionType();
			if (elementType == null) {
				if (descriptor.isRequired()) {
					throw new FatalBeanException("No element type declared for collection [" + type.getName() + "]");
				}
				return null;
			}
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType, descriptor);
			if (matchingBeans.isEmpty()) {
				if (descriptor.isRequired()) {
					raiseNoSuchBeanDefinitionException(elementType, "collection of " + elementType.getName(), descriptor);
				}
				return null;
			}
			if (autowiredBeanNames != null) {
				autowiredBeanNames.addAll(matchingBeans.keySet());
			}
			TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
			return converter.convertIfNecessary(matchingBeans.values(), type);
		}
		else if (Map.class.isAssignableFrom(type) && type.isInterface()) {
			Class<?> keyType = descriptor.getMapKeyType();
			if (keyType == null || !String.class.isAssignableFrom(keyType)) {
				if (descriptor.isRequired()) {
					throw new FatalBeanException("Key type [" + keyType + "] of map [" + type.getName() +
							"] must be assignable to [java.lang.String]");
				}
				return null;
			}
			Class<?> valueType = descriptor.getMapValueType();
			if (valueType == null) {
				if (descriptor.isRequired()) {
					throw new FatalBeanException("No value type declared for map [" + type.getName() + "]");
				}
				return null;
			}
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType, descriptor);
			if (matchingBeans.isEmpty()) {
				if (descriptor.isRequired()) {
					raiseNoSuchBeanDefinitionException(valueType, "map with value type " + valueType.getName(), descriptor);
				}
				return null;
			}
			if (autowiredBeanNames != null) {
				autowiredBeanNames.addAll(matchingBeans.keySet());
			}
			return matchingBeans;
		}
		else {
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
			if (matchingBeans.isEmpty()) {
				if (descriptor.isRequired()) {
					raiseNoSuchBeanDefinitionException(type, "", descriptor);
				}
				return null;
			}
			if (matchingBeans.size() > 1) {
				String primaryBeanName = determinePrimaryCandidate(matchingBeans, descriptor);
				if (primaryBeanName == null) {
					throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());
				}
				if (autowiredBeanNames != null) {
					autowiredBeanNames.add(primaryBeanName);
				}
				return matchingBeans.get(primaryBeanName);
			}
			// We have exactly one match.
			Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
			if (autowiredBeanNames != null) {
				autowiredBeanNames.add(entry.getKey());
			}
			return entry.getValue();
		}
	}

	/**
	 * Find bean instances that match the required type.
	 * Called during autowiring for the specified bean.
	 * @param beanName the name of the bean that is about to be wired
	 * @param requiredType the actual type of bean to look for
	 * (may be an array component type or collection element type)
	 * @param descriptor the descriptor of the dependency to resolve
	 * @return a Map of candidate names and candidate instances that match
	 * the required type (never {@code null})
	 * @throws BeansException in case of errors
	 * @see #autowireByType
	 * @see #autowireConstructor
	 */
	protected Map<String, Object> findAutowireCandidates(
			String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {

		String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
				this, requiredType, true, descriptor.isEager());
		Map<String, Object> result = new LinkedHashMap<String, Object>(candidateNames.length);
		for (Class<?> autowiringType : this.resolvableDependencies.keySet()) {
			if (autowiringType.isAssignableFrom(requiredType)) {
				Object autowiringValue = this.resolvableDependencies.get(autowiringType);
				autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
				if (requiredType.isInstance(autowiringValue)) {
					result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
					break;
				}
			}
		}
		for (String candidateName : candidateNames) {
			if (!candidateName.equals(beanName) && isAutowireCandidate(candidateName, descriptor)) {
				result.put(candidateName, getBean(candidateName));
			}
		}
		return result;
	}

	/**
	 * Determine the primary autowire candidate in the given set of beans.
	 * @param candidateBeans a Map of candidate names and candidate instances
	 * that match the required type, as returned by {@link #findAutowireCandidates}
	 * @param descriptor the target dependency to match against
	 * @return the name of the primary candidate, or {@code null} if none found
	 */
	protected String determinePrimaryCandidate(Map<String, Object> candidateBeans, DependencyDescriptor descriptor) {
		String primaryBeanName = null;
		String fallbackBeanName = null;
		for (Map.Entry<String, Object> entry : candidateBeans.entrySet()) {
			String candidateBeanName = entry.getKey();
			Object beanInstance = entry.getValue();
			if (isPrimary(candidateBeanName, beanInstance)) {
				if (primaryBeanName != null) {
					boolean candidateLocal = containsBeanDefinition(candidateBeanName);
					boolean primaryLocal = containsBeanDefinition(primaryBeanName);
					if (candidateLocal == primaryLocal) {
						throw new NoUniqueBeanDefinitionException(descriptor.getDependencyType(), candidateBeans.size(),
								"more than one 'primary' bean found among candidates: " + candidateBeans.keySet());
					}
					else if (candidateLocal && !primaryLocal) {
						primaryBeanName = candidateBeanName;
					}
				}
				else {
					primaryBeanName = candidateBeanName;
				}
			}
			if (primaryBeanName == null &&
					(this.resolvableDependencies.values().contains(beanInstance) ||
							matchesBeanName(candidateBeanName, descriptor.getDependencyName()))) {
				fallbackBeanName = candidateBeanName;
			}
		}
		return (primaryBeanName != null ? primaryBeanName : fallbackBeanName);
	}

	/**
	 * Return whether the bean definition for the given bean name has been
	 * marked as a primary bean.
	 * @param beanName the name of the bean
	 * @param beanInstance the corresponding bean instance
	 * @return whether the given bean qualifies as primary
	 */
	protected boolean isPrimary(String beanName, Object beanInstance) {
		if (containsBeanDefinition(beanName)) {
			return getMergedLocalBeanDefinition(beanName).isPrimary();
		}
		BeanFactory parentFactory = getParentBeanFactory();
		return (parentFactory instanceof DefaultListableBeanFactory &&
				((DefaultListableBeanFactory) parentFactory).isPrimary(beanName, beanInstance));
	}

	/**
	 * Determine whether the given candidate name matches the bean name or the aliases
	 * stored in this bean definition.
	 */
	protected boolean matchesBeanName(String beanName, String candidateName) {
		return (candidateName != null &&
				(candidateName.equals(beanName) || ObjectUtils.containsElement(getAliases(beanName), candidateName)));
	}

	/**
	 * Raise a NoSuchBeanDefinitionException for an unresolvable dependency.
	 */
	private void raiseNoSuchBeanDefinitionException(
			Class<?> type, String dependencyDescription, DependencyDescriptor descriptor)
			throws NoSuchBeanDefinitionException {

		throw new NoSuchBeanDefinitionException(type, dependencyDescription,
				"expected at least 1 bean which qualifies as autowire candidate for this dependency. " +
				"Dependency annotations: " + ObjectUtils.nullSafeToString(descriptor.getAnnotations()));
	}

	@Override
	public String toString() {
		StringBuilder sb = new StringBuilder(ObjectUtils.identityToString(this));
		sb.append(": defining beans [");
		sb.append(StringUtils.arrayToCommaDelimitedString(getBeanDefinitionNames()));
		sb.append("]; ");
		BeanFactory parent = getParentBeanFactory();
		if (parent == null) {
			sb.append("root of factory hierarchy");
		}
		else {
			sb.append("parent: ").append(ObjectUtils.identityToString(parent));
		}
		return sb.toString();
	}

	//---------------------------------------------------------------------
	// Serialization support
	//---------------------------------------------------------------------

	private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
		throw new NotSerializableException("DefaultListableBeanFactory itself is not deserializable - " +
				"just a SerializedBeanFactoryReference is");
	}

	protected Object writeReplace() throws ObjectStreamException {
		if (this.serializationId != null) {
			return new SerializedBeanFactoryReference(this.serializationId);
		}
		else {
			throw new NotSerializableException("DefaultListableBeanFactory has no serialization id");
		}
	}

	/**
	 * Minimal id reference to the factory.
	 * Resolved to the actual factory instance on deserialization.
	 */
	private static class SerializedBeanFactoryReference implements Serializable {

		private final String id;

		public SerializedBeanFactoryReference(String id) {
			this.id = id;
		}

		private Object readResolve() {
			Reference<?> ref = serializableFactories.get(this.id);
			if (ref == null) {
				throw new IllegalStateException(
						"Cannot deserialize BeanFactory with id " + this.id + ": no factory registered for this id");
			}
			Object result = ref.get();
			if (result == null) {
				throw new IllegalStateException(
						"Cannot deserialize BeanFactory with id " + this.id + ": factory has been garbage-collected");
			}
			return result;
		}
	}

	/**
	 * Serializable ObjectFactory for lazy resolution of a dependency.
	 */
	private class DependencyObjectFactory implements ObjectFactory<Object>, Serializable {

		private final DependencyDescriptor descriptor;

		private final String beanName;

		public DependencyObjectFactory(DependencyDescriptor descriptor, String beanName) {
			this.descriptor = new DependencyDescriptor(descriptor);
			this.descriptor.increaseNestingLevel();
			this.beanName = beanName;
		}

		public Object getObject() throws BeansException {
			return doResolveDependency(this.descriptor, this.descriptor.getDependencyType(), this.beanName, null, null);
		}
	}

	/**
	 * Serializable ObjectFactory for lazy resolution of a dependency.
	 */
	private class DependencyProvider extends DependencyObjectFactory implements Provider<Object> {

		public DependencyProvider(DependencyDescriptor descriptor, String beanName) {
			super(descriptor, beanName);
		}

		public Object get() throws BeansException {
			return getObject();
		}
	}

	/**
	 * Separate inner class for avoiding a hard dependency on the {@code javax.inject} API.
	 */
	private class DependencyProviderFactory {

		public Object createDependencyProvider(DependencyDescriptor descriptor, String beanName) {
			return new DependencyProvider(descriptor, beanName);
		}
	}

}

BeanFactory

下面深入到源码处去观察下spring container是如何完成任务的。在IOC模块中，大量使用了工厂设计模式来完成类的初始化。整个IOC中，最大的工厂接口便是BeanFactory，里面定义了了一些基本的方法去获取java bean，如下代码所示

package org.springframework.beans.factory;

import org.springframework.beans.BeansException;

/**
 * IOC中工厂类的基类，定义了一些列获取java bean对象的方法
 */
public interface BeanFactory {

	String FACTORY_BEAN_PREFIX = "&";
         /**根据名字获取java bean对象
	 * @param name the name of the bean to retrieve
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no bean definition
	 * with the specified name
	 * @throws BeansException if the bean could not be obtained
	 */
	Object getBean(String name) throws BeansException;

	/**
	 * 根据名字获取指定类型的java bean对象
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no such bean definition
	 * @throws BeanNotOfRequiredTypeException if the bean is not of the required type
	 * @throws BeansException if the bean could not be created
	 */
	<T> T getBean(String name, Class<T> requiredType) throws BeansException;

	/**
	 *根据类型获取指定类型的java bean对象，这里使用了泛型技术
	 * @return an instance of the single bean matching the required type
	 * @throws NoSuchBeanDefinitionException if no bean of the given type was found
	 * @throws NoUniqueBeanDefinitionException if more than one bean of the given type was found
	 * @since 3.0
	 * @see ListableBeanFactory
	 */
	<T> T getBean(Class<T> requiredType) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>Allows for specifying explicit constructor arguments / factory method arguments,
	 * overriding the specified default arguments (if any) in the bean definition.
	 * @param name the name of the bean to retrieve
	 * @param args arguments to use if creating a prototype using explicit arguments to a
	 * static factory method. It is invalid to use a non-null args value in any other case.
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no such bean definition
	 * @throws BeanDefinitionStoreException if arguments have been given but
	 * the affected bean isn't a prototype
	 * @throws BeansException if the bean could not be created
	 * @since 2.5
	 */
	Object getBean(String name, Object... args) throws BeansException;

	/**
	 * 判断IOC容器中是否有指定名字的java bean对象
	 * @param name the name of the bean to query
	 * @return whether a bean with the given name is present
	 */
	boolean containsBean(String name);

	/**
	 * 判断指定名字的java bean对象是否为单例模式
	 * @see #getBean
	 * @see #isPrototype
	 */
	boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

	/**
	 * 判断指定名字的java bean对象是否为原型设计模式
	 * @since 2.0.3
	 * @see #getBean
	 * @see #isSingleton
	 */
	boolean isPrototype(String name) throws NoSuchBeanDefinitionException;

	/**
	 * 判断指定名字的java bean对象是否为targetType的类型
	 * @since 2.0.1
	 * @see #getBean
	 * @see #getType
	 */
	boolean isTypeMatch(String name, Class<?> targetType) throws NoSuchBeanDefinitionException;

	/**
	 * 获取指定名字的java bean对象的类型
	 * @since 1.1.2
	 * @see #getBean
	 * @see #isTypeMatch
	 */
	Class<?> getType(String name) throws NoSuchBeanDefinitionException;

	/**获取指定名字的java bean对象的别名，一个对象可以有多个名字
	 * @param name the bean name to check for aliases
	 * @return the aliases, or an empty array if none
	 * @see #getBean
	 */
	String[] getAliases(String name);

}

当然，仅仅这几个普通的接口肯定是无法定义整个spring ioc需要的类的，但是BeanFactory作为基类，保持接口的精简是非常必要的，其他的BeanFactory根据特性继承该接口即可。下面贴出关于整个BeanFactory的继承体系。




这里面还有几个类没有截图进来，具体的大家可以使用Eclipse快捷键F4查看下BeanFactory的继承体系。不过从类的名字，我们大概可以猜到这些类或者接口的功能。

Bean

IOC是一个容器，管理系统的全部java bean对象。那么什么是java bean呢？JavaBean
是一种J***A语言写成的可重用组件。为写成JavaBean，类必须是具体的和公共的，并且具有无参数的构造器。JavaBean 通过提供符合一致性设计模式的公共方法将内部域暴露成员属性。众所周知，属性名称符合这种模式，其他Java 类可以通过自省机制发现和操作这些JavaBean 的属性。也就是我们常用的java
bean对象都有如下几个模块组成，具体详情可以参考百度百科java bean

构造方法
属性
方法
事件

在spring中，由于java bean对象都遵循着一定的规范，所以给反射提供了良好的先决条件。作为一个框架，经常会更具反射去动态的加载一些类，spring IOC同样也是利用了反射的技术。在spring中，会根据源数据来实例化一个java
bean，并且在实例化过程中注入依赖的类，或者是调用一些初始化、销毁方法。对于spring 容器中的bean，主要由如下几个属性

class
name	类名
scope	生存周期
constructor arguments	构造参数
properties	属性
autowiring mode	注入模式（byName, byType, auto）
lazy-initialization mode	延迟加载模式
initialization method	初始化时调用的方法
destruction method	摧毁时调用的方法

开发者掌握要点

在IOC模块，使用者主要需要掌握的就是如下几点：

bean依赖对象的注入方式（以那种方式注入bean，比如通过setter，constructor，field等）
bean依赖对象的查找方式，也就是根据哪一种方式来寻找依赖对象（no，byName，
byType，autowired）
选择使用那种configuration
metadata（源数据）来管理bean（主要有两种，一种是xml方式，一种是注解方式）

懒加载模式
bean对象实例化、或者摧毁时调用的方法
bean对象的生命周期（singleton，prototype，request，session，global，application，其中request，session，global，application为web专用）。在这里面，singleton代表的是单例，也就是一个class，spring
container中只用一个实例。prototype为原型模式，也就是一个class有多个实例，区别如下图所示

下面的表格为spring bean可配置的属性

源码下载

介绍完IOC的一些背景知识，下面使用具体的代码演示下系统是如何运行的。在demo中，使用注解的方式来管理configuration metadata（源信息）。为了让工程最小化，默认使用maven来管理系统的依赖jar。具体的大家可以直接下载源代码，github

总结

在spring container中，大量使用了设计模式，需要好好学习下。当然，也不仅仅是设计模式，还包含了面向对象汇总常用的一些概念。

依赖倒置法则的应用
java bean规范的使用
对象生命周期
工厂设计模式
面向接口编程