设计模式之单例模式

搞Java编程的时候，经常听人提起单例模式，但我当初并不在意，以为这纯理论我也用不着。今天在撸代码是跟进spring的源码。发现单例模式的应用就在身边呀。

 Spring的依赖注入（包括lazy-init方式）都是发生在AbstractBeanFactory的getBean里。getBean的doGetBean方法调用getSingleton进行bean的创建。lazy-init方式，在容器初始化时候进行调用，非lazy-init方式，在用户向容器第一次索要bean时进行调用。

代码如下：

protected Object getSingleton(String beanName, boolean allowEarlyReference) {    Object singletonObject = this.singletonObjects.get(beanName);    if (singletonObject == null) {     synchronized (this.singletonObjects) {      singletonObject = this.earlySingletonObjects.get(beanName);      if (singletonObject == null && allowEarlyReference) {       ObjectFactory singletonFactory = this.singletonFactories.get(beanName);       if (singletonFactory != null) {        singletonObject = singletonFactory.getObject();        this.earlySingletonObjects.put(beanName, singletonObject);        this.singletonFactories.remove(beanName);       }      }     }    }    return (singletonObject != NULL_OBJECT ? singletonObject : null);   }  

 在这里Spring并没有使用私有构造方法来创建bean，而是通过singletonFactory.getObject()返回具体beanName对应的ObjectFactory来创建bean。实际上是调用了AbstractAutowireCapableBeanFactory的doCreateBean方法，返回了BeanWrapper包装并创建的bean实例。

代码如下：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {          // Instantiate the bean.          BeanWrapper instanceWrapper = null;          if (mbd.isSingleton()) {              instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);          }          if (instanceWrapper == null) {              instanceWrapper = createBeanInstance(beanName, mbd, args);          }          final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);          Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);            // Allow post-processors to modify the merged bean definition.          synchronized (mbd.postProcessingLock) {              if (!mbd.postProcessed) {                  applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);                  mbd.postProcessed = true;              }          }            // Eagerly cache singletons to be able to resolve circular references          // even when triggered by lifecycle interfaces like BeanFactoryAware.          boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&                  isSingletonCurrentlyInCreation(beanName));          if (earlySingletonExposure) {              if (logger.isDebugEnabled()) {                  logger.debug("Eagerly caching bean '" + beanName +                          "' to allow for resolving potential circular references");              }              addSingletonFactory(beanName, new ObjectFactory() {                  public Object getObject() throws BeansException {                      return getEarlyBeanReference(beanName, mbd, bean);                  }              });          }....  

getEarlyBeanReference获取bean的所有后处理器，并进行处理。如果是SmartInstantiationAwareBeanPostProcessor类型，就进行处理，如果没有相关处理内容，就返回默认的实现。

protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {          Object exposedObject = bean;          if (bean != null && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {              for (BeanPostProcessor bp : getBeanPostProcessors()) {                  if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {                      SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;                      exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);                      if (exposedObject == null) {                          return exposedObject;                      }                  }              }          }          return exposedObject;      }  

那么何为单例呢，单例模式确保某个类只有一个实例，而且自行实例化并向整个系统提供这个实例。

单例模式可分为懒汉式和饿汉式：

懒汉式单例类

//懒汉式单例类.在第一次调用的时候实例化  2  public class Singleton { 3      //私有的默认构造子 4      private Singleton() {} 5      //注意，这里没有final     6      private static Singleton single=null; 7      //静态工厂方法  8      public synchronized  static Singleton getInstance() { 9           if (single == null) {  10               single = new Singleton();11           }  12          return single;13      }14  }

饿汉式单例类

//饿汉式单例类.在类初始化时，已经自行实例化  2  public class Singleton { 3      //私有的默认构造子 4      private Singleton() {} 5      //已经自行实例化  6      private static final Singleton single = new Singleton(); 7      //静态工厂方法  8      public static Singleton getInstance() { 9          return single;10      }11  }