85 - 剖析Spring的设计思想及原则（二）

2021-10-24  本文已影响0人  舍是境界

上文中，我们学习了 Spring 框架背后蕴藏的一些经典设计思想，比如约定优于配置、低侵入松耦合、模块化轻量级等等。我们可以将这些设计思想借鉴到其他框架开发中，在大的设计层面提高框架的代码质量，除了这些，实际上，可扩展也是大部分框架应该具备的一个重要特性。所谓的框架可扩展，我们之前也提到过，意思就是，框架使用者在不修改框架源码的情况下，基于扩展点定制扩展新的功能。

常用来实现扩展特性的设计模式有：观察者模式、模板模式、职责链模式、策略模式等。本文，我们再剖析 Spring 框架为了支持可扩展特性用的 2 种设计模式：观察者模式和模板模式。

观察者模式在 Spring 中的应用

// Event事件
public class DemoEvent extends ApplicationEvent {
  private String message;
  public DemoEvent(Object source, String message) {
    super(source);
  }
  public String getMessage() {
    return this.message;
  }
}
// Listener监听者
@Component
public class DemoListener implements ApplicationListener<DemoEvent> {
  @Override
  public void onApplicationEvent(DemoEvent demoEvent) {
    String message = demoEvent.getMessage();
    System.out.println(message);
  }
}
// Publisher发送者
@Component
public class DemoPublisher {
  @Autowired
  private ApplicationContext applicationContext;
  public void publishEvent(DemoEvent demoEvent) {
    this.applicationContext.publishEvent(demoEvent);
  }
}

public abstract class ApplicationEvent extends EventObject {
  private static final long serialVersionUID = 7099057708183571937L;
  private final long timestamp = System.currentTimeMillis();
  public ApplicationEvent(Object source) {
    super(source);
  }
  public final long getTimestamp() {
    return this.timestamp;
  }
}
public class EventObject implements java.io.Serializable {
    private static final long serialVersionUID = 5516075349620653480L;
    protected transient Object  source;
    public EventObject(Object source) {
        if (source == null)
            throw new IllegalArgumentException("null source");
        this.source = source;
    }
    public Object getSource() {
        return source;
    }
    public String toString() {
        return getClass().getName() + "[source=" + source + "]";
    }
}
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
  void onApplicationEvent(E var1);
}

public abstract class AbstractApplicationContext extends ... {
  private final Set<ApplicationListener<?>> applicationListeners;
  
  public AbstractApplicationContext() {
    this.applicationListeners = new LinkedHashSet();
    //...
  }
  
  public void publishEvent(ApplicationEvent event) {
    this.publishEvent(event, (ResolvableType)null);
  }
  public void publishEvent(Object event) {
    this.publishEvent(event, (ResolvableType)null);
  }
  protected void publishEvent(Object event, ResolvableType eventType) {
    //...
    Object applicationEvent;
    if (event instanceof ApplicationEvent) {
      applicationEvent = (ApplicationEvent)event;
    } else {
      applicationEvent = new PayloadApplicationEvent(this, event);
      if (eventType == null) {
        eventType = ((PayloadApplicationEvent)applicationEvent).getResolvableType();
      }
    }
    if (this.earlyApplicationEvents != null) {
      this.earlyApplicationEvents.add(applicationEvent);
    } else {
      this.getApplicationEventMulticaster().multicastEvent(
            (ApplicationEvent)applicationEvent, eventType);
    }
    if (this.parent != null) {
      if (this.parent instanceof AbstractApplicationContext) {
        ((AbstractApplicationContext)this.parent).publishEvent(event, eventType);
      } else {
        this.parent.publishEvent(event);
      }
    }
  }
  
  public void addApplicationListener(ApplicationListener<?> listener) {
    Assert.notNull(listener, "ApplicationListener must not be null");
    if (this.applicationEventMulticaster != null) {
    this.applicationEventMulticaster.addApplicationListener(listener);
    } else {
      this.applicationListeners.add(listener);
    }  
  }
  
  public Collection<ApplicationListener<?>> getApplicationListeners() {
    return this.applicationListeners;
  }
  
  protected void registerListeners() {
    Iterator var1 = this.getApplicationListeners().iterator();
    while(var1.hasNext()) {
      ApplicationListener<?> listener = (ApplicationListener)var1.next();     this.getApplicationEventMulticaster().addApplicationListener(listener);
    }
    String[] listenerBeanNames = this.getBeanNamesForType(ApplicationListener.class, true, false);
    String[] var7 = listenerBeanNames;
    int var3 = listenerBeanNames.length;
    for(int var4 = 0; var4 < var3; ++var4) {
      String listenerBeanName = var7[var4];
      this.getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
    }
    Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
    this.earlyApplicationEvents = null;
    if (earlyEventsToProcess != null) {
      Iterator var9 = earlyEventsToProcess.iterator();
      while(var9.hasNext()) {
        ApplicationEvent earlyEvent = (ApplicationEvent)var9.next();
        this.getApplicationEventMulticaster().multicastEvent(earlyEvent);
      }
    }
  }
}

public void multicastEvent(ApplicationEvent event) {
  this.multicastEvent(event, this.resolveDefaultEventType(event));
}
public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
  ResolvableType type = eventType != null ? eventType : this.resolveDefaultEventType(event);
  Iterator var4 = this.getApplicationListeners(event, type).iterator();
  while(var4.hasNext()) {
    final ApplicationListener<?> listener = (ApplicationListener)var4.next();
    Executor executor = this.getTaskExecutor();
    if (executor != null) {
      executor.execute(new Runnable() {
        public void run() {
          SimpleApplicationEventMulticaster.this.invokeListener(listener, event);
        }
      });
    } else {
      this.invokeListener(listener, event);
    }
  }
}

模板模式在 Spring 中的应用

public class DemoClass {
  //...
  
  public void initDemo() {
    //...初始化..
  }
}
// 配置：需要通过init-method显式地指定初始化方法
<bean id="demoBean" class="com.xzg.cd.DemoClass" init-method="initDemo"></bean>

public class DemoClass implements InitializingBean{
  @Override
  public void afterPropertiesSet() throws Exception {
    //...初始化...      
  }
}
// 配置：不需要显式地指定初始化方法
<bean id="demoBean" class="com.xzg.cd.DemoClass"></bean>

public interface BeanPostProcessor {
  Object postProcessBeforeInitialization(Object var1, String var2) throws BeansException;
  Object postProcessAfterInitialization(Object var1, String var2) throws BeansException;
}
Spring bean生命周期

小结

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