在之前的兩篇文章中，我們看到了一些在Spring框架中實(shí)現(xiàn)的設(shè)計(jì)模式。這一次我們會(huì)發(fā)現(xiàn)這個(gè)流行框架使用的3種新模式。本文將從描述兩個(gè)創(chuàng)意設(shè)計(jì)模式開(kāi)始：原型和對(duì)象池。最后我們將重點(diǎn)關(guān)注行為模式—>觀察者。

原型模式

這篇文章的第一個(gè)設(shè)計(jì)模式是原型。可以通過(guò)官方文檔查找有關(guān)Spring作用域中的bean作用域的文章中介紹了類(lèi)似的概念(prototype)。原型設(shè)計(jì)模式與有用相同名稱(chēng)的(prototype)作用域有點(diǎn)相似。此設(shè)計(jì)模式允許通過(guò)復(fù)制已存在的對(duì)象來(lái)創(chuàng)建一個(gè)對(duì)象的實(shí)例。副本應(yīng)該是真正的副本。這意味著新對(duì)象的所有屬性應(yīng)與復(fù)制對(duì)象的屬性相同。如果不清楚，比一個(gè)簡(jiǎn)單的JUnit案例更好的說(shuō)明：

public class PrototypeTest {

  @Test
  public void test() {
    Robot firstRobot = new Robot("Droid#1");
    Robot secondRobot = (Robot) firstRobot.clone();
    assertTrue("Cloned robot's instance can't be the same as the"
      +" source robot instance", 
      firstRobot != secondRobot);
    assertTrue("Cloned robot's name should be '"+firstRobot.getName()+"'"
      +" but was '"+secondRobot.getName()+"'", 
      secondRobot.getName().equals(firstRobot.getName()));
  }

}


class Robot implements Cloneable {
  private String name;

  public Robot(String name) {
    this.name = name;
  }

  public String getName() {
    return this.name;
  }

  protected Object clone() throws CloneNotSupportedException {
    return super.clone();
  }
}

在Spring中，在org.springframework.beans.factory.support.AbstractBeanFactory中使用一種特定的原型設(shè)計(jì)模式，它將初始化bean原型作用域。新對(duì)象基于配置文件中的bean定義。我們可以看到，在給定的例子中：

<bean id="shoppingCart" class="com.waitingforcode.data.ShoppingCart" scope="prototype">
  <property name="id" value="9"></property>
</bean>

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations={"applicationContext-test.xml"})
public class SpringPrototypeTest {

  @Autowired
  private BeanFactory beanFactory;

  @Test
  public void test() {
    ShoppingCart cart1 = (ShoppingCart) beanFactory.getBean("shoppingCart");
    assertTrue("Id of cart1 should be 9 but was "+cart1.getId(), 
      cart1.getId() == 9);
    cart1.setId(100);
    ShoppingCart cart2 = (ShoppingCart) beanFactory.getBean("shoppingCart");
    assertTrue("Id of cart2 should be 9 but was "+cart2.getId(), 
      cart2.getId() == 9);
    assertTrue("Id of second cart ("+cart2.getId()+") shouldn't be the same as the first one: "+cart1.getId(), 
      cart1.getId() != cart2.getId());
    cart2.setId(cart1.getId());
    assertTrue("Now (after cart2.setId(cart1.getId())), the id of second cart ("+cart2.getId()+") should be the same as the first one: "
      +cart1.getId(), cart1.getId() == cart2.getId());
    assertTrue("Both instance shouldn't be the same", cart1 != cart2);
  }

}

從前面的例子可以看出，ShoppingCart實(shí)例是直接從bean定義創(chuàng)建的。最初，cart1和cart2對(duì)象的id值為9.它在測(cè)試結(jié)束時(shí)被修改，以證明兩個(gè)引用都屬于兩個(gè)不同的對(duì)象。

對(duì)象池

Spring中使用的另一個(gè)模型是對(duì)象池設(shè)計(jì)模式。其主要目的在于在一個(gè)池中保存特定數(shù)量的對(duì)象，并根據(jù)需要重新使用。通過(guò)它，我們可以改善我們想要使用巨型對(duì)象的響應(yīng)時(shí)間。巨型意味著這些對(duì)象的構(gòu)造需要很多時(shí)間（例如：持有數(shù)據(jù)庫(kù)連接的對(duì)象），最好重用已經(jīng)存在的和未獲取的對(duì)象，而不是創(chuàng)建新對(duì)象。

Spring還使用線(xiàn)程池來(lái)管理其調(diào)度部分。一些示例位于org.springframework.scheduling.concurrent中。我們檢索數(shù)據(jù)庫(kù)（Spring JDBC）項(xiàng)目中的對(duì)象池的想法。數(shù)據(jù)庫(kù)連接池不是由Spring直接實(shí)現(xiàn)的，而是適用于Spring工作方式的項(xiàng)目，如C3P0或Jakarta Commons DBCP連接池。

觀察者

這里呈現(xiàn)的最后一個(gè)設(shè)計(jì)模式是觀察者。當(dāng)一個(gè)或幾個(gè)課程正在等待具體事件時(shí)可以使用它。觀察者模式由一個(gè)科目和觀察員名單組成。一個(gè)很好的例子就是GUI界面，其中點(diǎn)擊按鈕（按鈕是主題）會(huì)引起聽(tīng)眾（觀察者）啟動(dòng)的一些操作(再說(shuō)的直白點(diǎn)就是電影院一場(chǎng)電影這個(gè)subject,需要觀眾(也就是觀察者咯),電影產(chǎn)生的一些畫(huà)面產(chǎn)生的事件，比如恐怖 電影給男人女人帶來(lái)的不同的感官的感受，傳播到觀察者也就是觀眾的眼里所帶來(lái)的不一樣的反應(yīng)，這個(gè)中間一般會(huì)添加一個(gè)事件傳播者，在后面解釋Spring的例子的時(shí)候會(huì)說(shuō)到)，例如：打開(kāi)一個(gè)新頁(yè)面這個(gè)動(dòng)作。可以參考下面的例子：

public class ObserverTest {

  @Test
  public void test() {
    Observer pageOpener = new PageOpener();
    Observer register = new Register();
    Button btn = new Button();
    btn.addListener(pageOpener);
    btn.addListener(register);
    btn.clickOn();
    assertTrue("Button should be clicked but it wasn't", 
      btn.wasClicked());
    assertTrue("Page opener should be informed about click but it wasn't", 
      pageOpener.wasInformed());
    assertTrue("Register should be informed about click but it wasn't", 
      register.wasInformed());
  }

}

class Button {

  private boolean clicked;
  private List<observer> listeners;

  public List<observer> getListeners() {
    if (this.listeners == null) {
      this.listeners = new ArrayList<observer>();
    }
    return this.listeners;
  }

  public void addListener(Observer observer) {
    getListeners().add(observer);
  }

  public boolean wasClicked() {
    return this.clicked;
  }

  public void clickOn() {
    this.clicked = true;
    informAll();
  }

  private void informAll() {
    for (Observer observer : getListeners()) {
      observer.informAboutEvent();
    }
  }

}

abstract class Observer {
  protected boolean informed;

  public void informAboutEvent() {
    this.informed = true;
  }

  public boolean wasInformed() {
    return this.informed;
  }
}

class PageOpener extends Observer {

  @Override
  public void informAboutEvent() {
    System.out.println("Preparing download of new page");
    super.informAboutEvent();
  }

}

class Register extends Observer {

  @Override
  public void informAboutEvent() {
    System.out.println("Adding the action to register");
    super.informAboutEvent();
  }
}

可以看到，關(guān)于我們的Button實(shí)例點(diǎn)擊的事件被發(fā)送到所有的觀察者對(duì)象。從這些對(duì)象開(kāi)始下載頁(yè)面內(nèi)容，第二個(gè)將在事件的信息保存在注冊(cè)表中。在Spring中，觀察者設(shè)計(jì)模式用于將與應(yīng)用程序上下文相關(guān)的事件傳輸?shù)給rg.springframework.context.ApplicationListener的實(shí)現(xiàn)。要了解它們的實(shí)現(xiàn)方法，我們來(lái)看一下AbstractApplicationContext類(lèi)(老版本的代碼，新版本的請(qǐng)自行對(duì)照)：

public abstract class AbstractApplicationContext extends DefaultResourceLoader
  implements ConfigurableApplicationContext, DisposableBean {
  /** Statically specified listeners */
  private Set<applicationlistener<?>> applicationListeners = new LinkedHashSet<applicationlistener<?>>();

  // some other fields and methods
  @Override
  public void addApplicationListener(ApplicationListener<?> listener) {
    if (this.applicationEventMulticaster != null) {
      this.applicationEventMulticaster.addApplicationListener(listener);
    }
    else {//新版本這里直接咔嚓掉，上面的applicationEventMulticaster一旦為空，就會(huì)報(bào)錯(cuò)的
      this.applicationListeners.add(listener);
    }
  }

  /**
    * Return the list of statically specified ApplicationListeners.
    */
  public Collection<applicationlistener<?>> getApplicationListeners() {
    return this.applicationListeners;
  }

  /**
    * Add beans that implement ApplicationListener as listeners.
    * Doesn't affect other listeners, which can be added without being beans.
    */
  protected void registerListeners() {
    // Register statically specified listeners first.
    for (ApplicationListener<?> listener : getApplicationListeners()) {
      getApplicationEventMulticaster().addApplicationListener(listener);
    }
    // Do not initialize FactoryBeans here: We need to leave all regular beans
    // uninitialized to let post-processors apply to them!
    String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
    for (String lisName : listenerBeanNames) {
      getApplicationEventMulticaster().addApplicationListenerBean(lisName);
    }
  }
}

在提供的代碼中，監(jiān)聽(tīng)器在內(nèi)部添加到應(yīng)用程序上下文類(lèi)中，并且在registerListeners()方法之后，它們被注冊(cè)到由接口org.springframework.context.event.ApplicationEventMulticaster表示的適當(dāng)?shù)氖录嗦窂V播器(因?yàn)橛泻芏鄉(xiāng)isteners)。EventMulticaster負(fù)責(zé)管理不同的listener和向他們發(fā)布事件。

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
    private Executor taskExecutor;
    private ErrorHandler errorHandler;
    public SimpleApplicationEventMulticaster() {
    }
    public SimpleApplicationEventMulticaster(BeanFactory beanFactory) {
        this.setBeanFactory(beanFactory);
    }
    public void setTaskExecutor(Executor taskExecutor) {
        this.taskExecutor = taskExecutor;
    }
    protected Executor getTaskExecutor() {
        return this.taskExecutor;
    }
    public void setErrorHandler(ErrorHandler errorHandler) {
        this.errorHandler = errorHandler;
    }
    protected ErrorHandler getErrorHandler() {
        return this.errorHandler;
    }
    public void multicastEvent(ApplicationEvent event) {
        this.multicastEvent(event, this.resolveDefaultEventType(event));
    }
    //發(fā)布事件:通過(guò)池執(zhí)行任務(wù)的方式來(lái)做并發(fā)處理，這樣就把之前的對(duì)象池模式給利用上了
    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);
            }
        }
    }
...
}

這次我們講3種設(shè)計(jì)模式:用于在同一個(gè)調(diào)用作用域內(nèi)創(chuàng)建bean的原型，避免重新創(chuàng)建巨型對(duì)象的對(duì)象池，以及將應(yīng)用程序的上下文事件分派給適當(dāng)?shù)谋O(jiān)聽(tīng)器的觀察者。

原文：Spring框架中的設(shè)計(jì)模式(三)
轉(zhuǎn)載：極樂(lè)科技