Spring框架中的设计模式（五）

通过以前的4篇文章，我们看到Spring采用了大量的关于创建和结构方面的设计模式。本文将描述属于行为方面的两种设计模式：命令和访问者。前传：

命令模式

这篇文章描述的第一个行为设计模式是命令。它允许将请求封装在一个对象内并附加一个回调动作(每次遇到所所谓的回调大家就只需要理解为一个函数方法就好，省的去浪费那么多脑子)。请求被封装在命令对象之下，而请求的结果被发送到接收者。命令本身不是由调用者执行。为了直白了解其中的主要思想，想象一下管理服务器的情况(远程通过 ssh操作 Linux服务器)。管理员（ invoker）在命令行（ commands）中启动一些操作，将结果发送到服务器（接收器）。在这里,所有这一切都是由客户端的终端(也就是我们用的 xshell)来完成的。搞个 Demo来说明一下(对于命令，它的动作就是执行，对于管理员来讲，我们的动作其实就是一个回车，执不执行当然是管理员说的算了，执行交给命令对象了，服务器最后就是一个展示结果)：

public class CommandTest {

  // This test method is a client
  @Test
  public void test() {
    Administrator admin = new Administrator();
    Server server = new Server();

    // start Apache
    admin.setCommand(new StartApache(server));
    admin.typeEnter();

    // start Tomcat
    admin.setCommand(new StartTomcat(server));
    admin.typeEnter();

    // check executed commands
    int executed = server.getExecutedCommands().size();
    assertTrue("Two commands should be executed but only "+
      executed+ " were", executed == 2);
  }

}

// commands
abstract class ServerCommand {

  protected Server server;

  public ServerCommand(Server server) {
    this.server = server;
  }

  public abstract void execute();
}

class StartTomcat extends ServerCommand {

  public StartTomcat(Server server) {
    super(server);
  }

  @Override
  public void execute() {
    server.launchCommand("sudo service tomcat7 start");
  }
}

class StartApache extends ServerCommand {

  public StartApache(Server server) {
    super(server);
  }

  @Override
  public void execute() {
    server.launchCommand("sudo service apache2 start");
  }
}

// invoker
class Administrator {

  private ServerCommand command;

  public void setCommand(ServerCommand command) {
    this.command = command;
  }

  public void typeEnter() {
    this.command.execute();
  }

}

// receiver
class Server {

  // as in common terminals, we store executed commands in history
  private List<String> executedCommands = new ArrayList<String>();

  public void launchCommand(String command) {
    System.out.println("Executing: "+command+" on server");
    this.executedCommands.add(command);
  }

  public List<String> getExecutedCommands() {
    return this.executedCommands;
  }

}

测试应通过并打印两个命令：

Executing: sudo service apache2 start on server
Executing: sudo service tomcat7 start on server

命令模式不仅允许封装请求（ServerCommand）并将其传输到接收器（Server），而且还可以更好地处理给定的请求。在这里，这种更好的处理是通过存储命令的执行历史。在Spring中，我们在beanFactory后置处理器的特性中来找到指令设计模式的原理。要通过快速对它们进行定义，应用程序上下文会启动后置处理器，并可以用来对创建的bean进行一些操作（这里不打算细说了，具体的我后面会专门写一篇这方面的文章，来分析其中的源码细节）。

当我们将先前Demo里呈现的命令逻辑转换并对比到 Springbean工厂后处理器时，我们可以区分以下 actors：后置处理器bean(是指实现 BeanFactoryPostProcessor接口)是命令，org.springframework.context.support.PostProcessorRegistrationDelegate是调用者(它执行 postProcessBeanFactory方法注册所有的后置处理器bean，此处看下面第二段代码)和接收器org.springframework.beans.factory.config.ConfigurableListableBeanFactory可以在元素（bean）构造初始化之前修改它们（例如：在初始化bean之前可以更改属性）。

另外，回顾下上面的那个Demo，和我们的Demo中的命令历史管理一样。 PostProcessorRegistrationDelegate包含一个内部类 BeanPostProcessorChecker，它可以记录当一个bean不符合处理条件的情况。

可以观察 PostProcessorRegistrationDelegate中的两段代码:

/**
     * BeanPostProcessor that logs an info message when a bean is created during
     * BeanPostProcessor instantiation, i.e. when a bean is not eligible for
     * getting processed by all BeanPostProcessors.
     */
    private static class BeanPostProcessorChecker implements BeanPostProcessor {

        private static final Log logger = LogFactory.getLog(BeanPostProcessorChecker.class);

        private final ConfigurableListableBeanFactory beanFactory;

        private final int beanPostProcessorTargetCount;

        public BeanPostProcessorChecker(ConfigurableListableBeanFactory beanFactory, int beanPostProcessorTargetCount) {
            this.beanFactory = beanFactory;
            this.beanPostProcessorTargetCount = beanPostProcessorTargetCount;
        }

        @Override
        public Object postProcessBeforeInitialization(Object bean, String beanName) {
            return bean;
        }

        @Override
        public Object postProcessAfterInitialization(Object bean, String beanName) {
            if (bean != null && !(bean instanceof BeanPostProcessor) && !isInfrastructureBean(beanName) &&
                    this.beanFactory.getBeanPostProcessorCount() < this.beanPostProcessorTargetCount) {
                if (logger.isInfoEnabled()) {
                    logger.info("Bean '" + beanName + "' of type [" + bean.getClass() +
                            "] is not eligible for getting processed by all BeanPostProcessors " +
                            "(for example: not eligible for auto-proxying)");
                }
            }
            return bean;
        }

        private boolean isInfrastructureBean(String beanName) {
            if (beanName != null && this.beanFactory.containsBeanDefinition(beanName)) {
                BeanDefinition bd = this.beanFactory.getBeanDefinition(beanName);
                return RootBeanDefinition.ROLE_INFRASTRUCTURE == bd.getRole();
            }
            return false;
        }
    }

定义后的调用,用的就是 ConfigurableListableBeanFactory的实例(看 BeanPostProcessorChecker注释):

public static void registerBeanPostProcessors(
            ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

        String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

        // Register BeanPostProcessorChecker that logs an info message when
        // a bean is created during BeanPostProcessor instantiation, i.e. when
        // a bean is not eligible for getting processed by all BeanPostProcessors.
        int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
  //BeanPostProcessorChecker
        beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

        // Separate between BeanPostProcessors that implement PriorityOrdered,
        // Ordered, and the rest.
        List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
        List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
        List<String> orderedPostProcessorNames = new ArrayList<>();
        List<String> nonOrderedPostProcessorNames = new ArrayList<>();
        for (String ppName : postProcessorNames) {
            if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
                priorityOrderedPostProcessors.add(pp);
                if (pp instanceof MergedBeanDefinitionPostProcessor) {
                    internalPostProcessors.add(pp);
                }
            }
            else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }

        // First, register the BeanPostProcessors that implement PriorityOrdered.
        sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
        registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

        // Next, register the BeanPostProcessors that implement Ordered.
        List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
        for (String ppName : orderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            orderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        sortPostProcessors(beanFactory, orderedPostProcessors);
        registerBeanPostProcessors(beanFactory, orderedPostProcessors);

        // Now, register all regular BeanPostProcessors.
        List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
        for (String ppName : nonOrderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            nonOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

        // Finally, re-register all internal BeanPostProcessors.
        sortPostProcessors(beanFactory, internalPostProcessors);
        registerBeanPostProcessors(beanFactory, internalPostProcessors);

        // Re-register post-processor for detecting inner beans as ApplicationListeners,
        // moving it to the end of the processor chain (for picking up proxies etc).
        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
    }

总结一个过程就是，我要BeanFactory里面得到对象(也就是为了得到一个命令的执行结果)，那么，想要在得到对象的时候就已经实现了一些对其修改的想法，那么就通过后置处理器，也是就实现了后置处理器接口的beans(命令里可以通过传入不同的参数来得到不同结果，或者对命令的脚本进行修改)，然后还需要一个执行者(我们在做自动化运维的时候，不止操作一个脚本，这里的 PostProcessorRegistrationDelegate就是集中来管理这些的)，最后得到的结果就由 BeanFactory来展示咯。

访问者模式

接下来要介绍的一个行为设计模式是Visitor:抽象一点就是通过另一种类型的对象来使一个对象访问。在这个简短定义中，使用这个设计模式中的对象将被视为访问者或对象可被访问。第一个访问者要有可访问支持。这个模式的一个现实的例子可以是一个汽车质检员，他们检查一些汽车零件，比如轮子，制动器和发动机，以判断汽车质量是否合格。我们来做个JUnit测试用例：

public class VisitorTest {

  @Test
  public void test() {
    CarComponent car = new Car();
    Mechanic mechanic = new QualifiedMechanic();
    car.accept(mechanic);
    assertTrue("After qualified mechanics visit, the car should be broken",
      car.isBroken());
    Mechanic nonqualifiedMechanic = new NonQualifiedMechanic();
    car.accept(nonqualifiedMechanic);
    assertFalse("Car shouldn't be broken becase non qualified mechanic " +
      " can't see breakdowns", car.isBroken());
  }

}

// visitor
interface Mechanic {
  public void visit(CarComponent element);
  public String getName();
}

class QualifiedMechanic implements Mechanic {

  @Override
  public void visit(CarComponent element) {
    element.setBroken(true);
  }

  @Override
  public String getName() {
    return "qualified";
  }
}

class NonQualifiedMechanic implements Mechanic {

  @Override
  public void visit(CarComponent element) {
    element.setBroken(true);
  }

  @Override
  public String getName() {
    return "unqualified";
  }
}

// visitable
abstract class CarComponent {
  protected boolean broken;

  public abstract void accept(Mechanic mechanic);

  public void setBroken(boolean broken) {
    this.broken = broken;
  }

  public boolean isBroken() {
    return this.broken;
  }
}

class Car extends CarComponent {

  private boolean broken = false;
  private CarComponent[] components;

  public Car() {
    components = new CarComponent[] {
      new Wheels(), new Engine(), new Brake()
    };
  }

  @Override
  public void accept(Mechanic mechanic) {
    this.broken = false;
    if (mechanic.getName().equals("qualified")) {
      int i = 0;
      while (i < components.length && this.broken == false) {
        CarComponent component = components[i];
        mechanic.visit(component);
        this.broken = component.isBroken();
        i++;
      }
    }
    // if mechanic isn't qualified, we suppose that 
    // he isn't able to see breakdowns and so 
    // he considers the car as no broken 
    // (even if the car is broken)
  }

  @Override
  public boolean isBroken() {
          return this.broken;
  }
}

class Wheels extends CarComponent {

  @Override
  public void accept(Mechanic mechanic) {
    mechanic.visit(this);
  }
}

class Engine extends CarComponent {

  @Override
  public void accept(Mechanic mechanic) {
    mechanic.visit(this);
  }
}

class Brake extends CarComponent {

  @Override
  public void accept(Mechanic mechanic) {
    mechanic.visit(this);
  }
}

在这个例子中，我们可以看到他们有两个机制(访问者,其实就是免检和不免检)：合格和不合格。暴露于他们的可见对象是汽车。通过其接受方式，决定哪个角色应该适用于被访问者(通过代码 mechanic.getName().equals("qualified")来判断)。当访问者合格时，Car让他分析所有组件。如果访问者不合格，Car认为其干预是无用的，并且在方法 isBroken()中直接返回 false(其实就是为了达到一个免检的效果)。 Spring在beans配置中实现了访问者设计模式。为了观察，我们可以看看org.springframework.beans.factory.config.BeanDefinitionVisitor对象，该对象用于 解析bean元数据并将其解析为 String（例如：具有作用域或工厂方法名称的XML属性）或 Object（例如：构造函数定义中的参数）。已解析的值在与分析的bean关联的 BeanDefinition实例中进行判断设置。具体的源码请看 BeanDefinitionVisitor的代码片段：

/**
 * Traverse the given BeanDefinition object and the MutablePropertyValues
 * and ConstructorArgumentValues contained in them.
 * @param beanDefinition the BeanDefinition object to traverse
 * @see #resolveStringValue(String)
 */
public void visitBeanDefinition(BeanDefinition beanDefinition) {
  visitParentName(beanDefinition);
  visitBeanClassName(beanDefinition);
  visitFactoryBeanName(beanDefinition);
  visitFactoryMethodName(beanDefinition);
  visitScope(beanDefinition);
  visitPropertyValues(beanDefinition.getPropertyValues());
  ConstructorArgumentValues cas = beanDefinition.
    getConstructorArgumentValues();
  visitIndexedArgumentValues(cas.
    getIndexedArgumentValues());
  visitGenericArgumentValues(cas.
    getGenericArgumentValues());
}

protected void visitParentName(BeanDefinition beanDefinition) {
  String parentName = beanDefinition.getParentName();
  if (parentName != null) {
    String resolvedName = resolveStringValue(parentName);
    if (!parentName.equals(resolvedName)) {
      beanDefinition.setParentName(resolvedName);
    }
  }
}

在这种情况下，他们只是访问方式，没有对访问者做任何补充的控制(在Demo里对car的质检员做了控制)。这里访问包括分析给定 BeanDefinition的参数，并将其替换为已解析对象。

在最后一篇关于Spring中设计模式的文章中，我们发现了2种行为模式： 用于处理bean工厂的后置处理的命令模式和 用于将定义的bean参数转换为面向对象（String或Object的实例）参数的访问者模式。