• 欢迎访问 winrains 的个人网站!
  • 本网站主要从互联网整理和收集了与Java、网络安全、Linux等技术相关的文章,供学习和研究使用。如有侵权,请留言告知,谢谢!

Spring 框架中的设计模式(5)

Spring winrains 来源:一叶知秋 8个月前 (11-03) 27次浏览 0个评论

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。

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Spring bean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

作者:一叶知秋

来源:https://muyinchen.github.io/2017/08/01/Spring%E6%A1%86%E6%9E%B6%E4%B8%AD%E7%9A%84%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F(%E4%BA%94)/


版权声明:文末如注明作者和来源,则表示本文系转载,版权为原作者所有 | 本文如有侵权,请及时联系,承诺在收到消息后第一时间删除 | 如转载本文,请注明原文链接。
喜欢 (0)
发表我的评论
取消评论
表情 贴图 加粗 删除线 居中 斜体

Hi,您需要填写昵称和邮箱!

  • 昵称 (必填)
  • 邮箱 (必填)
  • 网址