Spring框架中的设计模式（五）

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。