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 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 ;
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。

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Spring框架中的设计模式（五）

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