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):
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public class CommandTest {
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// This test method is a client
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@Test
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public void test() {
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Administrator admin = new Administrator();
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Server server = new Server();
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// start Apache
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admin.setCommand(new StartApache(server));
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admin.typeEnter();
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// start Tomcat
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admin.setCommand(new StartTomcat(server));
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admin.typeEnter();
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// check executed commands
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int executed = server.getExecutedCommands().size();
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assertTrue("Two commands should be executed but only "+
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executed+ " were", executed == 2);
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}
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}
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// commands
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abstract class ServerCommand {
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protected Server server;
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public ServerCommand(Server server) {
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this.server = server;
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}
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public abstract void execute();
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}
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class StartTomcat extends ServerCommand {
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public StartTomcat(Server server) {
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super(server);
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}
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@Override
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public void execute() {
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server.launchCommand("sudo service tomcat7 start");
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}
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}
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class StartApache extends ServerCommand {
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public StartApache(Server server) {
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super(server);
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}
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@Override
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public void execute() {
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server.launchCommand("sudo service apache2 start");
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}
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}
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// invoker
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class Administrator {
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private ServerCommand command;
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public void setCommand(ServerCommand command) {
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this.command = command;
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}
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public void typeEnter() {
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this.command.execute();
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}
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}
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// receiver
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class Server {
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// as in common terminals, we store executed commands in history
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private List<String> executedCommands = new ArrayList<String>();
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public void launchCommand(String command) {
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System.out.println("Executing: "+command+" on server");
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this.executedCommands.add(command);
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}
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public List<String> getExecutedCommands() {
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return this.executedCommands;
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}
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}
测试应通过并打印两个命令:
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Executing: sudo service apache2 start on server
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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:
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/**
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* BeanPostProcessor that logs an info message when a bean is created during
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* BeanPostProcessor instantiation, i.e. when a bean is not eligible for
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* getting processed by all BeanPostProcessors.
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*/
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private static class BeanPostProcessorChecker implements BeanPostProcessor {
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private static final Log logger = LogFactory.getLog(BeanPostProcessorChecker.class);
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private final ConfigurableListableBeanFactory beanFactory;
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private final int beanPostProcessorTargetCount;
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public BeanPostProcessorChecker(ConfigurableListableBeanFactory beanFactory, int beanPostProcessorTargetCount) {
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this.beanFactory = beanFactory;
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this.beanPostProcessorTargetCount = beanPostProcessorTargetCount;
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}
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@Override
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public Object postProcessBeforeInitialization(Object bean, String beanName) {
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return bean;
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}
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@Override
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public Object postProcessAfterInitialization(Object bean, String beanName) {
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if (bean != null && !(bean instanceof BeanPostProcessor) && !isInfrastructureBean(beanName) &&
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this.beanFactory.getBeanPostProcessorCount() < this.beanPostProcessorTargetCount) {
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if (logger.isInfoEnabled()) {
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logger.info("Bean '" + beanName + "' of type [" + bean.getClass() +
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"] is not eligible for getting processed by all BeanPostProcessors " +
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"(for example: not eligible for auto-proxying)");
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}
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}
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return bean;
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}
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private boolean isInfrastructureBean(String beanName) {
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if (beanName != null && this.beanFactory.containsBeanDefinition(beanName)) {
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BeanDefinition bd = this.beanFactory.getBeanDefinition(beanName);
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return RootBeanDefinition.ROLE_INFRASTRUCTURE == bd.getRole();
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}
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return false;
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}
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}
定义后的调用,用的就是 ConfigurableListableBeanFactory的实例(看 BeanPostProcessorChecker注释):
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public static void registerBeanPostProcessors(
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ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
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String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
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// Register BeanPostProcessorChecker that logs an info message when
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// a bean is created during BeanPostProcessor instantiation, i.e. when
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// a bean is not eligible for getting processed by all BeanPostProcessors.
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int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
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//BeanPostProcessorChecker
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beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));
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// Separate between BeanPostProcessors that implement PriorityOrdered,
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// Ordered, and the rest.
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List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
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List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
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List<String> orderedPostProcessorNames = new ArrayList<>();
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List<String> nonOrderedPostProcessorNames = new ArrayList<>();
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for (String ppName : postProcessorNames) {
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if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
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BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
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priorityOrderedPostProcessors.add(pp);
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if (pp instanceof MergedBeanDefinitionPostProcessor) {
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internalPostProcessors.add(pp);
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}
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}
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else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
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orderedPostProcessorNames.add(ppName);
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}
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else {
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nonOrderedPostProcessorNames.add(ppName);
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}
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}
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// First, register the BeanPostProcessors that implement PriorityOrdered.
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sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
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registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);
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// Next, register the BeanPostProcessors that implement Ordered.
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List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
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for (String ppName : orderedPostProcessorNames) {
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BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
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orderedPostProcessors.add(pp);
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if (pp instanceof MergedBeanDefinitionPostProcessor) {
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internalPostProcessors.add(pp);
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}
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}
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sortPostProcessors(beanFactory, orderedPostProcessors);
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registerBeanPostProcessors(beanFactory, orderedPostProcessors);
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// Now, register all regular BeanPostProcessors.
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List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
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for (String ppName : nonOrderedPostProcessorNames) {
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BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
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nonOrderedPostProcessors.add(pp);
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if (pp instanceof MergedBeanDefinitionPostProcessor) {
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internalPostProcessors.add(pp);
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}
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}
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registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);
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// Finally, re-register all internal BeanPostProcessors.
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sortPostProcessors(beanFactory, internalPostProcessors);
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registerBeanPostProcessors(beanFactory, internalPostProcessors);
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// Re-register post-processor for detecting inner beans as ApplicationListeners,
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// moving it to the end of the processor chain (for picking up proxies etc).
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beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
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}
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:
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public class VisitorTest {
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@Test
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public void test() {
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CarComponent car = new Car();
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Mechanic mechanic = new QualifiedMechanic();
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car.accept(mechanic);
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assertTrue("After qualified mechanics visit, the car should be broken",
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car.isBroken());
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Mechanic nonqualifiedMechanic = new NonQualifiedMechanic();
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car.accept(nonqualifiedMechanic);
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assertFalse("Car shouldn't be broken becase non qualified mechanic " +
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" can't see breakdowns", car.isBroken());
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}
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}
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// visitor
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interface Mechanic {
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public void visit(CarComponent element);
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public String getName();
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}
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class QualifiedMechanic implements Mechanic {
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@Override
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public void visit(CarComponent element) {
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element.setBroken(true);
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}
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@Override
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public String getName() {
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return "qualified";
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}
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}
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class NonQualifiedMechanic implements Mechanic {
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@Override
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public void visit(CarComponent element) {
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element.setBroken(true);
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}
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@Override
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public String getName() {
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return "unqualified";
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}
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}
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// visitable
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abstract class CarComponent {
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protected boolean broken;
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public abstract void accept(Mechanic mechanic);
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public void setBroken(boolean broken) {
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this.broken = broken;
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}
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public boolean isBroken() {
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return this.broken;
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}
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}
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class Car extends CarComponent {
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private boolean broken = false;
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private CarComponent[] components;
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public Car() {
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components = new CarComponent[] {
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new Wheels(), new Engine(), new Brake()
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};
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}
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@Override
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public void accept(Mechanic mechanic) {
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this.broken = false;
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if (mechanic.getName().equals("qualified")) {
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int i = 0;
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while (i < components.length && this.broken == false) {
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CarComponent component = components ;
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mechanic.visit(component);
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this.broken = component.isBroken();
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i++;
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}
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}
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// if mechanic isn't qualified, we suppose that
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// he isn't able to see breakdowns and so
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// he considers the car as no broken
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// (even if the car is broken)
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}
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@Override
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public boolean isBroken() {
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return this.broken;
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}
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}
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class Wheels extends CarComponent {
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@Override
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public void accept(Mechanic mechanic) {
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mechanic.visit(this);
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}
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}
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class Engine extends CarComponent {
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@Override
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public void accept(Mechanic mechanic) {
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mechanic.visit(this);
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}
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}
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class Brake extends CarComponent {
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@Override
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public void accept(Mechanic mechanic) {
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mechanic.visit(this);
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}
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}
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:
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/**
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* Traverse the given BeanDefinition object and the MutablePropertyValues
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* and ConstructorArgumentValues contained in them.
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* @param beanDefinition the BeanDefinition object to traverse
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* @see #resolveStringValue(String)
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*/
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public void visitBeanDefinition(BeanDefinition beanDefinition) {
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visitParentName(beanDefinition);
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visitBeanClassName(beanDefinition);
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visitFactoryBeanName(beanDefinition);
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visitFactoryMethodName(beanDefinition);
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visitScope(beanDefinition);
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visitPropertyValues(beanDefinition.getPropertyValues());
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ConstructorArgumentValues cas = beanDefinition.
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getConstructorArgumentValues();
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visitIndexedArgumentValues(cas.
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getIndexedArgumentValues());
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visitGenericArgumentValues(cas.
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getGenericArgumentValues());
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}
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protected void visitParentName(BeanDefinition beanDefinition) {
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String parentName = beanDefinition.getParentName();
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if (parentName != null) {
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String resolvedName = resolveStringValue(parentName);
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if (!parentName.equals(resolvedName)) {
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beanDefinition.setParentName(resolvedName);
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}
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}
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}
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。