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2.2 命令模式

示例代码git地址 :

（1）概念

命令模式（Command Pattern）是一种数据驱动的设计模式，它属于行为型模式。请求以命令的形式包裹在对象中，并传给调用对象。调用对象寻找可以处理该命令的合适的对象，并把该命令传给相应的对象，该对象执行命令。

主要解决：在软件系统中，行为请求者与行为实现者通常是一种紧耦合的关系，但某些场合，比如需要对行为进行记录、撤销或重做、事务等处理时，这种无法抵御变化的紧耦合的设计就不太合适。

（2）适用场景

认为是命令的地方都可以使用命令模式，比如： 1、GUI 中每一个按钮都是一条命令。 2、模拟 CMD。

在某些场合，比如要对行为进行"记录、撤销/重做、事务"等处理，这种无法抵御变化的紧耦合是不合适的。在这种情况下，如何将"行为请求者"与"行为实现者"解耦？将一组行为抽象为对象，可以实现二者之间的松耦合。

注意事项:系统需要支持命令的撤销(Undo)操作和恢复(Redo)操作，也可以考虑使用命令模式，见命令模式的扩展。

（3）代码示例

关键代码：定义三个角色：1、received 真正的命令执行对象 2、Command 3、invoker 使用命令对象的入口

我们首先创建作为命令的接口 Order，然后创建作为请求的 Stock 类。实体命令类 BuyStock 和 SellStock，实现了 Order 接口，将执行实际的命令处理。创建作为调用对象的类 Broker，它接受订单并能下订单。

Broker 对象使用命令模式，基于命令的类型确定哪个对象执行哪个命令。CommandPatternDemo，我们的演示类使用 Broker 类来演示命令模式。

创建一个命令接口。

package com.alibaba.design.commandpattern;/** * @author zhouyanxiang * @create 2020-08-2020/8/1-8:57 */public interface Order {    void execute();}

创建一个请求类。

package com.alibaba.design.commandpattern;/** * @author zhouyanxiang * @create 2020-08-2020/8/1-8:57 */public class Stock {    private String name = "Tom";    private int quantity = 10;    public void buy(){        System.out.println("Stock [ Name: "+name+", Quantity: " + quantity +" ] bought");    }    public void sell(){        System.out.println("Stock [ Name: "+name+", Quantity: " + quantity +" ] sold");    }}

创建实现了 Order 接口的实体类。

• BuyStock

package com.alibaba.design.commandpattern;/** * @author zhouyanxiang * @create 2020-08-2020/8/1-8:59 */public class BuyStock implements Order {    private Stock abcStock;    public BuyStock(Stock abcStock){        this.abcStock = abcStock;    }    @Override    public void execute() {        abcStock.buy();    }}

• SellStock

package com.alibaba.design.commandpattern;/** * @author zhouyanxiang * @create 2020-08-2020/8/1-9:00 */public class SellStock implements Order {    private Stock abcStock;    public SellStock(Stock abcStock){        this.abcStock = abcStock;    }    @Override    public void execute() {        abcStock.sell();    }}

创建命令调用类。

package com.alibaba.design.commandpattern;import java.util.ArrayList;import java.util.List;/** * @author zhouyanxiang * @create 2020-08-2020/8/1-9:01 */public class Broker {    private List
   
     orderList = new ArrayList
    
     ();    public void takeOrder(Order order){        orderList.add(order);    }    public void placeOrders(){        for (Order order : orderList) {            order.execute();        }        orderList.clear();    }}
    
   

package com.alibaba.design.commandpattern;/** * @author zhouyanxiang * @create 2020-08-2020/8/1-9:02 */public class CommandPatternDemo {    public static void main(String[] args) {        Stock abcStock = new Stock();        BuyStock buyStockOrder = new BuyStock(abcStock);        SellStock sellStockOrder = new SellStock(abcStock);        Broker broker = new Broker();        broker.takeOrder(buyStockOrder);        broker.takeOrder(sellStockOrder);        broker.placeOrders();    }}

执行程序，输出结果：

（4）该模式在源码中的应用

典型的java.lang.Runable和线程池中都有用到命令模式

来看一下Runable的源码

Runable：任务抽象，也就是“命令”；线程池通过submit和execute调用

java.util.concurrent.ThreadPoolExecutor线程池中的源码

/**     * Executes the given task sometime in the future.  The task     * may execute in a new thread or in an existing pooled thread.     *     * If the task cannot be submitted for execution, either because this     * executor has been shutdown or because its capacity has been reached,     * the task is handled by the current {@code RejectedExecutionHandler}.     *     * @param command the task to execute     * @throws RejectedExecutionException at discretion of     *         {@code RejectedExecutionHandler}, if the task     *         cannot be accepted for execution     * @throws NullPointerException if {@code command} is null     */    public void execute(Runnable command) {
           if (command == null)            throw new NullPointerException();        /*         * Proceed in 3 steps:         *         * 1. If fewer than corePoolSize threads are running, try to         * start a new thread with the given command as its first         * task.  The call to addWorker atomically checks runState and         * workerCount, and so prevents false alarms that would add         * threads when it shouldn't, by returning false.         *         * 2. If a task can be successfully queued, then we still need         * to double-check whether we should have added a thread         * (because existing ones died since last checking) or that         * the pool shut down since entry into this method. So we         * recheck state and if necessary roll back the enqueuing if         * stopped, or start a new thread if there are none.         *         * 3. If we cannot queue task, then we try to add a new         * thread.  If it fails, we know we are shut down or saturated         * and so reject the task.         */        int c = ctl.get();        if (workerCountOf(c) < corePoolSize) {
               if (addWorker(command, true))                return;            c = ctl.get();        }        if (isRunning(c) && workQueue.offer(command)) {
               int recheck = ctl.get();            if (! isRunning(recheck) && remove(command))                reject(command);            else if (workerCountOf(recheck) == 0)                addWorker(null, false);        }        else if (!addWorker(command, false))            reject(command);    }    /**     * Initiates an orderly shutdown in which previously submitted     * tasks are executed, but no new tasks will be accepted.     * Invocation has no additional effect if already shut down.     *     * 
This method does not wait for previously submitted tasks to     * complete execution.  Use {@link #awaitTermination awaitTermination}     * to do that.     *     * @throws SecurityException {@inheritDoc}     */    public void shutdown() {
           final ReentrantLock mainLock = this.mainLock;        mainLock.lock();        try {
               checkShutdownAccess();            advanceRunState(SHUTDOWN);            interruptIdleWorkers();            onShutdown(); // hook for ScheduledThreadPoolExecutor        } finally {
               mainLock.unlock();        }        tryTerminate();    }

（5）命令模式的优缺点

• 优点：

  1、降低了系统耦合度。

  2、新的命令可以很容易添加到系统中去。

• 缺点：

  使用命令模式可能会导致某些系统有过多的具体命令类。

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