设计模式之命令模式

作用

将一个请求封装为一个对象，从而使你可以用不同的请求对客户进行参数化，对请求排队和记录请求日志，以及支持可撤销的操作。

原理

Invoker：调用者角色。

Command：命令角色，需要执行的所有命令都在这里，可以是接口或抽象类。

Receiver：接受者角色，知道如何实施和执行一个请求相关的操作。

ConcreteCommand：将一个接受者对象与一个动作绑定，调用接受者相应的操作，实现 execute。

案例

Invoker调用者

public class RemoteController {

    private Command[] onCommands;
    private Command[] offCommands;
    private Command undoCommand;

    public RemoteController() {
        onCommands = new Command[5];
        offCommands = new Command[5];
        for (int i = 0; i < 5; i++) {
            onCommands[i] = new NoCommand();
            offCommands[i] = new NoCommand();
        }
    }

    public void setCommand(int index, Command onCommand, Command offCommand) {
        this.onCommands[index] = onCommand;
        this.offCommands[index] = offCommand;
    }

    public void onButtonPushed(int index) {
        onCommands[index].execute();
        undoCommand = onCommands[index];
    }

    public void offButtonPushed(int index) {
        offCommands[index].execute();
        undoCommand = offCommands[index];
    }

    public void undoButtonPushed() {
        undoCommand.undo();
    }
}

Command命令

public interface Command {

    void execute();

    void undo();

}

Receiver

public class LightReceiver {

    public void on() {
        System.out.println("电灯打开了");
    }

    public void off() {
        System.out.println("电灯关闭了");
    }

}

ConcreteCommand

public class LightOnCommand implements Command {

    private LightReceiver lightReceiver;

    public LightOnCommand(LightReceiver lightReceiver) {
        this.lightReceiver = lightReceiver;
    }

    @Override
    public void execute() {
        lightReceiver.on();
    }

    @Override
    public void undo() {
        lightReceiver.off();
    }
}

public class LightOffCommand implements Command {

    private LightReceiver lightReceiver;

    public LightOffCommand(LightReceiver lightReceiver) {
        this.lightReceiver = lightReceiver;
    }

    @Override
    public void execute() {
        lightReceiver.off();
    }

    @Override
    public void undo() {
        lightReceiver.on();
    }
}

public class NoCommand implements Command {
    @Override
    public void execute() {

    }

    @Override
    public void undo() {

    }
}

调用

public class Client {

    public static void main(String[] args) {
        LightReceiver lightReceiver = new LightReceiver();
        LightOnCommand lightOnCommand = new LightOnCommand(lightReceiver);

        LightOffCommand lightOffCommand = new LightOffCommand(lightReceiver);
        RemoteController remoteController = new RemoteController();
        remoteController.setCommand(0, lightOnCommand, lightOffCommand);

        remoteController.onButtonPushed(0);
        remoteController.undoButtonPushed();
        System.out.println("--------------------");
        remoteController.offButtonPushed(0);
        remoteController.undoButtonPushed();
    }

}

源码

org.springframework.jdbc.core.JdbcTemplate

Invoker：org.springframework.jdbc.core.JdbcTemplate#execute(org.springframework.jdbc.core.StatementCallback)

Command：org.springframework.jdbc.core.StatementCallback

ConcreteCommand：org.springframework.jdbc.core.JdbcTemplate#query(java.lang.String, org.springframework.jdbc.core.ResultSetExtractor)下的内部类QueryStatementCallback