本文介绍访问者模式的概念和应用。

基本思想和原则

封装一些作用于某种数据结构中的各元素的操作，它可以在不改变数据结构的前提下定义作用于这些元素的新操作。

动机

如果我们要获取很多类的不同对象的信息时，可以考虑使用访问者模式。这可以将访问类对象信息这个操作和类本身分离。

实现

public abstract class Staff {
    private String name;
    private int age;
    private int salary;

    public Staff(String name, int age, int salary) {
        this.name = name;
        this.age = age;
        this.salary = salary;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return this.age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public int getSalary() {
        return this.salary;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }

    public abstract void accept(IVisitor visitor);
}

public class Empolyee extends Staff {
    private String job;

    public Empolyee(String name, int age, int salary) {
        super(name, age, salary);
    }

    public String getJob() {
        return this.job;
    }

    public void setJob(String job) {
        this.job = job;
    }

    public void accept(IVisitor visitor) {
        visitor.visit(this);
    }
}

public class Manager extends Staff {
    private int performance;

    public Manager(String name, int age, int salary) {
        super(name, age, salary);
    }

    public int getPerformance() {
        return this.performance;
    }

    public void setPerformance(int performance) {
        this.performance = performance;
    }

    public void accept(IVisitor visitor) {
        visitor.visit(this);
    }
}

public interface IVisitor {
    public void visit(Empolyee empolyee);
    public void visit(Manager manager);
}

    private String info = "";
    @Override
    public void visit(Empolyee empolyee) {
        this.info += this.getBasicInfo(empolyee) + " " + this.getEmpolyeeJob(empolyee) + "\n";
    }

    @Override
    public void visit(Manager manager) {
        this.info += this.getBasicInfo(manager) + " " + this.getManagerPerformance(manager) + "\n";
    }

    private String getBasicInfo(Staff staff) {
        return staff.getName() + " " + staff.getAge() + " " + staff.getSalary();
    }

    private String getEmpolyeeJob(Empolyee empolyee) {
        return empolyee.getJob();
    }

    private int getManagerPerformance(Manager manager) {
        return manager.getPerformance();
    }

    public void showStaffInfo() {
        System.out.println(this.info);
    }
}

public class BonusVisitor implements IVisitor {
    private final static int EMPOLYEE_COEFFICIENT = 2;
    private final static int MANAGER_COEFFICIENT = 4;
    private int totalBonus = 0;

    @Override
    public void visit(Empolyee empolyee) {
        this.totalBonus += empolyee.getSalary() * EMPOLYEE_COEFFICIENT;
    }

    @Override
    public void visit(Manager manager) {
        this.totalBonus += manager.getSalary() * MANAGER_COEFFICIENT;
    }

    public void showTotalBonus() {
        System.out.println("所有人的奖金总和是: " + this.totalBonus);
    }
}

public class Test {
    public static void main(String[] args) {
        InfoVisitor infoVisitor = new InfoVisitor();
        BonusVisitor bonusVisitor = new BonusVisitor();
        for (Staff staff:getStaffs()) {
            staff.accept(infoVisitor);
            staff.accept(bonusVisitor);
        }

        infoVisitor.showStaffInfo();
        bonusVisitor.showTotalBonus();
    }

    private static ArrayList<Staff> getStaffs() {
        ArrayList<Staff> staffs = new  ArrayList<Staff>();

        Empolyee empolyee1 = new Empolyee("Jack", 30, 8000);
        Empolyee empolyee2 = new Empolyee("Bob", 23, 4000);
        Empolyee empolyee3 = new Empolyee("Jane", 33, 9000);
        Manager manager1 = new Manager("John", 50, 20000);

        empolyee1.setJob("Sale");
        empolyee2.setJob("Engineer");
        empolyee3.setJob("Accounting");
        manager1.setPerformance(100000);

        staffs.add(empolyee1);
        staffs.add(empolyee2);
        staffs.add(empolyee3);
        staffs.add(manager1);

        return staffs;
    }
}

输出如下：

Jack 30 8000 Sale
Bob 23 4000 Engineer
Jane 33 9000 Accounting
John 50 20000 100000

所有人的奖金总和是: 122000

IVisitor声明了访问者的抽象接口，有两个具体的实现类：InfoVisitor和BonusVisitor，分别负责获取雇员的个人信息和计算奖金，这两个类的职责不同，一个打印信息，一个计算奖金，所以分为两个访问者类来实现没什么问题，实际开发中也提倡这么做。

优点

访问者模式符合单一职责原则，被访问者和访问者都可以各自独立发展，互不干扰。扩展性也不错，当需要增加新的访问需求时，只需要增加新的访问者实现类即可。

缺点

访问者模式必须了解一个具体类的细节，者违反了迪米特法则。另外有一种情况的扩展比较困难，当我在具体类中加入新的属性时，就需要修改访问者类，如果访问者类比较多，修改量会很大。访问者模式没有依赖抽象，而是依赖具体，这也违反了依赖倒置原则。