Visitor 模式心得

最近读到Visitor模式，还是一知半解的。偶然翻到Uncle Bob对该模式的推导过程，有所心得，和大家分享一下。 Uncle Bob 的链接是： http://butunclebob.com/ArticleS.UncleBob.VisitorVersusInstanceOf。个人觉得该模式用来操作复杂对象集合，特别适用于报表生成。因为报表的来源相对稳定(复杂数据集合)，但是表现形式却是千变万化。言归正传，我将该博客的内容按照自己的理解分享出来，如果有什么不对的地方，请指正。

首先有一个如下简单的场景，Bob先生的公司提供计算机方面的培训服务，对社会人士开设二门课程，一门是OOD, 一门是Java，java课程需要上机(结对编程，所以二个人用一台机器)，所以需要根据报名的时间计算具体的机器数目：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
    }

    abstract public int getComputersInUse(GregorianCalendar date);
}

public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    public int getComputersInUse(GregorianCalendar date) {
        return 0;
    }
}

public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    public int getComputersInUse(GregorianCalendar date) {
        int resources = 0;
        if (!date.before(startDate) && !date.after(endDate)) {
            resources = Math.round(students/2);
        }
        return resources;
    }
}

代码实现了，B先生很happy。不久客户说需要生一份报表，B先生想了5分钟，洋洋洒洒写下如下代码：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReportLine() {
        StringBuffer line = new StringBuffer();
        line.append(dateFormat.format(startDate.getTime())).append(" ")
                .append(courseName()).append(" ")
                .append(computersInUse()).append(" Computers\n");
        return line.toString();
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    protected abstract String courseName();
    protected abstract String computersInUse();
}

public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    @Override
    protected String courseName() {
        return "AOOD";
    }

    @Override
    protected String computersInUse() {
        return "0";
    }
}

public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    protected String courseName() {
        return "Java";
    }

    @Override
    protected String computersInUse() {
        return String.valueOf(Math.round(students/2));
    }
}

public class CourseResourceTracker<T extends Course> {

    Set<T> courses = new HashSet<>();

    public String generateReport() {
        StringBuilder reports = new StringBuilder();
        for (Iterator i = courses.iterator(); i.hasNext();) {
            T course = (T) i.next();
            reports.append(course.generateComputerReportLine()) ;
        }
        return reports.toString();
    }

    public void add(T course) {
        if (course == null) return;

        courses.add(course);
    }

    public void remove(T course) {
        if (course == null) return;

        courses.remove(course);
    }

    public static void main(String[] param) {
        CourseResourceTracker tracker = new CourseResourceTracker();
        tracker.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        tracker.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + tracker.generateReport());
    }
}

报表很简单， 打印结果如下， 格式为： 时间 + 名称 + 数量

Report is : 04/20/2015 AOOD 0 Computers
04/20/2015 Java 5 Computers

B洋洋得意，客户又来了新的需求，前面的报表太简单了，需要针对不同的课程打印不同的内容：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    abstract public int getComputersInUse(GregorianCalendar date);
}

public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    public int getComputersInUse(GregorianCalendar date) {
        return 0;
    }
}

public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    public int getComputersInUse(GregorianCalendar date) {
        int resources = 0;
        if (!date.before(startDate) && !date.after(endDate)) {
            resources = Math.round(students/2);
        }
        return resources;
    }

    public int getTotalComputers() {
        return students/2;
    }
}

public class CourseComputerReport {
    protected SimpleDateFormat dateFormat;

    public CourseComputerReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        if (courses == null || courses.size() == 0) return null;

        StringBuffer report = new StringBuffer();
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            if (course instanceof AOODCourse) {
                report.append("AOOD 0 Computers\n");
            } else if (course instanceof JavaCourse) {
                report.append("Java ");
                JavaCourse jc = (JavaCourse)course;
                report.append(String.valueOf(jc.getTotalComputers())).append(" Computers\n");
            }
        }
        return report.toString();
    }

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }
}

从上面的UML 类图可以看出，CourseComputerReport 因为对每门课程的format不一致，需要遍历的时候针对不同的课程设置不同的格式从而导致CourseReport和Course之前出现强耦合， 可以通过重构方法 generateComputerReport 使之更加合理：

public abstract class CourseReport {
    protected SimpleDateFormat dateFormat;

    public CourseReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        StringBuffer report = new StringBuffer();
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            if (course instanceof AOODCourse) {
                appendAOODLine((AOODCourse)course, report);
            } else if (course instanceof JavaCourse) {
                appendJavaLine((JavaCourse) course, report);
            }
        }
        return report.toString();
    }

    protected abstract void appendJavaLine(JavaCourse course, StringBuffer report);
    protected abstract void appendAOODLine(AOODCourse course, StringBuffer report);
}

public class CourseComputerReport extends CourseReport{

    protected void appendJavaLine(JavaCourse course, StringBuffer report) {
        report.append("Java ");
        report.append(String.valueOf(course.getTotalComputers())).append(" Computers\n");
    }

    protected void appendAOODLine(AOODCourse course, StringBuffer report) {
        report.append("AOOD 0 Computers\n");
    }

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }
}

老鸟L看了B的实现后，提出了自己的看法，为什么 CourseReport 需要依赖具体的Course， 能否将Course告诉reporter 而不是Reporter来判断具体的Course。 并将Visitor的经典类图随手抛给小B：

小B恍然大悟，修改代码如下：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    abstract void accept(CourseVisitor courseVisitor);

public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        courseVisitor.visit(this);
    }
}

public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        courseVisitor.visit(this);
    }

    public int getTotalComputers() {
        return students/2;
    }
}

public interface CourseVisitor {
    void visit(AOODCourse aoodCourse);
    void visit(JavaCourse javaCourse);
}

public class ReportCourseVisitor implements CourseVisitor {

    protected StringBuffer report;

    public ReportCourseVisitor(StringBuffer report) {
        this.report = report;

    }

    @Override
    public void visit(AOODCourse aoodCourse) {
        report.append("AOOD 0 Computers\n");
    }

    @Override
    public void visit(JavaCourse javaCourse) {
        report.append("Java ");
        report.append(String.valueOf(javaCourse.getTotalComputers())).append(" Computers\n");
    }
}

public abstract class CourseReport {

    protected SimpleDateFormat dateFormat;

    public CourseReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        StringBuffer report = new StringBuffer();
        CourseVisitor v = makeReportVisitor(report);
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            course.accept(v);
        }
        return report.toString();
    }

    protected abstract CourseVisitor makeReportVisitor(StringBuffer report);
}

public class CourseComputerReport extends CourseReport{

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }

    @Override
    protected CourseVisitor makeReportVisitor(StringBuffer report) {
        return new ReportCourseVisitor(report);
    }
}

老鸟L看了以后表示满意，同时指出如果添加新的Course，由于所有的course都依赖CourseVisitor，而courseVisitor需要添加新的接口，所有的course需要重新打包和编译。是否可以新建一个空接口CourseVisitor 使Course依赖这个接口从而达到隔离变化的目的。需要修改的是需要将courseVisitor Cast到具体的实现类，主要的代码实现是：

 @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof AOODCourseVisitor) {
            ((AOODCourseVisitor) courseVisitor).visit(this);
        }
    }

小B撸撸袖子，大笔一挥，修改代码如下：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    abstract void accept(CourseVisitor courseVisitor);
}

public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof AOODCourseVisitor) {
            ((AOODCourseVisitor) courseVisitor).visit(this);
        }
    }
}

public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof JavaCourseVisitor) {
            ((JavaCourseVisitor) courseVisitor).visit(this);
        }
    }

    public int getTotalComputers() {
        return students/2;
    }
}

public class AndroidCourse extends Course {

    private GregorianCalendar endDate;

    public AndroidCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof AndroidCourseVisitor) {
            ((AndroidCourseVisitor) courseVisitor).visit(this);
        }
    }

    public int getTotalComputers() {
        return students/2;
    }
}

public interface CourseVisitor {
}

public interface AndroidCourseVisitor {
    void visit(AndroidCourse androidCourse);
}

public interface AOODCourseVisitor {
    void visit(AOODCourse aoodCourse);
}

public interface JavaCourseVisitor {
    void visit(JavaCourse javaCourse);
}

public abstract class CourseReport {

    protected SimpleDateFormat dateFormat;

    public CourseReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        StringBuffer report = new StringBuffer();
        CourseVisitor v = makeCourseVisitor(report);
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            course.accept(v);
        }
        return report.toString();
    }

    protected abstract CourseVisitor makeCourseVisitor(StringBuffer report);
}

public class CourseComputerReport extends CourseReport {

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new AndroidCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }

    @Override
    protected CourseVisitor makeCourseVisitor(StringBuffer report) {
        return new ReportCourseVisitor(report);
    }
}

顺便说一下，上面使用的模式名称是Acyclic visitor， 是为了解决添加新的item 导致所有的item需要重新编译和打包的问题。