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。