Question

我是JAXB的新手，所以请在评论中保持宽容！无论如何，我每次运行程序时都会覆盖现有xml文件的问题，这不是我想要的。假设添加到现有的xml上。请帮忙！

CoursesApp.java：

package Courses;

import java.io.File;
import java.util.ArrayList;
import java.util.Scanner;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class CoursesApp {
    public static void main(String[] args) {
        int choice;
        String courseCode = "", professorName = "", groupIndex = "", classType = "";
        Scanner sc = new Scanner(System.in);

        try {

            File file = new File("C:\\Courselist.xml");
            JAXBContext jaxbContext = JAXBContext.newInstance(Courselist.class);

            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
            Courselist courselist = (Courselist) jaxbUnmarshaller.unmarshal(file);
            System.out.println(courselist.course.get(0).getclassType());

          } catch (JAXBException e) {
            e.printStackTrace();
          }

        Courselist courselist = new Courselist();
        Course course = new Course();

        do{ 
            System.out.println("(1) Add a student.\n" +
                               "(2) Add a course.\n" +
                               "(3) Exit.\n");
            System.out.print("Enter the number of your choice: ");
            choice = sc.nextInt();
            sc.nextLine();
            switch (choice) {
            case 1: 
                    break;
            case 2:
                System.out.println("Please enter course code:");
                courseCode = sc.nextLine();
                System.out.println("Please enter class type:");
                classType = sc.nextLine();
                System.out.println("Please enter group index:");
                groupIndex = sc.nextLine();
                System.out.println("Please enter professor name:");
                professorName = sc.nextLine();
                course.setcourseCode(courseCode);
                course.setclassType(classType);
                course.setgroupIndex(groupIndex);
                course.setprofessor(professorName);
                courselist.course.add(course);
                try{
                    File file = new File("C:\\Courselist.xml");
                    JAXBContext jaxbContext = JAXBContext.newInstance(Courselist.class);
                    Marshaller jaxbMarshaller = jaxbContext.createMarshaller();

                    jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

                    jaxbMarshaller.marshal(courselist, file);
                    jaxbMarshaller.marshal(courselist, System.out);

                }catch(JAXBException e)
                {
                    e.printStackTrace();
                }
                    break;
            case 3: //Modify course
                    break;
            default: System.out.println("Please enter a number between 1-3.\n");
                    break;
            }
        } while (choice!=3);
    }
}

Courselist.xml：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<courselist>
    <course>
        <classType>lect</classType>
        <courseCode>2002</courseCode>
        <groupIndex>12</groupIndex>
        <professor>james</professor>
    </course>
</courselist>

如果我添加另一个课程，请

到下面的那个：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<courselist>
    <course>
        <classType>lect</classType>
        <courseCode>2002</courseCode>
        <groupIndex>12</groupIndex>
        <professor>james</professor>
    </course>
    <course>
        <classType>Lab</classType>
        <courseCode>2001</courseCode>
        <groupIndex>1</groupIndex>
        <professor>john</professor>
    </course>
</courselist>

Answer 1

以为我已经回答了这个问题，但答案消失了......

首先不要这样做：

      } catch (JAXBException e) {
        e.printStackTrace();
      }

您正在捕获异常，将文本转储到stdout，然后继续，好像什么也没发生。这是IDE添加的无用样板文件，不执行。

由于您要根据文件是否已存在来调整您的行为，我建议将if块与File.exists一起使用。

Answer 2

    import java.io.File;
    import java.io.FileWriter;
    import java.io.IOException;
    import java.util.ArrayList;
    import java.util.Scanner;

    import java.util.logging.Level;
    import java.util.logging.Logger;
    import javax.xml.bind.JAXBContext;
    import javax.xml.bind.JAXBException;
    import javax.xml.bind.Marshaller;
    import javax.xml.bind.Unmarshaller;

    public class CoursesApp {

        int choice;
        String courseCode = "", professorName = "", groupIndex = "", classType = "";
        private boolean exists;
        Courselist courselist = new Courselist();
        Course course = new Course();

        public static void main(String[] args) {
            Scanner sc = new Scanner(System.in);
            Courselist courselist = new Courselist();
            Course course = new Course();
            try {

                File file = new File("C:\\Courselist.xml");
                JAXBContext jaxbContext = JAXBContext.newInstance(Courselist.class);

                Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
                courselist = (Courselist) jaxbUnmarshaller.unmarshal(file);
                System.out.println(courselist);

            } catch (JAXBException e) {
                e.printStackTrace();
            }
            //* OVER HERE *//
            CoursesApp ca = new CoursesApp();
            ca.process(0);
        }

        private void process(int choice) {

            //* PRINT MENU *//
            System.out.println("(1) Add a student.\n"
                    + "(2) Add a course.\n"
                    + "(3) Exit.\n");
            System.out.println("Enter the number of your choice: ");
            Scanner sc = new Scanner(System.in);
            choice = sc.nextInt();
            sc.nextLine();
            switch (choice) {
                case 1:
                    //ADD STUDENTS
                    break;
                case 2:
                    File file = new File("C:\\Course.xml");
                    if (file.exists()) {
                        choice = 4;
                        exists = true;
                    } else {
                        System.out.println("File doesnot exist press 4 to create or 3 to exit: ");
                        choice = sc.nextInt();
                        sc.nextLine();
                    }
                    /*ENTER COURSE DETAILS*/
                    if (choice == 4) {
                        System.out.println("Please enter course code:");
                        courseCode = sc.nextLine();
                        System.out.println("Please enter class type:");
                        classType = sc.nextLine();
                        System.out.println("Please enter group index:");
                        groupIndex = sc.nextLine();
                        System.out.println("Please enter professor name:");
                        professorName = sc.nextLine();
                        course.setcourseCode(courseCode);
                        course.setclassType(classType);
                        course.setgroupIndex(groupIndex);
                        course.setprofessor(professorName);
                        courselist.course.add(course);
                        System.out.println("Are there more records?\n Press 2(yes) or 3(n): ");
                        choice = sc.nextInt();
                        sc.nextLine();
                        if (choice == 3) {
                            /*END OF RECORDS SO GO AHEAD AND UNMARSHAL*/
                            unmarshal(file, courselist);
                        } else {
                            /*ADD MORE COURSE ITEMS*/
                            process(2);
                        }
                    } else {
                        /*FILE NOT EXIST AND USER SELECTED EXIT*/
                        System.exit(0);
                        break;
                    }
                    break;
                case 3:
                    System.exit(0);
                    break;
            }
        }

        private void unmarshal(File file, Courselist courselist) {
            {
                FileWriter fw = null;
                try {
                    JAXBContext jaxbContext =    JAXBContext.newInstance(Courselist.class);
                    Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
                    jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
                    StringBuffer str = new StringBuffer();
                    /*EXISTING FILE SO APPEND*/
                    if (exists) {
                        fw = new FileWriter(file, true);
                        jaxbMarshaller.marshal(courselist, fw);
                    } 
                    /*CREATE NEW FILE*/
                     else {
                        jaxbMarshaller.marshal(courselist, file);
                    }
                    jaxbMarshaller.marshal(courselist, System.out);
                } catch (IOException ex) {
                    Logger.getLogger(CoursesApp.class.getName()).log(Level.SEVERE, null, ex);
                } catch (JAXBException e) {
                    e.printStackTrace();
                } finally {
                    try {
                        fw.close();
                    } catch (IOException ex) {
                        Logger.getLogger(CoursesApp.class.getName()).log(Level.SEVERE, null, ex);
                    }
                }
            }
            process(0);//call and print options
        }
    }

这将解决覆盖问题，但您需要一种方法来防止重复xml声明。希望这有帮助

我基本上已经采用了提问者发布的代码并提取了一些代码来分离方法以便更好地理解。我还添加了检查以查看文件是否存在并附加到文件而不是覆盖内容。

Process（）方法用于用户输入并且需要执行操作。在unmarshal（）中，用户输入绑定到xml文件。

Answer 3

您需要使用JAXB 2.x Binder 抽象。 Detailed tutorial

Java / Jaxb帮助不要覆盖现有文件

3 个答案: