i/o from jsp to a file (via creating an object)

**Panoksa** · September 26th, 2012, 12:38 PM

Hello everyone,

I created 3 JSP pages: 1 - login, 2-displays 4 checkboxes with a submit, 3-displays the info submitted and also creates a file to store this information. I have a separate StudentElective.java where I created an object.
page.3 jsp looks like this:

<%  String knum = (String) session.getAttribute("knum");
    String mycourse = (String) session.getAttribute("course");
    String[] id = request.getParameterValues("id");%>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>Page 3</title>
    </head>
    <body>
        <h3>Thank you, <%=knum%>!</h3>
        You have selected
        <%
            int i;
            for (i = 0; i < id.length; i++) {
                out.print(id[i]);
                if (i == 0) {
                    out.print(" and ");
                }
            }
 
        %>!
        <br>
        Your application is now registered!
 
        <%  StudentElective se = new StudentElective();
            se.setKnum(knum);
            se.setCourse(mycourse);
            se.setElchoice(id);
 
 
 
            try {
                File DATAFILE = new File("c:/Users/k000000/Documents/studentData.dat");
                if (!DATAFILE.exists()) {DATAFILE.createNewFile();}
 
                FileOutputStream DATA = new FileOutputStream(DATAFILE);
                ObjectOutputStream SAVE = new ObjectOutputStream(DATA);
 
                SAVE.writeObject(se);
            } catch (Exception X) {
                System.out.print("\n\tFile Access Error. Could not SAVE.");
            }
 
        %>
    </body>
</html>

My object file:

public class StudentElective implements java.io.Serializable {
 
    private String knum,
            course;
    private String[] elchoice;
 
    public StudentElective() {
    }
 
    public StudentElective(String knum, String course, String[] elchoice) {
        this.knum = knum;
        this.course = course;
        this.elchoice = elchoice;
    }
 
    public String getCourse() {
        return course;
    }
 
    public String[] getElchoice() {
        return elchoice;
    }
 
    public String getKnum() {
        return knum;
    }
 
    public void setCourse(String course) {
        this.course = course;
    }
 
    public void setElchoice(String[] elchoice) {
        this.elchoice = elchoice;
    }
 
    public void setKnum(String knum) {
        this.knum = knum;
    }
       public void print() {
        System.out.print(this.knum);
        System.out.print(this.course);
        System.out.print(this.elchoice);
    }
 
}

I have 2 questions:
1. At the moment everytime I write new information into a file, my old info is gone.How can I modify my code so everytime I write a new info into a file it will be saved onto a new line. In other words, how can I open my file for multiple entries everytime new instance of object gets created?
2.I made an error somewhere when I was trying to pass selected checkboxes to an array in my object. Everytime I read the info from file created it shows something like

[Ljava.lang.String;@5456a499

instead of the names of my checkboxes.Where I made this mistake and how to fix it?

Thank you in advance for any help possible.

**Zaphod_b** · September 26th, 2012, 02:19 PM

Originally Posted by Panoksa

...
1. At the moment everytime I write new information into a file, my old info is gone.How can I modify my code so everytime I write a new info into a file it will be saved onto a new line.

Here's the thing: ObjectOutputStream objects are not designed to be appendable. Reference the ObjectOutputStream API

I mean, you can open the underlying FileOutputStream in appendable mode, but when you create an ObjectOutputStream object the constructor immediately writes a new Serializable header to the stream, which results in a corrupted stream at that point (the end of the old file) when you append things and then, later, try to read in the new stuff.

At least that what I observed. (I'm new to Java, and I would welcome any enlightenment here. It wouldn't be the first time I overlooked something that is "easy and obvious" to anyone with more than four months experience.)

I keep thinking that they could have made the ObjectInputStream readObject() "smarter," to get past this, but maybe that wasn't a good idea either. Maybe there is a Good Reason why things are as they are.

Anyhow...

Maybe you can read all of the previous stuff from the file and then create a brand new file and ObjectOutputStream and write the old stuff and then the new stuff. You can write as many objects as you want in a single session with a single new file, right? (By packaging the objects in a collection, I'm thinking. Maybe an ArrayList or some such thing?)

There is some discussion about a way to create appendable ObjectOutputStream objects is here: http://stackoverflow.com/questions/1...38141#12438141 I haven't tried it (because, for one thing, I couldn't think of a bullet-proof way of knowing how much stuff to read), but those guys always seem to be really on top of things.

Originally Posted by Panoksa

2....Everytime I read the info from file created it shows something like

System.out.println(se.getElChoice());
.
.
[Ljava.lang.String;@5456a499

Your object's elchoice is an array of Strings. If you use something like System.out.println(arrayName) for any kind of array, you get the name of the array and a hashcode. To print the contents of an array in a single print statement, you can use the Arrays static toString() method:

System.out.println(Arrays.toString(se.getElChoice()));

(You might want to change the print() method in your StudentElective class to something like this also.)

Cheers!

Z

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