Trouble reading objects from binary file in Java

Question

I'm trying to read the serialized data from a .dat file into its corresponding java class file. My .readObject() method keeps throwing ClassNotFoundException though. Why cant it read the data from the file? I'm trying to read it into a student class who's constructor accepts objects of its type then assigns the values of that object's variables to its variables.

public class StudentTest 
{
    private static ObjectInputStream OIS;
    public static void main(String[] args)
    {

        openFile();
        readFile();
        closeFile();
    }

    public static void openFile()
    {
        try
        {
            OIS=new ObjectInputStream(Files.newInputStream(Paths.get("C:\\Users\\Joey\\Desktop\\students.dat")));
        }
        catch(IOException e)
        {
            System.out.println("Error trying to read file. TERMINATING.");
            System.exit(1);
        }
    }

    public static void readFile()
    {
        try
        {
            while(true)
            {
                Student student=new Student((Student)OIS.readObject());
                System.out.printf("%s", student.toString());
            }
        }
        catch(ClassNotFoundException e)
        {
            System.out.println("Class not found. Terminating.");
            System.exit(1);
        }
        catch(EOFException e)
        {
            System.out.println("End of file reached.");
        }
        catch(IOException e)
        {
            System.out.println("Error reading from file. TERMINATING.");
            System.exit(1);
        }
    }

    public static void closeFile()
    {
        try
        {
            if(OIS!=null)
                OIS.close();
        }
        catch(IOException e)
        {
            System.out.println("IOEXCEPTION.");
            System.exit(1);
        }
    }
}

Here's the stack trace:

java.lang.ClassNotFoundException: files.Student
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at java.io.ObjectInputStream.resolveClass(Unknown Source)
at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at studentfiles.StudentTest.readFile(StudentTest.java:40)
at studentfiles.StudentTest.main(StudentTest.java:16)

Did you serialize `Student` into `students.dat` file using the same `Student` class, including the package name? Please post your exception stack trace. — Sergey Kalinichenko, Sep 17 '15 at 00:17
I downloaded the .dat file from my teacher and I created my own package to hold the Student.java file(That I also downloaded from my teacher) and this file. — ComicalGamer, Sep 17 '15 at 00:48
StudentTest [Java Application] studentfiles.StudentTest at localhost:52452 C:\Program Files\Java\jre1.8.0_31\bin\javaw.exe (Sep 16, 2015, 7:40:59 PM) Is this the exception stack trace? — ComicalGamer, Sep 17 '15 at 00:50
The problem is that `Student` from serialized file must match the `Student` compiled into your program, including the package name. It appears that there is a mismatch between what's in serialized file and in your program. — Sergey Kalinichenko, Sep 17 '15 at 00:51
Stack trace is something you need to print. Add `e.printStackTrace(System.out);` before the line that prints "Class not found. Terminating." — Sergey Kalinichenko, Sep 17 '15 at 00:53
`at java.lang.Class.forName(Unknown Source) at java.io.ObjectInputStream.resolveClass(Unknown Source) at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source) at java.io.ObjectInputStream.readClassDesc(Unknown Source) at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source) at java.io.ObjectInputStream.readObject0(Unknown Source) at java.io.ObjectInputStream.readObject(Unknown Source) at studentfiles.StudentTest.readFile(StudentTest.java:40) at studentfiles.StudentTest.main(StudentTest.java:16)` OK I got the stack trace, is this because of the package? — ComicalGamer, Sep 17 '15 at 01:00
@ComicalGamer The stack trace should always include the actual message that appears at the top of it. — user207421, Sep 17 '15 at 01:23

score 1 · Accepted Answer · answered Sep 17 '15 at 00:53

1

Your Student class is in a different package from what is expected. If you print the exception message that came with the ClassNotFoundException you will see what the expected package was.

And an error creating an ObjectInputStream or FileInputStream is not necessarily an 'error reading from file'. Don't make up your own error messages. Use the one that comes with the exception. Even when you have the right message, as with ClassNotFoundException, you're missing the actual class name that wasn't found, which was in the exception message. Don't do this.

answered Sep 17 '15 at 00:53

user207421

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Thanks your right I shouldn't use my own error messages. Especially when I'm just making up nonsense lol – ComicalGamer Sep 17 '15 at 01:29
So at the top of the stack trace it says "files.Student". Is this where it thinks my Student class is? In my project I have my Student class in the "studentfiles" package. – ComicalGamer Sep 17 '15 at 01:41
Correct, it should be in `files.` – user207421 Sep 17 '15 at 01:46

Trouble reading objects from binary file in Java

1 Answers1