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Thread: Error reading objects from file

  1. #1
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    Default Error reading objects from file

    Hi,
    I am trying to put objects in a file and later want to read them.
    I am using following code to add objects in file
    //for adding object in file
    public void addObject() throws IOException, ClassNotFoundException
    {
            ObjectOutputStream objs=new ObjectOutputStream(new FileOutputStream("x:\\ABCD1.dat",true));
            MyClass o1=new MyClass("abc", "abc");
            MyClass o2=new MyClass("cde", "pqr");
            objs.writeObject((Object)o1);
            objs.writeObject((Object)o2);
            objs.flush();
            objs.close();
    }

    And to read object from file i am using following code
    public void getObject() throws FileNotFoundException, IOException
    {
            ObjectInputStream obji=new ObjectInputStream(new FileInputStream("x:\\ABCD1.dat"));
            MyClass my=null;
            boolean flag=true;
            while(flag==true)
            {
                try
                {
                    my=(MyClass)obji.readObject();
                    System.out.println(""+my.str1);
                    System.out.println(""+my.str2);
                }
                catch(Exception e)
                {
                    System.out.println("Now in catch\n"+e);
                    flag=false;
                }
            }
            obji.close();
    }

    This works fine if i add two object on calling a addObject() function.
    But if i modify my addObject() function as shown below
    public void addObject() throws IOException, ClassNotFoundException
     {
            //adding object inside file
            ObjectOutputStream objs=new ObjectOutputStream(new FileOutputStream("x:\\ABCD1.dat",true));
            MyClass o1=new MyClass("abc", "abc");
            objs.writeObject((Object)o1);
            objs.flush();
            objs.close();
    }
    Now when i call this function twice from my main() function it adds two object in file(when i viewed file externally i can see two objects there).
    But when i call getObject() function it reads only one object from file and on trying to read second object it gives following error.
    java.io.StreamCorruptedException: invalid type code: AC
    No matter how many object i insert in a file it reads only first object and then gives above error.
    I am unable to understand what is going wrong in this please help.

    Thanks in advance.


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    Default Re: Error reading objects from file

    Can you post runnable code that demonstrates the error?

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    Default Re: Error reading objects from file

    Quote Originally Posted by prashant.6388 View Post
    ObjectOutputStream objs=new ObjectOutputStream(new FileOutputStream("x:\\ABCD1.dat",true));

    java.io.StreamCorruptedException: invalid type code: AC
    Sorry, there's an important quibble with object serialization: it doesn't work in "append" mode.
    The object serialization uses a well defined byte-level "protocol" and when you create an ObjectOutputStream, the first thing written is a special "header" that marks the stream as specific for serialization.
    If you "append" more times, the file will contain for example:

    [header] [objects......] [header] [objects......]

    When you read and arrive at the second header, the deserialization logic doesn't expect it ..... and fails, supposing that the stream is corrupted.


    Solution: rethink your application, you have to write and read all objects every time.
    Andrea, www.andbin.net SCJP 5 (91%) SCWCD 5 (94%)

    Useful links for Java beginners My new project Java Examples on Google Code

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    Default Re: Error reading objects from file

    Quote Originally Posted by GregBrannon View Post
    Can you post runnable code that demonstrates the error?
    First and second code snippet i have given for inserting and reading object works fine.
    I am using following code to run it.
    public static void main(String args[]) throws IOException, ClassNotFoundException
        {
            FileHandlingObject_Trial1 o1=new FileHandlingObject_Trial1();
            o1.addObject();
            o1.getObject();
        }

    Problem occurs when i modify my addObject() method as shown in second code snippet. getObject() method is same but it reads only one object this time.(in previous case it reads two objects correctly).
    For second time i am using following code to run it.
    public static void main(String args[]) throws IOException, ClassNotFoundException
    {
    FileHandlingObject_Trial1 o1=new FileHandlingObject_Trial1();
    o1.addObject();
    o1.addObject();

    o1.getObject();

    }

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    Default Re: Error reading objects from file

    I believe andbin answered your question.

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    Default Re: Error reading objects from file

    I can add an interesting thing. The main problem is given by that header. It's written or read/verified somewhere by the code into ObjectOutputStream / ObjectInputStream but fortunately this piece of code is well encapsulated in an appropriate method.

    ObjectOutputStream has a protected void writeStreamHeader()
    while
    ObjectInputStream has a protected void readStreamHeader()

    The write/read/check of the header is only here in these methods.
    Since these methods are protected and the classes non-final, technically it's perfectly possible to extend the classes and override these methods just to execute "nothing".

    If you want to choose this solution ... it's absolutely up to you. But beware that the stream will be a "non-standard" stream that, in general, doesn't comply with the object serialization protocol.
    Andrea, www.andbin.net SCJP 5 (91%) SCWCD 5 (94%)

    Useful links for Java beginners My new project Java Examples on Google Code

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    Talking Re: Error reading objects from file

    Hey guys thanks for your answers.
    What andbin said is correct each time it adds header before adding object.
    But i figured out that this happens because of following line of code.
    ObjectOutputStream objs=new ObjectOutputStream(new FileOutputStream("x:\\ABCD1.dat",true));

    if i remove this line from addObject() function and place it in void main i can add as many objects as i want.
    It reads object correct with same getObjectFunction().
    I am posting modified code.
    public static void main(String args[]) throws IOException, ClassNotFoundException
        {
            FileHandlingObject_Trial1 o1=new FileHandlingObject_Trial1();
            o1.objs=null;
            [COLOR="#FF0000"]o1.objs=new ObjectOutputStream(new FileOutputStream("x:\\ABCD1.dat",true));[/COLOR]
            o1.addObject();
            o1.addObject();
            o1.getObject();
     
        }
        public void addObject() throws IOException, ClassNotFoundException
        {
            //adding object inside file
    //        objs=new ObjectOutputStream(new FileOutputStream("x:\\ABCD1.dat"));
            MyClass o1=new MyClass("abc", "abc");
            objs.writeObject((Object)o1);
            objs.flush();
    //        objs.close();

    getObject function is same as stated previously.


    Thanks a lot for your help guys.

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    Default Re: Error reading objects from file

    Quote Originally Posted by prashant.6388 View Post
    But i figured out that this happens because of following line of code.
    ObjectOutputStream objs=new ObjectOutputStream(new FileOutputStream("x:\\ABCD1.dat",true));

    if i remove this line from addObject() function and place it in void main i can add as many objects as i want.
    But you have not solved much more .... you are still opening the file in "append" mode. If you run the app 2 times, you corrupt the stream as I have described.
    Andrea, www.andbin.net SCJP 5 (91%) SCWCD 5 (94%)

    Useful links for Java beginners My new project Java Examples on Google Code

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    Default Re: Error reading objects from file

    Quote Originally Posted by andbin View Post
    But you have not solved much more .... you are still opening the file in "append" mode. If you run the app 2 times, you corrupt the stream as I have described.
    I am completely agree with you. Opening file in append more corrupts file.
    I tried it.
    Thanks for your suggestions.

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