Appending to an Objectoutputstream

Appending to an ObjectOutputStream

Here's the trick: subclass ObjectOutputStream and override the writeStreamHeader method:

public class AppendingObjectOutputStream extends ObjectOutputStream {

  public AppendingObjectOutputStream(OutputStream out) throws IOException {
    super(out);
  }

  @Override
  protected void writeStreamHeader() throws IOException {
    // do not write a header, but reset:
    // this line added after another question
    // showed a problem with the original
    reset();
  }

}

To use it, just check whether the history file exists or not and instantiate either this appendable stream (in case the file exists = we append = we don't want a header) or the original stream (in case the file does not exist = we need a header).

Edit

I wasn't happy with the first naming of the class. This one's better: it describes the 'what it's for' rather then the 'how it's done'

Edit

Changed the name once more, to clarify, that this stream is only for appending to an existing file. It can't be used to create a new file with object data.

Edit

Added a call to reset() after this question showed that the original version that just overrode writeStreamHeader to be a no-op could under some conditions create a stream that couldn't be read.

Appending to ObjectOutputStream (writing multiple objects w/o closing stream)

Calling ObjectOutputStream.reset() after writing each object will fix this.

How can I append to an existing java.io.ObjectStream?

It is actually pretty easy to do. When you are adding to an existing stream you need to use a subclass of ObjectOutStream that overrides writeStreamHeader so that a second header is not written in the middle of the file. For example

class NoHeaderObjectOutputStream extends ObjectOutputStream {
  public NoHeaderObjectOutputStream(OutputStream os) {
    super(os);
  }
  protected void writeStreamHeader() {}
}

Then just use a standard ObjectInputStream to read the whole file.

Writing objects to file while appending

I think that the topic I created (It is solved now) is what you are looking for.

"You can't append to an existing file created with an ObjectOutputStream, at least not without effort. There is a trick somewhere about extending ObjectOutputStream and overriding the `writeStreamHeader() method so as not to write the stream header the second time (...)" - answered by EJP

Please have look for my topic here:

• Write and read multiple objects to file.

You can also check the link below

• Appending to an ObjectOutputStream

It is what you were looking for?

Append to ObjectOutputStream iteratively

Writing the two HYB objects to the ObjectOutputStream doesn't merge them into a single HYB object; the ObjectOutputStream still contains two HYB object, of which your code reads one. If you did a second call to readObject(), the second one would be retrieved and could be printed to the screen. So you could just wrap the readObject() and println() calls in a loop that reads/writes until there's nothing else to read from the stream.

Appending to a Serialization file (Java)

You are reading the same Object twice for each iteration

while ((in.readObject()) != null) {    // once
    TestClass testRead = (TestClass) in.readObject();    // twice

You can change your code to

 Object obj;
 while ((obj = in.readObject()) != null) {
       TestClass testRead = (TestClass) obj;
       System.out.println(testRead.username);
       System.out.println(testRead.age);
       System.out.println(testRead.address);
       System.out.println(testRead.phone);
 }

Note

To get it to work I changed to

  FileOutputStream f = new FileOutputStream("C:\\temp\\test.ser", true);

  ObjectOutputStream out2 = new ObjectOutputStream(f)
  {
            @Override 
            protected void writeStreamHeader() throws IOException {  
                   System.out.println("I am called");  
                   reset();
            }
  }
  ;
        
  out2.writeObject(test);
  out2.writeObject(test2);
  out2.close();

---

Related Topics