读取二进制文件直到结束

Question

I have a series of different object serialized into a binary file.How can I read the file until the end?

    try {
        ObjectInputStream reader = new ObjectInputStream(new FileInputStream(fname));

        Object obj = reader.readObject(); 

        if (obj instanceof Azienda) {
            Azienda a = (Azienda) obj;
            company.put(a.getCod(), a);
        } else if (obj instanceof Privato) {
            Privato p = (Privato) obj;
            privato.put(p.getCod(), p);
        }
    } catch (ClassNotFoundException cnfe) {
        cnfe.printStackTrace();
    } catch (FileNotFoundException ffe) {
        System.err.println("Error: the file was not found!");
    } catch (IOException ioe) {
        ioe.printStackTrace();
    }

in this way I read only once object per read.

when I read a text file I use null

Answer 1

try{
    while(true) {
       Object obj=reader.readObject()
     // do sth with object
    }
}catch(EOFException e){
//we expect it so ignore
}

there is no EOF check besides the exception when you read past it for ObjectInputStream so you'll have to use the code smell called exceptions for control flow

Answer 2

    ObjectInputStream reader = null;

    try {
        reader = new ObjectInputStream(new FileInputStream("sth"));

        Object obj = null;
        while ((obj = reader.readObject()) != null) {
            System.out.println(obj);
        }


    } catch (EOFException e) {
        System.out.println("finnished reading");
    } catch (ClassNotFoundException ex) {
        ex.printStackTrace();
    } catch (FileNotFoundException ex) {
        System.err.println("Error: the file was not found!");
    } catch (IOException ex) {
        ex.printStackTrace();
    } finally {
        reader.close();
    }

Answer 3

It seems that an EOFException is thrown when there is no more object to read from the stream. Unfortunately, it's not even documented. So, I see the following solutions:

• you read in a loop until you get this exception,
• you make it so that you know in advance the number of objects in the stream,
• you make it so that there is a marker object which marks the last object of the stream,
• you serialize (and unserialize) a unique object: a List<Object> containing all the objects. This last solution, of course prevents writing the objects on the fly to the stream, and forces you to have all the objects in memory before serializing them.

Answer 4

ObjectInputStream does not have a concrete method for checking for end-of-file.

But every read...() method of ObjectInputStream throws an EOFException when it tries to read past the end of file. Unfortunately this is not explicitly documented for readObject() , but it is for all the other methods ( readInt() etc.)

http://docs.oracle.com/javase/7/docs/api/java/io/ObjectInputStream.html