Serializing two or more Objects in Java [closed]

Question

Here I am creating a HashMap, adding it to a list, then serializing this list. Then closing the ObjectOutputStream and FileOutputStream. Then reopening the ObjectOutputStream and FileOutputStream and writing second list to it . While reading the file i am getting this error:

Exception in thread "main" java.io.StreamCorruptedException: invalid type code: AC
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1356)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
    at javaapplication27.NewMain4.main(NewMain4.java:60)
Java Result: 1

My code is this:

    HashMap<String, String> hm1 = new HashMap<String, String>(); 
    hm1.put("1", "A");
    hm1.put("2", "B");
    hm1.put("3", "C");

    ArrayList al1 = new ArrayList();
    al1.add(hm1); 

    HashMap<String, String> hm2 = new HashMap<String, String>();
    hm2.put("4", "A1");
    hm2.put("5", "B1");
    hm2.put("6", "C1");

    ArrayList al2 = new ArrayList();
    al2.add(hm2);

    FileOutputStream fos = new FileOutputStream("D:/sample.ser", true);
    ObjectOutputStream oos = new ObjectOutputStream(fos);
    oos.writeObject(al1);
    oos.close(); // closing the sreams
    fos.close();

    fos = new FileOutputStream("D:/sample.ser", true);
    oos = new ObjectOutputStream(fos);
    oos.writeObject(al2);


    oos.close();
    fos.close();


    FileInputStream fis = new FileInputStream("D:/sample.ser");
    ObjectInputStream ois = new ObjectInputStream(fis);
    ArrayList al = (ArrayList) ois.readObject();
    System.out.println("First: " + al);

    al = (ArrayList) ois.readObject(); // This is line 60 in my code
    System.out.println("Second: " + al);

1. What is the reason for the error?
2. What to do if I want to write two or more objects to a serialized file and read it back?
3. What to do if I have to serialize the objects to the same file e.g. sample.ser from different Java classes, and read all the objects back?

Answer 1

StreamCorruptedException is thrown when control information that was read from an object stream violates internal consistency checks.

I tested your code by commenting 2nd write and 2nd read; and it works fine. Also there is no problem in opening a closed stream again and then writing another object. The problem is occurring only when you try to read 2nd time. This is also evident from the StreamCorruptedException description.

Ideally its a good practice to make write/read calls as atomic in Serialization.