Convert ArrayList to byte array

Question

I have a code where I am converting array list to byte array and then saving that byte array as a BLOB in MySQL database. Below is code:-

Object temp = attributes.get(columnName);
if (temp instanceof List && temp != null) {
    List extraAttributes = (ArrayList) temp;
    resultStmt.setBytes(currentIndex, createByteArray(extraAttributes));    

The method createByteArray is defined as below:

 private byte [] createByteArray( Object obj)
    {
        byte [] bArray = null;
        try
        {
                ByteArrayOutputStream baos = new ByteArrayOutputStream();
                ObjectOutputStream objOstream = new ObjectOutputStream(baos);
                objOstream.writeObject(obj);
                bArray = baos.toByteArray();
        }
        catch (Exception e)
        {
      TraceDbLog.writeError("Problem in createByteArray", e);

        }

                return bArray;

    }

Well the above code was written earlier for writing HashMap to BLOB i am using same for converting ArrayList if HashMap to BLOB.

The problem which is occurring in read code when i am reading the blob.

 private Object readBytes (ResultSet rs, String columnName)
    throws SQLException
    {
        ObjectInputStream ois  = null;
        byte [] newArray;
        Object obj = null;

        try
        {
            newArray = rs.getBytes(columnName);

            ois = new ObjectInputStream (new ByteArrayInputStream(newArray));
            obj = ois.readObject ();
        }

In the read part the object is not coming as arrayList of hasMap and in debug perspective in eclipse eclipse is also not able to inspect the object which is coming.

I have also tried typecasting the object to List but still no success in getting the right response. Please tell me whether there is any flaw in reading/writing the above BLOB.

Answer 1

I have added sample coding for convert ArrayList to byte[].

One reasonable way would be to use UTF-8 encoding like DataOutputStream does for each string in the list. For a string it writes 2 bytes for the length of the UTF-8 encoding followed by the UTF-8 bytes.

This would be portable if you're not using Java on the other end. Here's an example of encoding and decoding an ArrayList:

// example input list
List<String> list = new ArrayList<String>();
list.add("foo");
list.add("bar");
list.add("baz");

// write to byte array
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream out = new DataOutputStream(baos);

for (String element : list) {
    out.writeUTF(element);
}
byte[] bytes = baos.toByteArray();

// read from byte array
ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
DataInputStream in = new DataInputStream(bais);
while (in.available() > 0) {
    String element = in.readUTF();
    System.out.println(element);
}