How to insert NULL in mysql especially INT dataType

You should use a PreparedStatement and use setNull(int, int):

String sql = "INSERT INTO temp(val) VALUES (?)";
PreparedStatement st = con.prepareStatement(sql);
if (/* int value is not null */) {
   st.setInt(1, value);
} else {
   set.setNull(1, Types.INTEGER);
}
count  = st.executeUpdate();

You should consider using prepared statements. If you do, you'll find information on how to deal with nulls here and elsewhere.

If you're 100% sure your val value is clean and won't cause SQL Injection (rare, but possible), then the "built string" approach needs to explicitly use null when defining the value:

String sql = "INSERT INTO temp (val) VALUES (";
if (val == null) {
  sql += "null";
} else {
  sql += val;
}
sql += ")";

As an alternative to Mark Rotteveel's answer, you can also use PreparedStatement.setObject(int, Object, int).

You specify the SQL Type (3rd Parameter), and if the object is null, it inserts a null automatically. It's faster and easier.

String sql = "INSERT INTO temp(val) VALUES (?)";
PreparedStatement st = con.prepareStatement(sql);
st.setObject(1, value, Types.INTEGER);
count  = st.executeUpdate();