legacy flat files for Java

Are you a Java Developer who needs to parse old times legacy flat files? You’re bored to write each time a new parser for these files? In this case, you’ve come to the right page. Legacy4j is a small Java library that helps you load fixed length files to POJOs and saves them the other way.

You can get the legacy4j library from my GitHub at http://github.com/agrison/legacy4j.

So, let‘s get started.

Hello world

Let’s say you have this file to read data from, and hopefully you have the legacy4j library available in your classpath.

Hello     Adam    20100427
Bonjour   Robert  20110912
BuongiornoMichele 20041108
Güten tag Karl    20100315

All you would have to do, is to define a Java object that will hold your data, and describe how data fits in it.

public class Hello {
   public String greeting;
   public String name;
   public Calendar meetDate;
   public String toString() {
      return String.format("hello[%s, %s, %3$tY-%3$tm-%3$td]", greeting, name,

And then call the library and ask it gently to read the file data into your object.

public class HelloTest {
   public static void main(String[] args) {
      ILegacyFile<Hello> file = LegacyFile.fileReader("file.txt", Hello.class);
      while (file.hasNext()) {
         Hello hello = file.next();
         System.out.println("> " + hello);

That’s it, you’ve read data from your file, with no need of any parser.

Here is the output of the Java program:

> hello[Hello     , Adam    , 2010-04-27]
> hello[Bonjour   , Robert  , 2011-09-12]
> hello[Buongiorno, Michele , 2004-11-08]
> hello[Güten tag , Karl    , 2010-03-15]

Field type annotations

The library makes huge use of Java annotations, that’s how it knows how to parse the file, and cut the data where it should.

A file is made of records (lines), a Java object needs to have the @FixedLengthRecordannotation, so that the library knows how to work with it.


The @FixedLengthField annotation indicates that a field is delimited in size, and its size can be set with it.

public @interface FixedLengthField {
   /** @return the field length. */
   public int value();


The @DateField annotation indicates that a field is a Date or a Calendar object, and that the data read should be converted to the suitable destination, using a custom date format.

public @interface DateField {
   /** @return the date format. */
   public String value() default "yyyyMMdd";


The @DecimalField annotation indicates that a field is one of int|Integer, double|Double, float|Float or BigDecimal and that the data read should be converted to the suitable destination, using a custom decimal format.

public @interface DecimalField {
   /** @return the lenth of int and decimal value. */
   public int[] value();
   /** @return the separator if any (must be of length 0 or 1). */
   public String separator() default "";


The @CustomField annotation indicates that a field is of a custom type and that the data read should be converted to the suitable custom type, using a custom object mapper.

public @interface CustomField {
   public Class<?> value();

Field feature annotations


The @TrimField indicates whether a field should be trimmed and in what direction (left, right or both which is the default value).

public @interface TrimField {
   public enum Type { Both, Left, Right }
   public Type value() defaults Type.Both;


The @QuoteField indicates whether a field should be quoted and with what characters (@[]@, (), {}, '' or "" which is the default value).

public @interface QuoteField {
   public enum Type {
      Brackets("[", "]"),
      Parenthesis("(", ")"),
      Braces("{", "}"),
      Quote("\"", "\""),
      SimpleQuote("'", "'");

      public String before, after;
      Type(String before, after) { this.before = before; this.after = after; }
   public Type value() defaults Type.Quote;

Sample code

The file to be parsed

0001{Alex    }12345678,9020121215Alexandre Grison

The Item entity

@FixedLengthRecord(ignoreMatching="^#.*|^-.*|^\\s+Page.*$", ignoreEmpty=true)
public class Item {
   public int id;
   public String name;
   @DecimalField({8, 2}) // size = 8 + 2 = 10
   public BigDecimal num1;
   @DecimalField(value={7, 4}, separator=",") // size = 7 + 4 + 1 = 12
   public Double num2;
   @DateField("yyyyMMdd") // size = "yyyyMMdd".length() = 8
   public Calendar cal;
   public Person person;
   public String toString() {
      return String.format("Item [id=%s, name='%s', num1=%s, num2=%s, cal=%s, person=%s]",
            id, name, num1, num2, new SimpleDateFormat("dd/MM/yyyy").format(cal.getTime()), person
   public String format() {
      return "";

The Person entity

public class Person {
   public String firstName, surName;
   public String toString() { return String.format("p[%s, %s]", firstName, surName); }

The Person custom mapper

public class PersonMapper implements FieldMapper {
   public int fill(Object inst, Field field, String value) {
      try {
         FixedLengthField flf = field.getAnnotation(FixedLengthField.class);
         if (flf == null)
            return ;

         if (field.getType() == Person.class) {
            Person p = new Person();
            p.firstName = value.substring(, 10).trim();
            p.surName = value.substring(10).trim();
            field.set(inst, p);

         return flf.value();
      } catch (Throwable e) {
         throw new RuntimeException(e);

   public String toString(Object inst, Field field) {
      try {
         Person p = (Person)field.get(inst);
         return String.format("%-10s%-10s", p.firstName, p.surName);
      } catch (Throwable e) {
         throw new RuntimeException(e);

The whole program

public class Test {
   public static void main(String[] args) {
      FixedLengthFile<Item> file = new FixedLengthFile<Item>("positionnal.txt", Item.class);
      EngineMappers.registerMapper(Person.class, new PersonMapper());
      while (file.hasNext()) {
         Item item = file.next();
         System.out.println("> " + item);
         System.out.println("= " + RecordConverter.toString(item));
      // print the items back
      Item item = null;
      while ((item = file.readNext()) != null) {
         System.out.println("> " + item);
         System.out.println("= " + RecordConverter.toString(item));

Alexandre Grison - //grison.me - @algrison