Converter Tutorial

# Simple Converter

## Setting up a simple example

This is the most basic converter... let's start with a simple Person:

    package com.thoughtworks.xstream.examples;

    public class Person {

            private String name;

            public String getName() {
                    return name;
            }

            public void setName(String name) {
                    this.name = name;
            }

    }

So let's create a person and convert it to XML...

    package com.thoughtworks.xstream.examples;

    import com.thoughtworks.xstream.XStream;
    import com.thoughtworks.xstream.io.xml.DomDriver;

    public class PersonTest {

            public static void main(String[] args) {
                    Person person = new Person();
                    person.setName("Guilherme");

                    XStream xStream = new XStream(new DomDriver());
                    System.out.println(xStream.toXML(person));
            }

    }

This results in a really ugly XML code which contains the full class name (including package)...

    <com.thoughtworks.xstream.examples.Person>
      <name>Guilherme</name>
    </com.thoughtworks.xstream.examples.Person>

So we make use of an 'alias' to change this full class name to something more 'human', for example 'person'.

    XStream xStream = new XStream(new DomDriver());
    xStream.alias("person", Person.class);
    System.out.println(xStream.toXML(person));

And the outcome is much easier to read (and smaller):

    <person>
      <name>Guilherme</name>
    </person>

Now that we have configured a simple class to play with, let's see what XStream converters can do for us...

## Creating a PersonConverter

Let's create a simple converter capable of:

1. telling its capable of converting Person's
2. translating a Person instance in XML
3. translate XML into a new Person

We begin creating the PersonConverter class and implementing the Converter interface:

    package com.thoughtworks.xstream.examples;

    import com.thoughtworks.xstream.converters.Converter;
    import com.thoughtworks.xstream.converters.MarshallingContext;
    import com.thoughtworks.xstream.converters.UnmarshallingContext;
    import com.thoughtworks.xstream.io.HierarchicalStreamReader;
    import com.thoughtworks.xstream.io.HierarchicalStreamWriter;

    public class PersonConverter implements Converter {

            public boolean canConvert(Class clazz) {
                    return false;
            }

            public void marshal(Object value, HierarchicalStreamWriter writer,
                            MarshallingContext context) {
            }

            public Object unmarshal(HierarchicalStreamReader reader,
                            UnmarshallingContext context) {
                    return null;
            }

    }

Now we tell whoever calls us that we can handle only Person's (and nothing else, including those classes which extends Person).

    public boolean canConvert(Class clazz) {
            return clazz.equals(Person.class);
    }

The second step is usually quite clean, unless you are dealing with generic converters.

The marshal method is responsible for translating an object to XML. It receives three arguments:

1. the object we are trying to convert
2. the writer were we should output the data
3. the current marshalling context

We start casting the object to person:

    Person person = (Person) value;

Now we can output the data... let's start creating a node called fullname and adding the person's name to it:

    writer.startNode("fullname");
    writer.setValue(person.getName());
    writer.endNode();

Quite simple huh?

    public void marshal(Object value, HierarchicalStreamWriter writer,
                    MarshallingContext context) {
            Person person = (Person) value;
            writer.startNode("fullname");
            writer.setValue(person.getName());
            writer.endNode();
    }

We could have called start/end node as many times as we would like (but remember to close everything you open)... and conversion usually takes place when calling the setValue method.

And now let's go to the unmarshal. We use the moveDown and moveUp methods to move in the tree hierarchy, so we can simply moveDown, read the value and moveUp.

                    Person person = new Person();
                    reader.moveDown();
                    person.setName(reader.getValue());
                    reader.moveUp();

Which gives us the following converter:

    package com.thoughtworks.xstream.examples;

    import com.thoughtworks.xstream.converters.Converter;
    import com.thoughtworks.xstream.converters.MarshallingContext;
    import com.thoughtworks.xstream.converters.UnmarshallingContext;
    import com.thoughtworks.xstream.io.HierarchicalStreamReader;
    import com.thoughtworks.xstream.io.HierarchicalStreamWriter;

    public class PersonConverter implements Converter {

            public boolean canConvert(Class clazz) {
                    return clazz.equals(Person.class);
            }

            public void marshal(Object value, HierarchicalStreamWriter writer,
                            MarshallingContext context) {
                    Person person = (Person) value;
                    writer.startNode("fullname");
                    writer.setValue(person.getName());
                    writer.endNode();
            }

            public Object unmarshal(HierarchicalStreamReader reader,
                            UnmarshallingContext context) {
                    Person person = new Person();
                    reader.moveDown();
                    person.setName(reader.getValue());
                    reader.moveUp();
                    return person;
            }

    }

Now let's register our converter and see how our application main method looks like:

    package com.thoughtworks.xstream.examples;

    import com.thoughtworks.xstream.XStream;
    import com.thoughtworks.xstream.io.xml.DomDriver;

    public class PersonTest {

            public static void main(String[] args) {
                    Person person = new Person();
                    person.setName("Guilherme");

                    XStream xStream = new XStream(new DomDriver());
                    xStream.registerConverter(new PersonConverter());
                    xStream.alias("person", Person.class);
                    System.out.println(xStream.toXML(person));
            }

    }

Did you notice how we registered our converter? It's a simple call to registerConverter:

    xStream.registerConverter(new PersonConverter());

The final result is:

    <person>
      <fullname>Guilherme</fullname>
    </person>

So you might say... that only changed my tree, I want to convert data!

Try using an attribute called fullname in the person tag instead of creating a new child node.

## An alternative for types with String representation

Let's enhance the Person with a String representation, that contains all necessary text to recreate the instance:

    package com.thoughtworks.xstream.examples;

    public class Person {

            private String name;

            public String getName() {
                    return name;
            }

            public void setName(String name) {
                    this.name = name;
            }

            public String toString() {
                    return getName();
            }
    }

In this case we can simplify our Converter to

    package com.thoughtworks.xstream.examples;

    import com.thoughtworks.xstream.converters.basic.AbstractSingleValueConverter;

    public class PersonConverter extends AbstractSingleValueConverter {

            public boolean canConvert(Class clazz) {
                    return clazz.equals(Person.class);
            }

            public Object fromString(String str) {
                    Person person = new Person();
                    person.setName(string);
                    return person;
            }

    }

But even nicer, our XML is also simplified (using the alias for the Person class). Since the String representation is complete, a nested element is not necessary anymore:

    <person>Guilherme</person>

Note, that in implementation of a SingleValueConverter is required for attributes, since these objects have to be represented by a single string only.

# Date Converter

Now that we know how the Converter interface works, let's create a simple calendar converter which uses the locale to convert the information.

Our converter will receive the Locale in its constructor and we will keep a reference to it in a member variable:

    package com.thoughtworks.xstream.examples;

    import java.util.Locale;

    import com.thoughtworks.xstream.converters.Converter;
    import com.thoughtworks.xstream.converters.MarshallingContext;
    import com.thoughtworks.xstream.converters.UnmarshallingContext;
    import com.thoughtworks.xstream.io.HierarchicalStreamReader;
    import com.thoughtworks.xstream.io.HierarchicalStreamWriter;

    public class DateConverter implements Converter {

            private Locale locale;

            public DateConverter(Locale locale) {
                    super();
                    this.locale = locale;
            }

            public boolean canConvert(Class clazz) {
                    return false;
            }

            public void marshal(Object value, HierarchicalStreamWriter writer,
                            MarshallingContext context) {
            }

            public Object unmarshal(HierarchicalStreamReader reader,
                            UnmarshallingContext context) {
                    return null;
            }

    }

Now let's convert anything which extends Calendar: means if instances of class clazz can be assigned to the Calendar class, they extends the abstract class Calendar:

    public boolean canConvert(Class clazz) {
            return Calendar.class.isAssignableFrom(clazz);
    }

Let's go for converting a Calendar in a localized string... we first cast the object to Calendar, extract its Date and then use a DateFormat factory method to get a date converter to our localized string.

    public void marshal(Object value, HierarchicalStreamWriter writer,
                    MarshallingContext context) {

            Calendar calendar = (Calendar) value;

            // grabs the date
            Date date = calendar.getTime();

            // grabs the formatter
            DateFormat formatter = DateFormat.getDateInstance(DateFormat.FULL,
                            this.locale);

            // formats and sets the value
            writer.setValue(formatter.format(date));

    }

And the other way around... in order to unmarshall, we create a GregorianCalendar, retrieves the localized DateFormat instance, parses the string into a Date and puts this date in the original GregorianCalendar:

    public Object unmarshal(HierarchicalStreamReader reader,
                    UnmarshallingContext context) {

            // creates the calendar
            GregorianCalendar calendar = new GregorianCalendar();

            // grabs the converter
            DateFormat formatter = DateFormat.getDateInstance(DateFormat.FULL,
                            this.locale);

            // parses the string and sets the time
            try {
                    calendar.setTime(formatter.parse(reader.getValue()));
            } catch (ParseException e) {
                    throw new ConversionException(e.getMessage(), e);
            }

            // returns the new object
            return calendar;

    }

Note 1: remember that some DateFormat implementations are not thread-safe, therefore don't put your formatter as a member of your converter.

Note 2: this implementation will convert other types of Calendar's to GregorianCalendar after save/load. If this is not what you want, change your canConvert method to return true only if class equals GregorianCalendar.

So we get the following converter:

    package com.thoughtworks.xstream.examples;

    import java.text.DateFormat;
    import java.text.ParseException;
    import java.util.Calendar;
    import java.util.Date;
    import java.util.GregorianCalendar;
    import java.util.Locale;

    import com.thoughtworks.xstream.converters.ConversionException;
    import com.thoughtworks.xstream.converters.Converter;
    import com.thoughtworks.xstream.converters.MarshallingContext;
    import com.thoughtworks.xstream.converters.UnmarshallingContext;
    import com.thoughtworks.xstream.io.HierarchicalStreamReader;
    import com.thoughtworks.xstream.io.HierarchicalStreamWriter;

    public class DateConverter implements Converter {

            private Locale locale;

            public DateConverter(Locale locale) {
                    super();
                    this.locale = locale;
            }

            public boolean canConvert(Class clazz) {
                    return Calendar.class.isAssignableFrom(clazz);
            }

            public void marshal(Object value, HierarchicalStreamWriter writer,
                            MarshallingContext context) {
                    Calendar calendar = (Calendar) value;
                    Date date = calendar.getTime();
                    DateFormat formatter = DateFormat.getDateInstance(DateFormat.FULL,
                                    this.locale);
                    writer.setValue(formatter.format(date));
            }

            public Object unmarshal(HierarchicalStreamReader reader,
                            UnmarshallingContext context) {
                    GregorianCalendar calendar = new GregorianCalendar();
                    DateFormat formatter = DateFormat.getDateInstance(DateFormat.FULL,
                                    this.locale);
                    try {
                            calendar.setTime(formatter.parse(reader.getValue()));
                    } catch (ParseException e) {
                            throw new ConversionException(e.getMessage(), e);
                    }
                    return calendar;
            }

    }

And let's try it out. We create a DateTest class with a main method:

1. creates a calendar (current date)
2. creates the XStream object
3. registers the converter with a Brazilian Portuguese locale
4. translates the object in XML

Well, we already know how to do all those steps... so let's go:

    package com.thoughtworks.xstream.examples;

    import java.util.Calendar;
    import java.util.GregorianCalendar;
    import java.util.Locale;

    import com.thoughtworks.xstream.XStream;
    import com.thoughtworks.xstream.io.xml.DomDriver;

    public class DateTest {

            public static void main(String[] args) {

                    // grabs the current date from the virtual machine
                    Calendar calendar = new GregorianCalendar();

                    // creates the xstream
                    XStream xStream = new XStream(new DomDriver());

                    // brazilian portuguese locale
                    xStream.registerConverter(new DateConverter(new Locale("pt", "br")));

                    // prints the result
                    System.out.println(xStream.toXML(calendar));

            }

    }

The result? Well... it depends, but it will be something like:

    <gregorian-calendar>Sexta-feira, 10 de Fevereiro de 2006</gregorian-calendar>

Note: we did not put any alias as gregorian-calendar is the default alias for GregorianCalendar.

And now let's try to unmarshal the result shown above:

    // loads the calendar from the string
    Calendar loaded = (Calendar) xStream
                    .fromXML("<gregorian-calendar>Sexta-feira, 10 de Fevereiro de 2006</gregorian-calendar>");

And print it using the system locale, short date format:

    // prints using the system defined locale
    System.out.println(DateFormat.getDateInstance(DateFormat.SHORT).format(
                    loaded.getTime()));

The result might be something like (if your system locale is American English):

    2/10/06

# Complex Converter

## Setting up another example

We already defined some classes, so let them glue together:

    package com.thoughtworks.xstream.examples;

    public class Birthday {

            private Person person;
            private Calendar date;
            private char gender;

            public Person getPerson() {
                    return person;
            }

            public void setPerson(Person person) {
                    this.person = person;
            }

            public Calendar getDate() {
                    return date;
            }

            public void setDate(Calendar date) {
                    this.date = date;
            }

            public char getGender() {
                    return gender;
            }

            public void setGenderMale() {
                    this.gender = 'm';
            }

            public void setGenderFemale() {
                    this.gender = 'f';
            }

    }

While XStream is capable of converting this class without any problem, we write our own custom converter just for demonstration. This time we want to reuse our already written converters for the Person and the Calendar and add an own attribute for the gender. The `canConvert` method is plain simple. We convert no derived classes this time, since they might have additional fields. But we reuse the converters registered in XStream for our member fields and handle `null` values:

    package com.thoughtworks.xstream.examples;

    import java.util.Calendar;

    import com.thoughtworks.xstream.converters.ConversionException;
    import com.thoughtworks.xstream.converters.Converter;
    import com.thoughtworks.xstream.converters.MarshallingContext;
    import com.thoughtworks.xstream.converters.UnmarshallingContext;
    import com.thoughtworks.xstream.io.HierarchicalStreamReader;
    import com.thoughtworks.xstream.io.HierarchicalStreamWriter;

    public class BirthdayConverter implements Converter {

            public boolean canConvert(Class clazz) {
                    return Birthday.class == clazz;
            }

            public void marshal(Object value, HierarchicalStreamWriter writer,
                            MarshallingContext context) {
                    Birthday birthday = (Birthday)value;
                    if (birthday.getGender() != '\0') {
                            writer.addAttribute("gender", Character.toString(birthday.getGender()));
                    }
                    if (birthday.getPerson() != null) {
                            writer.startNode("person");
                            context.convertAnother(birthday.getPerson());
                            writer.endNode();
                    }
                    if (birthday.getDate() != null) {
                            writer.startNode("birth");
                            context.convertAnother(birthday.getDate());
                            writer.endNode();
                    }
            }

            public Object unmarshal(HierarchicalStreamReader reader,
                            UnmarshallingContext context) {
                    Birthday birthday = new Birthday();
                    String gender = reader.getAttribute("gender");
                    if (gender != null) {
                            if (gender.length() > 0) {
                                    if (gender.char(0) == 'f') {
                                            birthday.setGenderFemale();
                                    } else if (gender.char(0) == 'm') {
                                            birthday.setFemale();
                                    } else {
                                            throw new ConversionException("Invalid gender value: " + gender);
                                    }
                            } else {
                                    throw new ConversionException("Empty string is invalid gender value");
                            }
                    }
                    while (reader.hasMoreChildren()) {
                            reader.moveDown();
                            if ("person".equals(reader.getNodeName())) {
                                    Person person = (Person)context.convertAnother(birthday, Person.class);
                                    birthday.setPerson(person);
                            } else if ("birth".equals(reader.getNodeName())) {
                                    Calendar date = (Calendar)context.convertAnother(birthday, Calendar.class);
                                    birthday.setDate(date);
                            }
                            reader.moveUp();
                    }
                    return birthday;
            }

    }

The unmarshal method ensures the valid value for the gender by throwing a ConversionException for invalid entries.

Note, that attributes will always have to be written and read first. You work on a stream and accessing the value of a tag or its members will close the surrounding tag (that is still active when the method is called).

If the implementation of `Birthday` ensures, that none of its fields could hold a `null` value and gender contains a valid value, then we could drop the `null` condition in the `marshal` method and in `unmarshal` we could omit the loop as well as the comparison of the tag names:

    package com.thoughtworks.xstream.examples;

    import java.util.Calendar;

    import com.thoughtworks.xstream.converters.Converter;
    import com.thoughtworks.xstream.converters.MarshallingContext;
    import com.thoughtworks.xstream.converters.UnmarshallingContext;
    import com.thoughtworks.xstream.io.HierarchicalStreamReader;
    import com.thoughtworks.xstream.io.HierarchicalStreamWriter;

    public class BirthdayConverter implements Converter {

            public boolean canConvert(Class clazz) {
                    return Birthday.class == clazz;
            }

            public void marshal(Object value, HierarchicalStreamWriter writer,
                            MarshallingContext context) {
                    Birthday birthday = (Birthday)value;
                    writer.addAttribute("gender", Character.toString(birthday.getGender()));
                    writer.startNode("person");
                    context.convertAnother(birthday.getPerson());
                    writer.endNode();
                    writer.startNode("birth");
                    context.convertAnother(birthday.getDate());
                    writer.endNode();
            }

            public Object unmarshal(HierarchicalStreamReader reader,
                            UnmarshallingContext context) {
                    Birthday birthday = new Birthday();
                    if (reader.getAttribute("gender").charAt(0) == 'm') {
                            birthday.setGenderMale();
                    } else {
                            birthday.setGenderFemale();
                    }
                    reader.moveDown();
                    Person person = (Person)context.convertAnother(birthday, Person.class);
                    birthday.setPerson(person);
                    reader.moveUp();
                    reader.moveDown();
                    Calendar date = (Calendar)context.convertAnother(birthday, Calendar.class);
                    birthday.setDate(date);
                    reader.moveUp();
                    return birthday;
            }

    }