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Type Conversion
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Added by Patrick Lightbody, last edited by Ted Husted on Feb 12, 2007 (view change)

Routine type conversion in the framework is transparent. Generally, all you need to do is ensure that HTML inputs have names that can be used in OGNL expressions. (HTML inputs are form elements and other GET/POST parameters.)

## A Simple Example

Type conversion is great for situations where you need to turn a String in to a more complex object. Because the web is type-agnostic (everything is a string in HTTP), WebWork's type conversion features are very useful. For instance, if you were prompting a user to enter in coordinates in the form of a string (such as "3, 22"), you could have WebWork do the conversion both from String to Point and from Point to String.

Using this "point" example, if your action (or another compound object in which you are setting properties on) has a corresponding ClassName-conversion.properties file, WebWork will use the configured type converters for conversion to and from strings. So turning "3, 22" in to new Point(3, 22) is done by merely adding the following entry to ClassName-conversion.properties (Note that the PointConverter should impl the ognl.TypeConverter interface):

point = com.acme.PointConverter

Your type converter should be sure to check what class type it is being requested to convert. Because it is used for both to and from strings, you will need to split the conversion method in to two parts: one that turns Strings in to Points, and one that turns Points in to Strings.

After this is done, you can now reference your point (using <ww:property value="post"/> in JSP or ${point} in FreeMarker) and it will be printed as "3, 22" again. As such, if you submit this back to an action, it will be converted back to a Point once again.

In some situations you may wish to apply a type converter globally. This can be done by editing the file xwork-conversion.properties in the root of your class path (typically WEB-INF/classes) and providing a property in the form of the class name of the object you wish to convert on the left hand side and the class name of the type converter on the right hand side. For example, providing a type converter for all Point objects would mean adding the following entry:

com.acme.Point = com.acme.PointConverter

| Type conversion should not be used as a substitute for i18n. It is not recommended to use this feature to print out properly formatted dates. Rather, you should use the i18n features of WebWork (and consult the JavaDocs for JDK's MessageFormat object) to see how a properly formatted date should be displayed.

The framework ships with a base helper class that simplifies converting to and from Strings, `org.apache.struts.action2.util.StrutsTypeConverter`. The helper class makes it easy to write type converters that handle converting objects to Strings as well as from Strings.

From the JavaDocs:

Base class for type converters used in Struts. This class provides two abstract methods that are used to convert both to and from strings – the critical functionality that is core to Struts's type coversion system.

Type converters do not have to use this class. It is merely a helper base class, although it is recommended that you use this class as it provides the common type conversion contract required for all web-based type conversion.

There's a hook (fall back method) called `performFallbackConversion` of which could be used to perform some fallback conversion if `convertValue` method of this failed. By default it just ask its super class (Ognl's DefaultTypeConverter) to do the conversion.

To allow the framework to recognize that a conversion error has occurred, throw an XWorkException or preferable a TypeConversionException.

## Built in Type Conversion Support

XWork will automatically handle the most common type conversion for you. This includes support for converting to and from Strings for each of the following:

- String

- boolean / Boolean

- char / Character

- int / Integer, float / Float, long / Long, double / Double

- dates - uses the SHORT format for the Locale associated with the current request

- arrays - assuming the individual strings can be coverted to the individual items

- collections - if not object type can be determined, it is assumed to be a String and a new ArrayList is created

Note that with arrays the type conversion will defer to the type of the array elements and try to convert each item individually. As with any other type conversion, if the conversion can't be performed the standard type conversion error reporting is used to indicate a problem occured while processing the type conversion.

## Relationship to Parameter Names

The best way to take advantage of the framework's type conversion is to utilize complete objects (ideally your domain objects directly). There is no need to capture form values using intermediate Strings and primitives and then convert those values to full objects in an Action method.

Here are some tips for leveraging the framework's type conversion capabilties:

- Use complex OGNL expressions - the framework will automatically take care of creating the actual objects for you.
- Use JavaBeans! The framework can only create objects if the objects obey the JavaBean specification and provide no-arg constructions, as well as getters and setters where appropriate.
- Remember that person.name will call getPerson().setName(), but if in order for the framework to create the Person object for you, a `setPerson` must also exist.
- For lists and maps, use index notation, such as people[0].name or friends['patrick'].name. Often these HTML form elements are being rendered inside a loop. For JSP Tags, use the iterator tag's status attribute. For FreeMarker Tags, use the special property ${foo_index}[].
- For multiple select boxes, it isn't possible to use index notation to name each individual item. Instead, name your element people.name and the framework will understand that it should create a new Person object for each selected item and set its name accordingly.

## Creating a Type Converter

To create a type converter one would need to extends StrutsTypeConverter.

    public class MyConverter extends StrutsTypeConverter {
        public Object convertFromString(Map context, String[] values, Class toClass) {
           .....
        }

        public String convertToString(Map context, Object o) {
           .....
        }
     }

| To allow Struts to recognize that a conversion error has occurred, the converter class need to throw XWorkException or preferably TypeConversionException.

## Advanced Type Conversion

The framework also handles advanced type conversion cases, like null property handling and converting values in Maps and Collections, and type conversion error handling.

### Null Property Handling

Null property handling will automatically create objects where null references are found.

Provided that the key #CREATE_NULL_OBJECTS is in the action context with a value of true (this key is set only during the execution of the com.opensymphony.xwork2.interceptor.ParametersInterceptor), OGNL expressions that have caused a NullPointerException will be temporarily stopped for evaluation while the system automatically tries to solve the null references by automatically creating the object.

The following rules are used when handling null references:

- If the property is declared exactly as a Collection or List, then an ArrayList shall be returned and assigned to the null references.

- If the property is declared as a Map, then a HashMap will be returned and assigned to the null references.

- If the null property is a simple bean with a no-arg constructor, it will simply be created using the {@link ObjectFactory#buildBean(java.lang.Class, java.util.Map)} method.

For example, if a form element has a text field named person.name and the expression person evaluates to null, then this class will be invoked. Because the person expression evaluates to a Person class, a new Person is created and assigned to the null reference. Finally, the name is set on that object and the overall effect is that the system automatically created a Person object for you, set it by calling setUsers() and then finally called getUsers().setName() as you would typically expect.

### Collection and Map Support

Collection and Map support provides intelligent null handling and type conversion for Java Collections.

The framework supports ways to discover the object type for elements in a collection. The discover is made via an ObjectTypeDeterminer. A default implementation is provided with the framework. The Javadocs explain how Map and Collection support is discovered in the `DefaultObjectTypeDeterminer`.

This ObjectTypeDeterminer looks at the Class-conversion.properties for entries that indicated what objects are contained within Maps and Collections. For Collections, such as Lists, the element is specified using the pattern Element_xxx, where xxx is the field name of the collection property in your action or object. For Maps, both the key and the value may be specified by using the pattern Key_xxx and Element_xxx, respectively.

From WebWork 2.1.x, the Collection_xxx format is still supported and honored, although it is deprecated and will be removed eventually.

Additionally, you can create your own custom `ObjectTypeDeterminer` by implementing the `ObjectTypeDeterminer` interface. There is also an optional ObjectTypeDeterminer that utilizes Java 5 generics. See the Annotations page for more information.

#### Indexing a collection by a property of that collection

It is also possible to obtain a unique element of a collection by passing the value of a given property of that element. By default, the property of the element of the collection is determined in Class-conversion.properties using `KeyProperty_xxx=yyy`, where xxx is the property of the bean Class that returns the collection and yyy is the property of the collection element that we want to index on.

For an example, see the following two classes:

MyAction.java

    /**
     * @return a Collection of Foo objects
     */
    public Collection getFooCollection()
    {
        return foo;
    }

Foo.java

    /**
     * @return a unique identifier
     */
    public Long getId()
    {
        return id;
    }

To enable type conversion, put the instruction `KeyProperty_fooCollection=id` in the `MyAction-conversion.properties` file. This technique allows use of the idiom `fooCollection(someIdValue)` to obtain the Foo object with value `someIdValue` in the Set `fooCollection`. For example, `fooCollection(22)` would return the Foo object in the `fooCollection` Collection whose `id` property value was 22.

This technique is useful, because it ties a collection element directly to its unique identifier. You are not forced to use an index. You can edit the elements of a collection associated to a bean without any additional coding. For example, parameter name `fooCollection(22).name` and value `Phil` would set name the Foo Object in the `fooCollection` Collection whose `id` property value was 22 to be Phil.

The framework automatically converts the type of the parameter sent in to the type of the key property using type conversion.

Unlike Map and List element properties, if `fooCollection(22)` does not exist, it will not be created. If you would like it created, use the notation `fooCollection.makeNew[index]` where index is an integer 0, 1, and so on. Thus, parameter value pairs `fooCollection.makeNew[0]=Phil` and `fooCollection.makeNew[1]=John` would add two new Foo Objects to `fooCollection --` one with name property value `Phil` and the other with name property value `Bar`. However, in the case of a Set, the `equals` and `hashCode` methods should be defined such that they don't only include the `id` property. Otherwise, one element of the null `id` properties Foos to be removed from the Set.

## An advanced example for indexed Lists and Maps

Here is the model bean used within the list. The KeyProperty for this bean is the `id` attribute.

MyBean.java

    public class MyBean implements Serializable {

        private Long id;
        private String name;

        public Long getId() {
            return id;
        }

        public void setId(Long id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

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

        public String toString() {
            return "MyBean{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    '}';
        }
    }

The Action has a `beanList` attribute initialized with an empty ArrayList.

MyBeanAction.java

    public class MyBeanAction implements Action {

        private List beanList = new ArrayList();
        private Map beanMap = new HashMap();

        public List getBeanList() {
            return beanList;
        }

        public void setBeanList(List beanList) {
            this.beanList = beanList;
        }

        public Map getBeanMap() {
            return beanMap;
        }

        public void setBeanMap(Map beanMap) {
            this.beanMap = beanMap;
        }

        public String execute() throws Exception {
            return SUCCESS;
        }
    }

These `conversion.properties` tell the TypeConverter to use MyBean instances as elements of the List.

MyBeanAction-conversion.properties

    KeyProperty_beanList=id
    Element_beanList=MyBean
    CreateIfNull_beanList=true

- When submitting this via a form, the `id` value is used as KeyProperty for the MyBean instances in the beanList.

- Notice the () notation! Do not use [] notation, which is for Maps only!

- The value for name will be set to the MyBean instance with this special id.

- The List does not have null values added for unavailable id values. This approach avoids the risk of OutOfMemoryErrors!

MyBeanAction.jsp

    <ww:iterator value="beanList" id="bean">
      <ww:textfield name="beanList(%{bean.id}).name" />
    </ww:iterator>

## Type Conversion Error Handling

Type conversion error handling provides a simple way to distinguish between an input validation problem and an input type conversion problem.

Any error that occurs during type conversion may or may not wish to be reported. For example, reporting that the input "abc" could not be converted to a number might be important. On the other hand, reporting that an empty string, "", cannot be converted to a number might not be important - especially in a web environment where it is hard to distinguish between a user not entering a value vs. entering a blank value.

By default, all conversion errors are reported using the generic i18n key xwork.default.invalid.fieldvalue, which you can override (the default text is Invalid field value for field "xxx", where xxx is the field name) in your global i18n resource bundle.

However, sometimes you may wish to override this message on a per-field basis. You can do this by adding an i18n key associated with just your action (Action.properties) using the pattern invalid.fieldvalue.xxx, where xxx is the field name.

It is important to know that none of these errors are actually reported directly. Rather, they are added to a map called conversionErrors in the ActionContext. There are several ways this map can then be accessed and the errors can be reported accordingly.

There are two ways the error reporting can occur:

1. Globally, using the Conversion Error Interceptor
2. On a per-field basis, using the conversion validator

By default, the conversion interceptor is included in `struts-default.xml` in the default stack. To keep conversion errors from reporting globally, change the interceptor stack, and add additional validation rules.

## Next: Interceptors