Type Conversion

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.)

Built in Type Conversion Support

Type Conversion is implemented by XWork.

• Enumerations
• BigDecimal and BigInteger

Relationship to Parameter Names

There is no need to capture form values using intermediate Strings and primitives. Instead, the framework can read from and write to properties of objects addressed via OGNL expressions and perform the appropriate type conversion for you.

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

• Use OGNL expressions - the framework will automatically take care of creating the actual objects for you.
• Use JavaBeans! The framework can only create objects that obey the JavaBean specification, provide no-arg constructions and include getters and setters where appropriate.
• Remember that person.name will call getPerson().setName(). If the framework creates the Person object for you, it remember that a setPerson method must also exist.
• The framework will not instantiate an object if an instance already exists. The PrepareInterceptor or action's constructor can be used to create target objects before type conversion.
• 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

Create a type converter by extending StrutsTypeConverter. The Converter's role is to convert a String to an Object and an Object to a String.

Applying a Type Converter to an Action

Create a file called 'ActionClassName-conversion.properties' in the same location of the classpath as the Action class itself resides.

Eg. if the action class name is MyAction, the action-level conversion properties file should be named 'MyAction-conversion.properties'. If the action's package is com.myapp.actions the conversion file should also be in the classpath at /com/myapp/actions/.

Within the conversion file, name the action's property and the Converter to apply to it:

Type conversion can also be specified via Annotations within the action.

Applying a Type Converter to a bean or model

When getting or setting the property of a bean, the framework will look for "classname-conversion.properties" in the same location of the classpath as the target bean. This is the same mechanism as used for actions.

Example: A custom converter is required for the Amount property of a Measurement bean. The Measurement class cannot be modified as its located within one of the application's dependencies. The action using Measurement implements ModelDriven<Measurement> so it cannot apply converters to the properties directly.
Solution: The conversion file needs to be in the same location of the classpath as Measurement. Create a directory in your source or resources tree matching the package of Measurement and place the converters file there.
eg. for com.acme.measurements.Measurement, create a file in the application source/resources at /com/acme/measurements/Measurement-conversion.properties:

Applying a Type Converter for an application

Application-wide converters can be specified in a file called xwork-conversion.properties located in the root of the classpath.

A Simple Example

The framework ships with a base helper class that simplifies converting to and from Strings, org.apache.struts2.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:

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.

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.

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:

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 John. 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.

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

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

• 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!

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.

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.

Common Problems

Null and Blank Values

Some properties cannot be set to null. Primitives like boolean and int cannot be null. If your action needs to or will accept null or blank values, use the object equivalents Boolean and Integer. Similarly, a blank string "" cannot be set on a primitive. At the time of writing, a blank string also cannot be set on a BigDecimal or BigInteger. Use server-side validation to prevent invalid values from being set on your properties (or handle the conversion errors appropriately).

Interfaces

The framework cannot instantiate an object if it can't determine an appropriate implementation. It recognizes well-known collection interfaces (List, Set, Map, etc) but cannot instantiate MyCustomInterface when all it sees is the interface. In this case, instantiate the target implementation first (eg. in a prepare method) or substitute in an implementation.

Generics and Erasure

The framework will inspect generics to determine the appropriate type for collections and array elements. However, in some cases Erasure can result in base types that cannot be converted (typically Object or Enum).

The following is an example of this problem:

Although to the developer the area.setUnits(enumValue) method only accepts a UnitsOfArea enumeration, due to erasure the signature of this method is actually setUnits(java.lang.Enum). The framework does not know that the parameter is a UnitsOfArea and when it attempts to instantiate the Enum an exception is thrown (java.lang.IllegalArgumentException: java.lang.Enum is not an enum type).

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