Apache Struts 2 Documentation > Home > Guides > Core Developers Guide > Type Conversion
Added by Patrick Lightbody, last edited by Matthieu Chase Heimer on May 25, 2008  (view change) show comment

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.

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.

  • 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.

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 needs to throw XWorkException or preferably TypeConversionException.

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:

# syntax: <propertyName> = <converterClassName>
point = com.acme.PointConverter 
person.phoneNumber = com.acme.PhoneNumberConverter

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:

# syntax: <propertyName>=<converterClassName>
amount=com.acme.converters.MyCustomBigDecimalConverter

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.

# syntax: <type> = <converterClassName>
java.math.BigDecimal = com.acme.MyBigDecimalConverter

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), Struts 2'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 Struts 2 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, Struts 2 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 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 <s:property value="point"/> 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 Struts 2 (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.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:

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.

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 ReflectionContextState#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 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.

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

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:

public abstract class Measurement<T extends Enum>
   public void setUnits(T enumValue) {...}
}

public class Area extends Measurement<UnitsOfArea> {
  @Override
  public void setUnits(UnitsOfArea enumValue){...}
}

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).

Next: Interceptors