org.springframework.web.bind

## Class WebDataBinder

- java.lang.Object
- - org.springframework.validation.DataBinder
  - - org.springframework.web.bind.WebDataBinder

- All Implemented Interfaces:
    PropertyEditorRegistry, TypeConverter
  Direct Known Subclasses:
    PortletRequestDataBinder, ServletRequestDataBinder, WebRequestDataBinder

      public class WebDataBinder
      extends DataBinder

  Special `DataBinder` for data binding from web request parameters to JavaBean objects. Designed for web environments, but not dependent on the Servlet API; serves as base class for more specific DataBinder variants, such as `ServletRequestDataBinder`.

  Includes support for field markers which address a common problem with HTML checkboxes and select options: detecting that a field was part of the form, but did not generate a request parameter because it was empty. A field marker allows to detect that state and reset the corresponding bean property accordingly. Default values, for parameters that are otherwise not present, can specify a value for the field other then empty.

  Since:
    1.2
  Author:
    Juergen Hoeller, Scott Andrews
  See Also:
    `DataBinder.registerCustomEditor(java.lang.Class<?>, java.beans.PropertyEditor)`, `DataBinder.setAllowedFields(java.lang.String...)`, `DataBinder.setRequiredFields(java.lang.String...)`, `setFieldMarkerPrefix(java.lang.String)`, `setFieldDefaultPrefix(java.lang.String)`, `ServletRequestDataBinder`

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static String` | `DEFAULT_FIELD_DEFAULT_PREFIX` Default prefix that field default parameters start with, followed by the field name: e.g.
    `static String` | `DEFAULT_FIELD_MARKER_PREFIX` Default prefix that field marker parameters start with, followed by the field name: e.g.

    - ### Fields inherited from class org.springframework.validation.DataBinder

      `DEFAULT_AUTO_GROW_COLLECTION_LIMIT, DEFAULT_OBJECT_NAME, logger`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `WebDataBinder(Object target)` Create a new WebDataBinder instance, with default object name.
    `WebDataBinder(Object target, String objectName)` Create a new WebDataBinder instance.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected void` | `bindMultipart(Map<String,List<MultipartFile>> multipartFiles, MutablePropertyValues mpvs)` Bind all multipart files contained in the given request, if any (in case of a multipart request).
    `protected void` | `checkFieldDefaults(MutablePropertyValues mpvs)` Check the given property values for field defaults, i.e.
    `protected void` | `checkFieldMarkers(MutablePropertyValues mpvs)` Check the given property values for field markers, i.e.
    `protected void` | `doBind(MutablePropertyValues mpvs)` This implementation performs a field default and marker check before delegating to the superclass binding process.
    `protected Object` | `getEmptyValue(String field, Class<?> fieldType)` Determine an empty value for the specified field.
    `String` | `getFieldDefaultPrefix()` Return the prefix for parameters that mark default fields.
    `String` | `getFieldMarkerPrefix()` Return the prefix for parameters that mark potentially empty fields.
    `boolean` | `isBindEmptyMultipartFiles()` Return whether to bind empty MultipartFile parameters.
    `void` | `setBindEmptyMultipartFiles(boolean bindEmptyMultipartFiles)` Set whether to bind empty MultipartFile parameters.
    `void` | `setFieldDefaultPrefix(String fieldDefaultPrefix)` Specify a prefix that can be used for parameters that indicate default value fields, having "prefix + field" as name.
    `void` | `setFieldMarkerPrefix(String fieldMarkerPrefix)` Specify a prefix that can be used for parameters that mark potentially empty fields, having "prefix + field" as name.

    - ### Methods inherited from class org.springframework.validation.DataBinder

      `addValidators, applyPropertyValues, bind, checkAllowedFields, checkRequiredFields, close, convertIfNecessary, convertIfNecessary, convertIfNecessary, findCustomEditor, getAllowedFields, getAutoGrowCollectionLimit, getBindingErrorProcessor, getBindingResult, getConversionService, getDisallowedFields, getInternalBindingResult, getObjectName, getPropertyAccessor, getPropertyEditorRegistry, getRequiredFields, getSimpleTypeConverter, getTarget, getTypeConverter, getValidator, getValidators, initBeanPropertyAccess, initDirectFieldAccess, isAllowed, isAutoGrowNestedPaths, isIgnoreInvalidFields, isIgnoreUnknownFields, registerCustomEditor, registerCustomEditor, replaceValidators, setAllowedFields, setAutoGrowCollectionLimit, setAutoGrowNestedPaths, setBindingErrorProcessor, setConversionService, setDisallowedFields, setExtractOldValueForEditor, setIgnoreInvalidFields, setIgnoreUnknownFields, setMessageCodesResolver, setRequiredFields, setValidator, validate, validate`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### DEFAULT_FIELD_MARKER_PREFIX

          public static final String DEFAULT_FIELD_MARKER_PREFIX

      Default prefix that field marker parameters start with, followed by the field name: e.g. "_subscribeToNewsletter" for a field "subscribeToNewsletter".

      Such a marker parameter indicates that the field was visible, that is, existed in the form that caused the submission. If no corresponding field value parameter was found, the field will be reset. The value of the field marker parameter does not matter in this case; an arbitrary value can be used. This is particularly useful for HTML checkboxes and select options.

      See Also:
        `setFieldMarkerPrefix(java.lang.String)`, Constant Field Values

    - #### DEFAULT_FIELD_DEFAULT_PREFIX

          public static final String DEFAULT_FIELD_DEFAULT_PREFIX

      Default prefix that field default parameters start with, followed by the field name: e.g. "!subscribeToNewsletter" for a field "subscribeToNewsletter".

      Default parameters differ from field markers in that they provide a default value instead of an empty value.

      See Also:
        `setFieldDefaultPrefix(java.lang.String)`, Constant Field Values

  - ### Constructor Detail

    - #### WebDataBinder

          public WebDataBinder(Object target)

      Create a new WebDataBinder instance, with default object name.
      Parameters:
        `target` - the target object to bind onto (or `null` if the binder is just used to convert a plain parameter value)
      See Also:
        `DataBinder.DEFAULT_OBJECT_NAME`

    - #### WebDataBinder

          public WebDataBinder(Object target,
                               String objectName)

      Create a new WebDataBinder instance.
      Parameters:
        `target` - the target object to bind onto (or `null` if the binder is just used to convert a plain parameter value)
        `objectName` - the name of the target object

  - ### Method Detail

    - #### setFieldMarkerPrefix

          public void setFieldMarkerPrefix(String fieldMarkerPrefix)

      Specify a prefix that can be used for parameters that mark potentially empty fields, having "prefix + field" as name. Such a marker parameter is checked by existence: You can send any value for it, for example "visible". This is particularly useful for HTML checkboxes and select options.

      Default is "_", for "_FIELD" parameters (e.g. "_subscribeToNewsletter"). Set this to null if you want to turn off the empty field check completely.

      HTML checkboxes only send a value when they're checked, so it is not possible to detect that a formerly checked box has just been unchecked, at least not with standard HTML means.

      One way to address this is to look for a checkbox parameter value if you know that the checkbox has been visible in the form, resetting the checkbox if no value found. In Spring web MVC, this typically happens in a custom `onBind` implementation.

      This auto-reset mechanism addresses this deficiency, provided that a marker parameter is sent for each checkbox field, like "_subscribeToNewsletter" for a "subscribeToNewsletter" field. As the marker parameter is sent in any case, the data binder can detect an empty field and automatically reset its value.

      See Also:
        `DEFAULT_FIELD_MARKER_PREFIX`

    - #### getFieldMarkerPrefix

          public String getFieldMarkerPrefix()

      Return the prefix for parameters that mark potentially empty fields.

    - #### setFieldDefaultPrefix

          public void setFieldDefaultPrefix(String fieldDefaultPrefix)

      Specify a prefix that can be used for parameters that indicate default value fields, having "prefix + field" as name. The value of the default field is used when the field is not provided.

      Default is "!", for "!FIELD" parameters (e.g. "!subscribeToNewsletter"). Set this to null if you want to turn off the field defaults completely.

      HTML checkboxes only send a value when they're checked, so it is not possible to detect that a formerly checked box has just been unchecked, at least not with standard HTML means. A default field is especially useful when a checkbox represents a non-boolean value.

      The presence of a default parameter preempts the behavior of a field marker for the given field.

      See Also:
        `DEFAULT_FIELD_DEFAULT_PREFIX`

    - #### getFieldDefaultPrefix

          public String getFieldDefaultPrefix()

      Return the prefix for parameters that mark default fields.

    - #### setBindEmptyMultipartFiles

          public void setBindEmptyMultipartFiles(boolean bindEmptyMultipartFiles)

      Set whether to bind empty MultipartFile parameters. Default is "true".

      Turn this off if you want to keep an already bound MultipartFile when the user resubmits the form without choosing a different file. Else, the already bound MultipartFile will be replaced by an empty MultipartFile holder.

      See Also:
        `MultipartFile`

    - #### isBindEmptyMultipartFiles

          public boolean isBindEmptyMultipartFiles()

      Return whether to bind empty MultipartFile parameters.

    - #### doBind

          protected void doBind(MutablePropertyValues mpvs)

      This implementation performs a field default and marker check before delegating to the superclass binding process.
      Overrides:
        `doBind` in class `DataBinder`
      Parameters:
        `mpvs` - the property values to bind, as MutablePropertyValues instance
      See Also:
        `checkFieldDefaults(org.springframework.beans.MutablePropertyValues)`, `checkFieldMarkers(org.springframework.beans.MutablePropertyValues)`

    - #### checkFieldDefaults

          protected void checkFieldDefaults(MutablePropertyValues mpvs)

      Check the given property values for field defaults, i.e. for fields that start with the field default prefix.

      The existence of a field defaults indicates that the specified value should be used if the field is otherwise not present.

      Parameters:
        `mpvs` - the property values to be bound (can be modified)
      See Also:
        `getFieldDefaultPrefix()`

    - #### checkFieldMarkers

          protected void checkFieldMarkers(MutablePropertyValues mpvs)

      Check the given property values for field markers, i.e. for fields that start with the field marker prefix.

      The existence of a field marker indicates that the specified field existed in the form. If the property values do not contain a corresponding field value, the field will be considered as empty and will be reset appropriately.

      Parameters:
        `mpvs` - the property values to be bound (can be modified)
      See Also:
        `getFieldMarkerPrefix()`, `getEmptyValue(String, Class)`

    - #### getEmptyValue

          protected Object getEmptyValue(String field,
                                         Class<?> fieldType)

      Determine an empty value for the specified field.

      Default implementation returns `Boolean.FALSE` for boolean fields and an empty array of array types. Else, `null` is used as default.

      Parameters:
        `field` - the name of the field
        `fieldType` - the type of the field
      Returns:
        the empty value (for most fields: null)

    - #### bindMultipart

          protected void bindMultipart(Map<String,List<MultipartFile>> multipartFiles,
                                       MutablePropertyValues mpvs)

      Bind all multipart files contained in the given request, if any (in case of a multipart request).

      Multipart files will only be added to the property values if they are not empty or if we're configured to bind empty multipart files too.

      Parameters:
        `multipartFiles` - Map of field name String to MultipartFile object
        `mpvs` - the property values to be bound (can be modified)
      See Also:
        `MultipartFile`, `setBindEmptyMultipartFiles(boolean)`