Module org.omnifaces
Package org.omnifaces.taghandler

Class ImportConstants

java.lang.Object
jakarta.faces.view.facelets.TagHandler
org.omnifaces.taghandler.ImportConstants
All Implemented Interfaces:
FaceletHandler
public class ImportConstants extends TagHandler

The <o:importConstants> taghandler allows the developer to have a mapping of all constant field values of the given fully qualified name of a type in the request scope. The constant field values are those public static final fields. This works for classes, interfaces and enums.

Usage

For example: 

 public class Foo {
     public static final String FOO1 = "foo1";
     public static final String FOO2 = "foo2";
 }

 public interface Bar {
     public String BAR1 = "bar1";
     public String BAR2 = "bar2";
 }

 public enum Baz {
     BAZ1, BAZ2;
 }

 public enum Faz implements Bar {
     FAZ1, FAZ2;
 }

The constant field values of the above types can be mapped into the request scope as follows: 

 <o:importConstants type="com.example.Foo" />
 <o:importConstants type="com.example.Bar" />
 <o:importConstants type="com.example.Baz" var="Bazzz" />
 <o:importConstants type="com.example.Faz" />
 ...
 #{Foo.FOO1}, #{Foo.FOO2}, #{Bar.BAR1}, #{Bar.BAR2}, #{Bazzz.BAZ1}, #{Bazzz.BAZ2}, #{Faz.FAZ1}, #{Faz.BAR2}
 ...
 <h:selectOneMenu>
     <f:selectItems value="#{Faz.values()}" /> <!-- FAZ1, FAZ2, BAR1, BAR2 -->
 </h:selectOneMenu>

The map is by default stored in the request scope by the simple name of the type as variable name. You can override this by explicitly specifying the var attribute, as demonstrated for com.example.Baz in the above example.

The resolved constants are by reference stored in the cache to improve retrieving performance. There is also a runtime (no, not compiletime as that's just not possible in EL) check during retrieving the constant value. If a constant value doesn't exist, then an IllegalArgumentException will be thrown.

Since version 4.3, you can use the loader attribute to specify an object whose class loader will be used to load the class specified in the type attribute. The class loader of the given object is resolved as specified in Utils.getClassLoader(Object). In the end this should allow you to use a more specific class when there are duplicate instances in the runtime classpath, e.g. via multiple (plugin) libraries.

Since version 4.6, when the class specified in the type attribute is an enum, such as Baz or Faz in the above example, then you can use #{Faz.members()} to exclusively access enum members rather than all constant field values. 

 <h:selectOneMenu>
     <f:selectItems value="#{Faz.members()}" /> <!-- FAZ1, FAZ2 -->
 </h:selectOneMenu>

JSF 2.3

JSF 2.3 also offers a <f:importConstants>, however it requires being placed in <f:metadata> which may not be appropriate when you intend to import constants only from a include, tagfile or a composite component.

Author:
Bauke Scholtz

  • Constructor Details

    • ImportConstants

      public ImportConstants(TagConfig config)
      The tag constructor.
      Parameters:
      config - The tag config.

  • Method Details

    • apply

      public void apply(FaceletContext context, UIComponent parent) throws IOException
      First obtain the constants of the class by its fully qualified name as specified in the type attribute from the cache. If it hasn't been collected yet and is thus not present in the cache, then collect them and store in cache. Finally set the constants in the request scope by the simple name of the type, or by the name as specified in the var attribute, if any.
      Throws:
      IOException