Is it possible to read the value of a annotation in java

Question

this is my code    Column columnName  firstname     private String firstName     Column columnName  lastname    private String lastName    public String getFirstName       return firstName       public void setFirstName String firstName      this firstName   firstName       public String getLastName       return lastName       public void setLastName String lastName      this lastName   lastName       is it possible to read the value of my annotation  Column columnName  xyz123   in another class

User · Answer

I ve never done it  but it looks like Reflection provides this  Field is an AnnotatedElement and so it has getAnnotation  This page has an example  copied below   quite straightforward if you know the class of the annotation and if the annotation policy retains the annotation at runtime  Naturally if the retention policy doesn t keep the annotation at runtime  you won t be able to query it at runtime   An answer that s since been deleted     provided a useful link to an annotations tutorial that you may find helpful  I ve copied the link here so people can use it   Example from this page   import java lang annotation Retention   import java lang annotation RetentionPolicy  import java lang reflect Method    Retention RetentionPolicy RUNTIME   interface MyAnno     String str       int val       class Meta      MyAnno str    Two Parameters   val   19    public static void myMeth String str  int i        Meta ob   new Meta         try         Class c   ob getClass           Method m   c getMethod  myMeth   String class  int class          MyAnno anno   m getAnnotation MyAnno class          System out println anno str           anno val           catch  NoSuchMethodException exc          System out println  Method Not Found                  public static void main String args          myMeth  test   10

User · Answer

For the few people asking for a generic method  this should help you  5 years later  p    For my below example  I m pulling the RequestMapping URL value from methods that have the RequestMapping annotation  To adapt this for fields  just change the  for  Method method  clazz getMethods      to  for  Field field  clazz getFields      And swap usage of RequestMapping for whatever annotation you are looking to read  But make sure that the annotation has  Retention RetentionPolicy RUNTIME    public static String getRequestMappingUrl final Class lt   gt  clazz  final String methodName           Only continue if the method name is not empty      if   methodName    null   amp  amp   methodName trim   length    gt  0                 RequestMapping tmpRequestMapping          String   tmpValues              Loop over all methods in the class          for  Method method  clazz getMethods                             If the current method name matches the expected method name  then keep going              if  methodName equalsIgnoreCase method getName                                      Try to extract the RequestMapping annotation from the current method                  tmpRequestMapping   method getAnnotation RequestMapping class                       Only continue if the current method has the RequestMapping annotation                  if  tmpRequestMapping    null                                           Extract the values from the RequestMapping annotation                      tmpValues   tmpRequestMapping value                            Only continue if there are values                      if   tmpValues    null   amp  amp   tmpValues length  gt  0                                                    Return the 1st value                          return tmpValues 0                                                                                 Since no value was returned  log it and return an empty string      logger error  Failed to find RequestMapping annotation value for method      methodName        return

User · Answer

Yes  if your Column annotation has the runtime retention   Retention RetentionPolicy RUNTIME   interface Column                you can do something like this  for  Field f  MyClass class getFields         Column column   f getAnnotation Column class      if  column    null         System out println column columnName         UPDATE   To get private fields use  Myclass class getDeclaredFields

User · Answer

In common case you have private access for fields  so you CAN T use getFields in reflection  Instead of this you should use getDeclaredFields  So  firstly  you should be aware if your Column annotation has the runtime retention    Retention RetentionPolicy RUNTIME   interface Column       After that you can do something like this   for  Field f  MyClass class getDeclaredFields         Column column   f getAnnotation Column class                     Obviously  you would like to do something with field - set new value using annotation value   Column annotation   f getAnnotation Column class   if  annotation    null        new PropertyDescriptor f getName    Column class  getWriteMethod   invoke          object          myCoolProcessing              annotation value                        So  full code can be looked like this   for  Field f   MyClass class getDeclaredFields          Column annotation   f getAnnotation Column class       if  annotation    null          new PropertyDescriptor f getName    Column class  getWriteMethod   invoke                  object                  myCoolProcessing                          annotation value

User · Answer

Of course it is  Here is a sample annotation    Target ElementType METHOD   Retention RetentionPolicy RUNTIME  public  interface TestAnnotation        String testText        And a sample annotated method   class TestClass         TestAnnotation testText  zyx       public void doSomething          And a sample method in another class that prints the value of the testText   Method   methods   TestClass class getMethods    for  Method m   methods        if  m isAnnotationPresent TestAnnotation class             TestAnnotation ta   m getAnnotation TestAnnotation class           System out println ta testText               Not much different for field annotations like yours   Cheerz

User · Answer

Elaborating to the answer of  Cephalopod  if you wanted all column names in a list you could use this oneliner   List lt String gt  columns            Arrays asList MyClass class getFields                   stream                  filter f - gt  f getAnnotation Column class   null                 map f - gt  f getAnnotation Column class  columnName                   collect Collectors toList

User · Answer

You can also use generic types  in my case  taking into account everything said before you can do something like   public class SomeTypeManager lt T gt         public SomeTypeManager T someGeneric               That s how you can achieve all previously said  with generic types          Annotation   an   someGeneric getClass   getAnnotations                Remember  that this will not equival at 100  to SomeClass class get          But can do the trick

User · Answer

While all the answers given so far are perfectly valid  one should also keep in mind the google reflections library for a more generic and easy approach to annotation scanning  e g    Reflections reflections   new Reflections  my project prefix      Set lt Field gt  ids   reflections getFieldsAnnotatedWith javax persistence Id class

User · Answer

one of the ways I used it    protected List lt Field gt  getFieldsWithJsonView Class sourceClass  Class jsonViewName       List lt Field gt  fields   new ArrayList lt  gt         for  Field field   sourceClass getDeclaredFields              JsonView jsonViewAnnotation   field getDeclaredAnnotation JsonView class           if jsonViewAnnotation  null               boolean jsonViewPresent   false              Class   viewNames   jsonViewAnnotation value                if jsonViewName  null  amp  amp  Arrays asList viewNames  contains jsonViewName                     fields add field                                     return fields

[java] Is it possible to read the value of a annotation in java?

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