Using JAXB to unmarshal marshal a List String

Question

I m trying to create a very simple REST server   I just have a test method that will return a List of Strings   Here s the code      GET  Path   test2   public List test2        List list new Vector        list add  a        list add  b        return list       It gives the following error    SEVERE  A message body writer for Java type  class java util Vector  and MIME media type  application octet-stream  was not found   I was hoping JAXB had a default setting for simple types like String  Integer  etc   I guess not   Here s what I imagined     lt Strings     lt String a lt  String     lt String b lt  String   lt  Strings    What s the easiest way to make this method work

User · Answer

I would ve saved time if I found Resteasy Jackson Provider sooner   Just add the Resteasy Jackson Provider JAR   No entity wrappers   No XML annotations   No custom message body writers

User · Answer

I used  LiorH s example and expanded it to      XmlRootElement name  List   public class JaxbList lt T       protected List lt T  list       public JaxbList          public JaxbList List lt T  list           this list list              XmlElement name  Item       public List lt T  getList            return list             Note  that it uses generics so you can use it with other classes than String   Now  the application code is simply          GET      Path   test2       public JaxbList test2            List list new Vector            list add  a            list add  b            return new JaxbList list            Why doesn t this simple class exist in the JAXB package   Anyone see anything like it elsewhere

User · Answer

I have encountered this pattern a few times  I found that the easiest way is to define an inner class with JaxB annotations   anyways  you ll probably want to define the root tag name   so your code would look something like this   GET  Path   test2   public Object test2       MyResourceWrapper wrapper   new MyResourceWrapper       wrapper  add  a       wrapper  add  b       return wrapper       XmlRootElement name  MyResource   private static class MyResourceWrapper           XmlElement name  Item          List lt String gt  list new ArrayList lt String gt            MyResourceWrapper              public void add String s   list add s         if you work with javax rs  jax-rs  I d return Response object with the wrapper set as its entity

User · Answer

GET  Path   test2   public Response test2       List lt String gt  list new Vector lt String gt        list add  a       list add  b        final GenericEntity lt List lt String gt  gt  entity   new GenericEntity lt List lt String gt  gt  list          return Response ok   entity entity  build

User · Answer

Finally I ve solved it using JacksonJaxbJsonProvider It requires few changes in your Spring context xml and Maven pom xml  In your Spring context xml add JacksonJaxbJsonProvider to the  lt jaxrs server gt     lt jaxrs server id  restService  address   resource  gt       lt jaxrs providers gt           lt bean class  org codehaus jackson jaxrs JacksonJaxbJsonProvider   gt       lt  jaxrs providers gt   lt  jaxrs server gt    In your Maven pom xml add    lt dependency gt       lt groupId gt org codehaus jackson lt  groupId gt       lt artifactId gt jackson-jaxrs lt  artifactId gt       lt version gt 1 9 0 lt  version gt   lt  dependency gt

User · Answer

In case anyone of you wants to write a list wrapper for lists containing elements of multiple classes and want to give an individual XmlElement name according to the Class type without Writing X Wrapper classes you could use the  XmlMixed annotation  By doing so JAXB names the items of the list according to the value set by the  XmlRootElement  When doing so you have to specify which classes could possibly be in the list using  XmlSeeAlso  Example   Possible Classes in the list   XmlRootElement name  user   public class User             XmlRootElement name  entry   public class LogEntry             Wrapper class   XmlRootElement name  records    XmlSeeAlso  User class  LogEntry class   public static class JaxbList lt T gt        protected List lt T gt  records       public JaxbList          public JaxbList List lt T gt  list           this records list              XmlMixed      public List lt T gt  getRecords            return records            Example   List l   new List    l add new User  userA     l add new LogEntry new UserB        XStream xStream   new XStream    String result   xStream toXML l     Result    lt records gt       lt user gt     lt  user gt       lt entry gt     lt  entry gt   lt  records gt    Alternatevily you could specify the XmlElement names directly inside the wrapper class using the  XmlElementRef annotation   XmlRootElement name  records    XmlSeeAlso  User class  LogEntry class   public static class JaxbList lt T gt        protected List lt T gt  records       public JaxbList          public JaxbList List lt T gt  list           this records list              XmlElementRefs            XmlElementRef name  item   type Object class            XmlElementRef name  user   type User class            XmlElementRef name  entry   type LogEntry class             public List lt T gt  getRecords            return records

User · Answer

If you are using maven in the jersey project add below in pom xml and update project dependencies so that Jaxb is able to detect model class and convert list to Media type application XML    lt dependency gt       lt groupId gt com sun xml bind lt  groupId gt       lt artifactId gt jaxb-core lt  artifactId gt       lt version gt 2 2 11 lt  version gt   lt  dependency gt

User · Answer

Make sure to add  XmlSeeAlso tag with your specific classes used inside JaxbList  It is very important else it throws HttpMessageNotWritableException

User · Answer

From a personal blog post  it is not necessary to create a specific JaxbList  lt  T  gt  object   Assuming an object with a list of strings    XmlRootElement public class ObjectWithList        private List lt String gt  list        XmlElementWrapper name  MyList        XmlElement     public List lt String gt  getList             return list             public void setList List lt String gt  list            this list   list             A JAXB round trip   public static void simpleExample   throws JAXBException        List lt String gt  l   new ArrayList lt String gt         l add  Somewhere        l add  This and that        l add  Something            Object with list     ObjectWithList owl   new ObjectWithList        owl setList l        JAXBContext jc   JAXBContext newInstance ObjectWithList class       ObjectWithList retr   marshallUnmarshall owl  jc        for  String s   retr getList              System out println s         System out println            Produces the following    lt  xml version  1 0  encoding  UTF-8  standalone  yes   gt   lt objectWithList gt       lt MyList gt           lt list gt Somewhere lt  list gt           lt list gt This and that lt  list gt           lt list gt Something lt  list gt       lt  MyList gt   lt  objectWithList gt

User · Answer

This can be done MUCH easier using wonderful XStream library  No wrappers  no annotations   Target XML   lt Strings gt     lt String gt a lt  String gt     lt String gt b lt  String gt   lt  Strings gt    Serialization   String alias can be avoided by using lowercase string tag  but I used OP s code   List  lt String gt  list   new ArrayList  lt String gt     list add  a    list add  b     XStream xStream   new XStream    xStream alias  Strings   List class   xStream alias  String   String class   String result   xStream toXML list     Deserialization  Deserialization into ArrayList   XStream xStream   new XStream    xStream alias  Strings   ArrayList class   xStream alias  String   String class   xStream addImplicitArray ArrayList class   elementData    List  lt String gt  result    List  lt String gt  xStream fromXML file     Deserialization into String    XStream xStream   new XStream    xStream alias  Strings   String   class   xStream alias  String   String class   String   result    String   xStream fromXML file     Note  that XStream instance is thread-safe and can be pre-configured  shrinking code amount to one-liners   XStream can also be used as a default serialization mechanism for JAX-RS service  Example of plugging XStream in Jersey can be found here

User · Answer

For a more general solution  for JAXB-XML serialization of any top level list   which only requires 1 new class to be written  check out the solution given in this question   Is it possible to programmatically configure JAXB   public class Wrapper lt T gt     private List lt T gt  items   new ArrayList lt T gt       XmlAnyElement lax true  public List lt T gt  getItems         return items          JAXBContext is thread safe and so create it in constructor or    setter or wherever       JAXBContext jc   JAXBContext newInstance Wrapper class  clazz         public String marshal List lt T gt  things  Class clazz         configure JAXB and marshaller        Marshaller m   jc createMarshaller      m setProperty Marshaller JAXB FORMATTED OUTPUT  true        Create wrapper based on generic list of objects   Wrapper lt T gt  wrapper   new Wrapper lt T gt  things     JAXBElement lt Wrapper gt  wrapperJAXBElement   new JAXBElement lt Wrapper gt  new QName clazz getSimpleName   toLowerCase    s    Wrapper class  wrapper      StringWriter result   new StringWriter        marshal    m marshal wrapperJAXBElement  result      return result toString

User · Answer

User1 s example worked well for me  But  as a warning  it won t work with anything other than simple String Integer types  unless you add an  XmlSeeAlso annotation    XmlRootElement name    List    XmlSeeAlso MovieTicket class  public class MovieTicketList       protected List lt MovieTicket gt  list    This works OK  although it prevents me from using a single generic list class across my entire application  It might also explain why this seemingly obvious class doesn t exist in the JAXB package

[java] Using JAXB to unmarshal/marshal a List<String>

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