Jackson JSON、不可变类和接口

Question

我正在使用 Jackson 的示例，并且在反序列化与不可变的类和接口一起使用时遇到了一些麻烦。

以下是我的代码：

package com.art.starter.jackson_starter;

import java.io.IOException;
import java.io.StringReader;
import java.io.StringWriter;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
/**  * Hello world!  *  */ public class App  {
    public static void main( String[] args ) throws JsonGenerationException, JsonMappingException, IOException
    {
        System.out.println( "Hello World!" );

        AddressImpl.AddressBuilder builder = new AddressImpl.AddressBuilder();
        NameImpl.Builder nameBuilder = new NameImpl.Builder();
        UserImpl.Builder userBuilder = new UserImpl.Builder();


        Name name = nameBuilder.first("FirstName")
                  .last("LastName")
                  .build();

        Address address =  builder.setCity("TestCity")
               .setCountry("TestCountry")
               .setState("PA")
               .setStreet("TestAddress")
               .setZip(123)
               .build();      

        User user = userBuilder.address(address)
                 .gender(User.Gender.MALE)
                 .isVerified(true)
                 .userImage(new byte[5])
                 .build();

        System.out.println(address);        
        System.out.println(name);
        System.out.println(user);

        StringWriter sw = new StringWriter();
        ObjectMapper mapper = new ObjectMapper();
        mapper.writeValue(sw, user);
        System.out.println(sw);


       StringReader sr = new StringReader("{\"address\":{\"state\":\"PA\",\"country\":\"TestCountry\",\"street\":\"TestAddress\",\"city\":\"TestCity\",\"zip\":123},\"verified\":true,\"gender\":\"MALE\",\"userImage\":\"AAAAAAA=\"}");

       /* 
          This line throws the Exception           
       */
       User user2 = mapper.readValue(sr, UserImpl.class);

       System.out.println(user2);
    } }

package com.art.starter.jackson_starter;

import java.util.Arrays;

import org.codehaus.jackson.annotate.JsonCreator;
import org.codehaus.jackson.annotate.JsonProperty;

public final class UserImpl implements User
{
   private final Address address;
   private final Gender gender;
   private final byte[] userImage;
   private final boolean isVerified;

   public static class Builder
   {
      private Address address;
      private Gender gender;
      //      private Name name;
      private byte[] userImage;
      private boolean isVerified;

      public Builder address(Address address)
      {
         this.address = address;
         return this;
      }

      public Builder gender(Gender gender)
      {
         this.gender = gender;
         return this;
      }

      //      public Builder name(Name name)
      //      {
      //         this.name = name;
      //         return this;
      //      }

      public Builder userImage(byte[] userImage)
      {
         this.userImage = userImage;
         return this;
      }

      public Builder isVerified(boolean isVerified)
      {
         this.isVerified = isVerified;
         return this;
      }

      public UserImpl build()
      {
         return new UserImpl(address, gender, userImage, isVerified);
      }
   }

   @JsonCreator
   public UserImpl(@JsonProperty("address") Address address, @JsonProperty("gender") Gender gender, @JsonProperty("userImage") byte[] userImage,
         @JsonProperty("verified") boolean isVerified)
   {
      super();
      this.address = address;
      this.gender = gender;
      this.userImage = userImage;
      this.isVerified = isVerified;
   }

   public Address getAddress()
   {
      return address;
   }

   public Gender getGender()
   {
      return gender;
       }

   public byte[] getUserImage()
   {
      return userImage;
   }

   public boolean isVerified()
   {
      return isVerified;
   }

   @Override
   public String toString()
   {
      StringBuilder builder2 = new StringBuilder();
      builder2.append("UserImpl [address=");
      builder2.append(address);
      builder2.append(", gender=");
      builder2.append(gender);
      builder2.append(", isVerified=");
      builder2.append(isVerified);
      builder2.append(", name=");
      builder2.append(", userImage=");
      builder2.append(Arrays.toString(userImage));
      builder2.append("]");
      return builder2.toString();
   }

}

package com.art.starter.jackson_starter;

import org.codehaus.jackson.annotate.JsonCreator;
import org.codehaus.jackson.annotate.JsonProperty;

public final class AddressImpl implements Address
{
   private final String city;
   private final String country;
   private final String street;
   private final String state;
   private final int zip;

   public static class AddressBuilder
   {
      private String city;
      private String country;
      private String street;
      private String state;
      private int zip;

      public AddressBuilder setCity(String city)
      {
         this.city = city;
         return this;
      }

      public AddressBuilder setCountry(String country)
      {
         this.country = country;
         return this;
      }

      public AddressBuilder setStreet(String street)
      {
         this.street = street;
         return this;
      }

      public AddressBuilder setState(String state)
      {
         this.state = state;
         return this;
      }

      public AddressBuilder setZip(int zip)
      {
         this.zip = zip;
         return this;
      }

      public AddressImpl build()
      {
         return new AddressImpl(city, country, street, state, zip);
      }

   }

   @JsonCreator
   public AddressImpl(@JsonProperty("city") String city, @JsonProperty("country") String country, @JsonProperty("street") String street,
         @JsonProperty("state") String state, @JsonProperty("zip") int zip)
   {
      this.city = city;
      this.country = country;
      this.street = street;
      this.state = state;
      this.zip = zip;
   }

   public String getCity()
   {
      return city;
   }

   public String getCountry()
   {
      return country;
   }

   public String getStreet()
   {
      return street;
   }

   public String getState()
   {
      return state;
   }

   public int getZip()
   {
      return zip;
   }

   @Override
   public String toString()
   {
      StringBuilder builder = new StringBuilder();
      builder.append("AddressImpl [city=");
      builder.append(city);
      builder.append(", country=");
      builder.append(country);
      builder.append(", state=");
      builder.append(state);
      builder.append(", street=");
      builder.append(street);
      builder.append(", zip=");
      builder.append(zip);
      builder.append("]");
      return builder.toString();
   }

}

问题似乎与地址有关。我得到这个异常：

Exception in thread "main" org.codehaus.jackson.map.JsonMappingException: Can not construct instance of com.art.starter.jackson_starter.Address, problem: abstract types can only be instantiated with additional type information
 at [Source: java.io.StringReader@785f8172; line: 1, column: 2]
    at org.codehaus.jackson.map.JsonMappingException.from(JsonMappingException.java:163)
    at org.codehaus.jackson.map.deser.StdDeserializationContext.instantiationException(StdDeserializationContext.java:212)
    at org.codehaus.jackson.map.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:97)
    at org.codehaus.jackson.map.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:230)
    at org.codehaus.jackson.map.deser.BeanDeserializer._deserializeUsingPropertyBased(BeanDeserializer.java:595)
    at org.codehaus.jackson.map.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:472)
    at org.codehaus.jackson.map.deser.BeanDeserializer.deserialize(BeanDeserializer.java:350)
    at org.codehaus.jackson.map.ObjectMapper._readMapAndClose(ObjectMapper.java:2391)
    at org.codehaus.jackson.map.ObjectMapper.readValue(ObjectMapper.java:1614)
    at com.art.starter.jackson_starter.App.main(App.java:56)

我确信这是因为 Jackson 无法解析 Address，它是 AddressImpl 的接口，而 AddressImpl 是具体的实现。我一直在浏览文档并查看了一些有关 @JsonDeserialize(as=AddressImpl.class) 的文章，但它不起作用。所以我很困惑。有没有人让这个工作过，甚至支持吗？

如果我将 UserImpl 类中的 Address 替换为 AddressImpl ，它会像冠军一样工作。

Answer 1

以防万一您还没有看过，这里有一个博客条目 讨论使用不可变对象和 Jackson。

但是您绝对应该能够使用 @JsonDeserialize(as=AddressImpl.class); ，方法是将其添加到 Address.java 接口（直接或使用 mix-ins），或者将其添加到领域或财产。需要注意的一件事是，对于反序列化，它必须位于您使用的访问器旁边；设置器（如果有），如果没有，则在字段旁边。注释尚未在访问者之间共享；因此，例如将其添加到“getter”将不起作用。

Jackson 1.8 最终还允许注册抽象到具体的类型（请参阅 http://jira.codehaus。 org/browse/JACKSON-464 了解更多详细信息）这可能是指示“AddressImpl”用于“Address”的最佳选项。