如何将数组反序列化为对象？

Question

The object I want to cast into looks like

public class AdDimension {
    private int width;
    private int height;

    public AdDimension() {
        // needed by Jackson
    }

    public AdDimension(final int width, final int height) {
        this.width = width;
        this.height = height;
    }
..
}

and it is used inside

public class Campaign {
    private String id;
    private List<AdDimension> adDimensions;

    public Campaign() {
        // needed by Jackson
    }
 ...
}

I try to deserialize in test as

@Test
public void testLoadCampaigns() throws IOException {
    final File campaignsFile = new File(getClass().getClassLoader().getResource("campaigns.json").getFile());
    final List<Campaign> campaigns = new JsonReader().loadCampaigns(campaignsFile);
    System.out.println(campaigns);
}

and my campaigns file look like

[
  {
    "id": 1,
    "targetedCountries": ["CA", "IT"],
    "targetedDomain": "apple.com",
    "adDimensions": [[300,250], [600,200]]
  }
]

When I run my test, I see

com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of com.org.site.messages.AdDimension out of START_ARRAY token
 at [Source: /Users/harit/IdeaProjects/site/target/test-classes/campaigns.json; line: 6, column: 22] (through reference chain: java.util.ArrayList[0]->com.org.scout.messages.Campaign["adDimensions"]->java.util.ArrayList[0])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
    at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:857)
    at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:853)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromArray(BeanDeserializerBase.java:1257)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:157)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:136)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:245)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:217)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:25)
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:520)
    at com.fasterxml.jackson.databind.deser.impl.FieldProperty.deserializeAndSet(FieldProperty.java:101)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:258)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:125)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:245)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:217)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:25)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3736)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2639)
    at com.org.harness.JsonReader.loadCampaigns(JsonReader.java:15)
    at com.org.harness.JsonReaderTest.testLoadCampaigns(JsonReaderTest.java:17)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:252)
    at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:141)
    at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:112)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.apache.maven.surefire.util.ReflectionUtils.invokeMethodWithArray(ReflectionUtils.java:189)
    at org.apache.maven.surefire.booter.ProviderFactory$ProviderProxy.invoke(ProviderFactory.java:165)
    at org.apache.maven.surefire.booter.ProviderFactory.invokeProvider(ProviderFactory.java:85)
    at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:115)
    at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:75)


Results :

Tests in error: 
  testLoadCampaigns(com.org.harness.JsonReaderTest): Can not deserialize instance of com.org.site.messages.AdDimension out of START_ARRAY token(..)

How can I read [[300,250], [600,200]] into List<AdDimension> ?

Answer 1

Jackson, by default, attempts to deserialize JSON array values into either Java array types or Collection types (eg. ArrayList ). But you're trying to make it deserialize a JSON array like [600,200] into a AdDimension object. This won't work out of the box.

You can change your JSON like other answers have suggested or you use a custom deserialization strategy.

Your first option is to define a custom JsonDeserializer (though you should use its sublcass StdDeserializer )

class AdDimensionDeserializer extends JsonDeserializer<AdDimension> {
    @Override
    public AdDimension deserialize(JsonParser parser, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        // you have complete control over the tokens in the JSON, so validate as you wish
        ArrayNode arrayNode = parser.readValueAsTree();
        return new AdDimension(arrayNode.get(0).asInt(), arrayNode.get(1).asInt());
    }
}

and register it as

@JsonDeserialize(using = AdDimensionDeserializer.class)
class AdDimension {

Jackson will instantiate AdDimensionDeserializer and use the instance to deserialize the JSON array elements into an instance(s) of AdDimension .

Another option is to use @JsonCreater

Marker annotation that can be used to define constructors and factory methods as one to use for instantiating new instances of the associated class.

For example

@JsonCreator
public AdDimension(int[] dimensions) {
    // again, validate as required
    this.width = dimensions[0];
    this.height = dimensions[1];
}

Jackson will find this constructor and use it to deserialize your JSON. It will first attempt to deserialize the text [300,250] into a int[] , which it obviously can, and then pass that to your constructor.

Answer 2

Maybe is a bad json format??

I think it should be like this

[
  {
    "id": 1,
    "targetedCountries": ["CA", "IT"],
    "targetedDomain": "apple.com",
    "adDimensions": [{"width":300, "height":250}, {"width":600, "height":200}]
  }
]

I can´t test it now, but it seems to be no OK.