Reputation: 7053
I have a large JSON file that is made up of structures that I am mapping into POJOs, and then storing in a Collection. The structure is similar to this:
[
{
"id": 1234,
"file": "C:\\Programs\\Program1.exe",
"exists": true
}
{
"id": 5678,
"file": "C:\\Programs\\Program2.exe",
"exists": false
}
...
]
Using the Jackson streaming API I have got all these structures read, and the POJOs stored in a Collection successfully. My POJO class looks like this:
public class Programs
{
@JsonProperty("id")
private Integer id;
@JsonProperty("file")
private String file;
@JsonProperty("exists")
private Boolean exists;
@JsonGetter("id")
public Integer getId()
{
return id;
}
@JsonGetter("file")
public String getFile()
{
return file;
}
@JsonGetter("exists")
public Boolean getExists()
{
return exists;
}
}
However, I want to omit any structures that have "exists"
set to false
during the deserialization process so that no POJO is ever created for them. So I wrote a custom deserializer with the help of this SO question [ How do I call the default deserializer from a custom deserializer in Jackson ], with my overridden deserialize
looking like:
@Override
public Programs deserialize(JsonParser parser, DeserializationContext context)
throws IOException
{
Programs programs = (Programs)defaultDeserializer.deserialize(parser, context);
if (!programs.getExists())
{
throw context.mappingException("[exists] value is false.");
}
return programs;
}
However, when I run some unit tests, I get the following error:
"Can not deserialize instance of java.util.ArrayList out of START_OBJECT token"
message was "Class com.myprogram.serializer.ProgramsJsonDeserializer
has no default (no arg) constructor"
(Adding a no arg constructor gives the error that StdDeserializer does not have a default constructor.)
Is this the correct approach to achieving what I am trying to do? And does anyone know why I get this error message?
Upvotes: 1
Views: 954
Reputation: 11637
I want to omit any structures that have "exists" set to false during the deserialization process so that no POJO is ever created for them.
I think your objective is to retrieve a list of Programs
instance that only have exists
set to true
after derserialization. A customized CollectionDeserializer
to filter those unwanted instance may help:
public class ProgramsCollectionHandler extends SimpleModule {
private static class ProgramsCollectionDeserializer extends CollectionDeserializer {
public ProgramsCollectionDeserializer(CollectionDeserializer deserializer) {
super(deserializer);
}
private static final long serialVersionUID = 1L;
@Override
public Collection<Object> deserialize(JsonParser parser, DeserializationContext context)
throws IOException, JsonProcessingException {
Collection<Object> result = super.deserialize(parser, context);
Collection<Object> filteredResult = new ArrayList<Object>();
for (Object o : result) {
if (o instanceof Programs) {
final Programs programs = (Programs) o;
if (programs.exists) {
filteredResult.add(programs);
}
}
}
return filteredResult;
}
@Override
public CollectionDeserializer createContextual(
DeserializationContext context,
BeanProperty property) throws JsonMappingException {
return new ProgramsCollectionDeserializer(super.createContextual(context, property));
}
}
private static final long serialVersionUID = 1L;
@Override
public void setupModule(Module.SetupContext context) {
super.setupModule(context);
context.addBeanDeserializerModifier(new BeanDeserializerModifier() {
@Override
public JsonDeserializer<?> modifyCollectionDeserializer(
DeserializationConfig config, CollectionType type,
BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
if (deserializer instanceof CollectionDeserializer) {
return new ProgramsCollectionDeserializer(
(CollectionDeserializer) deserializer);
}
return super.modifyCollectionDeserializer(config, type,
beanDesc, deserializer);
}
});
}
}
After that, your can register it into your object mapper:
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new ProgramsCollectionHandler());
"Can not deserialize instance of java.util.ArrayList out of START_OBJECT token" message was "Class com.myprogram.serializer.ProgramsJsonDeserializer has no default (no arg) constructor" (Adding a no arg constructor gives the error that StdDeserializer does not have a default constructor.)
This may be because your constructor cannot be accessed. For example, your deserializer is implemented as a non-static inner class.
Upvotes: 2