De-serialize List of POJOs rather than a single one at a time

Question

So I came across this tutorial for serializing a POJO to json and then de-serialize the json file back to the POJO. http://www.mkyong.com/java/how-to-convert-java-object-to-from-json-jackson/

he uses these helpful methods which worked for me but only for a single POJO in the file:

//1. Convert Java object to JSON format
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(new File("c:\\user.json"), user);

//2. Convert JSON to Java object
ObjectMapper mapper = new ObjectMapper();
User user = mapper.readValue(new File("c:\\user.json"), User.class);

How can I de-serialize a list of POJOs? My serialized file looks like the below:

[ {
  "name" : {
    "first" : "Wonder",
    "last" : "Woman"
  },
  "ssn" : "123-456-7890",
  "gender" : "FEMALE",
  "verified" : false
}, {
  "name" : {
    "first" : "Bat",
    "last" : "Man"
  },
  "ssn" : "321-456-0987",
  "gender" : "FEMALE",
  "verified" : true
}, {
  "name" : {
    "first" : "Super",
    "last" : "Man"
  },
  "ssn" : "321-654-1111",
  "gender" : "FEMALE",
  "verified" : true
} ]

possible duplicate of [Jackson - Json to POJO With Multiple Entries](http://stackoverflow.com/questions/7045836/jackson-json-to-pojo-with-multiple-entries) — Luca Basso Ricci, Aug 26 '13 at 20:49

Luke Willis · Accepted Answer · 2013-08-27T17:55:21.280

3

One option (probably the easiest) is to define a class that contains a list of Users:

public class Users
{
  public User[] users;
}

Then performing

ObjectMapper mapper = new ObjectMapper();
Users users = mapper.readValue(new File("c:\\user.json"), Users.class);

Another option would be to iterate over the json array, and capture each element of the array of users, then use ObjectMapper.readValue(String content, Class<T> valueType), like so:

ObjectMapper mapper = new ObjectMapper();
InputStream stream = new FileInputStream("c:\\user.json");
User user;

JsonNode json = mapper.readTree(stream);

//NOTE: calling json.isArray() should return true.
for (JsonNode userJson : json)
{
  user = mapper.readValue(userJson, User.class);
  // use the constructed user...
}

Note: I haven't tested the above, so let me know if it works or not.

edited Aug 27 '13 at 17:55

answered Aug 26 '13 at 20:14

Luke Willis

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Can you elaborate on option 2? Please provide a code example as I am new to Java. Thank you. – Horse Voice Aug 27 '13 at 12:35
@ImtiazAhmad elaborated. – Luke Willis Aug 27 '13 at 15:03
What is the InputStream from? Is it a jackson library or are you using java.io ? Also json.getElements() has a compilation error: `The method getElements() is undefined for the type JsonNode` – Horse Voice Aug 27 '13 at 17:47
@ImtiazAhmad Yes, the `InputStream` is out of `java.io`. I added a note, and removed the call to `getElements` entirely. – Luke Willis Aug 27 '13 at 17:56
`readValue()` is not compiling. It gives me the error: The method readValues(JsonParser, ResolvedType) in the type ObjectMapper is not applicable for the arguments (JsonNode, Class). Eclipse suggest, to change method call to `readValues()` but that doesnt work either. – Horse Voice Aug 27 '13 at 18:02
1

@ImtiazAhmad The API claims that [readValue(JsonNode, Class)](http://jackson.codehaus.org/1.7.9/javadoc/org/codehaus/jackson/map/ObjectMapper.html#readValue(org.codehaus.jackson.JsonNode,%20java.lang.Class)) exists, so I'm not sure why you're getting that. – Luke Willis Aug 27 '13 at 18:56

score 1 · Answer 2 · answered Aug 27 '13 at 19:10

1

Hmmh? Have you tried:

User[] users = mapper.readValue(new File("c:\\user.json"), User[].class);

answered Aug 27 '13 at 19:10

StaxMan

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score 0 · Answer 3 · answered Aug 26 '13 at 20:08

0

In your top level class, you have an array of people. something like this

    class People {
    public List<Person> persons;
}

answered Aug 26 '13 at 20:08

scphantm

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De-serialize List of POJOs rather than a single one at a time

3 Answers3