# Parsing Nested JSON String to CSV in Java ![parse.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1593162165362/qKu-SisUG.png) This is my first blog and I want to share my learnings and some observations/tips/challenges faced to dive into this topic. ** What is JSON?** JSON: It is a data format called JavaScript Object Notation, which is a standard text-based format storing and transporting data usually from server to webpage. It represents structured data in 2 format: 1. Object: an unordered collection of 0 or more key-value pairs separated by commas and represented inside { } braces 1. Array: an ordered collection of 0 or more values and represented inside [ ] brackets **Parsing JSON text:** When we parse this JSON text, it deserializes JSON string into objects may be map, dictionary, custom class object etc. POJO(plain old java object) It is a class with only private fields and public getter/setter methods. How to parse in Java?? There are various libraries available to parse JSON and some of them which are highly popular : **1. org.json** It is json-java library to convert json string to object using org.json.JSONObject class Pros: Just to get an attribute Cons: Its proof-of-concept with inefficient implementation No object de-serialization available **2.Gson** It is google library to convert JSON string to java object and vice-versa using either of JsonParser class or Gson class Pros: Convert JSON to POJO and vice-versa Need to create model with same JSON structure and is automatically filled where we can define custom field name using @SerializedName Cons: Doesn't support dynamic filtering of fields on levels other than root **3. Jackson** It is a library which support the use of JSON tree representation called JsonNode Convert JSON to POJO and vice-versa Need to create model with same JSON structure and is automatically filled where we can define custom field name using @JsonProperty *ObjectMapper package is available to do this binding of JSON to Object of com.fasterxml.jackson.JsonNode which is object representation of json string. It is used to extract value.* Overall, based on Benchmark, Jackson is 3-4 times better than org.json and twice better than Gson **Sample Json to parse** Let's call it sample.log ``` { "post/time" : 152, "post/bytes" : 6, "success" : true, "context" : { "postId" : "c8f0ef7", "nativePostIds" : "d1ebc", "siteId" : { " page-hits" : 123, " name" : "abc.com" } "users" : ["john","celia"] } } ``` **Code** ``` import com.fasterxml.jackson.annotation.JsonProperty; import com.fasterxml.jackson.databind.JsonNode; import com.fasterxml.jackson.databind.ObjectMapper; import java.io.IOException; public class ExtractFromJson { public static class Post { @JsonProperty("post/time") private int postDuration; @JsonProperty("post/bytes") private int postBytes; private String success; private Id context; private List users; public Post(){} //getter methods } public static class Id { private String postId; private Site siteId; public Id(){} //getter methods } public static class Site { private int page-hits; private String name; public Site(){} //getter methods } public static void main(String args[]) throws IOException { String json = "{\"posts/time\":152,\"post/bytes\":6,\"success\":true,\"context\":{\"postId\":\"c8f0ef7\",\"nativePostIds\":\"d1ebc\",{\"siteId\":{\"page-hits\":123,\"name\":\"abc.com\"}},\"users\":[\"john\",\"celia\"]}"; ObjectMapper mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERIES,false); Post mappedValue = mapper.readValue(json,Post.class); System.out.println(mappedValue.getPostDuration()); // **This is how we can handle nested fields** System.out.println(mappedValue.getContext().getSite().getPageHits()); ``` **Jackson Library is used ** I used Intellij and used Maven dependency for Jackson ``` com.fasterxml.jackson.core jackson-core 2.5.3 compile ``` **In case, we want to write results to CSV ** ``` FileWriter writer = new FileWriter("output/output.csv"); writer.write("PostDuration,PostBytes,PostId,Success,Page-Hits\n"); List outputData = new ArrayList<>(); outputData.add(mappedValue.getPostDuration()); outputData.add(mappedValue.getPostBytes()); outputData.add(mappedValue.getContext().getPostId()); outputData.add(mappedValue.getSuccess()); outputData.add(mappedValue.getContext().getSite().getPageHits()); String collect = outputData.stream().collect(Collectors.joining(",")); writer.write(collect); writer.write("\n"); writer.close(); ``` **Output CSV structure** ``` PostDuration,PostBytes,PostId,Success,Page-Hits 152 , 6 , c8f0ef7, true, 123 43 , 23, sdrinttq , true, 98 ``` **Exceptions faced** - Cannot resolve plugin org.apache.maven.plugins:maven-surefire-plugin:2.12.4 name P.S. We face this exception during build time Solution: ``` org.apache.maven.plugins maven-surefire-plugin 2.12.4 true ``` - Exception in thread "main" com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "field_name" (class ), not marked as ignorable (one known property: "context"]) Solution: Define the ObjectMapper object like this to avoid declaring unwanted fields ``` ObjectMapper mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERIES,false); ``` - IntelliJ - Error:java: release version 5 not supported Solution: this link is helpful [IntellijSolution](https://dev.to/techgirl1908/intellij-error-java-release- version-5-not-supported-376) **Tips when creating Model class!!!** 1. Declare it static if writing inner classes 2. when you see a JSON structure, create a class for every { } structure as I have explained in above code 3. while considering which library to use, we should consider performance ( This is the benchmark used to compare the performance of deserializer [https://github.com/fabienrenaud/java-json-benchmark](https://github.com/fabienrenaud/java-json-benchmark), ease of use and dependencies involved 4. JSON parser = new JSONParser() has been depricated Thanks for reading !!!