Read Json from URL

In this article, we will take a look at 3 different ways to read a json string from a url in java. Also, how to convert this json string to a java object.

1. Using Java API
Use java built-in classes to read json data from a URL.
java.net.URL class accepts the URL of json resource in string format. It has an openConnection() method which connects to the resource.
Once we have a connection, use its getInputStream() method to get access to input stream connected to the resource.
Using this input stream, we can read json data line by line using java BufferedReader class.
Example,

URL url = new URL("http://jsonplaceholder.typicode.com/posts/1");
URLConnection connection = url.openConnection();
BufferedReader reader = new BufferedReader(
                          new InputStreamReader(
                          connection.getInputStream()));
String line = null;
StringBuilder content = new StringBuilder();
while((line = reader.readLine()) != null) {
  content.append(line).append("\n");
}
System.out.println(content);

2. Using Apache Commons
Apache Commons IO library has a org.apache.commons.io.IOUtils class, having a method toString().
It accepts the URL of json resource as a URL object and returns the json data at this URL as a string. Example,

URL url = new URL("http://jsonplaceholder.typicode.com/posts/1");
String content = IOUtils.toString(url);
System.out.println(content);

Apache Commons IO can be added to a project using below Maven or Gradle dependency.

// Maven
<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.11.0</version>
</dependency>

// Gradle
implementation 'commons-io:commons-io:2.11.0'

3. Using Gson
Above methods read a json from URL to a string. But, you might want to convert a json string to a java object.
For this purpose, Google’s gson library will be handy.

It has a fromJson() method that accepts two arguments
A. Reader object that points to the json resource.
B. Class type to which the json string will be mapped.

Suppose the class to which the json string will be mapped is given below

class Post {
  private Integer id;

  private String title;

  // getters and setters
}

Remember that the fields or instance variables of the class should match with the keys of json string. Example,

Post post = new Gson().fromJson(
                    new InputStreamReader(url.openStream()), 
                    Post.class);

Convert json string to object
If you need to use the first two methods or you already have a json string and convert it to an object, then you can use below 2 methods
1. Using JSONObject
org.json is a java library that deals with json objects. It has an org.json.JSONObject class which converts a json string to json object. Example,

String jsonStr = "{\"id\":1, \"name\":\"Abc\"}";
JSONObject obj = new JSONObject(jsonStr);
String name = obj.getString("name");

Its constructor accepts a json string and returns a json object. This object can be used to get values using its get() methods supplying the name of a key.
2. Using ObjectMapper
Jackson library is the most popular java library for handling json objects. It has an ObjectMapper class, which converts a string to java object. Example,

ObjectMapper mapper = new ObjectMapper();
User user = mapper.readValue(content.toString(), User.class);

Remember that for this method to work, the java class to which the json string needs to be converted should have all the properties corresponding to the keys in json, else it will throw an error.
If you can’t add all the fields to the class, then use any methods outlined in this article to overcome this.

That is all on various methods to read json data from a url in java.

Read json string from url in java

Read Json from URL