How to read json

How to Read JSON from a File using Jackson

November 27, 2019 • Atta ✨

In this quick tutorial, you’ll learn how to read JSON data from a file by using the Jackson API. Jackson is a popular JSON processing library for reading, writing, and parsing JSON data in Java.

Dependencies

To add Jackson to your Gradle project, add the following dependency to build.gradle file:

For Maven, include the below dependency to your pom.xml file:

Read JSON File to a Java Map

Let us say you have the following JSON file called book.json :

You should see the following output printed on the console:

Read JSON File to a Java Object

Let us first create a simple Java class called Book.java to map the JSON object:

The following example demonstrates how you can read the above JSON file into a Book object by using the readValue() method:

The above code will output the following on the console:

Read JSON File to a List of Java Objects

Suppose we have the following JSON file called books.json that contains a JSON array:

You can now read a list of Book objects from the above JSON file using the same readValue() method as shown below:

Here is the output of the above code:

For more Jackson examples, check out the How to read and write JSON using Jackson in Java tutorial.

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Reading and Writing JSON Files in Java

октября 26, 2019 • Atta ✨

JSON (JavaScript Object Notation) is a light-weight, human-readable, and language-independent data exchange format. Originally derived from JavaScript in the early 2000s, JSON has quickly become a de-facto standard format for exchanging data between the servers and the clients.

JSON has only two data structures: objects and arrays. An object is an unordered set of zero or more key-value pairs. An array is a list of zero or more values. The values can be strings, numbers, booleans, null, nested objects, and arrays.

Almost all programming languages including Java, Python, Node.js, JavaScript, and Ruby provide code to generate and parse the JSON data. In this article, you’ll learn how to read and write JSON in Java.

Let us create a simple JSON file which contains a single object that describes a customer, as shown below:

Unfortunately, Java does not provide any native library for processing JSON. In this article, we will look at some of the best open-source libraries available for generating and parsing JSON in Java.

1. JSON.simple

JSON.simple is a simple library for processing JSON data in Java. It allows you to read, write, parse, and query JSON in full compliance with JSON specifications (RFC4627).

To add JSON.simple to your Gradle project, add the following dependency to build.gradle file:

For Maven, include the below dependency to your pom.xml file:

Write JSON to a File using JSON.simple

Here is an example that demonstrates how to use JSON.simple library to write JSON data to a file:

As you can see above, we are using the JsonObject and JsonArray classes to generating a customer object with personal details, projects, payment methods, and more. The JsonObject class is used to create a JSON object with unordered key-value pairs. The JsonArray class is used to create a JSON array with multiple objects.

Read JSON from a File using JSON.simple

Let us now read JSON from the file we just created above. JSON.simple provides the Jsoner.deserialize() static method for parsing a JSON file. Here is an example:

In the above example, we are creating a JsonObject object by explicitly casting the object returned by the Jsoner.deserialize() method. We then read the JSON objects one-by-one by casting them to their actual data type.

2. Jackson

Jackson is another popular Java library for reading and writing JSON data. Jackson provides the ObjectMapper class to convert Java Objects into their JSON representation. It can also be used to convert a JSON object back to an equivalent Java object.

Just add the following dependency to your Gradle project’s build.gradle file to include the Jackson support:

For Maven, add the below dependency to your pom.xml file:

Note: If you are looking to parse JSON data in Spring Boot, check out the Processing JSON Data in Spring Boot guide.

Write JSON to a File using Jackson

ObjectMapper provides the writeValue() method to convert a Java object to a JSON string. The following example shows how to use the Jackson library to write JSON data to a file:

As you can see above, we are using the writeValueAsString() method to serialize a map object to a JSON string. The generated JSON string is then written to the file.

Read JSON from a File using Jackson

Reading JSON from a file using Jackson is easier than the JSON.simple library. The ObjectMapper class can also be used to construct a hierarchical tree of nodes from JSON data. In the JSON tree model, you can access a specific node and read its value. In the tree model, each node is of type JsonNode which provides different methods to work with specific keys.

Here is an example that parses the customer.json file by using the Jackson library:

3. Gson

Gson is yet another Java library, developed and maintained by Google, for converting Java Objects into their JSON representation. You can also use it to convert a JSON string back to an equivalent Java object.

Gson provides simple toJson() and fromJson() methods that you can use to easily convert a Java object to and from JSON.

To add Gson to your Gradle project, add the below dependency to the build.gralde file:

For Maven, add the following dependency to your pom.xml file:

Write JSON to a File using Gson

The following example demonstrates how to use the toJson() method to convert a Java collection to a JSON string and then write it to a file:

The above code looks very much similar to Jackson’s code for writing JSON to a file. The only difference is we are now using the toJson() method from the Gson class for converting the collection to a JSON string.

Read JSON from a File using Gson

We can use the JsonParser class that comes with the Gson library to parse the JSON file we just created. Here is an example:

4. Moshi

Moshi is another powerful JSON library created by Square for Kotlin and Java. It makes it easy to parse JSON into Java objects as well as convert Java Objects into their JSON representation. Moshi has built-in support for reading and writing Java’s core data types including primitives, collections, strings, and enums.

If you want to use Moshi in a Gradle project, include the following dependency to your build.gradle file:

For Maven, add the below dependency to your pom.xml file:

Write JSON to a File using Moshi

Moshi provides the JsonWriter class that can be used to write a JSON encoded value to a stream, one token at a time, as shown below:

As you can see above, we have created an instance of JsonWriter to encode our data as JSON. We then called different methods on the write object to create a JSON object with nested objects and arrays.

Read JSON from a File using Moshi

Let us use the JsonReader class provided by Moshi to parse JSON from a file. It reads a JSON encode value as a stream of tokens. This stream includes both literal values (strings, numbers, booleans, and nulls) as well as the begin and end delimiters of objects and arrays. The tokens are traversed in depth-first order, the same order that they appear in the JSON document. Within JSON objects, name-value pairs are represented by a single token.

Here is an example:

Conclusion

That’s all folks for reading and writing JSON files in Java. There are no native libraries for efficiently processing JSON data in Java.

In this article, I have discussed four different open-source libraries for reading, writing, and parsing JSON data in Java. These libraries include JSON.simple, Jackson, Gson, and Moshi.

So what’s the best JSON library? Personally, I use and recommend Jackson. It is a suite of data processing tools for Java that can be used to parse not only JSON but other data formats too like CSV, XML, YAML, and more.

If you are looking to read and write XML, check out Reading and Writing XML in Java tutorial.

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How to Read JSON from a File using Gson

November 30, 2019 • Atta ✨

In this short article, you’ll learn how to read JSON data from a file by using the Gson library. Gson is a popular JSON processing library developed and maintained by Google for reading, writing, and parsing JSON data in Java.

Dependencies

To add Gson to your Gradle project, add the following dependency to build.gradle file:

For Maven, include the below dependency to your pom.xml file:

Read JSON File to a Java Map

Suppose you have the following JSON file called user.json :

You should see the following output printed on the console:

Read JSON File to a Java Object

Let us first create a simple Java class called User.java to map the JSON object:

The following example shows how you can read the above JSON file into a User object by using the fromJson() method:

You should see the following output printed on the console:

Read JSON File to a List of Java Objects

Let us say we have the following JSON file called users.json that contains a JSON array:

You can now read a list of User objects from the above JSON file by using the same fromJson() method, as shown below:

Here is the output of the above code:

For more Gson examples, check out the How to read and write JSON using Gson in Java tutorial.

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How To Read A JSON File With Python

Are you confronted with a JSON file and at a loss to know how to extract wanted information, or you don’t know about JSON but would like to? Today we’ll introduce the JSON format and use Python to access a JSON file. We’ll extract the data we need and we’ll use the Python JSON module designed specifically for this purpose. I’ll also mention some other options for achieving the same result.

As an added bonus I’ll also show you how to convert Python data-types into JSON format. So let’s get started.

Table of Contents

What Is A JSON file?

? JSON stands for Java Script Object Notation which is a human friendly text-format for exchanging data. Despite the word Java in the name, JSON is language independent and most major programming languages have the ability to generate and parse JSON formatted data.

As a Python user you will be familiar with the list and dictionary data-types, and the foundation of JSON uses two such structures.

You can see a dictionary style syntax using curly brackets and key : value pairs, and there is a square bracket notation that looks like a list (or an array).

So How Do You Read A JSON File?

Python has a module that encodes and decodes JSON format files. If you wish to read about it in the Python standard library, you can find it here.

In its basic usage the JSON module has two key read methods:

Let’s write some code to import JSON and convert the file to a Python format.

Using The json.load() Method

With that code we have now opened, read and converted the used_vehicle.json file to a Python dictionary and we’ve passed it to ‘stock’. Remember that if you are using a path name in the open() command, use a raw string by placing the letter r before the opening speech mark – here’s an example – open(r’C://PythonTutorial/used_vehicles.json’)

Now that we have our data opened and converted, let’s print it and see what we get.

With recognisable raw data we can now apply standard Python techniques to extract our desired information. We’ll break out the car and truck information into a more readable format.

Then finally into something approaching normal text-format.

Note the use of the \t character to insert tabs that will make the printed data look a little more tidy.

Remember that the code we just wrote used json.load() as we were reading from a file. If we had a JSON formatted string we’d be able to use json.loads() – so let’s do that also.

Using The json.loads() Method

With the following example we show a JSON string and then use the json.loads() command to convert the data into a Python object. In this example I’ve also imported pretty print to enable me to show the final output in a format that is somewhat readable, by using the pprint() command rather than the standard print() format.

So to drive the lesson home – remember that the singular load() method reads from a file and converts it to a Python object. The plural loads() method reads from a string and similarly returns a Python object.

Bonus: How Do We Write Data To A JSON File?

We saw in the read examples the use of load() vs loads() so unsurprisingly we have a similar syntax in a write scenario, just reversing the process. The two write methods are:

Let’s take a look at the code necessary for these tasks.

Using The json.dump() Method

First using the json.dump() method which writes to a file– note that I’ve opened a file using the ‘with’ command which prevents us needing to add an extra close() command at the end of the code block making the code tidier.

We also need to add the ‘w’ command to ‘write’ to the file, and finally we use the json.dump() syntax. Note I’ve included an optional ‘indent’ command to help make the new file more readable when opened, by indenting by four spaces.

If we then check the C:\PythonTutorials folder we find the following file, which I’ve opened in Notepad.

Using The json.dumps() Method

The json.dumps() command writes to a JSON string format, so I’ll code that and pass it to a variable which we’ll then print to see how it looks.

As you can see when comparing the two codes, the json.dumps() requires less lines of code. You’re simply passing your dictionary to the method which outputs the JSON string to our employees_json variable. You can then print that or utilise it elsewhere in your programme.

Another Option To Read JSON Files In Python

Python’s standard method of making HTTP requests is called, not surprisingly, requests. Using the get() method, you are able to download a web page and pass it to a response object. Within the requests module lies the ability to convert that response object, if it is in JSON format, into a Python dictionary datatype in the same way we have previously with the json module.

To install the requests library you will need to run the following command;

We then import requests into our code before constructing a get() request. Here’s the code;

You’ll see I’ve used pretty print again to make the output readable and I’ve also truncated the returned data to save space. In all other respects however, the results are very much the same as we’ve seen previously.

To Summarise

In this article we set out to understand the basics of a JSON file and we learned that it is a file type used for easy data interchange which is able to be read by most major programming languages.

JSON files use text in a very specific format, utilising structures similar to Python dictionary and list data-types.

We learned about a Python module called json which enables us to read JSON files and write them to Python objects using the load() and loads() methods, and similarly enables us to take Python data-types and write them to JSON format using the dump() and dumps() modules.

Finally we introduced the requests module which is a Python package used to handle HTTP requests, and which has a json method to convert the recovered JSON data to a Python object.

I trust this article helped with your understanding in how to read a JSON file. Thank you for reading.

David is a Python programmer and a technical writer creating in-depth articles for readers wanting uncomplicated explanations for topics made difficult by industry jargon. Also a woodworker, metalworker, landscape photographer, and pilot, he is freelance after 42 years in the corporate world. He has an MBA in Technology.

How to read JSON file in Python

What is a JSON file?

JSON stands for JavaScript Object Notation. It is commonly used for transmitting data in web applications( such as sending data from server to client to display on the web pages).

Sample JSON File

Read JSON file in Python

Python has an in-built package called json which can be used to work with JSON data and to read JSON files. The json module has many functions among which load() and loads() are used to read the json files.

load() − This function is used to parse or read a json file.

loads() − This function is used to parse a json string.

To use json module in python, we need to import it first. The json module is imported as follows −

Suppose we have json file named “persons.json” with contents as shown in Example 2 above. We want to open and read it using python. This can be done in following steps −

Import json module

Open the file using the name of the json file witn open() function

Open the file using the name of the json file witn open() function

Read the json file using load() and put the json data into a variable.

Use the data retrieved from the file or simply print it as in this case for simplicty.

Example

Output

Note:

Make sure the json file and the python program are saved in the same directory on your system, else an exception would be raised.