XML and JSON are the two most common formats for data interchange in the Web today. XML was created by the W3C in 1996, and JSON was publicly specified by Douglas Crockford in 2002. Although their purposes are not identical, they are frequently used to accomplish the same task, which is data interchange. Both have well-documented open standards on the Web (RFC 7159, RFC 4825), and both are human and machine-readable. Neither one is absolutely superior to the other, as each is better suited for different use cases.
XML Advantages
There are several advantages that XML has over JSON. One of the biggest differences between the two is that in XML you can put metadata into the tags in the form of attributes. With JSON, the programmer could accomplish the same goal that metadata achieves by making the entity an object and adding the attributes as members of the object. However, the way XML does it may often be preferable, given the slightly misleading nature of turning something into an object that is not one in the client program. For example, if your C++ program sends an int via JSON and needs metadata to be sent along with it, you would have to make it an object, with one name/value pair for the actual value of the int, and more name/value pairs for each attribute. The program that receives the JSON would read it as an object, when in fact it is not one. While this is a viable solution, it defies one of JSON’s key advantages: “JSON’s structures look like conventional programming language structures. No restructuring is necessary.”[2]
Although I will argue later that this can also be a drawback of XML, its mechanism to resolve name conflicts, prefixes, gives it power that JSON does not have. With prefixes, the programmer has the ability to name two different kinds of entities the same thing.[1] This would be advantageous in situations where the different entities should have the same name in the client program, perhaps if they are used in entirely different scopes.
Another advantage of XML is that most browsers render it in a highly readable and organized way. The tree structure of XML lends itself well to this formatting, and allows for browsers to let users to naturally collapse individual tree elements. This feature would be particularly useful in debugging.
One of the most significant advantages that XML has over JSON is its ability to communicate mixed content, i.e. strings that contain structured markup. In order to handle this with XML, the programmer need only put the marked-up text within a child tag of the parent in which it belongs. Similar to the metadata situation, since JSON only contains data, there is no such simple way to indicate markup. It would again require storing metadata as data, which could be considered an abuse of the format.
JSON Advantages
JSON has several advantages as well. One of the most obvious of these is that JSON is significantly less verbose than XML, because XML necessitates opening and closing tags (or in some cases less verbose self-closing tags), and JSON uses name/value pairs, concisely delineated by “{“ and “}” for objects, “[“ and “]” for arrays, “,” to separate pairs, and “:” to separate name from value. Even when zipped (using gzip), JSON is still smaller and it takes less time to zip it.[6] As determined by Sumaray and Makki as well as Nurseitov, Paulson, Reynolds, and Izurieta in their experimental findings, JSON outperforms XML in a number of ways. First, naturally following from its conciseness, JSON files that contain the same information as their XML counterparts are almost always significantly smaller, which leads to faster transmission and processing. Second, difference in size aside, both groups found that JSON was serialized and deserialized drastically faster than XML.[3][4] Third, the latter study determined that JSON processing outdoes XML in CPU resource utilization. They found that JSON used less total resources, more user CPU, and less system CPU. The experiment used RedHat machines, and RedHat claims that higher user CPU usage is preferable.[3] Unsurprisingly, the Sumaray and Makki study determined that JSON performance is superior to XML on mobile devices too.[4] This makes sense, given that JSON uses less resources, and mobile devices are less powerful than desktop machines.
Yet another advantage that JSON has over XML is that its representation of objects and arrays allows for direct mapping onto the corresponding data structures in the host language, such as objects, records, structs, dictionaries, hash tables, keyed lists, and associative arrays for objects, and arrays, vectors, lists, and sequences for arrays.[2] Although it is perfectly possible to represent these structures in XML, it is only as a function of the parsing, and it takes more code to serialize and deserialize properly. It also would not always be obvious to the reader of arbitrary XML what tags represent an object and what tags represent an array, especially because nested tags can just as easily be structured markup instead. The curly braces and brackets of JSON definitively show the structure of the data. However, this advantage does come with the caveat explained above, that the JSON can inaccurately represent the data if the need arises to send metadata.
Although XML supports namespaces and prefixes, JSON’s handling of name collisions is less verbose than prefixes, and arguably feels more natural with the program using it; in JSON, each object is its own namespace, so names may be repeated as long as they are in different scopes. This may be preferable, as in most programming languages members of different objects can have the same name, because they are distinguished by the names of the objects to which they belong.
Perhaps the most significant advantage that JSON has over XML is that JSON is a subset of JavaScript, so code to parse and package it fits very naturally into JavaScript code. This seems highly beneficial for JavaScript programs, but does not directly benefit any programs that use languages other than JavaScript. However, this drawback has been largely overcome, as currently the JSON website lists over 175 tools for 64 different programming languages that exist to integrate JSON processing. While I cannot speak to the quality of most of these tools, it is clear that the developer community has embraced JSON and has made it simple to use in many different platforms.
Purposes
Simply put, XML’s purpose is document markup. This is decidedly not a purpose of JSON, so XML should be used whenever this is what needs to be done. It accomplishes this purpose by giving semantic meaning to text through its tree-like structure and ability to represent mixed content. Data structures can be represented in XML, but that is not its purpose.
JSON’s purpose is structured data interchange. It serves this purpose by directly representing objects, arrays, numbers, strings, and booleans. Its purpose is distinctly not document markup. As described above, JSON does not have a natural way to represent mixed content.