In a previous post, I demonstrated how to compare JSON documents using Jackson and Gson, taking advantage of Java 8 streams and Guava for comparing the documents as flat maps.

In this post, I approach the comparison of JSON documents from another perspective, using JSON-P, also known as Java API for JSON Processing.

Table of contents

Representing the differences between two JSON documents

While the approach described in the previous post simply focus in listing the differences between two JSON documents, the approach described in this post focus in producing another JSON document that represents the differences between the two documents that have been compared. And the great thing about this is that the JSON document representing the differences can be applied to the first JSON document that has been compared, yielding the second JSON document that has been compared.

But before diving into the code to perform the comparison, let’s have a look at two standard formats that can be used to represent the differences between JSON documents: JSON Patch and JSON Merge Patch.

These formats are meant to represent set of instructions describing how the target document will be modified. Hence they are suitable for using as payload of PATCH requests (see this post for further details).

JSON Patch

JSON Patch is a format for expressing a sequence of operations to be applied to a JSON document. It is defined in the RFC 6902 and is identified by the application/json-patch+json media type.

The JSON Patch document represents an array of objects and each object represents a single operation to be applied to the target JSON document.

The evaluation of a JSON Patch document begins against a target JSON document and the operations are applied sequentially in the order they appear in the array. Each operation in the sequence is applied to the target document and the resulting document becomes the target of the next operation. The evaluation continues until all operations are successfully applied or until an error condition is encountered.

The operation objects must have exactly one op member, whose value indicates the operation to perform:

Operation Description
add Adds the value at the target location; if the value exists in the given location, it’s replaced
remove Removes the value at the target location
replace Replaces the value at the target location
move Removes the value at a specified location and adds it to the target location
copy Copies the value at a specified location to the target location
test Tests that a value at the target location is equal to a specified value

Any other values are considered errors.

JSON Merge Patch

JSON Merge Patch is a format that describes the changes to be made to a target JSON document using a syntax that closely mimics the document being modified. It is defined in the RFC 7396 is identified by the application/merge-patch+json media type.

The server processing a JSON Merge Patch document determine the exact set of changes being requested by comparing the content of the provided patch against the current content of the target document:

  • If the merge patch contains members that do not appear within the target document, those members are added.
  • If the target does contain the member, the value is replaced.
  • null values in the merge patch indicate that existing values in the target document are to be removed.
  • Other values in the target document will remain untouched.

JSON-P: Java API for JSON Processing

JSON-P, also known as Java API for JSON Processing, defines portable APIs under the javax.json package to parse, generate, transform, and query JSON using object model and streaming APIs.

JSON-P 1.0 was initially defined in the JSR 353 and brought what Oracle calls official support for JSON processing in Java EE. Later, in the JSR 374, JSON-P has evolved and introduced support for JSON Patch and JSON Merge Patch formats to Java EE.

Let’s have a quick look at the API to start getting familiar with it:

Type Description
Json Factory class for creating JSON processing objects
JsonPatch Represents an implementation of JSON Patch
JsonMergePatch Represents an implementation of JSON Merge Patch
JsonValue Represents an immutable JSON value that can be an object, an array, a number, a string, true, false or null
JsonStructure Super type for the two structured types in JSON: object and array

Having said that, it’s important to highlight that JSON-P is just a set of APIs and does nothing on it’s own. To actually work with it, we need an implementation. There are some interesting implementation around and, for this example, we’ll use Apache Johnzon:

<!-- JSON-P: Java API for JSON Processing (JSR 374) -->
<dependency>
    <groupId>javax.json</groupId>
    <artifactId>javax.json-api</artifactId>
    <version>${javax-json.version}</version>
</dependency>

<!-- Apache Johnzon: Implementation of the Java API for JSON Processing (JSR 374) -->
<dependency>
    <groupId>org.apache.johnzon</groupId>
    <artifactId>johnzon-core</artifactId>
    <version>${johnzon.version}</version>
</dependency>

For comparison purposes, JSON-P is to JPA as Apache Johnzon is to Hibernate.

And let me also highlight that our code won’t get tied to any implementation, as we will use only types from the JSON-P API.

Using JSON-P to create JSON documents representing the differences

The Json factory class can be used for creating JSON processing objects, and can be used to create representations of the differences between given source and target documents. And this document with the differences, when applied to the source document, yields the target document.

Creating a JSON Patch document with the differences

To create JSON Patch document with the differences between a source and target documents, we can use the createDiff() method:

JsonPatch diff = Json.createDiff(source, target);

Creating a JSON Merge Patch document with the differences

To create JSON Merge Patch document with the differences between a source and target documents, we can use the createMergeDiff() method:

JsonMergePatch mergeDiff = Json.createMergeDiff(source, target);

Example

For example purposes, let’s consider two JSON documents that represent details of a contact:

{
  "name": {
    "first": "John",
    "last": "Doe"
  },
  "address": null,
  "birthday": "1980-01-01",
  "company": "Acme",
  "occupation": "Software engineer",
  "phones": [
    {
      "number": "000000000",
      "type": "home"
    },
    {
      "number": "999999999",
      "type": "mobile"
    }
  ]
}
{
  "name": {
    "first": "Jane",
    "last": "Doe",
    "nickname": "Jenny"
  },
  "birthday": "1990-01-01",
  "occupation": null,
  "phones": [
    {
      "number": "111111111",
      "type": "mobile"
    }
  ],
  "favorite": true,
  "groups": [
    "close-friends",
    "gym"
  ]
}

Pretty printing JSON documents using JSON-P

For better visualization, we’ll pretty print the JSON documents using the following code:

System.out.println(format(diff.toJsonArray()));
System.out.println(format(mergeDiff.toJsonValue()));
public static String format(JsonValue jsonValue) {
    StringWriter stringWriter = new StringWriter();
    prettyPrint(jsonValue, stringWriter);
    return stringWriter.toString();
}

public static void prettyPrint(JsonValue jsonValue, Writer writer) {
    Map<String, Object> config = Collections.singletonMap(JsonGenerator.PRETTY_PRINTING, true);
    JsonWriterFactory writerFactory = Json.createWriterFactory(config);
    try (JsonWriter jsonWriter = writerFactory.createWriter(writer)) {
        jsonWriter.write(jsonValue);
    }
}

Producing a JSON Patch document with the differences

To produce a JSON Patch with the differences between two JSON documents we can use:

JsonValue source = Json.createReader(new StringReader(leftJsonDocument)).readValue();
JsonValue target = Json.createReader(new StringReader(rightJsonDocument)).readValue();

JsonPatch diff = Json.createDiff(source.asJsonObject(), target.asJsonObject());
System.out.println(format(diff.toJsonArray()));

And it will produce the following output:

[
  {
    "op":"replace",
    "path":"/name/first",
    "value":"Jane"
  },
  {
    "op":"add",
    "path":"/name/nickname",
    "value":"Jenny"
  },
  {
    "op":"remove",
    "path":"/address"
  },
  {
    "op":"replace",
    "path":"/birthday",
    "value":"1990-01-01"
  },
  {
    "op":"remove",
    "path":"/company"
  },
  {
    "op":"replace",
    "path":"/occupation",
    "value":null
  },
  {
    "op":"replace",
    "path":"/phones/0/number",
    "value":"111111111"
  },
  {
    "op":"replace",
    "path":"/phones/0/type",
    "value":"mobile"
  },
  {
    "op":"remove",
    "path":"/phones/1"
  },
  {
    "op":"add",
    "path":"/favorite",
    "value":true
  },
  {
    "op":"add",
    "path":"/groups",
    "value":[
      "close-friends",
      "gym"
    ]
  }
]

Producing a JSON Merge Patch document with the differences

To produce a JSON Merge Patch with the differences between two JSON documents we can use:

JsonValue source = Json.createReader(new StringReader(leftJsonDocument)).readValue();
JsonValue target = Json.createReader(new StringReader(rightJsonDocument)).readValue();

JsonMergePatch mergeDiff = Json.createMergeDiff(source, target);
System.out.println(format(mergeDiff.toJsonValue()));

And the output will be:

{
  "name":{
    "first":"Jane",
    "nickname":"Jenny"
  },
  "address":null,
  "birthday":"1990-01-01",
  "company":null,
  "occupation":null,
  "phones":[
    {
      "number":"111111111",
      "type":"mobile"
    }
  ],
  "favorite":true,
  "groups":[
    "close-friends",
    "gym"
  ]
}

Applying the patch documents

Consider the following code that applies JSON Patch to a document:

JsonPatch diff = ...
JsonValue patched = diff.apply(source.asJsonObject());
System.out.println(format(patched));

It produces:

{
  "name":{
    "last":"Doe",
    "nickname":"Jenny",
    "first":"Jane"
  },
  "groups":[
    "close-friends",
    "gym"
  ],
  "favorite":true,
  "occupation":null,
  "birthday":"1990-01-01",
  "phones":[
    {
      "number":"111111111",
      "type":"mobile"
    }
  ]
}

Now consider the following code that applies JSON Merge Patch to a document:

JsonMergePatch mergeDiff = ...
JsonValue patched = mergeDiff.apply(source);
System.out.println(format(patched));

It produces:

{
  "name":{
    "first":"Jane",
    "last":"Doe",
    "nickname":"Jenny"
  },
  "birthday":"1990-01-01",
  "phones":[
    {
      "number":"111111111",
      "type":"mobile"
    }
  ],
  "favorite":true,
  "groups":[
    "close-friends",
    "gym"
  ]
}

Different results when applying the patches

When the JSON Patch and the JSON Merge Patch documents are applied to the target document, the results are slightly different. Have a closer look to the documents shown above.

The order of the properties of the JSON object is different, but that shouldn’t be taken into account, as a JSON object is an unordered collection of zero or more name/value pairs, according to the RFC 8259:

An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

The key difference here is:

  • In the first example, the occupation property is null.
  • In the second example, the occupation property is omitted.

And it happens due to the null semantics on JSON Merge Patch. Quoting the RFC 7396 (highlight is mine):

If the target does contain the member, the value is replaced. Null values in the merge patch are given special meaning to indicate the removal of existing values in the target. […]

This design means that merge patch documents are suitable for describing modifications to JSON documents that primarily use objects for their structure and do not make use of explicit null values. The merge patch format is not appropriate for all JSON syntaxes.

References