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= Processing JSON

Groovy comes with integrated support for converting between Groovy objects and JSON. The classes dedicated to
JSON serialisation and parsing are found in the `groovy.json` package.

[[json_jsonslurper]]
== JsonSlurper

`JsonSlurper` is a class that parses JSON text or reader content into Groovy data structures (objects) such as maps, lists and
primitive types like `Integer`, `Double`, `Boolean` and `String`.

The class comes with a bunch of overloaded `parse` methods plus some special methods such as `parseText`,
`parseFile` and others. For the next example we will use the `parseText` method. It parses a JSON `String` and recursively converts it to a
list or map of objects. The other `parse*` methods are similar in that they return a JSON `String` but for different parameter
types.

[source,groovy]
----
include::{rootProjectDir}/subprojects/groovy-json/src/spec/test/json/JsonTest.groovy[tags=parse_text,indent=0]
----

Notice the result is a plain map and can be handled like a normal Groovy object instance. `JsonSlurper` parses the
given JSON as defined by the http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf[ECMA-404 JSON Interchange Standard]
plus support for JavaScript comments and dates.

In addition to maps `JsonSlurper` supports JSON arrays which are converted to lists.

[source,groovy]
----
include::{rootProjectDir}/subprojects/groovy-json/src/spec/test/json/JsonTest.groovy[tags=parse_list,indent=0]
----

The JSON standard supports the following primitive data types: string, number, object, `true`, `false` and `null`. `JsonSlurper`
converts these JSON types into corresponding Groovy types.

[source,groovy]
----
include::{rootProjectDir}/subprojects/groovy-json/src/spec/test/json/JsonTest.groovy[tags=parse_number,indent=0]
----

As `JsonSlurper` is returning pure Groovy object instances without any special JSON classes in the back, its usage
is transparent. In fact, `JsonSlurper` results conform to GPath expressions. GPath is a powerful expression language
that is supported by multiple slurpers for different data formats (`XmlSlurper` for XML being one example).

[NOTE]
For more details please have a look at the section on <<core-semantics.adoc#gpath_expressions,GPath expressions>>.

The following table gives an overview of the JSON types and the corresponding Groovy data types:

[cols="1,3" options="header"]
|===
|JSON
|Groovy

|string
|`java.lang.String`

|number
|`java.lang.BigDecimal` or `java.lang.Integer`

|object
|`java.util.LinkedHashMap`

|array
|`java.util.ArrayList`

|true
|`true`

|false
|`false`

|null
|`null`

|date
|`java.util.Date` based on the `yyyy-MM-dd'T'HH:mm:ssZ` date format
|===

[NOTE]
Whenever a value in JSON is `null`, `JsonSlurper` supplements it with the Groovy `null` value. This is in contrast to other
JSON parsers that represent a `null` value with a library-provided singleton object.

=== Parser Variants

`JsonSlurper` comes with a couple of parser implementations. Each parser fits different requirements, it could well be that for certain
scenarios the `JsonSlurper` default parser is not the best bet for all situations. Here is an overview of the shipped parser implementations:

* The `JsonParserCharArray` parser basically takes a JSON string and operates on the underlying character array. During value
conversion it copies character sub-arrays (a mechanism known as "chopping") and operates on them.

* The `JsonFastParser` is a special variant of the `JsonParserCharArray` and is the fastest parser. However, it is not the
default parser for a reason. `JsonFastParser` is a so-called index-overlay parser. During parsing of the given JSON `String` it
tries as hard as possible to avoid creating new char arrays or `String` instances. It keeps pointers to
the underlying original character array only. In addition, it defers object creation as late as possible. If parsed maps are
put into long-term caches care must be taken as the map objects might not be created and still consist of pointer to the
original char buffer only. However, `JsonFastParser` comes with a special chop mode which dices up the char buffer
early to keep a small copy of the original buffer. Recommendation is to use the `JsonFastParser` for JSON buffers
under 2MB and keeping the long-term cache restriction in mind.

* The `JsonParserLax` is a special variant of the `JsonParserCharArray` parser. It has similar performance characteristics as `JsonFastParser`
 but differs in that it isn't exclusively relying on the ECMA-404 JSON grammar. For example it allows for comments, no quote strings etc.

* The `JsonParserUsingCharacterSource` is a special parser for very large files. It uses a technique called "character windowing" to
parse large JSON files (large means files over 2MB size in this case) with constant performance characteristics.

The default parser implementation for `JsonSlurper` is `JsonParserCharArray`. The `JsonParserType` enumeration contains constants for
the parser implementations described above:

[cols="1,3" options="header"]
|===
|Implementation
|Constant

|`JsonParserCharArray`
|`JsonParserType#CHAR_BUFFER`

|`JsonFastParser`
|`JsonParserType#INDEX_OVERLAY`

|`JsonParserLax`
|`JsonParserType#LAX`

|`JsonParserUsingCharacterSource`
|`JsonParserType#CHARACTER_SOURCE`
|===

Changing the parser implementation is as easy as setting the `JsonParserType` with a call to `JsonSlurper#setType()`.

[source,groovy]
----
include::{rootProjectDir}/subprojects/groovy-json/src/spec/test/json/JsonTest.groovy[tags=set_type,indent=0]
----

== JsonOutput

`JsonOutput` is responsible for serialising Groovy objects into JSON strings. It can be seen as companion object to
<<json-userguide.adoc#json_jsonslurper,JsonSlurper>>, being a JSON parser.

`JsonOutput` comes with overloaded, static `toJson` methods. Each `toJson` implementation takes a different parameter type.
The static method can either be used directly or by importing the methods with a static import statement.

The result of a `toJson` call is a `String` containing the JSON code.

[source,groovy]
----
include::{rootProjectDir}/subprojects/groovy-json/src/spec/test/json/JsonTest.groovy[tags=json_output,indent=0]
----

`JsonOutput` does not only support primitive, maps or list data types to be serialized to JSON, it goes further and even
has support for serialising POGOs, that is, plain-old Groovy objects.

[source,groovy]
----
include::{rootProjectDir}/subprojects/groovy-json/src/spec/test/json/JsonTest.groovy[tags=json_output_pogo,indent=0]
----

As we saw in previous examples, the JSON output is not pretty printed per default. However, the `prettyPrint` method in `JsonOutput` comes
to rescue for this task.

[source,groovy]
----
include::{rootProjectDir}/subprojects/groovy-json/src/spec/test/json/JsonTest.groovy[tags=pretty_print,indent=0]
----

`prettyPrint` takes a `String` as single parameter; therefore, it can be applied on arbitrary JSON `String` instances, not only the result of
`JsonOutput.toJson`.

Another way to create JSON from Groovy is to use `JsonBuilder` or `StreamingJsonBuilder`. Both builders provide a
DSL which allows to formulate an object graph which is then converted to JSON.

[NOTE]
For more details on builders, have a look at the builders chapter which covers both <<core-domain-specific-languages.adoc#_jsonbuilder,JsonBuilder>>
and <<core-domain-specific-languages.adoc#_streamingjsonbuilder,StreamingJsonBuilder>>.
