Json Path 语法详解（Java）

Posted 2021-12-24 yanximin

Jayway JsonPath

A Java DSL for reading JSON documents.

Jayway JsonPath is a Java port of Stefan Goessner JsonPath implementation.

News

05 Jul 2017 - Released JsonPath 2.4.0

26 Jun 2017 - Released JsonPath 2.3.0

29 Feb 2016 - Released JsonPath 2.2.0

22 Nov 2015 - Released JsonPath 2.1.0

19 Mar 2015 - Released JsonPath 2.0.0

11 Nov 2014 - Released JsonPath 1.2.0

01 Oct 2014 - Released JsonPath 1.1.0

26 Sep 2014 - Released JsonPath 1.0.0

Getting Started

JsonPath is available at the Central Maven Repository. Maven users add this to your POM.

<dependency>
    <groupId>com.jayway.jsonpath</groupId>
    <artifactId>json-path</artifactId>
    <version>2.3.0</version>
</dependency>

If you need help ask questions at Stack Overflow. Tag the question ‘jsonpath‘ and ‘java‘.

JsonPath expressions always refer to a JSON structure in the same way as XPath expression are used in combination
with an XML document. The "root member object" in JsonPath is always referred to as $ regardless if it is an
object or array.

JsonPath expressions can use the dot–notation

$.store.book[0].title

or the bracket–notation

$[‘store‘][‘book‘][0][‘title‘]

Operators

Operator	Description
$	The root element to query. This starts all path expressions.
@	The current node being processed by a filter predicate.
*	Wildcard. Available anywhere a name or numeric are required.
..	Deep scan. Available anywhere a name is required.
.<name>	Dot-notated child
[‘<name>‘ (, ‘<name>‘)]	Bracket-notated child or children
[<number> (, <number>)]	Array index or indexes
[start:end]	Array slice operator
[?(<expression>)]	Filter expression. Expression must evaluate to a boolean value.

Functions

Functions can be invoked at the tail end of a path - the input to a function is the output of the path expression.
The function output is dictated by the function itself.

Function	Description	Output
min()	Provides the min value of an array of numbers	Double
max()	Provides the max value of an array of numbers	Double
avg()	Provides the average value of an array of numbers	Double
stddev()	Provides the standard deviation value of an array of numbers	Double
length()	Provides the length of an array	Integer

Filter Operators

Filters are logical expressions used to filter arrays. A typical filter would be [?(@.age > 18)] where @ represents the current item being processed. More complex filters can be created with logical operators && and ||. String literals must be enclosed by single or double quotes ([?(@.color == ‘blue‘)] or [?(@.color == "blue")]).

Operator	Description
==	left is equal to right (note that 1 is not equal to ‘1‘)
!=	left is not equal to right
<	left is less than right
<=	left is less or equal to right
>	left is greater than right
>=	left is greater than or equal to right
=~	left matches regular expression [?(@.name =~ /foo.*?/i)]
in	left exists in right [?(@.size in [‘S‘, ‘M‘])]
nin	left does not exists in right
subsetof	left is a subset of right [?(@.sizes subsetof [‘S‘, ‘M‘, ‘L‘])]
size	size of left (array or string) should match right
empty	left (array or string) should be empty

Path Examples

Given the json

    "store": 
        "book": [
            
                "category": "reference",
                "author": "Nigel Rees",
                "title": "Sayings of the Century",
                "price": 8.95
            ,
            
                "category": "fiction",
                "author": "Evelyn Waugh",
                "title": "Sword of Honour",
                "price": 12.99
            ,
            
                "category": "fiction",
                "author": "Herman Melville",
                "title": "Moby Dick",
                "isbn": "0-553-21311-3",
                "price": 8.99
            ,
            
                "category": "fiction",
                "author": "J. R. R. Tolkien",
                "title": "The Lord of the Rings",
                "isbn": "0-395-19395-8",
                "price": 22.99
            
        ],
        "bicycle": 
            "color": "red",
            "price": 19.95
        
    ,
    "expensive": 10

JsonPath (click link to try)	Result
$.store.book[*].author	The authors of all books
$..author	All authors
$.store.*	All things, both books and bicycles
$.store..price	The price of everything
$..book[2]	The third book
$..book[-2]	The second to last book
$..book[0,1]	The first two books
$..book[:2]	All books from index 0 (inclusive) until index 2 (exclusive)
$..book[1:2]	All books from index 1 (inclusive) until index 2 (exclusive)
$..book[-2:]	Last two books
$..book[2:]	Book number two from tail
$..book[?(@.isbn)]	All books with an ISBN number
$.store.book[?(@.price < 10)]	All books in store cheaper than 10
$..book[?(@.price <= $[‘expensive‘])]	All books in store that are not "expensive"
$..book[?(@.author =~ /.*REES/i)]	All books matching regex (ignore case)
$..*	Give me every thing
$..book.length()	The number of books

Reading a Document

The simplest most straight forward way to use JsonPath is via the static read API.

String json = "...";

List<String> authors = JsonPath.read(json, "$.store.book[*].author");

If you only want to read once this is OK. In case you need to read an other path as well this is not the way
to go since the document will be parsed every time you call JsonPath.read(...). To avoid the problem you can
parse the json first.

String json = "...";
Object document = Configuration.defaultConfiguration().jsonProvider().parse(json);

String author0 = JsonPath.read(document, "$.store.book[0].author");
String author1 = JsonPath.read(document, "$.store.book[1].author");

JsonPath also provides a fluent API. This is also the most flexible one.

String json = "...";

ReadContext ctx = JsonPath.parse(json);

List<String> authorsOfBooksWithISBN = ctx.read("$.store.book[?(@.isbn)].author");


List<Map<String, Object>> expensiveBooks = JsonPath
                            .using(configuration)
                            .parse(json)
                            .read("$.store.book[?(@.price > 10)]", List.class);

What is Returned When?

When using JsonPath in java its important to know what type you expect in your result. JsonPath will automatically
try to cast the result to the type expected by the invoker.

//Will throw an java.lang.ClassCastException    
List<String> list = JsonPath.parse(json).read("$.store.book[0].author")

//Works fine
String author = JsonPath.parse(json).read("$.store.book[0].author")

When evaluating a path you need to understand the concept of when a path is definite. A path is indefinite if it contains:

• .. - a deep scan operator
• ?(<expression>) - an expression
• [<number>, <number> (, <number>)] - multiple array indexes

Indefinite paths always returns a list (as represented by current JsonProvider).

By default a simple object mapper is provided by the MappingProvider SPI. This allows you to specify the return type you want and the MappingProvider will
try to perform the mapping. In the example below mapping between Long and Date is demonstrated.

String json = "\"date_as_long\" : 1411455611975";

Date date = JsonPath.parse(json).read("$['date_as_long']", Date.class);

If you configure JsonPath to use JacksonMappingProvider or GsonMappingProvider` you can even map your JsonPath output directly into POJO‘s.

Book book = JsonPath.parse(json).read("$.store.book[0]", Book.class);

To obtainin full generics type information, use TypeRef.

TypeRef<List<String>> typeRef = new TypeRef<List<String>>() ;

List<String> titles = JsonPath.parse(JSON_DOCUMENT).read("$.store.book[*].title", typeRef);

Predicates

There are three different ways to create filter predicates in JsonPath.

Inline Predicates

Inline predicates are the ones defined in the path.

List<Map<String, Object>> books =  JsonPath.parse(json)
                                     .read("$.store.book[?(@.price < 10)]");

You can use && and || to combine multiple predicates [?(@.price < 10 && @.category == ‘fiction‘)] ,
[?(@.category == ‘reference‘ || @.price > 10)].

You can use ! to negate a predicate [?(!(@.price < 10 && @.category == ‘fiction‘))].

Filter Predicates

Predicates can be built using the Filter API as shown below:

import static com.jayway.jsonpath.JsonPath.parse;
import static com.jayway.jsonpath.Criteria.where;
import static com.jayway.jsonpath.Filter.filter;
...
...

Filter cheapFictionFilter = filter(
   where("category").is("fiction").and("price").lte(10D)
);

List<Map<String, Object>> books =  
   parse(json).read("$.store.book[?]", cheapFictionFilter);

Notice the placeholder ? for the filter in the path. When multiple filters are provided they are applied in order where the number of placeholders must match
the number of provided filters. You can specify multiple predicate placeholders in one filter operation [?, ?], both predicates must match.

Filters can also be combined with ‘OR‘ and ‘AND‘

Filter fooOrBar = filter(
   where("foo").exists(true)).or(where("bar").exists(true)
);
   
Filter fooAndBar = filter(
   where("foo").exists(true)).and(where("bar").exists(true)
);

Roll Your Own

Third option is to implement your own predicates

Predicate booksWithISBN = new Predicate() 
    @Override
    public boolean apply(PredicateContext ctx) 
        return ctx.item(Map.class).containsKey("isbn");
    
;

List<Map<String, Object>> books = 
   reader.read("$.store.book[?].isbn", List.class, booksWithISBN);

Path vs Value

In the Goessner implementation a JsonPath can return either Path or Value. Value is the default and what all the examples above are returning. If you rather have the path of the elements our query is hitting this can be acheived with an option.

Configuration conf = Configuration.builder()
   .options(Option.AS_PATH_LIST).build();

List<String> pathList = using(conf).parse(json).read("$..author");

assertThat(pathList).containsExactly(
    "$['store']['book'][0]['author']",
    "$['store']['book'][1]['author']",
    "$['store']['book'][2]['author']",
    "$['store']['book'][3]['author']");

Tweaking Configuration

Options

When creating your Configuration there are a few option flags that can alter the default behaviour.

DEFAULT_PATH_LEAF_TO_NULL


This option makes JsonPath return null for missing leafs. Consider the following json

[
   
      "name" : "john",
      "gender" : "male"
   ,
   
      "name" : "ben"
   
]

Configuration conf = Configuration.defaultConfiguration();

//Works fine
String gender0 = JsonPath.using(conf).parse(json).read("$[0]['gender']");
//PathNotFoundException thrown
String gender1 = JsonPath.using(conf).parse(json).read("$[1]['gender']");

Configuration conf2 = conf.addOptions(Option.DEFAULT_PATH_LEAF_TO_NULL);

//Works fine
String gender0 = JsonPath.using(conf2).parse(json).read("$[0]['gender']");
//Works fine (null is returned)
String gender1 = JsonPath.using(conf2).parse(json).read("$[1]['gender']");

ALWAYS_RETURN_LIST


This option configures JsonPath to return a list even when the path is definite.

Configuration conf = Configuration.defaultConfiguration();

//Works fine
List<String> genders0 = JsonPath.using(conf).parse(json).read("$[0]['gender']");
//PathNotFoundException thrown
List<String> genders1 = JsonPath.using(conf).parse(json).read("$[1]['gender']");

SUPPRESS_EXCEPTIONS


This option makes sure no exceptions are propagated from path evaluation. It follows these simple rules:

• If option ALWAYS_RETURN_LIST is present an empty list will be returned
• If option ALWAYS_RETURN_LIST is NOT present null returned

JsonProvider SPI

JsonPath is shipped with three different JsonProviders:

• JsonSmartJsonProvider (default)
• JacksonJsonProvider
• JacksonJsonNodeJsonProvider
• GsonJsonProvider
• JsonOrgJsonProvider

Changing the configuration defaults as demonstrated should only be done when your application is being initialized. Changes during runtime is strongly discouraged, especially in multi threaded applications.

Configuration.setDefaults(new Configuration.Defaults() 

    private final JsonProvider jsonProvider = new JacksonJsonProvider();
    private final MappingProvider mappingProvider = new JacksonMappingProvider();
      
    @Override
    public JsonProvider jsonProvider() 
        return jsonProvider;
    

    @Override
    public MappingProvider mappingProvider() 
        return mappingProvider;
    
    
    @Override
    public Set<Option> options() 
        return EnumSet.noneOf(Option.class);
    
);

Note that the JacksonJsonProvider requires com.fasterxml.jackson.core:jackson-databind:2.4.5 and the GsonJsonProvider requires com.google.code.gson:gson:2.3.1 on your classpath.

Cache SPI

In JsonPath 2.1.0 a new Cache SPI was introduced. This allows API consumers to configure path caching in a way that suits their needs. The cache must be configured before it is accesses for the first time or a JsonPathException is thrown. JsonPath ships with two cache implementations

• com.jayway.jsonpath.spi.cache.LRUCache (default, thread safe)
• com.jayway.jsonpath.spi.cache.NOOPCache (no cache)

If you want to implement your own cache the API is simple.

CacheProvider.setCache(new Cache() 
    //Not thread safe simple cache
    private Map<String, JsonPath> map = new HashMap<String, JsonPath>();

    @Override
    public JsonPath get(String key) 
        return map.get(key);
    

    @Override
    public void put(String key, JsonPath jsonPath) 
        map.put(key, jsonPath);
    
);