JsonPath用法
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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
$
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.
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")]
).
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}
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 fineString 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 fineString gender0 = JsonPath.using(conf).parse(json).read("$[0]['gender']");//PathNotFoundException thrownString gender1 = JsonPath.using(conf).parse(json).read("$[1]['gender']");Configuration conf2 = conf.addOptions(Option.DEFAULT_PATH_LEAF_TO_NULL);//Works fineString 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 fineList<String> genders0 = JsonPath.using(conf).parse(json).read("$[0]['gender']");//PathNotFoundException thrownList<String> genders1 = JsonPath.using(conf).parse(json).read("$[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); }});
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