Java json api google

Java Gson + JSON Tutorial with Examples

Gson is google API that effectively coverts JSON into corresponding java object and java object into JSON string. Gson provides options to print JSON in well-defined way. In our example I will provide a complete demo how to use Gson. We will see how to create Gson object and how API can be used to convert between java object and JSON.

Gson Jar to Resolve Dependency

<dependency> <groupId>com.google.code.gson</groupId> <artifactId>gson</artifactId> <version>2.2.4</version> </dependency>

Ways To Create GSON Object

There are two way to create Gson object.

1. This is the simple way and we get all default settings for Gson.

2. GsonBuilder is also used to get instance of Gson that facilitate to set other settings. Suppose we want pretty printing of Json, then we can create Gson object as below.

Gson gson = new GsonBuilder().setPrettyPrinting().create();

From JSON to Java Object Using GSON API

Gson has fromJson() method that accepts JSON string and a class. This method converts JSON into the object of specified class. In our example we have a person data as JSON and person class. Find the JSON string which we will use to create Peron object.
Person JSON

< 'id': 10, 'name': 'Ram', 'address': , 'mobileNums': [ 111111, 222222 ] >

package com.concretepage.gson; public class Address < private String city; private String zip; public String getCity() < return city; >public void setCity(String city) < this.city = city; >public String getZip() < return zip; >public void setZip(String zip) < this.zip = zip; >>

Now find the person clas. Person class has all fields for person JSON. Person class field name and JSON key name must match so that Gson can find the field name to set the value from JSON.
Person.java

package com.concretepage.gson; public class Person < private int id; private String name; private Address address; private long[] mobileNums; public int getId() < return id; >public void setId(int id) < this.id = id; >public String getName() < return name; >public void setName(String name) < this.name = name; >public Address getAddress() < return address; >public void setAddress(Address address) < this.address = address; >public long[] getMobileNums() < return mobileNums; >public void setMobileNums(long[] mobileNums) < this.mobileNums = mobileNums; >>

Now find the main class which will use Gson API to convert JSON into person Object.
JsonToJavaObject.java

package com.concretepage.gson; import com.google.gson.Gson; public class JsonToJavaObject < public static void main(String[] args) < String jsonStr= "< 'id': 10, 'name': 'Ram', 'address': ,'mobileNums': [ 111111, 222222 ]>"; Gson gson = new Gson(); Person person = gson.fromJson(jsonStr, Person.class); System.out.println(person.getName()); System.out.println(person.getAddress().getCity()); long mobNums[] = person.getMobileNums(); System.out.println(mobNums[0]); > >

Now person object has all data which been declared in JSON format. Find the output for some person field.

From Java Object to JSON using GSON API

Gson has toJson() method that accepts an object and returns the corresponding JSON string. In our example we will create a person object and then we will print its JSON format.
JavaObjectToJson.java

package com.concretepage.gson; import com.google.gson.Gson; public class JavaObjectToJson < private static void createPerson(Person person)< person.setId(15); person.setName("Shyam"); Address address = new Address(); address.setCity("Allahabad"); address.setZip("230043"); person.setAddress(address); long[] mobNums = ; person.setMobileNums(mobNums); > public static void main(String[] args) < Person person = new Person(); Gson gson = new Gson(); createPerson(person); System.out.println(gson.toJson(person)); >>

Pretty Printing for JSON Using Gson

When we create instance of Gson using GsonBuilder , we can set different format settings. Gson by default prints compact output of JSON which will be in one line. We can set Pretty Printing and the Gson will print JSON in well formatted. Change main method as below in JavaObjectToJson.java

public static void main(String[] args)

Источник

JSON

A fully pluggable JSON streaming library abstraction allows you to take advantage of the native platform’s built-in JSON library support (for example the JSON library that is built into Android Honeycomb). The streaming library enables you to write optimized code for efficient memory usage that minimizes parsing and serialization time.

A big advantage of this JSON library is that the choice of low-level streaming library is fully pluggable. There are three built-in choices, all of which extend JsonFactory . You can easily plug in your own implementation.

• JacksonFactory : Based on the popular Jackson library, which is considered the fastest in terms of parsing/serialization speed. Our JSON library provides JsonFactory implementations based on Jackson 2.
• GsonFactory : Based on the Google GSON library, which is a lighter-weight option (small size) that is also fairly fast, though not as fast as Jackson.
• AndroidJsonFactory ( @Beta ): Based on the JSON library built into Android Honeycomb (SDK 3.0) and higher, and that is identical to the Google GSON library.

User-defined JSON data models

User-defined JSON data models allow you to define Plain Old Java Objects (POJOs) and define how the library parses and serializes them to and from JSON. The code snippets below are part of a more complete example, YouTube sample, which demonstrates these concepts.

Example

The following JSON snippet shows the relevant fields of a typical YouTube video search:

 "kind": "youtube#searchListResponse", "pageInfo":  "totalResults": 1000000, "resultsPerPage": 5 >, "items": [  "kind": "youtube#searchResult", "id":  "kind": "youtube#video", "videoId": "e6Tudp5lqt8" >, "snippet":  "publishedAt": "2020-06-25T23:18:43Z", "channelId": "UCKwGZZMrhNYKzucCtTPY2Nw", "title": "Video 1 Title", "description": "Video 1 Description", "thumbnails":  "default":  "url": "https://i.ytimg.com/vi/e6Tudp5lqt8/default.jpg", "width": 120, "height": 90 >, "medium":  "url": "https://i.ytimg.com/vi/e6Tudp5lqt8/mqdefault.jpg", "width": 320, "height": 180 >, "high":  "url": "https://i.ytimg.com/vi/e6Tudp5lqt8/hqdefault.jpg", "width": 480, "height": 360 > > > >,  "kind": "youtube#searchResult", "id":  "kind": "youtube#video", "videoId": "o-NtLpiMpw0" >, "snippet":  "publishedAt": "2020-06-25T17:28:52Z", "channelId": "UClljAz6ZKy0XeViKsohdjqA", "title": "Video Title 2", "description": "Video 2 Description", "thumbnails":  "default":  "url": "https://i.ytimg.com/vi/o-NtLpiMpw0/default.jpg", "width": 120, "height": 90 >, "medium":  "url": "https://i.ytimg.com/vi/o-NtLpiMpw0/mqdefault.jpg", "width": 320, "height": 180 >, "high":  "url": "https://i.ytimg.com/vi/o-NtLpiMpw0/hqdefault.jpg", "width": 480, "height": 360 > > > >,  "kind": "youtube#searchResult", "id":  "kind": "youtube#video", "videoId": "TPAahzXZFZo" >, "snippet":  "publishedAt": "2020-06-26T15:45:00Z", "channelId": "UCR4Yfr8HAZJd9X24dwuAt1Q", "title": "Video 3 Title", "description": "Video 3 Description", "thumbnails":  "default":  "url": "https://i.ytimg.com/vi/TPAahzXZFZo/default.jpg", "width": 120, "height": 90 >, "medium":  "url": "https://i.ytimg.com/vi/TPAahzXZFZo/mqdefault.jpg", "width": 320, "height": 180 >, "high":  "url": "https://i.ytimg.com/vi/TPAahzXZFZo/hqdefault.jpg", "width": 480, "height": 360 > > > >,  "kind": "youtube#searchResult", "id":  "kind": "youtube#video", "videoId": "gBL-AelsdFk" >, "snippet":  "publishedAt": "2020-06-24T15:24:06Z", "channelId": "UCFHZHhZaH7Rc_FOMIzUziJA", "title": "Video 4 Title", "description": "Video 4 Description", "thumbnails":  "default":  "url": "https://i.ytimg.com/vi/gBL-AelsdFk/default.jpg", "width": 120, "height": 90 >, "medium":  "url": "https://i.ytimg.com/vi/gBL-AelsdFk/mqdefault.jpg", "width": 320, "height": 180 >, "high":  "url": "https://i.ytimg.com/vi/gBL-AelsdFk/hqdefault.jpg", "width": 480, "height": 360 > > > >,  "kind": "youtube#searchResult", "id":  "kind": "youtube#video", "videoId": "9ofe8axKjH0" >, "snippet":  "publishedAt": "2020-06-26T11:59:32Z", "channelId": "UCtNpbO2MtsVY4qW23WfnxGg", "title": "Video 5 Title", "description": "Video 5 Description", "thumbnails":  "default":  "url": "https://i.ytimg.com/vi/9ofe8axKjH0/default.jpg", "width": 120, "height": 90 >, "medium":  "url": "https://i.ytimg.com/vi/9ofe8axKjH0/mqdefault.jpg", "width": 320, "height": 180 >, "high":  "url": "https://i.ytimg.com/vi/9ofe8axKjH0/hqdefault.jpg", "width": 480, "height": 360 > > > > ] > 

Here’s one possible way to design the Java data classes to represent this:

public static class ListResponse  @Key("items") private ListSearchResult> searchResults; @Key private PageInfo pageInfo; public ListSearchResult> getSearchResults()  return searchResults; > public PageInfo getPageInfo()  return pageInfo; > > public static class PageInfo  @Key private long totalResults; @Key private long resultsPerPage; public long getTotalResults()  return totalResults; > public long getResultsPerPage()  return resultsPerPage; > > public static class SearchResult  @Key private String kind; @Key("id") private VideoId videoId; @Key private Snippet snippet; public String getKind()  return kind; > public VideoId getId()  return videoId; > public Snippet getSnippet()  return snippet; > > public static class VideoId  @Key private String kind; @Key private String videoId; public String getKind()  return kind; > public String getVideoId()  return videoId; > > public static class Snippet  @Key private String publishedAt; @Key private String channelId; @Key private String title; @Key private String description; public String getPublishedAt()  return publishedAt; > public String getChannelId()  return channelId; > public String getTitle()  return title; > public String getDescription()  return description; > public MapString, Thumbnail> getThumbnails()  return thumbnails; > > public static class Thumbnail  @Key private String url; @Key private long width; @Key private long height; public String getUrl()  return url; > public long getWidth()  return width; > public long getHeight()  return height; > > 

A fully supported HTTP JSON parser makes it easy to parse HTTP responses to objects of these user defined classes:

public static ListResponse parseJson(HttpResponse httpResponse) throws IOException  ListResponse listResponse = httpResponse.parseAs(ListResponse.class); if (listResponse.getSearchResults().isEmpty())  System.out.println("No results found."); > else  for (SearchResult searchResult : listResponse.getSearchResults())  System.out.println(); System.out.println("-----------------------------------------------"); System.out.println("Kind: " + searchResult.getKind()); System.out.println("Video ID: " + searchResult.getId().getVideoId()); System.out.println("Title: " + searchResult.getSnippet().getTitle()); System.out.println("Description: " + searchResult.getSnippet().getDescription()); > > return listResponse; > 

Key annotation

Use the @Key annotation to indicate fields that need to be parsed from or serialized to JSON. By default, the @Key annotation uses the Java field name as the JSON key. To override this, specify the value of the @Key annotation.

Fields that don’t have the @Key annotation are considered internal data and are not parsed from or serialized to JSON.

Visibility

Visibility of the fields does not matter, nor does the existence of the getter or setter methods. So for example, the following alternative representation for VideoId would work in the example given above:

public static class VideoId  @Key public String kind; @Key public String videoId; > 

GenericJson

Normally only the fields you declare are parsed when a JSON response is parsed. The actual Google+ activity feed response contains a lot of content that we are not using in our example. The JSON parser skips that other content when parsing the response from Google+.

To retain the other content, declare your class to extend GenericJson . Notice that GenericJson implements Map , so we can use the get and put methods to set JSON content. See Youtube sample for an example of how it was used in the Snippet class above.

Map

The JSON library supports any implementation of Map , which works similarly to GenericJson . The downside, of course, is that you lose the static type information for the fields.

JSON null

One advantage of this JSON library is its ability to support JSON nulls and distinguish them from undeclared JSON keys. Although JSON nulls are relatively rare, when they do occur they often cause confusion.

Google+ doesn’t use JSON null values, so the following example uses fictitious JSON data to illustrate what can happen:

 "items": [  "id": "1", "value": "some value" >,  "id": "2", "value": null >  "id": "3" > ] > 

We might represent each item as follows:

public class Item  @Key public String id; @Key public String value; > 

For items 2 and 3, what should be in the value field? The problem is that there is no obvious way in Java to distinguish between a JSON key that is undeclared and a JSON key whose value is JSON null. This JSON library solves the problem by using Java null for the common case of an undeclared JSON key, and a special “magic” instance of String ( Data.NULL_STRING ) to identify it as a JSON null rather than a normal value.

The following example shows how you might take advantage of this functionality:

private static void show(ListItem> items)  for (Item item : items)  System.out.println("ID: " + item.id); if (item.value == null)  System.out.println("No Value"); > else if (Data.isNull(item.value))  System.out.print("Null Value"); > else  System.out.println("Value: '" + item.value + "'"); > > > 

Источник