How to parse JSON in Java

2020-01-22 10:00发布

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问题:

I have the following JSON text. How can I parse it to get the values of pageName, pagePic, post_id, etc.?

{
   "pageInfo": {
         "pageName": "abc",
         "pagePic": "http://example.com/content.jpg"
    },
    "posts": [
         {
              "post_id": "123456789012_123456789012",
              "actor_id": "1234567890",
              "picOfPersonWhoPosted": "http://example.com/photo.jpg",
              "nameOfPersonWhoPosted": "Jane Doe",
              "message": "Sounds cool. Can't wait to see it!",
              "likesCount": "2",
              "comments": [],
              "timeOfPost": "1234567890"
         }
    ]
}

回答1:

The org.json library is easy to use. Example code below:

import org.json.*;


JSONObject obj = new JSONObject(" .... ");
String pageName = obj.getJSONObject("pageInfo").getString("pageName");

JSONArray arr = obj.getJSONArray("posts");
for (int i = 0; i < arr.length(); i++)
{
    String post_id = arr.getJSONObject(i).getString("post_id");
    ......
}

You may find more examples from: Parse JSON in Java

Downloadable jar: http://mvnrepository.com/artifact/org.json/json

回答2:

For the sake of the example lets assume you have a class Person with just a name.

private class Person {
    public String name;

    public Person(String name) {
        this.name = name;
    }
}

Google GSON (Maven)

My personal favourite as to the great JSON serialisation / de-serialisation of objects.

Gson g = new Gson();

Person person = g.fromJson("{\"name\": \"John\"}", Person.class);
System.out.println(person.name); //John

System.out.println(g.toJson(person)); // {"name":"John"}

Update

If you want to get a single attribute out you can do it easily with the Google library as well:

JsonObject jsonObject = new JsonParser().parse("{\"name\": \"John\"}").getAsJsonObject();

System.out.println(jsonObject.get("name").getAsString()); //John

Org.JSON (Maven)

If you don't need object de-serialisation but to simply get an attribute, you can try org.json (or look GSON example above!)

JSONObject obj = new JSONObject("{\"name\": \"John\"}");

System.out.println(obj.getString("name")); //John

Jackson (Maven)

ObjectMapper mapper = new ObjectMapper();
Person user = mapper.readValue("{\"name\": \"John\"}", Person.class);

System.out.println(user.name); //John

回答3:

If one wants to create Java object from JSON and vice versa, use GSON or JACKSON third party jars etc.

//from object to JSON 
Gson gson = new Gson();
gson.toJson(yourObject);

// from JSON to object 
yourObject o = gson.fromJson(JSONString,yourObject.class);

But if one just want to parse a JSON string and get some values, (OR create a JSON string from scratch to send over wire) just use JaveEE jar which contains JsonReader, JsonArray, JsonObject etc. You may want to download the implementation of that spec like javax.json. With these two jars I am able to parse the json and use the values.

These APIs actually follow the DOM/SAX parsing model of XML.
```
Response response = request.get(); // REST call 
    JsonReader jsonReader = Json.createReader(new StringReader(response.readEntity(String.class)));
    JsonArray jsonArray = jsonReader.readArray();
    ListIterator l = jsonArray.listIterator();
    while ( l.hasNext() ) {
          JsonObject j = (JsonObject)l.next();
          JsonObject ciAttr = j.getJsonObject("ciAttributes");
```

回答4:

quick-json parser is very straightforward, flexible, very fast and customizable. Try it

Features:

Compliant with JSON specification (RFC4627)
High-Performance JSON parser
Supports Flexible/Configurable parsing approach
Configurable validation of key/value pairs of any JSON Hierarchy
Easy to use # Very small footprint
Raises developer friendly and easy to trace exceptions
Pluggable Custom Validation support - Keys/Values can be validated by configuring custom validators as and when encountered
Validating and Non-Validating parser support
Support for two types of configuration (JSON/XML) for using quick-JSON validating parser
Requires JDK 1.5
No dependency on external libraries
Support for JSON Generation through object serialisation
Support for collection type selection during parsing process

It can be used like this:

JsonParserFactory factory=JsonParserFactory.getInstance();
JSONParser parser=factory.newJsonParser();
Map jsonMap=parser.parseJson(jsonString);

回答5:

A - Explanation

You can use Jackson libraries, for binding JSON String into POJO (Plain Old Java Object) instances. POJO is simply a class with only private fields and public getter/setter methods. Jackson is going to traverse the methods (using reflection), and maps the JSON object into the POJO instance as the field names of the class fits to the field names of the JSON object.

In your JSON object, which is actually a composite object, the main object consists o two sub-objects. So, our POJO classes should have the same hierarchy. I'll call the whole JSON Object as Page object. Page object consist of a PageInfo object, and a Post object array.

So we have to create three different POJO classes;

Page Class, a composite of PageInfo Class and array of Post Instances
PageInfo Class
Posts Class

The only package I've used is Jackson ObjectMapper, what we do is binding data;

com.fasterxml.jackson.databind.ObjectMapper

The required dependencies, the jar files is listed below;

jackson-core-2.5.1.jar
jackson-databind-2.5.1.jar
jackson-annotations-2.5.0.jar

Here is the required code;

B - Main POJO Class : Page

package com.levo.jsonex.model;

public class Page {

    private PageInfo pageInfo;
    private Post[] posts;

    public PageInfo getPageInfo() {
        return pageInfo;
    }

    public void setPageInfo(PageInfo pageInfo) {
        this.pageInfo = pageInfo;
    }

    public Post[] getPosts() {
        return posts;
    }

    public void setPosts(Post[] posts) {
        this.posts = posts;
    }

}

C - Child POJO Class : PageInfo

package com.levo.jsonex.model;

public class PageInfo {

    private String pageName;
    private String pagePic;

    public String getPageName() {
        return pageName;
    }

    public void setPageName(String pageName) {
        this.pageName = pageName;
    }

    public String getPagePic() {
        return pagePic;
    }

    public void setPagePic(String pagePic) {
        this.pagePic = pagePic;
    }

}

D - Child POJO Class : Post

package com.levo.jsonex.model;

public class Post {

    private String post_id;
    private String actor_id;
    private String picOfPersonWhoPosted;
    private String nameOfPersonWhoPosted;
    private String message;
    private int likesCount;
    private String[] comments;
    private int timeOfPost;

    public String getPost_id() {
        return post_id;
    }

    public void setPost_id(String post_id) {
        this.post_id = post_id;
    }

    public String getActor_id() {
        return actor_id;
    }

    public void setActor_id(String actor_id) {
        this.actor_id = actor_id;
    }

    public String getPicOfPersonWhoPosted() {
        return picOfPersonWhoPosted;
    }

    public void setPicOfPersonWhoPosted(String picOfPersonWhoPosted) {
        this.picOfPersonWhoPosted = picOfPersonWhoPosted;
    }

    public String getNameOfPersonWhoPosted() {
        return nameOfPersonWhoPosted;
    }

    public void setNameOfPersonWhoPosted(String nameOfPersonWhoPosted) {
        this.nameOfPersonWhoPosted = nameOfPersonWhoPosted;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public int getLikesCount() {
        return likesCount;
    }

    public void setLikesCount(int likesCount) {
        this.likesCount = likesCount;
    }

    public String[] getComments() {
        return comments;
    }

    public void setComments(String[] comments) {
        this.comments = comments;
    }

    public int getTimeOfPost() {
        return timeOfPost;
    }

    public void setTimeOfPost(int timeOfPost) {
        this.timeOfPost = timeOfPost;
    }

}

E - Sample JSON File : sampleJSONFile.json

I've just copied your JSON sample into this file and put it under the project folder.

{
   "pageInfo": {
         "pageName": "abc",
         "pagePic": "http://example.com/content.jpg"
    },
    "posts": [
         {
              "post_id": "123456789012_123456789012",
              "actor_id": "1234567890",
              "picOfPersonWhoPosted": "http://example.com/photo.jpg",
              "nameOfPersonWhoPosted": "Jane Doe",
              "message": "Sounds cool. Can't wait to see it!",
              "likesCount": "2",
              "comments": [],
              "timeOfPost": "1234567890"
         }
    ]
}

F - Demo Code

package com.levo.jsonex;

import java.io.File;
import java.io.IOException;
import java.util.Arrays;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.levo.jsonex.model.Page;
import com.levo.jsonex.model.PageInfo;
import com.levo.jsonex.model.Post;

public class JSONDemo {

    public static void main(String[] args) {
        ObjectMapper objectMapper = new ObjectMapper();

        try {
            Page page = objectMapper.readValue(new File("sampleJSONFile.json"), Page.class);

            printParsedObject(page);
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    private static void printParsedObject(Page page) {
        printPageInfo(page.getPageInfo());
        System.out.println();
        printPosts(page.getPosts());
    }

    private static void printPageInfo(PageInfo pageInfo) {
        System.out.println("Page Info;");
        System.out.println("**********");
        System.out.println("\tPage Name : " + pageInfo.getPageName());
        System.out.println("\tPage Pic  : " + pageInfo.getPagePic());
    }

    private static void printPosts(Post[] posts) {
        System.out.println("Page Posts;");
        System.out.println("**********");
        for(Post post : posts) {
            printPost(post);
        }
    }

    private static void printPost(Post post) {
        System.out.println("\tPost Id                   : " + post.getPost_id());
        System.out.println("\tActor Id                  : " + post.getActor_id());
        System.out.println("\tPic Of Person Who Posted  : " + post.getPicOfPersonWhoPosted());
        System.out.println("\tName Of Person Who Posted : " + post.getNameOfPersonWhoPosted());
        System.out.println("\tMessage                   : " + post.getMessage());
        System.out.println("\tLikes Count               : " + post.getLikesCount());
        System.out.println("\tComments                  : " + Arrays.toString(post.getComments()));
        System.out.println("\tTime Of Post              : " + post.getTimeOfPost());
    }

}

G - Demo Output

Page Info;
****(*****
    Page Name : abc
    Page Pic  : http://example.com/content.jpg
Page Posts;
**********
    Post Id                   : 123456789012_123456789012
    Actor Id                  : 1234567890
    Pic Of Person Who Posted  : http://example.com/photo.jpg
    Name Of Person Who Posted : Jane Doe
    Message                   : Sounds cool. Can't wait to see it!
    Likes Count               : 2
    Comments                  : []
    Time Of Post              : 1234567890

回答6:

Almost all the answers given requires a full deserialization of the JSON into a Java object before accessing the value in the property of interest. Another alternative, which does not go this route is to use JsonPATH which is like XPath for JSON and allows traversing of JSON objects.

It is a specification and the good folks at JayWay have created a Java implementation for the specification which you can find here: https://github.com/jayway/JsonPath

So basically to use it, add it to your project, eg:

<dependency>
    <groupId>com.jayway.jsonpath</groupId>
    <artifactId>json-path</artifactId>
    <version>${version}</version>
</dependency>

and to use:

String pageName = JsonPath.read(yourJsonString, "$.pageInfo.pageName");
String pagePic = JsonPath.read(yourJsonString, "$.pageInfo.pagePic");
String post_id = JsonPath.read(yourJsonString, "$.pagePosts[0].post_id");

etc...

Check the JsonPath specification page for more information on the other ways to transverse JSON.

回答7:

You could use Google Gson.

Using this library you only need to create a model with the same JSON structure. Then the model is automatically filled in. You have to call your variables as your JSON keys, or use @SerializedName if you want to use different names.

For your example:

JSON:

{
    "pageInfo": {
        "pageName": "abc",
        "pagePic": "http://example.com/content.jpg"
    }
    "posts": [
        {
            "post_id": "123456789012_123456789012",
            "actor_id": "1234567890",
            "picOfPersonWhoPosted": "http://example.com/photo.jpg",
            "nameOfPersonWhoPosted": "Jane Doe",
            "message": "Sounds cool. Can't wait to see it!",
            "likesCount": "2",
            "comments": [],
            "timeOfPost": "1234567890"
        }
    ]
}

Model:

class MyModel {

    private PageInfo pageInfo;
    private ArrayList<Post> posts = new ArrayList<>();
}

class PageInfo {

    private String pageName;
    private String pagePic;
}

class Post {

    private String post_id;

    @SerializedName("actor_id") // <- example SerializedName
    private String actorId;

    private String picOfPersonWhoPosted;
    private String nameOfPersonWhoPosted;
    private String message;
    private String likesCount;
    private ArrayList<String> comments;
    private String timeOfPost;
}

Now you can parse using Gson library:

MyModel model = gson.fromJson(jsonString, MyModel.class);

Remember to import the library in the app Gradle file

implementation 'com.google.code.gson:gson:2.8.6' // or earlier versions

You can generate model from JSON automatically using online tools like this.

回答8:

Use minimal-json which is very fast and easy to use. You can parse from String obj and Stream.

Sample data:

{
  "order": 4711,
  "items": [
    {
      "name": "NE555 Timer IC",
      "cat-id": "645723",
      "quantity": 10,
    },
    {
      "name": "LM358N OpAmp IC",
      "cat-id": "764525",
      "quantity": 2
    }
  ]
}

Parsing:

JsonObject object = Json.parse(input).asObject();
int orders = object.get("order").asInt();
JsonArray items = object.get("items").asArray();

Creating JSON:

JsonObject user = Json.object().add("name", "Sakib").add("age", 23);

Maven:

<dependency>
  <groupId>com.eclipsesource.minimal-json</groupId>
  <artifactId>minimal-json</artifactId>
  <version>0.9.4</version>
</dependency>

回答9:

I believe the best practice should be to go through the official Java JSON API which are still work in progress.

回答10:

The below example shows how to read the text in the question, represented as the "jsonText" variable. This solution uses the Java EE7 javax.json API (which is mentioned in some of the other answers). The reason I've added it as a separate answer is that the following code shows how to actually access some of the values shown in the question. An implementation of the javax.json API would be required to make this code run. The full package for each of the classes required was included as I didn't want to declare "import" statements.

javax.json.JsonReader jr = 
    javax.json.Json.createReader(new StringReader(jsonText));
javax.json.JsonObject jo = jr.readObject();

//Read the page info.
javax.json.JsonObject pageInfo = jo.getJsonObject("pageInfo");
System.out.println(pageInfo.getString("pageName"));

//Read the posts.
javax.json.JsonArray posts = jo.getJsonArray("posts");
//Read the first post.
javax.json.JsonObject post = posts.getJsonObject(0);
//Read the post_id field.
String postId = post.getString("post_id");

Now, before anyone goes and downvotes this answer because it doesn't use GSON, org.json, Jackson, or any of the other 3rd party frameworks available, it's an example of "required code" per the question to parse the provided text. I am well aware that adherence to the current standard JSR 353 was not being considered for JDK 9 and as such the JSR 353 spec should be treated the same as any other 3rd party JSON handling implementation.

回答11:

Since nobody mentioned it yet, here is a beginning of a solution using Nashorn (JavaScript runtime part of Java 8, but deprecated in Java 11).

Solution

private static final String EXTRACTOR_SCRIPT =
    "var fun = function(raw) { " +
    "var json = JSON.parse(raw); " +
    "return [json.pageInfo.pageName, json.pageInfo.pagePic, json.posts[0].post_id];};";

public void run() throws ScriptException, NoSuchMethodException {
    ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
    engine.eval(EXTRACTOR_SCRIPT);
    Invocable invocable = (Invocable) engine;
    JSObject result = (JSObject) invocable.invokeFunction("fun", JSON);
    result.values().forEach(e -> System.out.println(e));
}

Performance comparison

I wrote JSON content containing three arrays of respectively 20, 20 and 100 elements. I only want to get the 100 elements from the third array. I use the following JavaScript function to parse and get my entries.

var fun = function(raw) {JSON.parse(raw).entries};

Running the call a million times using Nashorn takes 7.5~7.8 seconds

(JSObject) invocable.invokeFunction("fun", json);

org.json takes 20~21 seconds

new JSONObject(JSON).getJSONArray("entries");

Jackson takes 6.5~7 seconds

mapper.readValue(JSON, Entries.class).getEntries();

In this case Jackson performs better than Nashorn, which performs much better than org.json. Nashorn API is harder to use than org.json's or Jackson's. Depending on your requirements Jackson and Nashorn both can be viable solutions.

回答12:

This blew my mind with how easy it was. You can just pass a String holding your JSON to the constructor of a JSONObject in the default org.json package.

JSONArray rootOfPage =  new JSONArray(JSONString);

Done. Drops microphone. This works with JSONObjects as well. After that, you can just look through your hierarchy of Objects using the get() methods on your objects.

回答13:

There are many JSON libraries available in Java.

The most notorious ones are: Jackson, GSON, Genson, FastJson and org.json.

There are typically three things one should look at for choosing any library:

Performance
Ease of use (code is simple to write and legible) - that goes with features.
For mobile apps: dependency/jar size

Specifically for JSON libraries (and any serialization/deserialization libs), databinding is also usually of interest as it removes the need of writing boiler-plate code to pack/unpack the data.

For 1, see this benchmark: https://github.com/fabienrenaud/java-json-benchmark I did using JMH which compares (jackson, gson, genson, fastjson, org.json, jsonp) performance of serializers and deserializers using stream and databind APIs. For 2, you can find numerous examples on the Internet. The benchmark above can also be used as a source of examples...

Quick takeaway of the benchmark: Jackson performs 5 to 6 times better than org.json and more than twice better than GSON.

For your particular example, the following code decodes your json with jackson:

public class MyObj {

    private PageInfo pageInfo;
    private List<Post> posts;

    static final class PageInfo {
        private String pageName;
        private String pagePic;
    }

    static final class Post {
        private String post_id;
        @JsonProperty("actor_id");
        private String actorId;
        @JsonProperty("picOfPersonWhoPosted")
        private String pictureOfPoster;
        @JsonProperty("nameOfPersonWhoPosted")
        private String nameOfPoster;
        private String likesCount;
        private List<String> comments;
        private String timeOfPost;
    }

    private static final ObjectMapper JACKSON = new ObjectMapper();
    public static void main(String[] args) throws IOException {
        MyObj o = JACKSON.readValue(args[0], MyObj.class); // assumes args[0] contains your json payload provided in your question.
    }
}

Let me know if you have any questions.

回答14:

In addition to other answers, I recomend this online opensource service jsonschema2pojo.org for quick generating Java classes from json or json schema for GSON, Jackson 1.x or Jackson 2.x. For example, if you have:

{
   "pageInfo": {
         "pageName": "abc",
         "pagePic": "http://example.com/content.jpg"
    }
    "posts": [
         {
              "post_id": "123456789012_123456789012",
              "actor_id": 1234567890,
              "picOfPersonWhoPosted": "http://example.com/photo.jpg",
              "nameOfPersonWhoPosted": "Jane Doe",
              "message": "Sounds cool. Can't wait to see it!",
              "likesCount": 2,
              "comments": [],
              "timeOfPost": 1234567890
         }
    ]
}

The jsonschema2pojo.org for GSON generated:

@Generated("org.jsonschema2pojo")
public class Container {
    @SerializedName("pageInfo")
    @Expose
    public PageInfo pageInfo;
    @SerializedName("posts")
    @Expose
    public List<Post> posts = new ArrayList<Post>();
}

@Generated("org.jsonschema2pojo")
public class PageInfo {
    @SerializedName("pageName")
    @Expose
    public String pageName;
    @SerializedName("pagePic")
    @Expose
    public String pagePic;
}

@Generated("org.jsonschema2pojo")
public class Post {
    @SerializedName("post_id")
    @Expose
    public String postId;
    @SerializedName("actor_id")
    @Expose
    public long actorId;
    @SerializedName("picOfPersonWhoPosted")
    @Expose
    public String picOfPersonWhoPosted;
    @SerializedName("nameOfPersonWhoPosted")
    @Expose
    public String nameOfPersonWhoPosted;
    @SerializedName("message")
    @Expose
    public String message;
    @SerializedName("likesCount")
    @Expose
    public long likesCount;
    @SerializedName("comments")
    @Expose
    public List<Object> comments = new ArrayList<Object>();
    @SerializedName("timeOfPost")
    @Expose
    public long timeOfPost;
}

回答15:

If you have some Java class(say Message) representing the JSON string(jsonString), you can use Jackson JSON library with:

Message message= new ObjectMapper().readValue(jsonString, Message.class);

and from message object you can fetch any of its attribute.

回答16:

There are many open source libraries present to parse JSON content to an object or just to read JSON values. Your requirement is just to read values and parsing it to custom object. So org.json library is enough in your case.

Use org.json library to parse it and create JsonObject:

JSONObject jsonObj = new JSONObject(<jsonStr>);

Now, use this object to get your values:

String id = jsonObj.getString("pageInfo");

You can see a complete example here:

How to parse JSON in Java

回答17:

Gson is easy to learn and implement, what we need to know are following two methods

toJson() – Convert Java object to JSON format
fromJson() – Convert JSON into Java object

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import com.google.gson.Gson;

public class GsonExample {
    public static void main(String[] args) {

    Gson gson = new Gson();

    try {

        BufferedReader br = new BufferedReader(
            new FileReader("c:\\file.json"));

        //convert the json string back to object
        DataObject obj = gson.fromJson(br, DataObject.class);

        System.out.println(obj);

    } catch (IOException e) {
        e.printStackTrace();
    }

    }
}

回答18:

Please do something like this:

JSONParser jsonParser = new JSONParser();
JSONObject obj = (JSONObject) jsonParser.parse(contentString);
String product = (String) jsonObject.get("productId");

回答19:

You can use the Gson Library to parse the JSON string.

Gson gson = new Gson();
JsonObject jsonObject = gson.fromJson(jsonAsString, JsonObject.class);

String pageName = jsonObject.getAsJsonObject("pageInfo").get("pageName").getAsString();
String pagePic = jsonObject.getAsJsonObject("pageInfo").get("pagePic").getAsString();
String postId = jsonObject.getAsJsonArray("posts").get(0).getAsJsonObject().get("post_id").getAsString();

You can also loop through the "posts" array as so:

JsonArray posts = jsonObject.getAsJsonArray("posts");
for (JsonElement post : posts) {
  String postId = post.getAsJsonObject().get("post_id").getAsString();
  //do something
}

回答20:

Read the following blog post, JSON in Java.

This post is a little bit old, but still I want to answer you question.

Step 1: Create a POJO class of your data.

Step 2: Now create a object using JSON.

Employee employee = null;
ObjectMapper mapper = new ObjectMapper();
try{
    employee =  mapper.readValue(newFile("/home/sumit/employee.json"), Employee.class);
} 
catch (JsonGenerationException e){
    e.printStackTrace();
}

For further reference you can refer to the following link.

回答21:

{
   "pageInfo": {
         "pageName": "abc",
         "pagePic": "http://example.com/content.jpg"
    },
    "posts": [
         {
              "post_id": "123456789012_123456789012",
              "actor_id": "1234567890",
              "picOfPersonWhoPosted": "http://example.com/photo.jpg",
              "nameOfPersonWhoPosted": "Jane Doe",
              "message": "Sounds cool. Can't wait to see it!",
              "likesCount": "2",
              "comments": [],
              "timeOfPost": "1234567890"
         }
    ]
}

Java code :

JSONObject obj = new JSONObject(responsejsonobj);
String pageName = obj.getJSONObject("pageInfo").getString("pageName");

JSONArray arr = obj.getJSONArray("posts");
for (int i = 0; i < arr.length(); i++)
{
    String post_id = arr.getJSONObject(i).getString("post_id");
    ......etc
}

回答22:

Top answers on this page use too simple examples like object with one property (e.g. {name: value}). I think that still simple but real life example can help someone.

So this is the JSON returned by Google Translate API:

{
  "data": 
     {
        "translations": 
          [
            {
              "translatedText": "Arbeit"
             }
          ]
     }
}

I want to retrieve the value of "translatedText" attribute e.g. "Arbeit" using Google's Gson.

Two possible approaches:

Retrieve just one needed attribute

String json  = callToTranslateApi("work", "de");
JsonObject jsonObject = new JsonParser().parse(json).getAsJsonObject();
return jsonObject.get("data").getAsJsonObject()
        .get("translations").getAsJsonArray()
        .get(0).getAsJsonObject()
        .get("translatedText").getAsString();

Create Java object from JSON

class ApiResponse {
    Data data;      
    class Data {
        Translation[] translations;         
        class Translation {
            String translatedText;
        }
     }
 }

...

 Gson g = new Gson();
 String json =callToTranslateApi("work", "de");
 ApiResponse response = g.fromJson(json, ApiResponse.class);
 return response.data.translations[0].translatedText;

回答23:

First you need to select an implementation library to do that.

The Java API for JSON Processing (JSR 353) provides portable APIs to parse, generate, transform, and query JSON using object model and streaming APIs.

The reference implementation is here: https://jsonp.java.net/

Here you can find a list of implementations of JSR 353:

What are the API that does implement JSR-353 (JSON)

And to help you decide... I found this article as well:

http://blog.takipi.com/the-ultimate-json-library-json-simple-vs-gson-vs-jackson-vs-json/

If you go for Jackson, here is a good article about conversion between JSON to/from Java using Jackson: https://www.mkyong.com/java/how-to-convert-java-object-to-from-json-jackson/

Hope it helps!

回答24:

You can use Jayway JsonPath. Below is a GitHub link with source code, pom details and good documentation.

https://github.com/jayway/JsonPath

Please follow the below steps.

Step 1: Add the jayway JSON path dependency in your class path using Maven or download the JAR file and manually add it.

<dependency>
            <groupId>com.jayway.jsonpath</groupId>
            <artifactId>json-path</artifactId>
            <version>2.2.0</version>
</dependency>

Step 2: Please save your input JSON as a file for this example. In my case I saved your JSON as sampleJson.txt. Note you missed a comma between pageInfo and posts.

Step 3: Read the JSON contents from the above file using bufferedReader and save it as String.

BufferedReader br = new BufferedReader(new FileReader("D:\\sampleJson.txt"));

        StringBuilder sb = new StringBuilder();
        String line = br.readLine();

        while (line != null) {
            sb.append(line);
            sb.append(System.lineSeparator());
            line = br.readLine();
        }
        br.close();
        String jsonInput = sb.toString();

Step 4: Parse your JSON string using jayway JSON parser.

Object document = Configuration.defaultConfiguration().jsonProvider().parse(jsonInput);

Step 5: Read the details like below.

String pageName = JsonPath.read(document, "$.pageInfo.pageName");
String pagePic = JsonPath.read(document, "$.pageInfo.pagePic");
String post_id = JsonPath.read(document, "$.posts[0].post_id");

System.out.println("$.pageInfo.pageName " + pageName);
System.out.println("$.pageInfo.pagePic " + pagePic);
System.out.println("$.posts[0].post_id " + post_id);

The output will be:

$.pageInfo.pageName = abc
$.pageInfo.pagePic = http://example.com/content.jpg
$.posts[0].post_id  = 123456789012_123456789012

回答25:

I have JSON like this:

{
   "pageInfo": {
         "pageName": "abc",
         "pagePic": "http://example.com/content.jpg"
    }
}

Java class

class PageInfo {

    private String pageName;
    private String pagePic;

    // Getters and setters
}

Code for converting this JSON to a Java class.

    PageInfo pageInfo = JsonPath.parse(jsonString).read("$.pageInfo", PageInfo.class);

Maven

<dependency>
    <groupId>com.jayway.jsonpath</groupId>
    <artifactId>json-path</artifactId>
    <version>2.2.0</version>
</dependency>

回答26:

jsoniter (jsoniterator) is a relatively new and simple json library, designed to be simple and fast. All you need to do to deserialize json data is

JsonIterator.deserialize(jsonData, int[].class);

where jsonData is a string of json data.

Check out the official website for more information.

回答27:

One can use Apache @Model annotation to create Java model classes representing structure of JSON files and use them to access various elements in the JSON tree. Unlike other solutions this one works completely without reflection and is thus suitable for environments where reflection is impossible or comes with significant overhead.

There is a sample Maven project showing the usage. First of all it defines the structure:

@Model(className="RepositoryInfo", properties = {
    @Property(name = "id", type = int.class),
    @Property(name = "name", type = String.class),
    @Property(name = "owner", type = Owner.class),
    @Property(name = "private", type = boolean.class),
})
final class RepositoryCntrl {
    @Model(className = "Owner", properties = {
        @Property(name = "login", type = String.class)
    })
    static final class OwnerCntrl {
    }
}

and then it uses the generated RepositoryInfo and Owner classes to parse the provided input stream and pick certain information up while doing that:

List<RepositoryInfo> repositories = new ArrayList<>();
try (InputStream is = initializeStream(args)) {
    Models.parse(CONTEXT, RepositoryInfo.class, is, repositories);
}

System.err.println("there is " + repositories.size() + " repositories");
repositories.stream().filter((repo) -> repo != null).forEach((repo) -> {
    System.err.println("repository " + repo.getName() + 
        " is owned by " + repo.getOwner().getLogin()
    );
})

That is it! In addition to that here is a live gist showing similar example together with asynchronous network communication.

回答28:

We can use the JSONObject class to convert a JSON string to a JSON object, and to iterate over the JSON object. Use the following code.

JSONObject jObj = new JSONObject(contents.trim());
Iterator<?> keys = jObj.keys();

while( keys.hasNext() ) {
  String key = (String)keys.next();
  if ( jObj.get(key) instanceof JSONObject ) {           
    System.out.println(jObj.getString(String key));
  }
}

回答29:

You can use DSM stream parsing library for parsing complex json and XML document. DSM parse data only once and not load all data into memory.

Let's say we have Page class to deserialize given json data.

Page class

public class Page {
    private String pageName;
    private String pageImage;
    private List<Sting> postIds;

    // getter/setter

}

Create a yaml Mapping file.

result:
  type: object     # result is array
  path: /posts
  fields:
    pageName:
        path: /pageInfo/pageName
    pageImage:
        path: /pageInfo/pagePic
    postIds:
      path: post_id
      type: array

Use DSM to extract fields.

DSM dsm=new DSMBuilder(new File("path-to-yaml-config.yaml")).create(Page.class);
Page page= (Page)dsm.toObject(new path-to-json-data.json");

page variable serialized to json:

{
  "pageName" : "abc",
  "pageImage" : "http://example.com/content.jpg",
  "postIds" : [ "123456789012_123456789012" ]
}

DSM is very good for complex json and xml.

回答30:

You can use JsonNode for a structured tree representation of your JSON string. It's part of the rock solid jackson library which is omnipresent.

ObjectMapper mapper = new ObjectMapper();
JsonNode yourObj = mapper.readTree("{\"k\":\"v\"}");

标签： java json parsing

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How to parse JSON in Java

问题:

回答1:

回答2:

Google GSON (Maven)

Org.JSON (Maven)

Jackson (Maven)

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A - Explanation

B - Main POJO Class : Page

C - Child POJO Class : PageInfo

D - Child POJO Class : Post

E - Sample JSON File : sampleJSONFile.json

F - Demo Code

G - Demo Output

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