我有以下JSON文本。我如何解析它以获得pageName, pagePic, post_id等的值?
{
"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"
}
]
}
由于还没有人提到它,这里是一个使用Nashorn (Java 8的JavaScript运行时部分,但在Java 11中已弃用)的解决方案的开始。
解决方案
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));
}
性能比较
我编写的JSON内容包含三个数组,分别为20、20和100个元素。我只想从第三个数组中获取100个元素。我使用下面的JavaScript函数来解析和获取我的条目。
var fun = function(raw) {JSON.parse(raw).entries};
使用Nashorn运行一百万次调用需要7.5~7.8秒
(JSObject) invocable.invokeFunction("fun", json);
org。Json需要20~21秒
new JSONObject(JSON).getJSONArray("entries");
杰克逊用时6.5~7秒
mapper.readValue(JSON, Entries.class).getEntries();
在这种情况下,Jackson的性能比Nashorn好,后者的性能比org.json好得多。
Nashorn API比org更难使用。json或Jackson的。根据您的需求,Jackson和Nashorn都是可行的解决方案。
您可以使用DSM流解析库来解析复杂的json和XML文档。DSM只解析一次数据,不会将所有数据加载到内存中。
假设我们有一个Page类来反序列化给定的json数据。
页面类
public class Page {
private String pageName;
private String pageImage;
private List<Sting> postIds;
// getter/setter
}
创建一个yaml Mapping文件。
result:
type: object # result is array
path: /posts
fields:
pageName:
path: /pageInfo/pageName
pageImage:
path: /pageInfo/pagePic
postIds:
path: post_id
type: array
使用DSM提取字段。
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");
页面变量序列化为json:
{
"pageName" : "abc",
"pageImage" : "http://example.com/content.jpg",
"postIds" : [ "123456789012_123456789012" ]
}
DSM非常适合处理复杂的json和xml。
Quick-json解析器非常简单,灵活,快速,可定制。试一试
特点:
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
它可以这样使用:
JsonParserFactory factory=JsonParserFactory.getInstance();
JSONParser parser=factory.newJsonParser();
Map jsonMap=parser.parseJson(jsonString);
可以使用Apache @Model注释创建表示JSON文件结构的Java模型类,并使用它们访问JSON树中的各种元素。与其他解决方案不同,该解决方案完全没有反射,因此适用于不可能反射或开销很大的环境。
有一个示例Maven项目展示了这种用法。首先它定义了结构:
@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 {
}
}
然后它使用生成的RepositoryInfo和Owner类来解析所提供的输入流,并在此过程中获取某些信息:
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()
);
})
就是这样!除此之外,这里还有一个生动的要点,展示了类似的例子以及异步网络通信。
您需要使用JsonNode和来自jackson库的ObjectMapper类来获取Json树的节点。在pom.xml中添加以下依赖项以获得对Jackson类的访问权。
<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.5</version>
</dependency>
你应该尝试下面的代码,这将工作:
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
class JsonNodeExtractor{
public void convertToJson(){
String filepath = "c:\\data.json";
ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readTree(filepath);
// create a JsonNode for every root or subroot element in the Json String
JsonNode pageInfoRoot = node.path("pageInfo");
// Fetching elements under 'pageInfo'
String pageName = pageInfoRoot.path("pageName").asText();
String pagePic = pageInfoRoot.path("pagePic").asText();
// Now fetching elements under posts
JsonNode postsNode = node.path("posts");
String post_id = postsNode .path("post_id").asText();
String nameOfPersonWhoPosted = postsNode
.path("nameOfPersonWhoPosted").asText();
}
}