如何使用 Jackson Databind 反序列化以下 JSON？

Question

JSON 看起来像这样，并且可以在数组中包含 none、one 或 more 元素。

{
   "players":[
      {
         "SteamId":"hidden",
         "CommunityBanned":false,
         "VACBanned":false,
         "NumberOfVACBans":0,
         "DaysSinceLastBan":0,
         "NumberOfGameBans":0,
         "EconomyBan":"none"
      },
      {
         "SteamId":"hidden",
         "CommunityBanned":false,
         "VACBanned":false,
         "NumberOfVACBans":0,
         "DaysSinceLastBan":0,
         "NumberOfGameBans":0,
         "EconomyBan":"none"
      }
   ]
}

这是我迄今为止尝试过的，但没有成功，我只是捕获了异常。

// I fetch the JSON from a specific API through an HTTP request.
HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
          .GET()
          .header("accept", "application/json")
          .uri(URI.create(URL))
          .build();
HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());

// ... and try to deserialize it
ObjectMapper mapper = new ObjectMapper();
PlayersWrapper players = mapper.readValue(response.body(), PlayersWrapper.class); // <-- fails here.
players.getPlayerBans().forEach(System.out::println);

PlayersWrapper.java 经过一番谷歌搜索后，我看到了一些以某种奇特方式使用包装类的解决方案，它用于的 JSON 比我的要复杂一些，尽管有两到三层的嵌套。我的更简单。

...

public class PlayersWrapper {
    List<PlayerBan> players = new ArrayList<PlayerBan>();
}

PlayerBan.java

...

public class PlayerBan {
    private String SteamId;
    private boolean CommunityBanned;
    private boolean VACBanned;
    private int NumberOfVACBans;
    private int DaysSinceLastBan;
    private int NumberOfGameBans;
    private String EconomyBan;
}

堆栈跟踪

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "players" (class com.domain.steamfinder.model.PlayersWrapper), not marked as ignorable (one known property: "playerBans"])
 at [Source: (String)"{"players":[{"SteamId":"hidden","CommunityBanned":false,"VACBanned":false,"NumberOfVACBans":0,"DaysSinceLastBan":0,"NumberOfGameBans":0,"EconomyBan":"none"}]}"; line: 1, column: 13] (through reference chain: com.domain.steamfinder.model.PlayersWrapper["players"])
        at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:61)
        at com.fasterxml.jackson.databind.DeserializationContext.handleUnknownProperty(DeserializationContext.java:1127)
        at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:1989)
        at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1700)
        at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownVanilla(BeanDeserializerBase.java:1678)
        at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:319)
        at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:176)
        at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322)
        at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4674)
        at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3629)
        at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3597)
        at com.domain.steamfinder.finder.FinderService.findUser(FinderService.java:48)
        at com.domain.steamfinder.finder.FinderController.find(FinderController.java:27)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205)
        at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150)
        at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117)
        at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895)
        at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808)
        at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1067)
        at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963)
        at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
        at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:655)
        at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
        at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
        at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
        at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:540)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357)
        at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:382)
        at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
        at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:895)
        at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1732)
        at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
        at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
        at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.base/java.lang.Thread.run(Thread.java:829)

编辑：原始问题得到解决，这要归功于指出包装类中的错误命名的答案。出现了一个新问题，请参阅上面的更新代码并在下面查看新的堆栈跟踪。

堆栈跟踪

`Unrecognized field "SteamId" (class com.domain.steamfinder.model.PlayerBan), not marked as ignorable 
(7 known properties: "economyBan", "numberOfGameBans", "communityBanned", "steamId", "numberOfVACBans", "daysSinceLastBan", "vacbanned"]) 

... 

(through reference chain: com.domain.steamfinder.model.PlayersWrapper["players"]->java.util.ArrayList[0]->com.domain.steamfinder.model.PlayerBan["SteamId"])`

Answer 1

您可以将变量名称更改为 players 以匹配 JSON，因为这就是错误所抱怨的原因

public class PlayersWrapper {
    List<PlayerBan> players = new ArrayList<PlayerBan>();
}

Answer 2

您的代码存在一些问题，您的异常并不能说明一切，因为 Jackson 停在第一个问题上。

第一个显然是 json 文件有一个名为 players 的数组，但在您的班级中，您将列表命名为 playerBans。这可以通过多种方式解决：您可以将List<PlayerBan> playerBans 变量重命名为players，或者使用@JsonProperty 注释变量：

@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
public class PlayersWrapper {
    @JsonProperty("players")
    private List<PlayerBan> playerBans;
}

您可能已经注意到@JsonAutoDetect 注释。这将是一个问题，主要是在您的PlayerBan 类中，因为所有成员变量都是私有的。您可以通过使用 @JsonAutoDetect 注释您的班级来告诉 Jackson 识别私人成员。

此外，PlayerBan 类会出现与PlayersWrapper 类类似的问题，这意味着某些成员的命名与 json 文件中出现的名称不同。你也可以在那里使用@JsonProperty，或者你可以重命名成员变量。

@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)的解释：

我们需要这个的原因是 Jackson 默认不会自动检测类的私有成员。自动检测的最小可见性取决于上下文（通常 Jackson 会自动检测公共成员），但我们可以调整得更广泛，这意味着也会检测到受保护和私有字段。有关其他可见性选项，请参阅JsonAutoDetect.Visibility docs。