Java REST API：将 ArrayList<T> 作为 JSON 响应问题返回

Question

我需要简单地返回一个对象的 ArrayList 作为我的 Rest API 响应，但我不断收到此错误：

Java 类的消息体编写器 java.util.Collections$SingletonList 和 MIME 媒体类型 未找到应用程序/json。

我的代码：

@Path("/plans")
public class PlanAPI {

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    @Path("/getplans/{type}")
    public Response GetAllPlans(@PathParam("type") String type) {

        PlanDAO dao = new PlanDAO();
        ArrayList<UserPlan> plans = dao.GetAllPlans_ByType(type);

        return Response.status(200).entity(plans).build();
    }
}

我得到的错误：

com.sun.jersey.spi.container.ContainerResponse logException
SEVERE: Mapped exception to response: 500 (Internal Server Error)
javax.ws.rs.WebApplicationException: com.sun.jersey.api.MessageException: A message body writer for Java class java.util.Collections$SingletonList, and Java type java.util.List<java.util.ArrayList<com.dinuka.planapp.model.UserPlan>>, and MIME media type application/json was not found.

    at com.sun.jersey.spi.container.ContainerResponse.write(ContainerResponse.java:284)
    at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1510)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1419)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1409)
    at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:409)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:558)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:733)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
    at com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:128)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:63)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:50)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectRequest(DevAppServerModulesFilter.java:366)
    at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectModuleRequest(DevAppServerModulesFilter.java:349)
    at com.google.appengine.tools.development.DevAppServerModulesFilter.doFilter(DevAppServerModulesFilter.java:116)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
    at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:98)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:513)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:547)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
Caused by: com.sun.jersey.api.MessageException: A message body writer for Java class java.util.Collections$SingletonList, and Java type java.util.List<java.util.ArrayList<com.dinuka.planapp.model.UserPlan>>, and MIME media type application/json was not found.

用户计划类：

public class UserPlan {

    String ID = "";
    String Name = "";
    String Description = "";

    //Getter Setter and Constructors
}

当我调用 API 时，我收到上述错误。我做错了什么？

Answer 1

@XmlRootElement 
@XmlType(propOrder = {ID, Name, Description})
public class UserPlan {

String ID = "";
String Name = "";
String Description = "";

//Getter Setter and Constructors
}

尝试为 UserPlan 类添加@XmlRootElement @XmlType

String json = new Gson().toJson(plans);

@GET
@Produces(MediaType.APPLICATION_JSON)
@Path("/getplans/{type}")
public String GetAllPlans(@PathParam("type") String type) {

    PlanDAO dao = new PlanDAO();
    ArrayList<UserPlan> plans = dao.GetAllPlans_ByType(type);

    return new Gson().toJson(plans);
}

Answer 2

这段代码有两个问题：

1. POJO (UserPlan) 不对应球衣支持的任何built-in provider。通常，提供者（或 messagebodyreader/writer）将处理对象到相应 MediaType 的转换。因此，create a Provider 为您定义的每个自定义类或将该类转换为预处理类型。为您的 POJO 添加@XmlRootElement 注释使其代表JAXBElement，默认提供程序将对其起作用。

2. 要使用泛型集合类型，请使用：

ArrayList<UserPlan> plans = dao.GetAllPlans_ByType(type); GenericEntity entity = new GenericEntity<List<UserPlan>>(plans) {}; return Response.ok(entity).build();

Answer 3

如果您在项目中使用 maven，请取消注释 pom.xml 文件中的以下行，然后重新启动服务器。它会正常工作。

    <dependency>
        <groupId>org.glassfish.jersey.media</groupId>
        <artifactId>jersey-media-moxy</artifactId>
    </dependency>