Java REST API：返回ArrayList <T> 作為JSON響應問題

Question

我只需要返回一個對象的ArrayList作為我的Rest API響應，但我一直收到此錯誤：

找不到Java類java.util.Collections $ SingletonList和MIME媒體類型application / json的消息正文編寫器。

我的代碼：

@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();
    }
}

我得到的錯誤：

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.

UserPlan類別：

public class UserPlan {

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

    //Getter Setter and Constructors
}

調用API時，出現上述錯誤。 我究竟做錯了什么？

Answer 1

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

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

//Getter Setter and Constructors
}

嘗試為UserPlan類添加@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

此代碼有兩件事是錯誤的：

1. POJO（ UserPlan ）不對應於jersey支持的任何內置提供程序 。 通常，提供者（或messagebodyreader / writer）將處理對象到相應MediaType的轉換。 因此，要么為您定義的每個自定義類創建一個Provider ，要么將該類轉換為預先處理的類型。 為您的POJO添加@XmlRootElement批注使它表示一個JAXBElement ，默認提供程序將對其起作用。

2. 要使用通用集合類型，請使用：

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

Answer 3

如果您在項目中使用Maven，請取消注釋pom.xml文件中的以下行，然后重新啟動服務器。 它將正常工作。

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