请耐心等待，因为这是我第一次学习如何使用网络服务。
我正在尝试构建一个简单的系统，当给定参数（String id）时，Web服务返回一个JSON对象。我有点关注此代码库作为指南https://o7planning.org/en/11207/simple-crud-example-with-java-restful-web-service
问题：当我运行服务并转到URL http://localhost:8080/RESTfulServiceLIT_war_exploded/users/ {id}时，我会在glassfish日志中收到NullPointer异常。它应该只显示相对于从id返回的用户对象的JSON对象。
这是UserDAO类代码
import java.util.*;
public class UserDAO {
private static final Map<String, User> userMap = new HashMap<String, User>();
static {
initUsers();
}
private static void initUsers() {
User u1 = new User("K1", "Student 1", "Student");
User u2 = new User("K2", "Student 2", "Student");
User u3 = new User("K3", "Student 3", "Student");
User u4 = new User("S1", "Staff 1", "Staff");
User u5 = new User("S2", "Staff 2", "Staff");
User u6 = new User("S3", "Staff 3", "Staff");
userMap.put(u1.getId(), u1);
userMap.put(u2.getId(), u2);
userMap.put(u3.getId(), u3);
userMap.put(u4.getId(), u4);
userMap.put(u5.getId(), u5);
userMap.put(u6.getId(), u6);
}
public static User getUser(String id) {
return userMap.get(id);
}
public static List<User> getAllUsers() {
Collection<User> c = userMap.values();
List<User> list = new ArrayList<User>();
list.addAll(c);
return list;
}
List<User> list;
}
以下是具有其余服务的Main类
import javax.print.attribute.standard.Media;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import java.util.ArrayList;
import java.util.List;
import javax.ws.rs.core.MediaType;
// The Java class will be hosted at the URI path "/helloworld"
@Path("/users")
public class Main {
@GET
@Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
@Path("/{id}")
public User getUser(@PathParam("id") String id) {
return UserDAO.getUser(id);
}
// @GET
// @Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
// @Path{"/all"}
// public List<User> getUsers_JSON(){
// List<User> listOfUsers = UserDAO.getAllUsers();
// return listOfUsers;
// }
}
这是服务器日志
[2018-05-22T20:09:17.937+0100] [glassfish 4.1] [WARNING] [] [javax.enterprise.web] [tid: _ThreadID=35 _ThreadName=http-listener-1(3)] [timeMillis: 1527016157937] [levelValue: 900] [[
StandardWrapperValve[MyApplication]: Servlet.service() for servlet MyApplication threw exception
java.lang.NullPointerException
at org.eclipse.persistence.jaxb.rs.MOXyJsonProvider.writeTo(MOXyJsonProvider.java:944)
at org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.invokeWriteTo(WriterInterceptorExecutor.java:265)
at org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.aroundWriteTo(WriterInterceptorExecutor.java:250)
at org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:162)
at org.glassfish.jersey.server.internal.JsonWithPaddingInterceptor.aroundWriteTo(JsonWithPaddingInterceptor.java:106)
at org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:162)
at org.glassfish.jersey.server.internal.MappableExceptionWrapperInterceptor.aroundWriteTo(MappableExceptionWrapperInterceptor.java:86)
at org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:162)
at org.glassfish.jersey.message.internal.MessageBodyFactory.writeTo(MessageBodyFactory.java:1130)
at org.glassfish.jersey.server.ServerRuntime$Responder.writeResponse(ServerRuntime.java:683)
at org.glassfish.jersey.server.ServerRuntime$Responder.processResponse(ServerRuntime.java:424)
at org.glassfish.jersey.server.ServerRuntime$Responder.process(ServerRuntime.java:414)
at org.glassfish.jersey.server.ServerRuntime$2.run(ServerRuntime.java:312)
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:271)
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:267)
at org.glassfish.jersey.internal.Errors.process(Errors.java:315)
at org.glassfish.jersey.internal.Errors.process(Errors.java:297)
at org.glassfish.jersey.internal.Errors.process(Errors.java:267)
at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:317)
at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:292)
at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:1139)
at org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:460)
at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:386)
at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:334)
at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:221)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:416)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:283)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:459)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:167)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:283)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:200)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:132)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:111)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:536)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:591)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571)
at java.lang.Thread.run(Thread.java:748)
]]
用户类
public class User {
private String id;
private String name;
private String type;
User(){}
User(String id, String name, String type){
this.setId(id);
this.setName(name);
this.setType(type);
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
}
答案 0 :(得分：0)
网址：
http://localhost:8080/RESTfulServiceLIT_war_exploded/users/{id}
是Jersey用来说明如何访问数据的路径规范。如果您只是在网络浏览器中输入该字符串，那么您将获得NullPointerException。
键入时会发生什么：
http://localhost:8080/RESTfulServiceLIT_war_exploded/users/K1