找不到媒体类型=application/xml、类型=class java.util.HashMap$Values 的 MessageBodyWriter

Question

我的 API 方法无法将对象转换为 XML，但它可以转换为 JSON。当方法 return 类型是 Collection (MAP) 时，它会转换为 XML 而不会出现任何运行时异常（未找到 MessageBodyWriter），但是当使用 Response 作为 return 类型时，它会导致运行时异常。在这里我提到代码。我认为我的代码没有任何问题，但依赖项有问题。

package lk.ac.jfn.vau.MyDBapi.Model;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Department {
    private long Id;
    private String Name;
    private String Location;
    
    public Department() {
        
    }
    
    public Department(long id, String name, String location) {
        super();
        Id = id;
        Name = name;
        Location = location;
    }

    public long getId() {
        return Id;
    }

    public void setId(long id) {
        Id = id;
    }

    public String getName() {
        return Name;
    }

    public void setName(String name) {
        Name = name;
    }

    public String getLocation() {
        return Location;
    }

    public void setLocation(String location) {
        Location = location;
    }
    
    

}

package lk.ac.jfn.vau.MyDBapi.Repo;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class Repo {
    private static final String DB_Driver = "com.mysql.jdbc.Driver";
    private static final String DB_Connection = "jdbc:mysql://localhost:3306/";
    private static final String DB_Name = "departmentapi";
    private static final String DB_User = "root";
    private static final String DB_Password = "";

    public static void getDriver() {
        try {
            Class.forName(DB_Driver);
            System.out.println("Driver found");
        } catch (ClassNotFoundException e) {
            System.out.println("Driver not found " + e.getMessage());
        }
    }

    public static Connection getConnection() {
        Connection connection = null;

        try {
            connection = DriverManager.getConnection(DB_Connection + DB_Name, DB_User, DB_Password);
            System.out.println("Database Connected");
        } catch (SQLException e) {
            System.out.println(e.getMessage());
        }

        return connection;
    }

    public ResultSet get(String query, Connection connection) {
        ResultSet resultSet = null;
        try {
                Statement statement = connection.createStatement();
                resultSet = statement.executeQuery(query);

        } catch (SQLException e) {
            System.out.println(e.getMessage());
        }
        return resultSet;
    }
}

package lk.ac.jfn.vau.MyDBapi.Repo;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

import lk.ac.jfn.vau.MyDBapi.Model.Department;

public class DepartmentRepo extends Repo {
    Connection connection=null;
    public DepartmentRepo() {
        getDriver();
        connection = getConnection();
    }

    Map<Long, Department>map=new HashMap<Long, Department>();
    
    public Collection<Department> getAll(){
        ResultSet resultSet=get("select * from department", connection);
        try {
            while(resultSet.next()) {
                Department department=new Department(resultSet.getLong(1), resultSet.getString(2), resultSet.getString(3));
                map.put(department.getId(), department);
            }
        } catch (SQLException e) {
            System.out.println(e.getMessage());
        }
        
        return map.values();
    }
    
}

package lk.ac.jfn.vau.MyDBapi;

import java.util.Collection;

import javax.ws.rs.Consumes;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import javax.ws.rs.core.Response.Status;

import lk.ac.jfn.vau.MyDBapi.Model.Department;
import lk.ac.jfn.vau.MyDBapi.Repo.DepartmentRepo;

@Path("/dept")
@Produces(value = {MediaType.APPLICATION_JSON,MediaType.APPLICATION_XML})
@Consumes(MediaType.APPLICATION_JSON)
public class DepartmentResource {
    private DepartmentRepo repo=new DepartmentRepo();
    @GET
    public Response getDepartments() {
        Collection<Department> departments = repo.getAll();
        if(!departments.isEmpty()) {
            return Response.status(Status.FOUND).entity(departments).build();
        }
        return Response.status(Status.NOT_FOUND).entity("Nothing Found").build();
    }

}

POM.xml

<dependencies>
        <dependency>
            <groupId>org.glassfish.jersey.containers</groupId>
            <artifactId>jersey-container-servlet-core</artifactId>
            <!-- use the following artifactId if you don't need servlet 2.x compatibility -->
            <!-- artifactId>jersey-container-servlet</artifactId -->
        </dependency>
        <dependency>
            <groupId>org.glassfish.jersey.media</groupId>
            <artifactId>jersey-media-json-jackson</artifactId>
        </dependency>
        <dependency>
            <groupId>org.glassfish.jaxb</groupId>
            <artifactId>jaxb-runtime</artifactId>
            <version>2.3.2</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.34</version>
        </dependency>
    </dependencies>

Answer 1

使用 JAXB（XML 提供程序），您需要使用 GenericEntity。

GenericEntity<Collection<Department>> entity
    = new GenericEntity<Collection<Department>>(departments){};
return Response.ok(entity).build();

您需要这样做的原因是 JAXB 需要知道类型信息才能序列化。当你只是 return 泛型实体时，而不是 Response，泛型类型信息在方法签名中

@GET
public Collection<Department> getDepartments() {}

当您使用 Response 时，由于类型擦除，通用信息不可用。所以 JAX-RS 允许的是使用 GenericEntity。但是你不能只使用一个实例，因为类型擦除不允许这样做。您实际上需要使用匿名 class

new GenericEntity<Collection<Department>>(departments){}

注意最后的大括号。这是一个匿名 class。通过反射，你可以从这个匿名class.

中得到泛型类型参数

找不到媒体类型=application/xml、类型=class java.util.HashMap$Values 的 MessageBodyWriter

MessageBodyWriter not found for media type=application/xml, type=class java.util.HashMap$Values

java

rest

jax-rs

jersey