title: org.hibernate.AnnotationException: Unknown mappedBy in: error
body: <p>I am trying one to one relationships using annotations.<br>
  This is the code.</p>
  
  <pre><code>package test.hibernate;
  
  @Entity
  @Table(name="EMPLOYEE")
  
  public class Employee {
      @Id
      @GeneratedValue
      @Column(name="EMP_ID")
      private Long empID;
  
      @Column(name="fName")
      private String fName;
  
      @Column(name="lName")
      private String lName;
  
      @OneToOne(mappedBy="employee", cascade=CascadeType.ALL)
      private Address address;
  
  
      public Employee(/* Long empID, */String fName, String lName) {
          super();
          // this.empID = empID;
          this.fName = fName;
          this.lName = lName;
      }
  
      public Employee() {
          super();
      }
  
      public Long getEmpID() {
          return empID;
      }
  
      public void setEmpID(Long empID) {
          this.empID = empID;
      }
  
      public String getfName() {
          return fName;
      }
  
      public void setfName(String fName) {
          this.fName = fName;
      }
  
      public String getlName() {
          return lName;
      }
  
      public void setlName(String lName) {
          this.lName = lName;
      }
  
      public Address getAddress() {
          return address;
      }
  
      public void setAddress(Address address) {
          this.address = address;
      }
  }
  
  
  package test.hibernate;
  
  @NamedNativeQueries({ @NamedNativeQuery(name = "addressQuery", query = "from Address", resultClass = Address.class) })
  
  public class Address {
  
      @Column(name = "EMP_ID")
      @GenericGenerator(name = "gen", strategy = "foreign", parameters = @Parameter(name = "property", value = "employee"))
      private Long empID;
  
      @Column(name = "ADDRESS")
      private String address;
  
      @Column(name = "CITY")
      private String city;
  
      @OneToOne
      @PrimaryKeyJoinColumn
      private Employee employee;
  
      public Address() {
          super();
          // TODO Auto-generated constructor stub
      }
  
      public Address(/* Long empID, */String address, String city) {
          super();
          // this.empID = empID;
          this.address = address;
          this.city = city;
      }
  
      public Long getEmpID() {
          return empID;
      }
  
      public void setEmpID(Long empID) {
          this.empID = empID;
      }
  
      public String getAddress() {
          return address;
      }
  
      public void setAddress(String address) {
          this.address = address;
      }
  
      public String getCity() {
          return city;
      }
  
      public void setCity(String city) {
          this.city = city;
      }
  
      public Employee getEmployee() {
          return employee;
      }
  
      public void setEmployee(Employee employee) {
          this.employee = employee;
      }
  
  
  }
  
  }
  </code></pre>
  
  <p>I am getting the following exception:</p>
  
  <pre><code>Hibernate one to many (XML Mapping)
  log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).
  log4j:WARN Please initialize the log4j system properly.
  org.hibernate.AnnotationException: Unknown mappedBy in: test.hibernate.Employee.address, referenced property unknown: test.hibernate.Address.employee
      at org.hibernate.cfg.OneToOneSecondPass.doSecondPass(OneToOneSecondPass.java:129)
      at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1127)
      at org.hibernate.cfg.AnnotationConfiguration.secondPassCompile(AnnotationConfiguration.java:296)
      at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1283)
      at test.hibernate.HibernateUtil.openSession(HibernateUtil.java:14)
      at test.hibernate.Test.main(Test.java:11)
  Exception in thread "main" java.lang.NullPointerException
      at test.hibernate.HibernateUtil.openSession(HibernateUtil.java:19)
      at test.hibernate.Test.main(Test.java:11)
  </code></pre>
  
  <p>What is wrong with my code? Can someone help?</p>