springmvc hibernate返回级联实体的json格式报错?报错

Could not write JSON: No serializer found for class org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) (through reference chain: org.springframework.validation.support.BindingAwareModelMap["course"]->com.thon.entity.course.Course["createBy"]->com.thon.entity.system.User_$$_jvstdcb_0["handler"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: No serializer found for class org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) (through reference chain: org.springframework.validation.support.BindingAwareModelMap["course"]->com.thon.entity.course.Course["createBy"]->com.thon.entity.system.User_$$_jvstdcb_0["handler"])

一方实体

@JsonIgnoreProperties(ignoreUnknown=true)
public class Course extends IdEntity {
	
	private static final long serialVersionUID = 2118439396249691137L;
	private String name;
	private User createBy;
	private List<CourseTag> courseTags;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@OneToMany(fetch = FetchType.LAZY, mappedBy = "course",cascade = CascadeType.ALL)
	public List<CourseTag> getCourseTags() {
		return courseTags;
	}

	public void setCourseTags(List<CourseTag> courseTags) {
		this.courseTags = courseTags;
	}

	@ManyToOne(fetch = FetchType.LAZY)
	public User getCreateBy() {
		return createBy;
	}

	public void setCreateBy(User createBy) {
		this.createBy = createBy;
	}
}

@JsonIgnoreProperties(ignoreUnknown=true)
public class CourseTag extends IdEntity {

	private static final long serialVersionUID = 7790694935170125722L;
	private Integer region;
	private Integer tagId;
	private Course course;
	
	public Integer getRegion() {
		return region;
	}
	
	public void setRegion(Integer region) {
		this.region = region;
	}
	
	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "courseId")
	public Course getCourse() {
		return course;
	}
	
	public void setCourse(Course course) {
		this.course = course;
	}

	public Integer getTagId() {
		return tagId;
	}

	public void setTagId(Integer tagId) {
		this.tagId = tagId;
	}
	
}