JPA标注要注意的地方

 这些标注尽量都要写到get方法的头上，我无意中把下面的标注写到了定义变量的头上时候，发生了错误。

报告错误：java.lang.NullPointerException

         怎么回事，刚刚我试了下把@Id放到getId()方法的头上结果也报上面那样的错误，然后我再把@Id又放回到private Integer id;的头上结果就没错了。这个问题要待解决下...(我个人认为都放到get的头上，但是id要放到定义的头上，这个我也不知道是什么原因)

private Set<SiteChannel> siteChannelCollection = new HashSet<SiteChannel>();

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "site")
public Set<SiteChannel> getSiteChannelCollection() {
	return siteChannelCollection;
}
public void setSiteChannelCollection(Set<SiteChannel> siteChannelCollection) {
	this.siteChannelCollection = siteChannelCollection;
}

 

private Site site;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "site_id")
public Site getSite() {
	return site;
}
public void setSite(Site site) {
	this.site = site;
}

 

@Id
@GeneratedValue(strategy = IDENTITY)
@Column(name = "id", unique = true, nullable = false)
public Long getId() {
	return this.id;
}
public void setId(Long id) {
	this.id = id;
}

 

@Temporal(TemporalType.TIMESTAMP)
@Column(name = "created", length = 19)
public Date getCreated() {
	return this.created;
}
public void setCreated(Date created) {
	this.created = created;
}

 

参考的例子Site.java
package cn.ctit.cms.entity;

// Generated Feb 3, 2009 4:08:09 AM by Hibernate Tools 3.2.2.GA

import java.util.Date;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Transient;

import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

import org.hibernate.annotations.Parent;
import org.hibernate.validator.Length;
import org.hibernate.validator.Min;
import org.hibernate.validator.NotNull;
import org.hibernate.validator.Pattern;

import cn.ctit.cms.entity.pay.Biller;
import cn.ctit.cms.entity.pay.Joinoption;

/**
 * Site generated by hbm2java
 */
@Entity
@Table(name = "site", catalog = "cms")
public class Site implements java.io.Serializable {

	private Long id;
	private String name;
	private String description;
	private String siteUrl;
	private Short siteType;
	private String ioUrl;
	private String ioService;
	private Integer ioPort;
	private String ioUser;
	private String ioPass;
	private String defaultFormat;
	private Boolean enable = true;
	private Short status;
	private Date created;
	private Date modified;
	private Long serverId;
	private String template;
	private Set<SiteChannel> siteChannels = new HashSet<SiteChannel>(0);
	private Set<SiteSchedule> siteSchedules = new HashSet<SiteSchedule>(0);
	private Set<Joinoption> joinoptions = new HashSet<Joinoption>(0);
	private Set<Biller> billers = new HashSet<Biller>(0);
	//NATS
	private String nats_siteid;

	public Site() {
	}

	public Site(String name, String description, String siteUrl,
			Short siteType, String ioUrl, String ioService, Integer ioPort,
			String ioUser, String ioPass, String defaultFormat, Boolean enable,
			Short status, Date created, Date modified,
			Set<SiteChannel> siteChannels, Set<SiteSchedule> siteSchedules,Long serverId) {
		this.name = name;
		this.description = description;
		this.siteUrl = siteUrl;
		this.siteType = siteType;
		this.ioUrl = ioUrl;
		this.ioService = ioService;
		this.ioPort = ioPort;
		this.ioUser = ioUser;
		this.ioPass = ioPass;
		this.defaultFormat = defaultFormat;
		this.enable = enable;
		this.status = status;
		this.created = created;
		this.modified = modified;
		this.siteChannels = siteChannels;
		this.siteSchedules = siteSchedules;
		this.serverId = serverId;
	}

	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "id", unique = true, nullable = false)
	public Long getId() {
		return this.id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	@NotNull
	@Column(name = "name", length = 100)
	@Length(max = 100)
	public String getName() {
		return this.name;
	}

	public void setName(String name) {
		this.name = name;
	}
	@Column(name = "description", length = 200)
	@Length(max = 200)
	public String getDescription() {
		return this.description;
	}

	public void setDescription(String description) {
		this.description = description;
	}
	
	@NotNull
	@Column(name = "site_url", length = 200)
	@Length(max = 200)
	public String getSiteUrl() {
		return this.siteUrl;
	}
	public void setSiteUrl(String siteUrl) {
		this.siteUrl = siteUrl;
	}
	@Column(name = "site_type")
	public Short getSiteType() {
		return this.siteType;
	}

	public void setSiteType(Short siteType) {
		this.siteType = siteType;
	}
	@NotNull
	@Column(name = "io_url", length = 200)
	@Length(max = 200)
	public String getIoUrl() {
		return this.ioUrl;
	}
	public void setIoUrl(String ioUrl) {
		this.ioUrl = ioUrl;
	}
	@Pattern(regex="[a-zA-Z]+[0-9]*",message="Please enter right value : transform!")
	@Column(name = "io_service", length = 50)
	@Length(max = 50)
	public String getIoService() {
		return this.ioService;
	}
	public void setIoService(String ioService) {
		this.ioService = ioService;
	}
	@Min(value=1)
	@Column(name = "io_port")
	public Integer getIoPort() {
		return this.ioPort;
	}
	public void setIoPort(Integer ioPort) {
		this.ioPort = ioPort;
	}
	@NotNull
	@Column(name = "io_user", length = 50)
	@Length(max = 50)
	public String getIoUser() {
		return this.ioUser;
	}

	public void setIoUser(String ioUser) {
		this.ioUser = ioUser;
	}
	@NotNull
	@Column(name = "io_pass", length = 50)
	@Length(max = 50)
	public String getIoPass() {
		return this.ioPass;
	}

	public void setIoPass(String ioPass) {
		this.ioPass = ioPass;
	}
	
	@Column(name = "default_format", length = 200)
	@Length(max = 200)
	public String getDefaultFormat() {
		return this.defaultFormat;
	}

	public void setDefaultFormat(String defaultFormat) {
		this.defaultFormat = defaultFormat;
	}
	
	@Column(name = "enable")
	public Boolean getEnable() {
		return this.enable;
	}

	public void setEnable(Boolean enable) {
		this.enable = enable;
	}
	
	@Column(name = "status")
	public Short getStatus() {
		return this.status;
	}

	public void setStatus(Short status) {
		this.status = status;
	}
	
	@Temporal(TemporalType.TIMESTAMP)
	@Column(name = "created", length = 19)
	public Date getCreated() {
		return this.created;
	}

	public void setCreated(Date created) {
		this.created = created;
	}
	
	@Temporal(TemporalType.TIMESTAMP)
	@Column(name = "modified", length = 19)
	public Date getModified() {
		return this.modified;
	}

	public void setModified(Date modified) {
		this.modified = modified;
	}

	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "site")
	public Set<SiteChannel> getSiteChannels() {
		return this.siteChannels;
	}

	public void setSiteChannels(Set<SiteChannel> siteChannels) {
		this.siteChannels = siteChannels;
	}

	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "site")
	public Set<SiteSchedule> getSiteSchedules() {
		return this.siteSchedules;
	}

	public void setSiteSchedules(Set<SiteSchedule> siteSchedules) {
		this.siteSchedules = siteSchedules;
	}

	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "site")
	public Set<Joinoption> getJoinoptions() {
		return joinoptions;
	}

	public void setJoinoptions(Set<Joinoption> joinoptions) {
		this.joinoptions = joinoptions;
	}

	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "site")
	public Set<Biller> getBillers() {
		return billers;
	}

	public void setBillers(Set<Biller> billers) {
		this.billers = billers;
	}
	
	@Column(name = "server_id")
	public Long getServerId() {
		return this.serverId;
	}

	public void setServerId(Long serverId) {
		this.serverId = serverId;
	}

	@Column(name = "nats_siteid")
	public String getNats_siteid() {
		return nats_siteid;
	}

	public void setNats_siteid(String nats_siteid) {
		this.nats_siteid = nats_siteid;
	}
	
	//@Transient
	public String getTemplate() {
		return template;
	}

	public void setTemplate(String template) {
		this.template = template;
	}

}