休眠性能提示：脏收集效果

在使用Hibernate作为ORM开发服务器和嵌入式应用程序8年后，我全力以赴地寻求提高Hibernate性能的解决方案，阅读博客和参加会议，我决定与您分享这几年获得的知识。

这是更多新帖子中的第一篇：

去年，我以Devoxx的身份参加了演讲，但我也参加了有关休眠反模式的 Patrycja Wegrzynowicz会议。 在该演示中， Patrycja向我们展示了一种反模式，这种模式令我震惊，因为事实证明它预料到了意外情况。

我们将看到当Hibernate检测到一个肮脏的集合并应该重新创建它时所产生的效果。

让我们从将要使用的模型开始，只有两个与一对多关联相关的类： 

@Entity
public class Starship {private Long id;@Id @GeneratedValue(strategy=GenerationType.SEQUENCE) public Long getId() {return id;}public void setId(Long id) {this.id = id;}private Date launched;@Temporal(TemporalType.DATE)  public Date getLaunched() {return launched;}public void setLaunched(Date launched) {this.launched = launched;}private String registry;@Column(unique=true, nullable=false) public String getRegistry() {return registry;}public void setRegistry(String registry) {this.registry = registry;}private StarshipClassEnum starshipClassEnum;@Enumerated public StarshipClassEnum getStarshipClassEnum() {return starshipClassEnum;}public void setStarshipClassEnum(StarshipClassEnum starshipClassEnum) {this.starshipClassEnum = starshipClassEnum;}private AffiliationEnum affiliationEnum;@Enumerated public AffiliationEnum getAffiliationEnum() {return affiliationEnum;}public void setAffiliationEnum(AffiliationEnum affiliationEnum) {this.affiliationEnum = affiliationEnum;}private Physics physics;@Embedded public Physics getPhysics() {return physics;}public void setPhysics(Physics physics) {this.physics = physics;}private List<Officer> officers = new ArrayList<Officer>();@OneToMany(cascade={CascadeType.ALL}) public List<Officer> getOfficers() {return Collections.unmodifiableList(officers);}protected void setOfficers(List<Officer> officers) {this.officers = officers;}public void addOfficer(Officer officer) {officer.setStarship(this);this.officers.add(officer);}public Starship() {super();}public Starship(String registry) {setRegistry(registry);}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result+ ((registry == null) ? 0 : registry.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Starship other = (Starship) obj;if (registry == null) {if (other.registry != null)return false;} else if (!registry.equals(other.registry))return false;return true;}
}
@Entity
public class Officer {private Long id;@Id @GeneratedValue(strategy=GenerationType.SEQUENCE)public Long getId() {return id;}protected void setId(Long id) {this.id = id;}private String name;@Column(unique=true, nullable=false) public String getName() {return this.name;}public void setName(String name) {this.name = name;}private SpeciesEnum speciesEnum;@Enumerated public SpeciesEnum getSpeciesEnum() {return speciesEnum;}public void setSpeciesEnum(SpeciesEnum speciesEnum) {this.speciesEnum = speciesEnum;}private PlanetEnum homePlanet;@Enumerated public PlanetEnum getHomePlanet() {return homePlanet;}public void setHomePlanet(PlanetEnum homePlanet) {this.homePlanet = homePlanet;}private AffiliationEnum affiliationEnum;@Enumerated public AffiliationEnum getAffiliationEnum() {return affiliationEnum;}public void setAffiliationEnum(AffiliationEnum affiliationEnum) {this.affiliationEnum = affiliationEnum;}private RankEnum rank;@Enumerated @NotNull public RankEnum getRank() {return rank;}public void setRank(RankEnum rank) {this.rank = rank;}private Starship starship; @ManyToOne public Starship getStarship() {return starship;}protected void setStarship(Starship starship) {this.starship = starship;}public Officer() {super();}public Officer(String name, RankEnum rank) {setName(name);setRank(rank);}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Officer other = (Officer) obj;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}}

在上一堂课中，我们应注意三个重点：

  • 我们在属性级别而不是字段级别进行注释。
  • @ OneToMany和@ ManyToOne使用默认选项（ 级联定义除外）
  • 星际飞船类的军官getter返回一个不变的列表。

为了测试模型配置，我们将创建一个测试，该测试创建并保留一个Starship和七个高级管理人员 ，并在不同的Transaction和EntityManager中找到创建的Starship 。 

@ContextConfiguration
@RunWith(SpringJUnit4ClassRunner.class)
public class StarshipPersistenceTests {@Injectprivate EntityManagerFactory entityManagerFactory;@Testpublic void testSaveOrderWithItems() throws Exception {Starship starship = createData();findStarship(starship);}private Starship createData() {EntityManager entityManager = entityManagerFactory.createEntityManager();EntityTransaction transaction = entityManager.getTransaction();transaction.begin();Physics physics = physics().height(137.5D).length(642.5D).power("Wrap reactor").width(467.0D).build();Calendar launched = Calendar.getInstance();launched.set(2363, 9, 4);Starship starship = starship().registry("NCC-1701-D").physics(physics).launched(launched.getTime()).starshipClass(StarshipClassEnum.GALAXY).affiliation(AffiliationEnum.STARFLEET).build();Officer jeanLucPicard = officer().name("Jean-Luc Picard").rank(RankEnum.CAPTAIN).affiliation(AffiliationEnum.STARFLEET).homePlanet(PlanetEnum.EARTH).speciment(SpeciesEnum.HUMAN).build();starship.addOfficer(jeanLucPicard);Officer williamRiker = officer().name("William Riker").rank(RankEnum.COMMANDER).affiliation(AffiliationEnum.STARFLEET).homePlanet(PlanetEnum.EARTH).speciment(SpeciesEnum.HUMAN).build();starship.addOfficer(williamRiker);Officer data = officer().name("Data").rank(RankEnum.LIEUTENANT_COMMANDER).affiliation(AffiliationEnum.STARFLEET).homePlanet(PlanetEnum.OMICRON_THETA).speciment(SpeciesEnum.ANDROID).build();starship.addOfficer(data);Officer geordiLaForge = officer().name("Geordi La Forge").rank(RankEnum.LIEUTENANT).affiliation(AffiliationEnum.STARFLEET).homePlanet(PlanetEnum.EARTH).speciment(SpeciesEnum.HUMAN).build();starship.addOfficer(geordiLaForge);Officer worf = officer().name("Worf").rank(RankEnum.LIEUTENANT).affiliation(AffiliationEnum.STARFLEET).homePlanet(PlanetEnum.QONOS).speciment(SpeciesEnum.KLINGON).build();starship.addOfficer(worf);Officer beverlyCrusher = officer().name("Beverly Crusher").rank(RankEnum.COMMANDER).affiliation(AffiliationEnum.STARFLEET).homePlanet(PlanetEnum.EARTH).speciment(SpeciesEnum.HUMAN).build();starship.addOfficer(beverlyCrusher);Officer deannaTroi = officer().name("Deanna Troi").rank(RankEnum.COMMANDER).affiliation(AffiliationEnum.STARFLEET).homePlanet(PlanetEnum.BETAZED).speciment(SpeciesEnum.BETAZOID).build();starship.addOfficer(deannaTroi);entityManager.persist(starship);transaction.commit();entityManager.close();return starship;}private void findStarship(Starship starship) {EntityManager entityManager = this.entityManagerFactory.createEntityManager();EntityTransaction transaction = entityManager.getTransaction();transaction.begin();System.out.println("Before Find");Starship newStarship = entityManager.find(Starship.class, starship.getId());System.out.println("After Find Before Commit");transaction.commit();System.out.println("After commit");entityManager.close();}
}

现在我们已经创建了这个测试，我们可以运行它，并且我们将观察Hibernate控制台的输出。 

Hibernate: insert into Starship (affiliationEnum, launched, height, length, power, width, registry, starshipClassEnum, id) values (?, ?, ?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)Before Find Starship By IdHibernate: select starship0_.id as id1_0_, starship0_.affiliationEnum as affiliat2_1_0_, starship0_.launched as launched1_0_, starship0_.height as height1_0_, starship0_.length as length1_0_, starship0_.power as power1_0_, starship0_.width as width1_0_, starship0_.registry as registry1_0_, starship0_.starshipClassEnum as starship9_1_0_ from Starship starship0_ where starship0_.id=?After Find Starship By Id and Before CommitHibernate: select officers0_.Starship_id as Starship1_1_2_, officers0_.officers_id as officers2_2_, officer1_.id as id0_0_, officer1_.affiliationEnum as affiliat2_0_0_, officer1_.homePlanet as homePlanet0_0_, officer1_.name as name0_0_, officer1_.rank as rank0_0_, officer1_.speciesEnum as speciesE6_0_0_, officer1_.starship_id as starship7_0_0_, starship2_.id as id1_1_, starship2_.affiliationEnum as affiliat2_1_1_, starship2_.launched as launched1_1_, starship2_.height as height1_1_, starship2_.length as length1_1_, starship2_.power as power1_1_, starship2_.width as width1_1_, starship2_.registry as registry1_1_, starship2_.starshipClassEnum as starship9_1_1_ from Starship_Officer officers0_ inner join Officer officer1_ on officers0_.officers_id=officer1_.id left outer join Starship starship2_ on officer1_.starship_id=starship2_.id where officers0_.Starship_id=?
Hibernate: delete from Starship_Officer where Starship_id=?
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)After commit

查看在第一次提交期间（持久对象）和在第二次事务提交期间（查找Starship ）执行的查询数。 在忽略序列生成器的总和中，我们可以计数22个inserts ，2个selects和1个delete ，这在我们仅创建8个对象和1个通过主键查找时很不错。

此时，让我们检查为什么执行这些SQL查询：

前八个插页不可避免。 通过将数据插入数据库需要它们。

接下来的七都需要插入，因为我们已经注释getOfficers财产没有的mappedBy属性。 如果我们仔细查看Hibernate文档，它会指出“在不描述任何物理映射的情况下，将使用具有连接表的单向一对多 ”。

下一组查询甚至更陌生，第一个选择语句是通过id查找Starship，但是我们已经创建的这些数据删除和插入是什么？

在提交期间， Hibernate通过比较对象引用来验证集合属性是否脏。 当一个集合被标记为脏集合时， Hibernate需要重新创建整个集合，甚至包含相同的对象。 在本例中，当我们要招募军官时，我们要返回一个不同的收集实例，具体来说是不可修改的列表，因此Hibernate认为军官的收集是肮脏的。

由于使用了联接表，因此应重新创建Starship_Officer表，删除先前插入的元组并插入新的元组（尽管它们具有相同的值）。

让我们尝试解决此问题。 我们首先映射一个双向的一对多关联，并以多对一的一方为拥有方。 

private List<Officer> officers = new ArrayList<Officer>();
@OneToMany(mappedBy="starship", cascade={CascadeType.ALL}) public  List<Officer> getOfficers() {return Collections.unmodifiableList(officers);}
protected void setOfficers(List<Officer> officers) {this.officers = officers;}
public void addOfficer(Officer officer) {this.officers.add(officer);}

现在，我们再次重新运行相同的测试，并再次检查输出。 

Hibernate: insert into Starship (affiliationEnum, launched, height, length, power, width, registry, starshipClassEnum, id) values (?, ?, ?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)Before Find Starship By IdHibernate: select starship0_.id as id1_0_, starship0_.affiliationEnum as affiliat2_1_0_, starship0_.launched as launched1_0_, starship0_.height as height1_0_, starship0_.length as length1_0_, starship0_.power as power1_0_, starship0_.width as width1_0_, starship0_.registry as registry1_0_, starship0_.starshipClassEnum as starship9_1_0_ from Starship starship0_ where starship0_.id=?After Find Starship By Id and Before CommitHibernate: select officers0_.starship_id as starship7_1_1_, officers0_.id as id1_, officers0_.id as id0_0_, officers0_.affiliationEnum as affiliat2_0_0_, officers0_.homePlanet as homePlanet0_0_, officers0_.name as name0_0_, officers0_.rank as rank0_0_, officers0_.speciesEnum as speciesE6_0_0_, officers0_.starship_id as starship7_0_0_ from Officer officers0_ where officers0_.starship_id=?After commit

尽管我们已将SQL语句的数量从25个减少到10个，但仍然有一个不必要的查询，即第二个事务的commit部分中的查询。 为什么如果默认情况下军官是懒惰的（ JPA规范），而我们又没有让军官进行交易，那么Hibernate会在“军官”表上执行选择吗？ 由于与先前配置相同的原因，返回的集合具有不同的Java标识符，因此Hibernate将其标记为新实例化的集合，但是现在显然不再需要连接表操作。 我们减少了查询数量，但是仍然存在性能问题。 可能我们需要其他解决方案，而该解决方案不是最明显的解决方案，我们不会返回Hibernate返回的集合对象，我们稍后可能会对此进行扩展，但是我们将更改批注的位置。

我们要做的是将映射位置从属性方法更改为使用字段映射。 简单来说，我们将所有注释移至类属性，而不是getter上 。 

@Entity
public class Starship {@Id @GeneratedValue(strategy=GenerationType.SEQUENCE) private Long id;public Long getId() {return id;}protected void setId(Long id) {this.id = id;}@Temporal(TemporalType.DATE) private Date launched;public Date getLaunched() {return launched;}public void setLaunched(Date launched) {this.launched = launched;}...@OneToMany(mappedBy="starship", cascade={CascadeType.ALL}) private List<Officer> officers = new ArrayList<Officer>();public List<Officer> getOfficers() {return Collections.unmodifiableList(officers);}protected void setOfficers(List<Officer> officers) {this.officers = officers;}public void addOfficer(Officer officer) {officer.setStarship(this);this.officers.add(officer);}public Starship() {super();}public Starship(String registry) {setRegistry(registry);}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result+ ((registry == null) ? 0 : registry.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Starship other = (Starship) obj;if (registry == null) {if (other.registry != null)return false;} else if (!registry.equals(other.registry))return false;return true;} 
}
@Entity
public class Officer {@Id @GeneratedValue(strategy=GenerationType.SEQUENCE) private Long id;public Long getId() {return id;}protected void setId(Long id) {this.id = id;}@Column(unique=true, nullable=false) private String name;public String getName() {return this.name;}public void setName(String name) {this.name = name;}@Enumerated private SpeciesEnum speciesEnum;public SpeciesEnum getSpeciesEnum() {return speciesEnum;}public void setSpeciesEnum(SpeciesEnum speciesEnum) {this.speciesEnum = speciesEnum;}...@ManyToOne private Starship starship; public Starship getStarship() {return starship;}protected void setStarship(Starship starship) {this.starship = starship;}public Officer() {super();}public Officer(String name, RankEnum rank) {setName(name);setRank(rank);}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Officer other = (Officer) obj;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}
}

最后，我们将再次运行测试，看看会发生什么： 

Hibernate: insert into Starship (affiliationEnum, launched, height, length, power, width, registry, starshipClassEnum, id) values (?, ?, ?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)Before Find
Hibernate: select starship0_.id as id1_0_, starship0_.affiliationEnum as affiliat2_1_0_, starship0_.launched as launched1_0_, starship0_.height as height1_0_, starship0_.length as length1_0_, starship0_.power as power1_0_, starship0_.width as width1_0_, starship0_.registry as registry1_0_, starship0_.starshipClassEnum as starship9_1_0_ from Starship starship0_ where starship0_.id=?After Find Before Commit
After commit

为什么使用属性映射Hibernate在提交期间运行查询并且不执行使用字段映射？ 提交事务后，Hibernate执行刷新以使基础持久性存储与内存中保持的可持久状态同步。 当使用属性映射时，Hibernate调用getter / setter方法来同步数据，对于getOfficers方法，它将返回一个脏集合（由于进行了unmodifiableList调用）。 另一方面，当我们使用字段映射时， Hibernate直接获取字段，因此收集不被认为是肮脏的，并且不需要重新创建。

但是我们还没有完成，我想您想知道为什么我们还没有从getter中删除Collections.unmodifiableList，而是返回Hibernate集合？ 是的，我同意您的意见，我们很快完成了工作，更改看起来像@ OneToMany（cascade = {CascadeType.ALL} ）public List <Officer> getOfficers（）{ 但是返回原始集合最终会导致封装问题，实际上我们的封装已损坏！ 我们可以将任何所需的内容添加到可变列表中； 我们可以将不受控制的更改应用于对象的内部状态。

使用unmodifiableList是避免破坏封装的一种方法，但是我们当然可以对公共访问和休眠访问使用不同的访问器，而不用调用Collections.unmodifiableList方法。

考虑到我们今天所看到的，我建议您使用始终字段注释而不是属性映射，我们将避免很多意外。

希望您发现这篇文章有用。

此示例的屏幕截图：

下载代码

参考： Hibernate性能提示： JCG合作伙伴的 脏回收效应   在一个罐子统治他们所有博客的亚历克斯·索托。


翻译自: https://www.javacodegeeks.com/2012/03/hibernate-performance-tips-dirty.html

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