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休眠性能提示：脏收集效果

發(fā)布時間：2023/12/3 编程问答 29 豆豆

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在使用Hibernate作為ORM開發(fā)服務(wù)器和嵌入式應(yīng)用程序8年后，我全力以赴地尋求提高Hibernate性能的解決方案，閱讀博客和參加會議，我決定與您分享這幾年獲得的知識。

這是更多新帖子中的第一篇：

去年，我以Devoxx的身份參加了演講，但我也參加了有關(guān)休眠反模式的 Patrycja Wegrzynowicz會議。在該演示中， Patrycja向我們展示了一種反模式，這種模式令我震驚，因為事實證明它預(yù)料到了意外情況。

我們將看到當(dāng)Hibernate檢測到一個骯臟的集合并應(yīng)該重新創(chuàng)建它時所產(chǎn)生的效果。

讓我們從將要使用的模型開始，只有兩個與一對多關(guān)聯(lián)相關(guān)的類：

@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;}}

在上一堂課中，我們應(yīng)注意三個重點：

我們在屬性級別而不是字段級別進行注釋。
@ OneToMany和@ ManyToOne使用默認選項（級聯(lián)定義除外）
星際飛船類的軍官getter返回一個不變的列表。

為了測試模型配置，我們將創(chuàng)建一個測試，該測試創(chuàng)建并保留一個Starship和七個高級管理人員，并在不同的Transaction和EntityManager中找到創(chuàng)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();} }

現(xiàn)在我們已經(jīng)創(chuàng)建了這個測試，我們可以運行它，并且我們將觀察Hibernate控制臺的輸出。

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間（持久對象）和在第二次事務(wù)提交期間（查找Starship ）執(zhí)行的查詢數(shù)。在忽略序列生成器的總和中，我們可以計數(shù)22個inserts ，2個selects和1個delete ，這在我們僅創(chuàng)建8個對象和1個通過主鍵查找時很不錯。

此時，讓我們檢查為什么執(zhí)行這些SQL查詢：

前八個插頁不可避免。通過將數(shù)據(jù)插入數(shù)據(jù)庫需要它們。

接下來的七都需要插入，因為我們已經(jīng)注釋getOfficers財產(chǎn)沒有的mappedBy屬性。如果我們仔細查看Hibernate文檔，它會指出“在不描述任何物理映射的情況下，將使用具有連接表的單向一對多 ”。

下一組查詢甚至更陌生，第一個選擇語句是通過id查找Starship，但是我們已經(jīng)創(chuàng)建的這些數(shù)據(jù)刪除和插入是什么？

在提交期間， Hibernate通過比較對象引用來驗證集合屬性是否臟。當(dāng)一個集合被標(biāo)記為臟集合時， Hibernate需要重新創(chuàng)建整個集合，甚至包含相同的對象。在本例中，當(dāng)我們要招募軍官時，我們要返回一個不同的收集實例，具體來說是不可修改的列表，因此Hibernate認為軍官的收集是骯臟的。

由于使用了聯(lián)接表，因此應(yīng)重新創(chuàng)建Starship_Officer表，刪除先前插入的元組并插入新的元組（盡管它們具有相同的值）。

讓我們嘗試解決此問題。我們首先映射一個雙向的一對多關(guān)聯(lián)，并以多對一的一方為擁有方。

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);}

現(xiàn)在，我們再次重新運行相同的測試，并再次檢查輸出。

盡管我們已將SQL語句的數(shù)量從25個減少到10個，但仍然有一個不必要的查詢，即第二個事務(wù)的commit部分中的查詢。為什么如果默認情況下軍官是懶惰的（ JPA規(guī)范），而我們又沒有讓軍官進行交易，那么Hibernate會在“軍官”表上執(zhí)行選擇嗎？由于與先前配置相同的原因，返回的集合具有不同的Java標(biāo)識符，因此Hibernate將其標(biāo)記為新實例化的集合，但是現(xiàn)在顯然不再需要連接表操作。我們減少了查詢數(shù)量，但是仍然存在性能問題。可能我們需要其他解決方案，而該解決方案不是最明顯的解決方案，我們不會返回Hibernate返回的集合對象，我們稍后可能會對此進行擴展，但是我們將更改批注的位置。

我們要做的是將映射位置從屬性方法更改為使用字段映射。簡單來說，我們將所有注釋移至類屬性，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，我們將再次運行測試，看看會發(fā)生什么：

為什么使用屬性映射Hibernate在提交期間運行查詢并且不執(zhí)行使用字段映射？提交事務(wù)后，Hibernate執(zhí)行刷新以使基礎(chǔ)持久性存儲與內(nèi)存中保持的可持久狀態(tài)同步。當(dāng)使用屬性映射時，Hibernate調(diào)用getter / setter方法來同步數(shù)據(jù)，對于getOfficers方法，它將返回一個臟集合（由于進行了unmodifiableList調(diào)用）。另一方面，當(dāng)我們使用字段映射時， Hibernate直接獲取字段，因此收集不被認為是骯臟的，并且不需要重新創(chuàng)建。

但是我們還沒有完成，我想您想知道為什么我們還沒有從getter中刪除Collections.unmodifiableList，而是返回Hibernate集合？是的，我同意您的意見，我們很快完成了工作，更改看起來像@ OneToMany（cascade = {CascadeType.ALL} ）public List <Officer> getOfficers（）{ 但是返回原始集合最終會導(dǎo)致封裝問題，實際上我們的封裝已損壞！我們可以將任何所需的內(nèi)容添加到可變列表中；我們可以將不受控制的更改應(yīng)用于對象的內(nèi)部狀態(tài)。

使用unmodifiableList是避免破壞封裝的一種方法，但是我們當(dāng)然可以對公共訪問和休眠訪問使用不同的訪問器，而不用調(diào)用Collections.unmodifiableList方法。

考慮到我們今天所看到的，我建議您使用始終字段注釋而不是屬性映射，我們將避免很多意外。

希望您發(fā)現(xiàn)這篇文章有用。

此示例的屏幕截圖：

下載代碼

參考： Hibernate性能提示： JCG合作伙伴的臟回收效應(yīng) ? 在一個罐子統(tǒng)治他們所有博客的亞歷克斯·索托。

翻譯自: https://www.javacodegeeks.com/2012/03/hibernate-performance-tips-dirty.html

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