这是我上一个问题 How to model packages, versions and licenses? 的后续问题。
这是我的数据库设置。
V1__create_table_license.sql
CREATE TABLE IF NOT EXISTS license (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
reference TEXT NOT NULL,
is_deprecated_license_id BOOLEAN NOT NULL,
reference_number INTEGER NOT NULL,
license_id TEXT NOT NULL,
is_osi_approved BOOLEAN NOT NULL
);
INSERT INTO license
("name",reference,is_deprecated_license_id,reference_number,license_id,is_osi_approved)
VALUES
('MIT License','./MIT.json',false,275,'MIT',true);
V2__create_npm_package.sql
CREATE TABLE IF NOT EXISTS npm_package (
id BIGSERIAL PRIMARY KEY,
name TEXT NOT NULL,
description TEXT NOT NULL
);
INSERT INTO npm_package
(name, description)
VALUES
('react', 'React is a JavaScript library for building user interfaces.'),
('react-router-dom', 'DOM bindings for React Router'),
('typescript', 'TypeScript is a language for application scale JavaScript development'),
('react-dom', 'React package for working with the DOM.');
V3__create_npm_version.sql
CREATE TABLE IF NOT EXISTS npm_package_version (
npm_package_id BIGINT NOT NULL REFERENCES npm_package,
version TEXT NOT NULL,
license_id INTEGER NOT NULL REFERENCES license,
UNIQUE(npm_package_id, version)
)
这是我的 Java 对象。
License.java
@Entity
public class License {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
private String reference;
private Boolean isDeprecatedLicenseId;
private Integer referenceNumber;
private String name;
private String licenseId;
private Boolean isOsiApproved;
}
LicenseRepository.java
public interface LicenseRepository extends JpaRepository<License, Integer> {
License findByLicenseIdIgnoreCase(String licenseId);
}
NpmPackage.java
@Entity
public class NpmPackage {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
private String description;
@OneToMany(mappedBy = "npmPackage", cascade = CascadeType.ALL, orphanRemoval = true)
private List<NpmPackageVersion> versions = new ArrayList<>();
public NpmPackage() {}
public void addVersion(NpmPackageVersion version) {
this.versions.add(version);
version.setNpmPackage(this);
}
public void removeVersion(NpmPackageVersion version) {
this.versions.remove(version);
version.setNpmPackage(null);
}
}
@Entity
public class NpmPackageVersion {
public NpmPackageVersion() {}
public NpmPackageVersion(String version, License license) {
this.setVersion(version);
this.license = license;
}
@EmbeddedId private NpmPackageIdVersion npmPackageIdVersion = new NpmPackageIdVersion();
@MapsId("npmPackageId")
@ManyToOne(fetch = FetchType.LAZY)
private NpmPackage npmPackage;
@ManyToOne(fetch = FetchType.LAZY)
private License license;
@Embeddable
public static class NpmPackageIdVersion implements Serializable {
private static final long serialVersionUID = 3357194191099820556L;
private Long npmPackageId;
private String version;
// ...
}
public String getVersion() {
return this.npmPackageIdVersion.version;
}
public void setVersion(String version) {
this.npmPackageIdVersion.version = version;
}
}
MyRunner.java
@Component
class MyRunner implements CommandLineRunner {
@Autowired LicenseRepository licenseRepository;
@Autowired NpmPackageRepository npmPackageRepository;
@Override
// @Transactional
public void run(String... args) throws Exception {
// get license from database
var license = licenseRepository.findByLicenseIdIgnoreCase("mit");
// get package from db
var dbPackage = npmPackageRepository.findByNameIgnoreCase("react");
var version = new NpmPackageVersion("1.0.0", license);
dbPackage.addVersion(version);
npmPackageRepository.save(dbPackage);
}
}
在我之前的问题中,我得到了使用 fetch = FetchType.EAGER
的答案,但后来我了解到这并不理想。我想使用延迟获取。
@OneToMany(mappedBy = "npmPackage", fetch = FetchType.EAGER, cascade = CascadeType.ALL, orphanRemoval = true)
private List<NpmPackageVersion> versions = new ArrayList<>();
所以我删除了预先获取并遇到了错误。
<块引用>org.hibernate.LazyInitializationException:无法延迟初始化角色集合:com.example.bom.NpmPackage.NpmPackage.versions,无法初始化代理 - 无会话
使用 @Transactional
注释一切正常。为什么会这样?我试图在网上阅读所有内容,但我仍然不明白。我知道数据库会话在某个时候关闭了,我想知道这到底是怎么回事。我也想知道我是否可以做些什么,例如在添加另一个版本之前,我尝试获取所有版本以确保它们已加载。
那么我真的必须使用 @Transactional
还是有其他解决方案?我只是想了解正在进行的“魔法”:)
非常感谢!
答案 0 :(得分:1)
当您使用 FetchType.LAZY
时,当您找到实体时,Hibernate ORM 并不会真正返回已初始化的集合。关联将成为代理,当您需要访问集合时,Hibernate ORM 将查询数据库并获取它。
要实现这一点,实体(NpmPackage
)需要处于托管状态。如果实体不受管理并且您尝试访问惰性关联(在本例中为 versions
),您将获得 LazyInitializationException
。
在您的示例中,当您使用 @Transactional
时,实体在方法的持续时间内保持受管理状态。没有它,一旦您从 findByNameIgnoreCase
返回,它就无法管理。
如果您知道需要关联 versions
,您还可以使用 fetch join 查询来获取 NpmPackage
:
from NpmPackage p left join fetch p.versions where p.name=:name
这样关联会保持惰性,但您可以通过单个查询获得它。