How can I leverage Hibernate and Spring annotations to establish and manage one-to-many, many-to-one, and many-to-many relationships between entities?

Using Annotations to Create Relationships in Hibernate and Spring

One-to-Many Relationships

• Uni-directional: The owning class, Foo, maintains a list of Bars. In the database, Bars will have a foreign key to Foo.

  <code class="java">@Entity
  public class Foo {
    @OneToMany
    private List<bar> bars;
  }
  
  @Entity
  public class Bar {
    @ManyToOne
    @JoinColumn(name="fooId")
    private Foo foo;
  }</bar></code>

• Bidirectional: Both Foo and Bar maintain references to each other.

  <code class="java">@Entity
  public class Foo {
    @OneToMany(mappedBy="foo")
    private List<bar> bars;
  }
  
  @Entity
  public class Bar {
    @ManyToOne
    @JoinColumn(name="fooId")
    private Foo foo;
  }</bar></code>

Many-to-One Relationships

• Uni-directional: The owning class, Bar, has a reference to Foo. In the database, Foo will have a foreign key to Bar.

  <code class="java">@Entity
  public class Bar {
    @ManyToOne
    @JoinColumn(name="fooId")
    private Foo foo;
  }
  
  @Entity
  public class Foo {
    // No corresponding mapping to Bar
  }</code>

• Bidirectional: Both Foo and Bar maintain references to each other.

  <code class="java">@Entity
  public class Bar {
    @ManyToOne
    @JoinColumn(name="fooId")
    private Foo foo;
  }
  
  @Entity
  public class Foo {
    @OneToMany(mappedBy="foo")
    private List<bar> bars;
  }</bar></code>

Many-to-Many Relationships

• Using a bridge table: Creates a join table to store the relationships.

  <code class="java">@Entity
  public class Foo {
    @ManyToMany
    @JoinTable(name="FOO_BAR",
        joinColumns = @JoinColumn(name="fooId"),
        inverseJoinColumns = @JoinColumn(name="barId"))
    private List<bar> bars;
  }
  
  @Entity
  public class Bar {
    @ManyToMany
    @JoinTable(name="FOO_BAR",
        joinColumns = @JoinColumn(name="barId"),
        inverseJoinColumns = @JoinColumn(name="fooId"))
    private List<foo> foos;
  }</foo></bar></code>

Configuration Options

• orphanRemoval: When true, orphaned entities (those not referenced by any parent) will be deleted on flush.
• fetchType: Controls the type of fetch strategy used for lazy loading of collections.
• cascade: Specifies the operations that are cascaded from parent to child entities.

Troubleshooting

• LazyInitializationException: Occurs when attempting to access a lazily loaded collection without first initializing it.
• Performance Issues: Overuse of Eager Fetching can degrade performance.
• orphanRemoval: Use sparingly to avoid unnecessary database deletions.

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