@OneToMany(fetch = FetchType.EAGER) // Игнорируется, если есть @LazyCollection
@LazyCollection(LazyCollectionOption.TRUE)
private List<Order> orders;

@OneToMany
@LazyCollection(LazyCollectionOption.EXTRA)
private Set<Comment> comments; // Для comments.size() не загружает все комментарии

@OneToMany
@LazyCollection(LazyCollectionOption.FALSE) // Аналог FetchType.EAGER
private List<Post> posts;

import java.util.HashMap;

public class Task140425 {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();
        map.put(null, 1);
        map.put("null", 2);
        map.put(null, 3);
        
        System.out.println(map.get(null) + " " + map.get("null"));
    }
}

spring.jpa.properties.hibernate.bytecode.provider=javassist
Без этого NO_PROXY может не работать.

spring.jpa.properties.hibernate.enable_lazy_load_no_trans=true  # Не рекомендуется!
(Может привести к N+1 и утечкам памяти.)

@Entity
public class Order {
    @ManyToOne(fetch = FetchType.LAZY) // Стандартный LAZY
    @LazyToOne(LazyToOneOption.PROXY)  // Явное указание прокси
    private User user;
}

@Entity
public class Employee {
    @OneToOne(fetch = FetchType.LAZY)
    @LazyToOne(LazyToOneOption.NO_PROXY) // Требует инструментирования
    private ParkingSpot parkingSpot;
}

@Entity
@Loader(namedQuery = "findUserById")
@NamedQuery(name = "findUserById", query = "SELECT u FROM User u WHERE u.id = ?1")
public class User { ... }

@Entity
@Loader(query = "SELECT * FROM users WHERE user_id = ?1 AND is_active = true")
public class User { ... }

@Entity
@Loader(query = """
    SELECT u.* FROM users u 
    LEFT JOIN user_settings s ON u.id = s.user_id 
    WHERE u.id = ?1
    """)
public class User { ... }

@Entity
@Loader(query = "CALL load_user_by_id(:id)")
public class User { ... }

import java.util.HashMap;
import java.util.Map;

public class Task150425 {
    public static void main(String[] args) {
        Map<StringBuilder, Integer> map = new HashMap<>();
        StringBuilder key = new StringBuilder("Key");
        map.put(key, 1);
        
        key.append("Modified");
        System.out.println(map.get(key));
        System.out.println(map.get(new StringBuilder("Key")));
    }
}

@AnyMetaDef(
    name = "myMetaDef",
    metaType = "string",
    idType = "long",
    metaValues = {
        @MetaValue(targetEntity = Book.class, value = "BOOK"),
        @MetaValue(targetEntity = Movie.class, value = "MOVIE")
    }
)

@Entity
public class User {
    @Id
    private Long id;

    @ManyToAny
    @AnyMetaDef(
        name = "itemMetaDef",
        metaType = "string",
        idType = "long",
        metaValues = {
            @MetaValue(targetEntity = Book.class, value = "BOOK"),
            @MetaValue(targetEntity = Movie.class, value = "MOVIE")
        }
    )
    @JoinTable(
        name = "user_items",
        joinColumns = @JoinColumn(name = "user_id"),
        inverseJoinColumns = @JoinColumn(name = "item_id")
    )
    @Column(name = "item_type") // Дискриминатор
    private List<Object> items; // Может содержать Book и Movie
}

@Entity
public class Review {
    @Id
    private Long id;

    @Any(metaDef = "contentMetaDef", metaColumn = @Column(name = "content_type"))
    @JoinColumn(name = "content_id")
    private Object content; // Может быть Book или Movie
}

@AnyMetaDef(
    name = "mediaMetaDef",
    metaType = "string",  // Тип столбца-дискриминатора (varchar)
    idType = "long",      // Тип ID целевой сущности
    metaValues = {
        @MetaValue(targetEntity = Book.class, value = "BOOK"),
        @MetaValue(targetEntity = Movie.class, value = "MOVIE")
    }
)

@Entity
public class Library {
    @Id
    private Long id;

    @ManyToAny
    @AnyMetaDef(
        name = "mediaMetaDef",
        metaType = "string",
        idType = "long",
        metaValues = {
            @MetaValue(targetEntity = Book.class, value = "BOOK"),
            @MetaValue(targetEntity = Movie.class, value = "MOVIE")
        }
    )
    @JoinTable(
        name = "library_items",
        joinColumns = @JoinColumn(name = "library_id"),
        inverseJoinColumns = @JoinColumn(name = "item_id")
    )
    @Column(name = "item_type") // Столбец для дискриминатора
    private List<Object> items; // Может содержать Book и Movie
}

@Entity
public class Review {
    @Id
    private Long id;

    @Any(metaDef = "mediaMetaDef")
    @JoinColumn(name = "content_id")
    @Column(name = "content_type") // Дискриминатор
    private Object content; // Book или Movie
}

import java.util.HashMap;
import java.util.Map;

class Employee160425 {
    String name;
    
    Employee160425(String name) { this.name = name; }
    
    @Override
    public boolean equals(Object o) {
        return this.name.equals(((Employee160425)o).name);
    }
    
    @Override
    public int hashCode() {
        return name.length();
    }
}

public class Task160425 {
    public static void main(String[] args) {
        Map<Employee160425, String> map = new HashMap<>();
        Employee160425 e1 = new Employee160425("John");
        Employee160425 e2 = new Employee160425("Jane");
        
        map.put(e1, "Developer");
        map.put(e2, "Manager");
        
        e1.name = "Johnny";
        
        System.out.println(map.get(e1));
        System.out.println(map.get(new Employee160425("John")));
    }
}