I will try to answer the question of "why". Imagine a situation where you have a huge database with a lot of columns in a table, and your project/system uses tools to generate entities from database. (Hibernate has those, etc...) Now, suppose that by your business logic you need a particular field NOT to be persisted. You have to "configure" your entity in a particular way. While Transient keyword works on an object - as it behaves within a java language, the @Transient only designed to answer the tasks that pertains only to persistence tasks.

Java的transient关键字用于表示字段不被序列化，而JPA的@Transient注释用于表示字段不被持久化到数据库中，也就是说，它们的语义是不同的。

因为它们有不同的含义。@Transient注释告诉JPA提供者不要持久化任何(非瞬态的)属性。另一个告诉序列化框架不要序列化某个属性。您可能希望有一个@Transient属性并仍然序列化它。

正如其他人所说，@Transient用于标记不应该持久化的字段。请看这个简短的例子:

public enum Gender { MALE, FEMALE, UNKNOWN }

@Entity
public Person {
    private Gender g;
    private long id;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    public long getId() { return id; }
    public void setId(long id) { this.id = id; }

    public Gender getGender() { return g; }    
    public void setGender(Gender g) { this.g = g; }

    @Transient
    public boolean isMale() {
        return Gender.MALE.equals(g);
    }

    @Transient
    public boolean isFemale() {
        return Gender.FEMALE.equals(g);
    }
}

当这个类被提供给JPA时，它持久化了性别和id，但不会尝试持久化helper布尔方法——如果没有@Transient，底层系统会抱怨实体类Person缺少setMale()和setFemale()方法，因此根本不会持久化Person。

如果你只是想要一个字段不会被持久化，无论是transient还是@Transient工作。但问题是，既然“瞬态”已经存在，为什么要@Transient。

因为@Transient字段仍然会被序列化!

假设你创建一个实体，做一些cpu消耗的计算来得到一个结果，这个结果不会保存在数据库中。但是您希望将实体发送给其他Java应用程序以通过JMS使用，那么您应该使用@Transient，而不是JavaSE关键字transient。因此，运行在其他vm上的接收器可以节省重新计算的时间。

I will try to answer the question of "why". Imagine a situation where you have a huge database with a lot of columns in a table, and your project/system uses tools to generate entities from database. (Hibernate has those, etc...) Now, suppose that by your business logic you need a particular field NOT to be persisted. You have to "configure" your entity in a particular way. While Transient keyword works on an object - as it behaves within a java language, the @Transient only designed to answer the tasks that pertains only to persistence tasks.