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.

通俗地说，如果在实体的属性上使用@Transient注释:该属性将被单独挑选出来，不会保存到数据库中。实体内对象的其余属性仍将被保存。

我保存对象到数据库使用jpa存储库内置的保存方法，这样:

userRoleJoinRepository.save(user2);

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。

对于Kotlin开发人员，请记住Java transient关键字将成为内置的Kotlin @Transient注释。因此，如果您在实体中使用JPA @Transient，请确保您有JPA导入:

import javax.persistence.Transient

目的不同:

transient关键字和@Transient注释有两个不同的目的:一个处理序列化，一个处理持久性。作为程序员，我们经常把这两个概念结合在一起，但这通常是不准确的。持久性是指状态的特性，它比创造它的过程更持久。Java中的序列化是指将对象的状态编码/解码为字节流的过程。

关键字transient是一个比@Transient更强的条件:

如果字段使用了transient关键字，当对象转换为字节流时，该字段将不会被序列化。此外，由于JPA将标记有transient关键字的字段视为具有@Transient注释，因此JPA也不会持久化该字段。

另一方面，当对象被序列化时，单独带注释的@Transient字段将被转换为字节流，但JPA不会持久化它。因此，瞬态关键字是比@Transient注释更强的条件。

例子

This begs the question: Why would anyone want to serialize a field that is not persisted to the application's database? The reality is that serialization is used for more than just persistence. In an Enterprise Java application there needs to be a mechanism to exchange objects between distributed components; serialization provides a common communication protocol to handle this. Thus, a field may hold critical information for the purpose of inter-component communication; but that same field may have no value from a persistence perspective.

例如，假设在服务器上运行一个优化算法，并假设该算法需要几个小时才能完成。对于客户来说，拥有最新的解决方案集非常重要。因此，客户端可以订阅服务器，并在算法执行阶段定期接收更新。这些更新是使用ProgressReport对象提供的:

@Entity
public class ProgressReport implements Serializable{

    private static final long serialVersionUID = 1L;

    @Transient
    long estimatedMinutesRemaining;
    String statusMessage;
    Solution currentBestSolution;

}

Solution类可能看起来像这样:

@Entity
public class Solution implements Serializable{

    private static final long serialVersionUID = 1L;

    double[][] dataArray;
    Properties properties;
}

服务器将每个ProgressReport保存到它的数据库中。服务器并不关心如何持久化estimatedMinutesRemaining，但是客户端肯定关心这个信息。因此，estimatedMinutesRemaining使用@Transient进行注释。当最终的解决方案由算法定位时，它将由JPA直接持久化，而不使用ProgressReport。

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.