Why does JPA have a @Transient annotation?

Question

Java has the transientkeyword. Why does JPA have @Transient instead of simply using the already existing java keyword?

Solution 1

Java's transient keyword is used to denote that a field is not to be serialized, whereas JPA's @Transient annotation is used to indicate that a field is not to be persisted in the database, i.e. their semantics are different.

Solution 2

Because they have different meanings. The @Transient annotation tells the JPA provider to not persist any (non-transient) attribute. The other tells the serialization framework to not serialize an attribute. You might want to have a @Transient property and still serialize it.

Solution 3

As others have said, @Transient is used to mark fields which shouldn't be persisted. Consider this short example:

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);
    }
}

When this class is fed to the JPA, it persists the gender and id but doesn't try to persist the helper boolean methods - without @Transient the underlying system would complain that the Entity class Person is missing setMale() and setFemale() methods and thus wouldn't persist Person at all.

Solution 4

Purpose is different:

The transient keyword and @Transient annotation have two different purposes: one deals with serialization and one deals with persistence. As programmers, we often marry these two concepts into one, but this is not accurate in general. Persistence refers to the characteristic of state that outlives the process that created it. Serialization in Java refers to the process of encoding/decoding an object's state as a byte stream.

The transient keyword is a stronger condition than @Transient:

If a field uses the transient keyword, that field will not be serialized when the object is converted to a byte stream. Furthermore, since JPA treats fields marked with the transient keyword as having the @Transient annotation, the field will not be persisted by JPA either.

On the other hand, fields annotated @Transient alone will be converted to a byte stream when the object is serialized, but it will not be persisted by JPA. Therefore, the transient keyword is a stronger condition than the @Transient annotation.

Example

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.

For example, suppose an optimization algorithm is run on a server, and suppose this algorithm takes several hours to complete. To a client, having the most up-to-date set of solutions is important. So, a client can subscribe to the server and receive periodic updates during the algorithm's execution phase. These updates are provided using the ProgressReport object:

@Entity
public class ProgressReport implements Serializable{

    private static final long serialVersionUID = 1L;

    @Transient
    long estimatedMinutesRemaining;
    String statusMessage;
    Solution currentBestSolution;

}

The Solution class might look like this:

@Entity
public class Solution implements Serializable{

    private static final long serialVersionUID = 1L;

    double[][] dataArray;
    Properties properties;
}

The server persists each ProgressReport to its database. The server does not care to persist estimatedMinutesRemaining, but the client certainly cares about this information. Therefore, the estimatedMinutesRemaining is annotated using @Transient. When the final Solution is located by the algorithm, it is persisted by JPA directly without using a ProgressReport.

Solution 5

If you just want a field won't get persisted, both transient and @Transient work. But the question is why @Transient since transient already exists.

Because @Transient field will still get serialized!

Suppose you create a entity, doing some CPU-consuming calculation to get a result and this result will not save in database. But you want to sent the entity to other Java applications to use by JMS, then you should use @Transient, not the JavaSE keyword transient. So the receivers running on other VMs can save their time to re-calculate again.

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Comments

[Dilum Ranatunga]

Yes, the semantics are different. But why was JPA designed this way?

[Jawher]

Not sure I'm understanding you, but have a look at "Pascal Thivent"'s answer ;)

[Kdeveloper]

This is handy because you might not want to store the data in the database, but you do want to store it in the JPA Chaching system that uses serialization for store/restore of entities.

[DataNucleus]

What "JPA Caching system" that uses serialisation for store/restore of entities ? a JPA implementation can cache an object in any way they wish, and serialisation doesn't enter into it.

[Satish Sharma]

@Jawher , here for transient not presistant means to not to presist any value or it will insert default value for that attribute.

[40pro]

@psp can you explain more on why/how it could cause unspecified behavior? Thanks!

[psp]

@40Plot the specification states so

[Honza Zidek]

This should be IMHO the accepted answer as it is much more explaining that the current accepted one...

[Austin]

@DilumRanatunga I tried to address your question in my (late) answer below.

[Dilum Ranatunga]

If they actually are different concerns, surely there exists a different word that captures the nuances. Why overload the term? As a starter suggestion, @Unpersisted.

[Harsh Kanakhara]

can you please provide example to make it more clear ?

[Fernando Pie]

The value of trasient after the entity is persisted continue in the cache?

[Austin]

I personally like @Ephemeral. According to Merriam Webster: When ephemeral was first printed in English in the 1600s, "it was a scientific term applied to short-term fevers, and later, to organisms (such as insects and flowers) with very short life spans. Soon after that, it acquired an extended sense referring to anything fleeting and short-lived (as in "ephemeral pleasures")."

[Kalpesh Soni]

is there an annotation that jpa will treat as transient but jackson will not?

[neXus]

What I also like about this answer is that it mentions that JPA regards transient fields as implicitly having the @Transient annotation. So if you use the transient keyword to prevent serialization of a field then it will not end up in the database either.

[jfajunior]

Thanks for the answer Pascal. As a note from your comment: "You might want to have a @Transient property and still serialize it." (That was what I was looking for) I also want to add that the contrary is not true. If one set a variable as transient, than you cannot persist it.

[Jack]

c'mon.. do you really use equals() to compare enums? It's like writing new Integer(3).equals(new Integer(3)) to compare numbers

[Esko]

@Jack C'mon, you're replying to 12 year old answer. I haven't touched Java since 2016 even, I'm a full stack Clojurist these days.

[Jack]

Just in case you're returning to Java again :P