Question

我已经阅读了@transient和transient关键字之间的讨论：Why does JPA have a @Transient annotation?

但是当我使用java关键字而不是@Transient表示法创建某个字段瞬态时，这些字段不会在创建表的表中创建。这是为什么？

这是我的persistence.xml：

<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
    version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
    <persistence-unit name="someDB" transaction-type="RESOURCE_LOCAL">
        <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
        <class>somewhere.classnameA</class>
        <class>somewhere.classnameB</class>
        <properties>
            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url"
                value="jdbc:mysql://localhost:3306/project" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="" />

            <!-- EclipseLink should create the database schema automatically -->
            <property name="eclipselink.ddl-generation" value="create-or-extend-tables" />
            <property name="eclipselink.ddl-generation.output-mode"
                value="both" />
        </properties>

这是一个示例实体：

import java.sql.Timestamp;

import com.google.gwt.user.client.rpc.IsSerializable;

@Entity
public class Session implements IsSerializable{

    @Id
    @Basic(optional = false)
    @Column(length = 36)
    private String sessionID;

    @Version
    @Basic(optional = false)
    transient private Timestamp lastModification;

    @Basic(optional = false)
    transient private Timestamp expireTime;

    @OneToOne(optional = false)
    private User user;

    protected Session(){

    }

    // constructor server side
    public Session(String sessionID, User user, Timestamp expireTime){
        this.sessionID = sessionID;
        this.user = user;
        this.expireTime = expireTime;
    }

    public String getSessionID() {
        return sessionID;
    }

    public void setSessionID(String sessionID) {
        this.sessionID = sessionID;
    }

    public Timestamp getLastModification() {
        return lastModification;
    }

    public void setLastModification(Timestamp lastModification) {
        this.lastModification = lastModification;
    }

    public Timestamp getExpireTime() {
        return expireTime;
    }

    public void setExpireTime(Timestamp expireTime) {
        this.expireTime = expireTime;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    @Override
    @Transient
    public String toString() {
        String userID = (user != null) ? String.valueOf(user.getUserID()) : "?";
        return String.format("(%s)%s", userID, sessionID);
    }


}

注意：在上面的文件中，我删除了一些不重要的导入。在生成的表中，只有两个字段，即SESSIONID和USER_USERID。我还使用了持久性api 1.0

Answer 1

从JPA的角度来看，注释和修饰符完全等效，两者都表示该字段不是持久的。见JSR 220 Specification的第2.1.1段，其中说：

If the entity has field-based access, the persistence provider runtime accesses
instance variables directly. All non-transient instance variables that are not
annotated with the Transient annotation are persistent.

Answer 2

瞬态字段不参与持久性，它们的值永远不会存储在数据库中，类似于Java中不参与序列化的瞬态字段

Answer 3

ANSWER来自书 - Pro JPA掌握JPA

.. 我们使用了transient修饰符而不是@Transient注释，这样如果Employee（例如一个实体）从一个VM序列化到另一个VM，那么该字段将重新初始化以对应于新VM的区域设置。如果跨序列化应保留非持久性值，则应使用注释而不是修饰符。

JPA的transient关键字无法按预期工作

3 个答案: