java - Hibernate脏检查查询

Question

我正在阅读 Java Persistence with Hibernate 第二版。我在书中找到了以下引文，但我不太清楚。

Another issue to consider is dirty checking. Hibernate automatically detects state changes in order to synchronize the updated state with the database. It’s usually safe to return a different instance from the getter method than the instance passed by Hibernate to the setter. Hibernate compares them by value—not by object identity—to determine whether the attribute’s persistent state needs to be updated. For example, the following getter method doesn’t result in unnecessary SQL UPDATEs:

public String getFirstName(){
    return new String(firstName);
}

我的问题是为什么返回一个新的 String 对象不会进行不必要的 SQL 更新，它与仅返回 firstName 有什么不同？

当我尝试运行 getFirstName 时，我没有看到任何更新查询被触发。

请告诉我，因为我不清楚。

下面是代码:

package org.prashdeep.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;


@Entity
public class User {


    protected Long id;

    @Id
    @GeneratedValue()
    public Long getId() { // Optional but useful
        return id;
    }

    protected String firstName;

    protected String lastName;

    public void setId(Long id) {
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {

        return lastName;
    }

    public void setLastName(String lastName) {
        System.out.println("Called from the setter of lastname");
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                '}';
    }
}

SaveUserEntity.java

package org.prashdeep.executor;

import bitronix.tm.resource.jdbc.PoolingDataSource;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataBuilder;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Environment;
import org.hibernate.resource.transaction.backend.jta.internal.JtaTransactionCoordinatorBuilderImpl;
import org.hibernate.service.ServiceRegistry;
import org.prashdeep.common.DatabaseProduct;
import org.prashdeep.model.User;

import javax.naming.Context;
import javax.naming.InitialContext;
import javax.transaction.Status;
import javax.transaction.UserTransaction;
import java.util.List;


public class SaveUserEntity {

    protected Context context;
    protected PoolingDataSource dataSource;
    public static String DATASOURCE_NAME = "myDS";
    public DatabaseProduct databaseProduct;


    public UserTransaction getUserTransaction() {
        try {
            return (UserTransaction) context.lookup("java:comp/UserTransaction");
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }

    public void rollback() {
        UserTransaction tx = getUserTransaction();
        try {
            if (tx.getStatus() == Status.STATUS_ACTIVE ||
                    tx.getStatus() == Status.STATUS_MARKED_ROLLBACK)
                tx.rollback();
        } catch (Exception ex) {
            System.err.println("Rollback of transaction failed, trace follows!");
            ex.printStackTrace(System.err);
        }
    }

    protected SessionFactory createSessionFactory()  throws Exception{

        this.dataSource = new PoolingDataSource();
        dataSource.setUniqueName(DATASOURCE_NAME);
        dataSource.setMinPoolSize(1);
        dataSource.setMaxPoolSize(5);
        dataSource.setPreparedStatementCacheSize(10);
        dataSource.setIsolationLevel("READ_COMMITTED");
        dataSource.setClassName("org.h2.jdbcx.JdbcDataSource");
        dataSource.setAllowLocalTransactions(true);
        dataSource.getDriverProperties().put(
                "URL",
                "jdbc:h2:mem:test"
        );


        dataSource.getDriverProperties().put("user", "sa");
        context = new InitialContext();
        dataSource.init();


        /*
            This builder helps you create the immutable service registry with
            chained method calls.
         */
        StandardServiceRegistryBuilder serviceRegistryBuilder =
                new StandardServiceRegistryBuilder();

        /*
            Configure the services registry by applying settings.
         */
        serviceRegistryBuilder
                .applySetting("hibernate.connection.datasource", "myDS")
                .applySetting("hibernate.format_sql", "true")
                .applySetting("hibernate.show_sql", "true")
                .applySetting("hibernate.hbm2ddl.auto", "create-drop");

        // Enable JTA (this is a bit crude because Hibernate devs still believe that JTA is
        // used only in monstrous application servers and you'll never see this code).
        serviceRegistryBuilder.applySetting(
                Environment.TRANSACTION_COORDINATOR_STRATEGY,
                JtaTransactionCoordinatorBuilderImpl.class
        );
        ServiceRegistry serviceRegistry = serviceRegistryBuilder.build();

        /*
            You can only enter this configuration stage with an existing service registry.
         */
        MetadataSources metadataSources = new MetadataSources(serviceRegistry);

        /*
            Add your persistent classes to the (mapping) metadata sources.
         */
        metadataSources.addAnnotatedClass(
                org.prashdeep.model.User.class
        );

        // Add hbm.xml mapping files
        // metadataSources.addFile(...);

        // Read all hbm.xml mapping files from a JAR
        // metadataSources.addJar(...)

        MetadataBuilder metadataBuilder = metadataSources.getMetadataBuilder();

        Metadata metadata = metadataBuilder.build();


        SessionFactory sessionFactory = metadata.buildSessionFactory();

        return sessionFactory;
    }




   public static void main(String args[]) throws Exception {
       SaveUserEntity obj = new SaveUserEntity();

        SessionFactory sessionFactory = obj.createSessionFactory();
        try {
            {
                /*
                    Get access to the standard transaction API <code>UserTransaction</code> and
                    begin a transaction on this thread of execution.
                 */
                UserTransaction tx = obj.getUserTransaction();
                tx.begin();

                /*
                    Whenever you call <code>getCurrentSession()</code> in the same thread you get
                    the same <code>org.hibernate.Session</code>. It's bound automatically to the
                    ongoing transaction and is closed for you automatically when that transaction
                    commits or rolls back.
                 */
                Session session = sessionFactory.getCurrentSession();

                User user = new User();
                user.setFirstName("Pradeep");
                user.setLastName("Kumar");

                /*
                    The native Hibernate API is very similar to the standard Java Persistence API and most methods
                    have the same name.
                 */
                session.persist(user);



                /*
                    Hibernate synchronizes the session with the database and closes the "current"
                    session on commit of the bound transaction automatically.
                 */
               tx.commit();
            }

            {
                UserTransaction tx = obj.getUserTransaction();
                tx.begin();


                /*
                    A Hibernate criteria query is a type-safe programmatic way to express queries,
                    automatically translated into SQL.
                 */
                List<User> users =
                        sessionFactory.getCurrentSession().createCriteria(
                                User.class
                        ).list();
                // SELECT * from MESSAGE
                users.get(0).setFirstName("Praveen");

                System.out.println(users.get(0).getFirstName());


                tx.commit();
            }

        } finally {
            obj.rollback();
        }
    }
}

Answer 1

假设在数据库中，您有一条 firstName 为 FOO 的记录。在某些时候，该记录会作为持久实体加载，因此您现在拥有一个 User 类型的 Java 对象，其中 getFirstName() 返回一个字符串 "FOO".

当您使用 setFirstName("BAR"); 修改持久对象时，下次提交时，将导致 SQL UPDATE。这是因为Hibernate会比较“数据库状态”和“内存状态”，它会发现有区别(FOO变成了BAR)，然后他就会启动SQL 更新。到目前为止都是标准的。

现在 Hibernate 手册试图指出的一点是，这种比较是基于 equals 而不是 ==。在我们的示例中，它将检查字符串值 FOO 和 BAR 是否相等(概念上是 oldValue.equals(newValue))，而不是返回相同的 String 对象 (oldValue == newValue)。

因此，他们的示例是:您可以返回 new String(firstName)，它是一个不同的 String 对象，但 equals 是相同的。

希望这对您有帮助。