JPA和Hibernate对@Transactional的使用是什么?

问题描述

我正在学习如何使用JPA和Hibernate和MySQL数据库创建REST API，并且看到了@Transactional注释.有人可以解释此注释的用途吗?

I'm learning about how to create REST API with JPA and Hibernate and a MySQL database and I see this @Transactional annotation. Can someone explain what is the use of this annotation?

例如，我有一个简单的DAO类:

For example I have this simple DAO class:

@Repository
public class EmployeeDAOHibernateImpl implements EmployeeDAO {

    // define field for entitymanager
    private EntityManager entityManager;

    // set up constructor injection
    @Autowired
    public EmployeeDAOHibernateImpl(EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    @Override
    @Transactional
    public List<Employee> findAll() {

        // get the current hibernate session
        Session currentSession = entityManager.unwrap(Session.class);

        // create a query
        Query<Employee> theQuery = 
                currentSession.createQuery("from Employee", Employee.class);

        // execute query and get result list
        List<Employee> employees = theQuery.getResultList();

        // return the results
        return employees;
    }

}

您可以看到用于findAll()方法的@Transactional，但是如果删除此@Transactional，则会得到相同的输出...那么，此@Transactional的用途是什么?

You can see the @Transactional used for findAll() method, but if I delete this @Transactional I get the same output... then what is the use of this @Transactional?

推荐答案

@Transactional批注用于在事务中执行某些方法/类(=内部的所有方法)的情况.

@Transactional annotation is used when you want the certain method/class(=all methods inside) to be executed in a transaction.

假设用户A希望将100 $转移给用户B.会发生什么:

Let's assume user A wants to transfer 100$ to user B. What happens is:

1. 我们将A的帐户减少了100 $
2. 我们将100美元添加到B的帐户中

让我们假设在执行1)之后和执行2)之前引发了异常.现在我们会有某种不一致的地方，因为A损失了100美元，而B一无所获.交易意味着全部或全部.如果方法中某处引发异常，则更改不会持久保存在数据库中.发生rollback之类的事情.

Let's assume the exception is thrown after succeeding 1) and before executing 2). Now we would have some kind of inconsistency because A lost 100$ while B got nothing.Transactions means all or nothing. If there is an exception thrown somewhere in the method, changes are not persisted in the database. Something called rollback happens.

如果未指定@Transactional，则每个数据库调用将处于不同的事务中.

If you don't specify @Transactional, each DB call will be in a different transaction.

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