关于JPA规范中 EQL 查询的一点建议！

最新推荐文章于 2025-02-23 16:19:14 发布

ivor_zhou

最新推荐文章于 2025-02-23 16:19:14 发布

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文章标签： JPA C C++ C# SQL

本文探讨了在使用JPA 2.0作为ORM技术时遇到的问题：如何在不依赖构造函数的情况下，通过EQL SQL语句创建Java类实例。作者提出了一种利用getter方法而非构造函数的方法，并给出了具体的实现案例。

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When I use JPA as ORM technology in my project,I have some problem.

Following is my problem.

[b]FROM JPA2.0 SPEC
4.8.2Constructor Expressions in the SELECT Clause

A constructor may be used in the SELECT list to return an instance of a Java class. The specified class is not required to be an entity or to be mapped to the database. The constructor name must be fully qualified.

If an entity class name is specified as the constructor name in the SELECT NEW clause, the resulting entity instances are in the new state.
For example,

SELECT NEW com.acme.example.CustomerDetails(c.id, c.status, o.count)
FROM Customer c JOIN c.orders o
WHERE o.count>100[/b]

We can write EQL SQL like that

[color=blue] SELECT NEW com.acme.example.CustomerDetails(c.id, c.status as cdStatus, o.count)
FROM Customer c JOIN c.orders o
WHERE o.count>100[/color]

[color=darkred] without having appropriate Constructor.(Because in real environment,We should randomly access entity's properties.)
instead,JPA implmentation can invoke the get Mehtod.[/color]

(like the native sql
select c.id,c.status as cdStatus,o.count from cust c join order o on(c.id=o.cust_id) where o.count>100 random select clause)

class CustomerDetails
{
private String _id;
private String _status;
private String _count;

public CustomerDetails()
{
}

public void setId(String id)
{
_id = id;
}

public String getId()
{
return _id;
}

public void setCdStatus(String status)
{
_status = status;
}

public String getCdStatus()
{
return _status;
}

public void setCount(String count)
{
_count = count;
}

public String getCount()
{
return _count;
}

}

我曾经发给JSR委员会，没回音，到javaeye上请教高手，我的想法是否可行！