如何使JPA应用程序访问不同的数据库? [英] How can I make a JPA application access different databases?

问题描述

我正在编写一个Java SE(桌面)应用程序，该应用程序必须访问不同的数据库，所有这些数据库都将具有相同的数据模型(相同的架构，表等).我想重用在每个数据库前面的Java EE应用程序中已经使用过的JPA实体.

I'm writing a Java SE (desktop) application that has to access different databases all of which will have the same data model (same schema, tables, etc.). I want to reuse the JPA Entities that I already use in a Java EE application that front each database.

要重用现有的entity.jar文件，我必须使用具有resource_local数据源的另一个persistence.xml将其重新打包.那是构建时间上的不便，但不是一个大问题.

To reuse the existing entity.jar file I'll have to repackage it with a different persistence.xml that has a resource_local data source. That's an build time inconvenience but not a big problem.

问题是我的桌面应用程序将只能使用persistence.xml文件中定义的数据源.我可以定义多个持久性单元并选择在运行时使用哪个持久性单元，但是当添加新数据库时，我将不得不更改persistence.xml并更新所有桌面二进制文件.

The problem is that my desktop application will be limited to using the datasource defined in the persistence.xml file. I could define multiple persistence units and select which one to use at runtime, but when a new database is added I'll have to change the persistence.xml and update all the desktop binaries.

我希望能够在每个用户可以配置的.properties文件中定义新的数据源.在运行时是否有任何方法可以覆盖或添加到persistence.xml中声明的持久性单元?

I'd like to be able to define new data sources in a .properties file that each user could configure. Is there any way to override or add to the persistence units declared in the persistence.xml at runtime?

我不想仅使用Web服务接口来构建Java EE应用程序以支持该桌面应用程序. Java EE应用程序具有不同的用途，我想将桌面功能保留在桌面应用程序中.

I don't want to build out the Java EE applications with web service interfaces just to support this desktop application. The Java EE applications have a different purpose and I want to keep the desktop functionality in the desktop application.

谢谢.

推荐答案

您可以在运行时通过提供属性来创建EntityManagerFactory.

You can create EntityManagerFactory at runtime by providing properties.

Map<String, Object> properties = new HashMap<String, Object>();

properties.put(TRANSACTION_TYPE, PersistenceUnitTransactionType.RESOURCE_LOCAL.name());
properties.put(JDBC_DRIVER, driver);
properties.put(JDBC_URL, db_url);
properties.put(JDBC_USER, "userName");
properties.put(JDBC_PASSWORD, "password");

EntityManagerFactory factory = Persistence.createEntityManagerFactory("PERSISTENT_UNIT_NAME", properties);

您还可以尝试拥有一个属性文件，该属性文件将在运行时从中加载到map中.因此，它将使数据库配置与代码脱钩.

Also you can try having a property file from which properties will be loaded at runtime into map. Therefore it will decouple the database configuration from code.

属性键(JDBC_URL等)是供应商特定的，应相应地替换它们.

Edit : The property keys(JDBC_URL etc) are vendor specific, they should be replaced accordingly.

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