JAVA：如何从web.xml加载数据库URL？

Question

I'm using persistence API and want to load jdbc URL from web.xml. URL should be a context parameter of servlet. I can't find how to construct EntityManagerFactory not using persistence.xml. May be I should create PersistenceUnit in servlet and set some parameters? Can you give me some short example?

Thank you

Answer 1

you can use method createEntityManagerFactory(String persistenceUnitName, Map properties) of class javax.persistence.Persistence . and insert all your parameters in the map

Answer 2

I personally find it the cleanest way to define a datasource in the servlet container usng JNDI and refer to that from the persistence.xml.

This also solves one configuration issue when migrating from test-uat-prod as I can use the same datasource name on the machines.

Yeah, I know JNDI is not popular, but this works nice and it avoids having to manipulate hte web.xml which is also wrapped in the war.

Answer 3

Whether you use a datasource or not, the JPA configuration goes in the persistence.xml , not in the web.xml . If you want to use JPA, provide a persistence.xml .

Answer 4

You can retrieve any value from the web.xml by using env-entry

<env-entry>
    <env-entry-name>dbUrl</env-entry-name>
    <env-entry-type>java.lang.String</env-entry-type>
    <env-entry-value>jdbc:url:goes:here</env-entry-value>
</env-entry>

And in java

try {
    Context ctx = new InitialContext();
    String dbUrl= (String) ctx.lookup("java:comp/env/dbUrl");

    //... create your connection
} catch (Exception e ) {

}

Answer 5

Define an init-param in your web.xml that points to your database url. For example, for MySQL, it would be something like this:

<init-param>
    <param-name>DB_URL</param-name>
    <param-value>jdbc:mysql:///MY_DB</param-value>
</init-param>

Then get this value inside your Web app (in a Servlet) using something like this:

String myDbUrl = getServletConfig().getInitParameter("DB_URL");

In JSP files you can get the value in this way:

String myDbUrl  = config.getInitParameter("DB_URL");