我应该如何在基于 servlet 的应用程序中连接到 JDBC 数据库/数据源? [英] How should I connect to JDBC database / datasource in a servlet based application?

问题描述

我有一个外部 MySQL 服务器，它已经设置好并且运行良好.我在 Eclipse 中创建了一个数据库连接，可以在数据源资源管理器选项卡中查看数据库.

I have an external MySQL server that's set up and working fine. I created a database connection in Eclipse and can view the database in the Data Source Explorer tab.

现在，我有一个需要访问该数据库的 servlet.我该怎么做?有没有办法引用在数据源资源管理器中创建的数据库连接，或者我必须定义两次?

Now, I have a servlet that needs to access that database. How do I do it? Is there a way to reference that database connection created in the data source explorer, or do I have to define everything twice?

另外，打开连接的最佳方式是什么?我已经包含了 mysql-connector-java-5.1.11-bin.jar 文件，并且我发现了两种有效的方法:

Also, what's the best way to open the connection? I've got the mysql-connector-java-5.1.11-bin.jar file included, and I've found two methods that work:

MysqlDataSource d = new MysqlDataSource();
d.setUser("user");
d.setPassword("pass");
d.setServerName("hostname.com");
d.setDatabaseName("db");
Connection c = d.getConnection();

和

Connection c = DriverManager.getConnection("jdbc:mysql://hostname.com/db","user","pass");

两者都不是最优的，因为首先，它们都使用硬编码的字符串来处理所有事情.这是一个Java EE web app项目，那么有没有好放连接数据的地方呢?或者有没有办法放弃所有这些而只使用数据源浏览器中的连接?

Neither is optimal, because first of all, they both use hard-coded strings for everything. This is a Java EE web app project, so is there a good place to put connection data? Or is there a way to forgo all that and just use the connection in the data source explorer?

推荐答案

一种常见的做法是将其配置为 DataSource 在 servlet 容器 有问题.它将为您提供连接池设施，这将大大提高性能.另外一个常见的做法是将一些配置文件中的原始设置外部化，这些文件放在类路径中.

A common practice is to configure this as a DataSource in the servlet container in question. It will provide you connection pooling facilities which will greatly improve performance. Also a common practice is to externalize the raw settings in some configuration file which is been placed in the classpath.

如果您使用 Tomcat 作为 servletcontainer，您需要根据其 JNDI 文档.你会看到有几种方法.最简单的方法是在你的动态 web 项目的 webcontent 中创建一个 /META-INF/context.xml(要清楚，/META-INF 是相同的级别作为 web 应用程序的 /WEB-INF)并用以下内容填充它:

In case you're using Tomcat as servletcontainer, you need to configure the datasource as per its JNDI documentation. You'll see that there are several ways. Easiest way is to create a /META-INF/context.xml in the webcontent of your dynamic web project (to be clear, the /META-INF is at the same level as the /WEB-INF of the webapp) and fill it with something like:

<?xml version="1.0" encoding="UTF-8"?>

<Context>
    <Resource
        name="jdbc/db" type="javax.sql.DataSource"
        maxActive="100" maxIdle="30" maxWait="10000" 
        url="jdbc:mysql://hostname.com/db"
        driverClassName="com.mysql.jdbc.Driver"
        username="user" password="pass"
    />
</Context>

这大致意味着Tomcat服务器应该创建一个JNDI名称为jdbc/db的数据源，最多有100个活动连接，最多30个空闲连接和一个在从应用程序返回连接之前的最长等待时间为 10000 毫秒(实际上:由应用程序关闭，因此应用程序在获取连接和关闭连接之间有 10 秒的时间).设置的剩余部分对您来说应该是熟悉且不言自明的；这些是 JDBC 设置.

This roughly means that Tomcat server should create a datasource with the JNDI name jdbc/db with a maximum of 100 active connections, a maximum of 30 idle connections and a maximum wait time of 10000 milliseconds before a connection should be returned from your application (actually: closed by your application, so your application has 10 seconds time between acquiring the connection and closing the connection). The remnant of the settings should be familiar and self-explaining enough to you; those are the JDBC settings.

最后在您的 web 项目中，编辑文件 /WEB-INF/web.xml 以添加以下条目:

Finally in your web project, edit the file /WEB-INF/web.xml to add the following entry:

<resource-env-ref>
    <resource-env-ref-name>jdbc/db</resource-env-ref-name>
    <resource-env-ref-type>javax.sql.DataSource</resource-env-ref-type>
</resource-env-ref>

这大致意味着 Web 应用程序应该使用名为 jdbc/db 的服务器提供的数据源.

This roughly means that the webapplication should use the server-provided datasource with the name jdbc/db.

然后将您的连接管理器更改为如下所示:

Then change your connection manager to something like this:

private DataSource dataSource;

public Database(String jndiname) {
    try {
        dataSource = (DataSource) new InitialContext().lookup("java:comp/env/" + jndiname);
    } catch (NamingException e) {
        // Handle error that it's not configured in JNDI.
        throw new IllegalStateException(jndiname + " is missing in JNDI!", e);
    }
}

public Connection getConnection() {
    return dataSource.getConnection();
}

..并用 new Database("jdbc/db") 替换所有 Class.forName(driver) 调用并替换所有 DriverManager.getConnection() 由 database.getConnection() 调用.如有必要，您可以从一些配置文件(属性文件?).

..and replace all Class.forName(driver) calls by new Database("jdbc/db") and replace all DriverManager.getConnection() calls by database.getConnection(). You can if necessary obtain the value jdbc/db from some config file (Properties file?).

或者，通过 DataSource">@Resource 容器管理工件内的注释，例如 @WebServlet servlet 类:

Alternatively, inject the DataSource via the @Resource annotation inside a container managed artifact, such as a @WebServlet servlet class:

@Resource(name="jdbc/db")
private DataSource dataSource;

应该是这样.只需使用上述更改部署您的 web 应用程序并运行它.不要忘记将数据库 JDBC 驱动程序放在 Tomcat/lib 或将其路径添加到 Tomcat/conf/catalina 的 shared.loader 属性.properties，因为加载 JDBC 驱动程序的责任现在从 web 应用程序转移到服务器.有关更多提示和其他基本 JDBC/JNDI 示例，您可以找到 这篇文章 也很有用.

That should be it. Just deploy your webapplication with the above changes and run it. Don't forget to place the database JDBC driver in the Tomcat/lib or to add its path to the shared.loader property of Tomcat/conf/catalina.properties, because the responsibility of loading the JDBC driver is now moved from the webapplication to the server. For more hints and other basic JDBC/JNDI examples you may find this article useful as well.

• 如何在Eclipse Web项目中安装JDBC驱动程序而无需面对java.lang.ClassNotFoundException
• 我在哪里必须要为Tomcat的连接池放置JDBC驱动吗?
• 在多线程系统中使用静态 java.sql.Connection 实例是否安全?
• 显示 JDBC 结果集在 JSP 页面的 HTML 中使用 MVC 和 DAO 模式
• 如何在 JSP 页面中从数据库中检索和显示图像?

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