blocks|key|253556|text|Class.forName(JDBC_DRIVER)调用将在DriverManager中注册您的JDBC驱动程序，因此您可以通过url对其进行寻址，例如"jdbc:odbc:Database“等等……|type|unstyled|depth|inlineStyleRanges|entityRanges|data|253557|通常，driver类具有如下静态初始化代码，该代码在Class.forName()上调用：|253558|+public+class+Driver+implements+java.sql.Driver+{
+++static+{
+++++try+{
++++++++DriverManager.registerDriver(new+Driver());
+++++}+catch+(SQLException+E)+{
++++++++throw+new+RuntimeException("Can't+register+driver!");
+++++}
+++}
+}|code-block|syntax|javascript|253559|您仍然需要将JDBC驱动程序jar放入类路径中。|253560|作为另一种选择，您可以使用特定于数据库的DataSource，然后您可以声明性地指定数据源类型，例如在Spring上下文中或在Web+server+JNDI中。这是一个example。|offset|length|253561|entityMap|0|LINK|mutability|MUTABLE|url|http://www.java2s.com/Code/Java/Spring/SetupDataSourceforMySQL.htm^0|0|0|0|0|2C|7|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|10|8|@]|9|@$M|11|N|12|1|13]]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

The Class.forName(JDBC_DRIVER) call will register your JDBC driver in the DriverManager, so you can address it by url, such as "jdbc:odbc:Database" and so on...

Usually the driver class has static initialization code like this, which is invoked on Class.forName():

<pre><code> public class Driver implements java.sql.Driver {
 static {
 try {
 DriverManager.registerDriver(new Driver());
 } catch (SQLException E) {
 throw new RuntimeException("Can't register driver!");
 }
 }
 }
</code></pre>

You still have to put JDBC driver jar into the classpath.

As an alternative, you can use database specific DataSource, then you can declaratively specify the datasource type, for example in Spring context or in your Web server JNDI. Here is an <a href="http://www.java2s.com/Code/Java/Spring/SetupDataSourceforMySQL.htm" rel="noreferrer">example</a>.

blocks|key|249963|text|首先:使用现代的JDBC驱动程序和当前的JDK+(至少是Java6)，不再需要调用Class.forName()。使用service+provider+mechanism的JDBC+driver+classes+are+now+located。您应该能够简单地删除该调用，并保持代码的其余部分不变，它应该可以继续工作。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|249964|如果您没有使用当前的JDK+(或者如果您的JDBC驱动程序没有设置适当的文件来使用该机制)，那么需要使用registerDriver向DriverManager注册该驱动程序。该方法通常从实际驱动程序类的静态初始化块调用，该块在第一次加载类时触发，因此发出Class.forName()可确保驱动程序自行注册(如果尚未完成注册)。|249965|而且，无论您是使用Class.forName()还是新的服务提供者机制，类路径上总是需要JDBC驱动程序(或者至少在运行时可以通过一些ClassLoader获得)。|249966|t1；dr：是的，该Class.forName()调用的唯一用途是确保驱动程序已注册。如果您使用当前的JDK和当前的JDBC驱动程序，则此调用应该不再是必需的。|BOLD|249967|entityMap|0|LINK|mutability|MUTABLE|url|http://download.oracle.com/javase/7/docs/technotes/guides/jar/jar.html#Service%2520Provider|1|http://download.oracle.com/javase/7/docs/api/java/sql/DriverManager.html|2|http://download.oracle.com/javase/7/docs/api/java/sql/DriverManager.html#registerDriver%2528java.sql.Driver%2529^0|15|F|1N|Q|0|2E|Z|1|0|1G|E|1V|D|3K|F|1G|E|2|0|9|F|1V|B|0|0|5|A|F|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]|$9|14|A|15|1|16]]|E|$]]|$1|F|3|G|5|6|7|17|8|@$9|18|A|19|B|C]|$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]]|D|@$9|1E|A|1F|1|1G]]|E|$]]|$1|H|3|I|5|6|7|1H|8|@$9|1I|A|1J|B|C]|$9|1K|A|1L|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|1M|8|@$9|1N|A|1O|B|L]|$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|1R|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]|W|$5|P|Q|R|E|$S|X]]]]

First of: with modern JDBC drivers and a current JDK (at least Java 6) the call to <code>Class.forName()</code> is no longer necessary. <a href="http://download.oracle.com/javase/7/docs/api/java/sql/DriverManager.html" rel="noreferrer">JDBC driver classes are now located</a> using the <a href="http://download.oracle.com/javase/7/docs/technotes/guides/jar/jar.html#Service%20Provider" rel="noreferrer">service provider mechanism</a>. You should be able to simply remove that call and leave the rest of the code unchanged and it should continue to work.

If you're not using a current JDK (or if you have a JDBC driver that does not have the appropriate files set up to use that mechanism) then the driver needs to be registered with the <code>DriverManager</code> using <a href="http://download.oracle.com/javase/7/docs/api/java/sql/DriverManager.html#registerDriver%28java.sql.Driver%29" rel="noreferrer"><code>registerDriver</code></a>. That method is usually called from the static initializer block of the actual driver class, which gets triggered when the class is first loaded, so issuing the <code>Class.forName()</code> ensures that the driver registers itself (if it wasn't already done).

And no matter if you use <code>Class.forName()</code> or the new service provider mechanism, you will always need the JDBC driver on the classpath (or available via some <code>ClassLoader</code> at runtime, at least).

tl;dr: yes, the only use of that <code>Class.forName()</code> call is to ensure the driver is registered. If you use a current JDK and current JDBC drivers, then this call should no longer be necesary.

blocks|key|3147492|text|将一个类放在类路径中并不足以让类加载器加载它。并且必须加载驱动程序类，以确保将其注册到JDBC+API。顺便说一句，为了让Class.forName工作，驱动程序类必须在类路径中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|BOLD|entityRanges|data|3147493|entityMap^0|1P|D|2A|7|0^^$0|@$1|2|3|4|5|6|7|I|8|@$9|J|A|K|B|C]|$9|L|A|M|B|D]]|E|@]|F|$]]|$1|G|3|-4|5|6|7|N|8|@]|E|@]|F|$]]]|H|$]]

Putting a class in the classpath is not sufficient to have it loaded by the class loader. And the driver class must be loaded to ensure that it's registered to the JDBC API. BTW, for <code>Class.forName</code> to work, the driver class must be in the classpath.

blocks|key|3147528|text|在许多工业应用程序中，我们希望通过以属性文件/配置文件的形式提取这些信息，将数据访问层从其余代码中抽象出来。在这些情况下，我们可能需要像您一样使用一些东西。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3147529|例如，我们可以使用JDBC接口类来编写访问数据库的代码，您可以通过选择要用作数据库的技术(例如ojdbc驱动程序或mysql+jdbc驱动程序等)来配置属性。在这些情况下，您可以使用这样的方法装入类。|3147530|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

In many industrial applications, we would like to abstract the data access layer from the rest of code, by pulling such information in a form of a property file/configuration file. In those cases, we might need to use something as you have done.

For e.g. we can use the JDBC interface classes to write the code to access the database, where you could configure the properties by selecting which technology you want to use as a database (e.g. ojdbc driver, or mysql jdbc driver, etc.) In those cases you can load the class using such method.

I understand that class loading is useful for load the class at runtime with its class name.
However while using JDBC in our project we know which driver we are going to use and mostly driver manager string is hard coded.
My question is: Why are we loading driver using <code>Class.forName(&quot;JDBC_DRIVER&quot;)</code> here? 
Why can't we go ahead adding the driver in class path? since we know which driver jar we are going to use.
I believe <code>Class.forName(JDBC_DRIVER)</code> will load the Driver into <code>DriverManager</code>. Is it the only reason?
Edit 1:
The <a href="http://docs.oracle.com/javase/6/docs/api/index.html?overview-summary.html" rel="noreferrer"><code>DriverManager</code> API doc</a> states that
<blockquote>
As part of its(DriverManager) initialization, the DriverManager class will attempt to load the driver classes referenced in the &quot;jdbc.drivers&quot; system property.
Applications no longer need to explictly load JDBC drivers using <code>Class.forName()</code>. Existing programs which currently load JDBC drivers using <code>Class.forName()</code> will continue to work without modification.
</blockquote>
Then when I use other than oracle driver; do I need to change the driver name string in system property?

What is the purpose of 'Class.forName("MY_JDBC_DRIVER")'?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我知道类装入对于在运行时用其类名装入类很有用。然而，在我们的项目中使用JDBC时，我们知道要使用哪个驱动程序，而且大多数驱动程序管理器字符串都是硬编码的。我的问题是:为什么我们要在这里使用Class.forName("JDBC_DRIVER")加载驱动程序？为什么我们不能在类路径中添加驱动程序呢？因为我们知道要使用哪个...

问'Class.forName("MY_JDBC_DRIVER")‘的用途是什么？
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问'Class.forName("MY_JDBC_DRIVER")‘的用途是什么？
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