关于java：简单来说，什么是工厂？

In simplest terms, what is a factory?

什么是工厂？我为什么要用？

你熟悉JDBC吗？它是一个抽象的工厂。这是一个很好的现实例子。

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// Factory method. Loads the driver by given classname. It actually returns a
// concrete Class<Driver>. However, we don't need it here, so we just ignore it.
// It can be any driver class name. The MySQL one here is just an example.
// Under the covers, it will do DriverManager.registerDriver(new Driver()).
Class.forName("com.mysql.jdbc.Driver");

// Abstract factory. This lets the driver return a concrete connection for the
// given URL. You can just declare it against java.sql.Connection interface.
// Under the covers, the DriverManager will find the MySQL driver by URL and call
// driver.connect() which in turn will return new ConnectionImpl().
Connection connection = DriverManager.getConnection(url);

// Abstract factory. This lets the driver return a concrete statement from the
// connection. You can just declare it against java.sql.Statement interface.
// Under the covers, the MySQL ConnectionImpl will return new StatementImpl().
Statement statement = connection.createStatement();

// Abstract factory. This lets the driver return a concrete result set from the
// statement. You can just declare it against java.sql.ResultSet interface.
// Under the covers, the MySQL StatementImpl will return new ResultSetImpl().
ResultSet resultSet = statement.executeQuery(sql);

您的代码中不需要有一行特定于JDBC驱动程序的import。你不需要做任何事情。你只需申报所有针对java.sql.*的东西。你不需要自己动手。您只需要从一个抽象工厂获得它作为标准API的一部分。

如果将JDBC驱动程序类名称命名为可以在外部配置的变量(例如，属性文件)，并编写ANSI兼容SQL查询，那么您就不必为世界所知的每一个数据库供应商和/或JDBC驱动程序重写、重新编译、重建和重新分配Java应用程序。您只需在运行时类路径中删除所需的JDBC驱动程序JAR文件，并提供一些(属性)文件的配置，而不需要更改任何Java代码行，只要您想切换DB或在不同的DB上重用应用程序。

这就是接口和抽象工厂的力量。

另一个已知的真实世界例子是JavaEE。将"JDBC"与"JavaEE"和"JDBC驱动程序"替换为"JavaEE应用服务器"(FielF飞、TomEE、GelasFISH、Reland等)。

参见：

Class#forName()和DriverManager#getConnection()的工作原理是什么？
JavaEE究竟是什么？
维基百科：工厂方法模式
维基百科：抽象工厂模式

简单地说，工厂是一个面向对象的设计模式，它处理创建对象，而不指定要创建的对象的确切类。

在维基百科中有一个很好的理由来使用它：

The creation of an object often requires complex processes not
appropriate to include within a composing object. The object's
creation may lead to a significant duplication of code, may require
information not accessible to the composing object, may not provide a
sufficient level of abstraction, or may otherwise not be part of the
composing object's concerns. The factory method design pattern handles
these problems by defining a separate method for creating the objects,
which subclasses can then override to specify the derived type of
product that will be created.