Java工厂模式

    工厂模式不仅要求有一个能够创建新对象的方法，还要求能够是客户代码无需了解应该具体实例化哪个类。具体说有三个特征：

    1. 该方法创建一个新对象;

    2. 该方法的返回类型为一个抽象类或接口;

    3. 有若干个类实现了上述接口;

    我们来看下数据库连接中使用的工厂模式的例子，由于数据库连接可能使用JDBC也可能是数据源的方式获取一个数据库连接，好吧，我们设计一个连接接口，把数据库连接的细节隐藏起来。

 

public interface BuildConnection {
	Connection getConnection() throws SQLException;
}

 

 

public class TestFactory {
	private BuildConnection con;

	public TestFactory(String driver, String url, String userid, String password)
			throws ClassNotFoundException {
		con = new DriverManagerConnection(driver, url, userid, password);

	}

	public TestFactory(String factory, String lookupURL, String lookupName)
			throws ClassNotFoundException {
		con = new DataSourceConnection(factory, lookupURL, lookupName);

	}

	public BuildConnection getConnection() {
		return con;
	}

    上面已经搞好工厂了，只要传递进去变量，就可以返回BuildConnection对象了。

 

public class DriverManagerConnection implements BuildConnection {
	static private Set<String> set = new HashSet<String>();
	private String driver;
	private String url;
	private String userid;
	private String password;
		
	public DriverManagerConnection(String newDriver, String newUrl, String newUserid, String newPassword) throws ClassNotFoundException {
		// save the input parameters
		driver = newDriver;
		url = newUrl;
		userid = newUserid;
		password = newPassword;
		
		// load the driver
		loadDriver();
	}
		
	private void loadDriver() throws ClassNotFoundException {
		if (!set.contains(driver)) {
			Class.forName(driver); // load the driver
			set.add(driver);	   // add it to the collection	
		}
	}

	public Connection getConnection() throws SQLException {
		return DriverManager.getConnection(url, userid, password);
	}		
}

 

public class DataSourceConnection implements BuildConnection {
	private String factory;
	private String lookupURL;
	private String lookupName;
	private Context context;
	private DataSource dataSource;
	
	public DataSourceConnection(String newFactory, String newLookupURL,  String newLookupName) {
		factory = newFactory;
		lookupURL = newLookupURL;
		lookupName = newLookupName;
	}

	private DataSource getDataSource() throws NamingException {
		if (dataSource == null) {
			Context context = getContext();
			dataSource = (DataSource) context.lookup(lookupName);
		}
		return dataSource;
	}

	private Context getContext() throws NamingException {
		if (context == null) {
			Hashtable<String, String> env = new Hashtable<String, String>(5);
			env.put(Context.INITIAL_CONTEXT_FACTORY, factory);
			env.put(Context.PROVIDER_URL,lookupURL);
//			env.put("java.naming.factory.initial", factory);
//			env.put("java.naming.provider.url",lookupURL);

			context = new InitialContext(env);
		}
		return context; 
	}

	public Connection getConnection() throws SQLException {
		Connection connection = null;
		try {
			connection = getDataSource().getConnection();
		} catch (NamingException ne) {
			throw new SQLException("DataSource Unavailable");
		}
		return connection;
	}

}

    对于用户来说，用户只要了解有TestFactory和BuildConnection类就可以了，不需要了解实现BuildConnection接口的各种类。