tomcat连接池问题

// 在此输入java代码
/*
 * 数据源管理器，可以管理多个数据源
 * 提高 JNDI 查找的速度
 */

package comm.ds;

import java.util.HashMap;
import java.util.Map;

import javax.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.sql.DataSource;

public class DataSourceManager {

	private static DataSourceManager instance;

	private Map dataSources;
	private Context context;

	// This is private, and can't be instantiated directly
	private DataSourceManager() throws NamingException {
		dataSources = new HashMap();

		// Get the context for caching purposes
		context = new InitialContext();

		/**
		 * In non-J2EE applications, you might need to load up
		 *   a properties file and get this context manually. I've
		 *   kept this simple for demonstration purposes.
		 */
	}

	public static DataSourceManager getInstance() throws NamingException {
		// Not completely thread-safe, but good enough 
		// (see note in article)
		if (instance == null) {
			instance = new DataSourceManager();
		}
		return instance;
	}

	public DataSource lookup(String jndiName)
		throws NamingException {

		// See if we already have this interface cached
		DataSource ds = (DataSource) dataSources.get(jndiName);

		// If not, look up with the supplied JNDI name
		if (ds == null) {
			ds =(DataSource) context.lookup(jndiName);
			
			// If this is a new ref, save for caching purposes
			dataSources.put(jndiName, ds);
		}
		return ds;
	}
}

Connection con = null;
con = DataSourceManager.getInstance().lookup("java:comp/env/jdbc/...").getConnection();
if ( null == con ) ...