本文主要是介绍实现可扩展的DAO，本文给出实现DAO的编程思想。，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
开发环境：本文使用Tomcat，Struts和MySQL。
为实现可扩展的DAO，本文将使用JNDI连接数据库，并将JNDI保存在XML文件里。同时也将调用sql语句的类 名保存在XML文件里。
例如：
1 dao-config.xml。该文件可以配置多个数据库的JNDI。在DAO初始化时，会将这些信息存入对象里。JNDI名为java:comp/env/jdbc/Colimas
<!--DOCTYPE dao SYSTEM "/DTD/dao.dtd"-->
<dao>
<dao-config id="base" type="jdbc">
<dao-param name="datasource">java:comp/env/jdbc/Colimas</dao-param>
</dao-config>
</dao>
2 dao-declaration.xml。该文件保存处理某数据库表的类名。
<dao>
<dao-object name="daoCBI">
<use-dao id="base"/>
<class>com.nova.colimas.data.sql.SQLTCBI</class>
</dao-object>
<dao-object name="daoUBI">
<use-dao id="base"/>
<class>com.nova.colimas.data.sql.SQLTUBI</class>
</dao-object>
<dao-object name="daoURM">
<use-dao id="base"/>
<class>com.nova.colimas.data.sql.SQLTURM</class>
</dao-object>
</dao>
3 配置JNDI。修改Tomcat的server.xml。在<host..>的<Context..>里面添加
<Resource name="jdbc/Colimas" auth="Container" type="javax.sql.DataSource"
maxActive="100" maxIdle="30" maxWait="10000"
username="root" password="197913" driverClassName="com.mysql.jdbc.Driver" url="jdbc:mysql://localhost:3306/Colimas?autoReconnect=true"/>
网上的很多文章的JNDI配置方法都不一样。这个对MySQL 5.0的配置方法。另外要到www.mysql.com 里下载最新的jdbc driver: mysql-connector-java-3.1.10-bin.jar
拷贝到$(TOMCATHOME)/common/lib。
4 配置web.xml。在你的web app里调用jndi需要添加
<resource-ref>
<description>DB Connection</description>
<res-ref-name>jdbc/Colimas</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
到web.xml。
5 初始化DAO配置信息。
在StartupServlet里装载Dao的配置信息。
public class StartupServlet extends Action {
/**
* List of DAO files
*/
public final static String PARAM_DAO = "dao";
public ActionForward execute(ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response)
throws Exception{
// Get list of DAO files
initDAO();
logger.info("init DAO successfully");
return mapping.findForward("success");
}
/**
* Initialization of DAO environment
*/
private void initDAO() throws ServletException
{
//Constants.DAO的值为两配置文件名：/resources/config/dao-config.xml,/resources/config/dao-declaration.xml
StringTokenizer st = new StringTokenizer (Constants.DAO, ",");
while (st.hasMoreTokens())
{
String name = st.nextToken();
// Get associated resource
//获得配置文件的URL。
URL url = getClass().getResource(name);
if (url == null)
{
throw new ServletException ("Cannot find resource for " + name);
}
else
{
try
{
//使用DAO工厂加载配置文件信息。
DAOFactory.addConfiguration (url);
}
catch (DAOFactoryException ex)
{
throw new ServletException ("Cannot initialize DAO", ex);
}
}
}
}
}
6 DAO工厂类 DAOFactory
根据输入的dao-object name来实例化DAO对象。
public abstract class DAOFactory {
//得到DAOObject
public static DAOObject getDAOObject(String daoName, DAOContext daoContext) throws DAOFactoryException
{
// Gets the DAO object declaration
// Gets the associated factory
// Creates DAO object
return factory.newDAOObject(objectDeclaration);
}
/**
* @see com.ibm.services.epricer.framework.dao.DAOFactory#newDAOObject(DAOObjectDeclaration)
*/
protected DAOObject newDAOObject(DAOObjectDeclaration objectDeclaration) throws DAOFactoryException
{
// Gets class
Class objectClass = (Class) classes.get(objectDeclaration.getName());
if (objectClass == null)
{
synchronized (classes)
{
try
{
objectClass = Class.forName(objectDeclaration.getClassName());
}
catch (ClassNotFoundException e)
{
throw new DAOFactoryException (this, "Cannot instantiate the DAO object class " + objectDeclaration.getClassName(), e);
}
classes.put(objectDeclaration.getName(), objectClass);
}
}
try
{
//初始化DAOObject，并将自己传入DAOObject。
DAOObject object = (DAOObject) objectClass.newInstance();
object.init(this);
return object;
}
catch (Exception ex)
{
throw new DAOFactoryException(this, "Cannot create DAO object " + objectDeclaration.getName(), ex);
}
}
/**
* Add a configuration file for the DAO
*/
public synchronized static void addConfiguration(URL url) throws DAOFactoryException
{}
}
实现JDBC的DAOFactory
public class JDBCDAOFactory extends DAOFactory {
/**
* Factory parameter, JDBC name of the Data source (example : jdbc/DataSource)
*/
public final static String PARAM_DATASOURCE = "datasource";
/**
* JDBC Data Source
*/
private DataSource dataSource;
private Logger logger;
/**
* Initializes the Data source.
*/
protected void init(Map parameters) throws DAOFactoryException
{
logger = Logger.getLogger(this.getClass());
// Gets the data source name
String dsName = (String)parameters.get (PARAM_DATASOURCE);
if (dsName == null)
{
throw new DAOFactoryException (this, "Cannot find 'datasource' parameter to initialize the DAO factory.");
}
else
{
try
{
// JNDI context
Context initialContext = new InitialContext ();
// Gets the data source
dataSource = (DataSource) initialContext.lookup(dsName);
}
catch (NamingException ex)
{
throw new DAOFactoryException (this, "Cannot find JDBC Data Source with JNDI name " + dsName, ex);
}
}
}
/**
* Gets a connection from the data source.
* First, this method try to get the registered connection from the <code>context</code>.
* If not found, a new connection is created and then registered into the <code>context</code>.
* @see JDBCDAOSource
* @param context Current context for DAO accesses
* @return The DAO source to use (for this factory, it is an instance of <code>JDBCDAOSource</code>
* @throws DAOFactoryException If the connection cannot be created.
*/
public DAOSource getDAOSource(DAOContext context) throws DAOFactoryException
{
// Gets the DAO source from the context
DAOSource source = context.getDAOSource(this);
if (source != null)
{
return source;
}
else
{
try
{
// Creates the connection
Connection connection = dataSource.getConnection();
connection.setAutoCommit(false);
connection.setTransactionIsolation(Connection.TRANSACTION_READ_COMMITTED);
// Creates the DAO source
source = new JDBCDAOSource (connection);
// Registers it
context.setDAOSource(this, source);
// Returns it
return source;
}
catch (SQLException ex)
{
ex.printStackTrace();
throw new DAOFactoryException (this, "Cannot get a new connection", ex);
}
}
}
}
其中DAOContext内根据用户的不同保存用户相关的数据库链接。初始时JDBCFactory会建立一个连接然后存入DAOContext中。
7 调用DAO Object
try{
//daoURM为DAO Object名称
SQLTURM urm=(SQLTURM)DAOFactory.getDAOObject(“daoURM”,new DAOContext(this.userbean,new HashMap()));
urm.callData(new DAOContext(this.userbean,new HashMap()));
}catch(DAOException e){
logger.error(e);
}
其中SQLTURM为DAOObject
public class SQLTURM extends JDBCDAOObject {
/**
*
* @param m_context context may contain projectid
*/
public Object callData(DAOContext m_context) {
logger = Logger.getLogger(this.getClass());
context = m_context;
PreparedStatement ps=null;
try{
//获得连接
connection = getConnection(context);
String sql = getQuery(context, Queries.SQL_USER_INSERT);
try{
ps = connection.prepareStatement(sql);
//执行sql
}finally{
ps.close();
}
}
catch (SQLException ex){
ex.printStackTrace();
logger.equals(ex);
}catch(DAOException e){
e.printStackTrace();
logger.error(e);
}
return null;
}
}
public class JDBCDAOObject extends DAOObject {
/**
* Gets the current connection.
*/
protected Connection getConnection (DAOContext context) throws DAOFactoryException
{
//首先获得jdbc Factory，然后获得daosource，最后获得daosource的connection。
JDBCDAOSource source = (JDBCDAOSource)getDAOFactory().getDAOSource(context);
return source.getConnection();
}
}
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