querytag.java

Servlet与JSP核心编程原码,适合刚做软件开发的程序员参考！

package examples;

import javax.servlet.jsp.tagext.*;
import javax.servlet.jsp.*;
import java.io.*;
import java.sql.*;
import java.sql.Connection;

/**
 * A query tag.
 *
 * Make a query using a connection.  Either use the connection from
 * the enclosing connection tag, or from a connection attribute that points
 * into a PageContext entry
 *
 * (A) Either
 * <connection id="con01" ....> ... </connection>
 * <query id="balances" connection="con01">
 *   SELECT account, balance FROM acct_table
 *	where customer_number = <%=request.custno %>
 * </query>
 *
 * (B) Or
 * <connection ...>
 *   <query id="balances">
 *     SELECT account, balance FROM acct_table
 *	where customer_number = <%=request.custno %>
 *   </query>
 * </connection>
 *
 * For this example, unlike connection, we will require an id.
 *
 * All JDBC code is pretty much faked
 */

public class QueryTag extends BodyTagSupport {
  
    /**
     * Create a new QueryTag
     */
    public QueryTag() {
	super();
    }

    /**
     * Attributes for this action:
     *   id (inherited)
     *  connection
     */

    /**
     * Connection: the id of the connection object
     * Setter and getter methods
     */

    public void setConnection(String s) {
	connectionId = s;
    }
    public String getConnection() {
	return connectionId;
    }


    /**
     * process the start Tag
     * Get attributes, locate any objects that are required to be in
     * the pagecontext.
     *
     * @return evaluate the body
     */
    public int doStartTag() throws JspTagException {
	String cid = getConnection();
	if (cid != null) {
	    // there is a connection id, use it
	    connection = (Connection) pageContext.getAttribute(cid);
	} else {
	    ConnectionTag tag =
		(ConnectionTag) findAncestorWithClass(this,
						      ConnectionTag.class);
	    if (tag == null) {
		throw new JspTagException("a query without a connection attribute must be nested within a ConnectionTag tag");
	    }
	    connection = tag.getConnection();
	}
	if (connection == null) {
	    throw new JspTagException("could not find connection object");
	}
	return EVAL_BODY_TAG;
    }

    /**
     * Process after body evaluation
     * Extract the String from the nested stream; do a query;
     * save away the value into a field instance.
     *
     * @return whether the body should be reevaluated
     */
    public int doAfterBody() throws JspTagException {
	BodyContent body = getBodyContent();
	    
	// get the body as string
	String query = body.getString();

	// clean up
	body.clearBody();

	try {
	    Statement stmt = connection.createStatement();
	    result = stmt.executeQuery(query);
	} catch (SQLException ex) {
	    throw new JspTagException("trouble in execution");
	}
	return SKIP_BODY;
    }

    /**
    * End, create the object
    */
    public int doEndTag() {
	pageContext.setAttribute(getId(), result);
	return EVAL_PAGE;
    }

    // variables
    private ResultSet result;	// the JDBC result set
    private Connection connection; // the JDBC connection object
    private String connectionId; // the connection attribute
}