| [ Team LiB ] |
|
Making Your Application More ModularMaintaining a JSP-based application can be a lot of work when the JavaServer Pages aren't well organized. Over time, they tend to grow and end up with an ugly mixture of HTML and Java code that can be almost unreadable. In most cases, you can reorganize the pages and make use of the <jsp:include> tag to split your page into manageable segments. A Reusable HTML Table ModuleOne of the really handy things you can do with the <jsp:include> tag is create reusable modules. How many times have you had to take an array or a vector of objects and display them in an HTML table? Depending on the kind of applications you have done, you may have had to do this many times. You can make a servlet or a JSP that takes an array or a vector of data and displays the data in a table. Then, every time you need to show a table, you can call the servlet. Listing 10.10 shows a servlet that uses the Reflection API to fetch data from either an array or a vector of objects and then displays the data in a table. It uses request.getAttribute to get the array/vector but uses request.getParameter to retrieve any other options. Listing 10.10 Source Code for TableServlet.java
package examples;
import javax.servlet.*;
import java.io.*;
import java.util.*;
import java.lang.reflect.*;
/** This class uses the Reflection API to fetch data from an array or
* a vector and put it in a table. */
public class TableServlet extends GenericServlet
{
public static final Class[] NO_PARAMS = new Class[0];
public void service(ServletRequest request, ServletResponse response)
throws IOException, ServletException
{
// First, get the parameters for the TABLE, TR, TD, and TH options.
String tableOptions = request.getParameter("tableOptions");
if (tableOptions == null) tableOptions = "";
String trOptions = request.getParameter("trOptions");
if (trOptions == null) trOptions = "";
String tdOptions = request.getParameter("tdOptions");
if (tdOptions == null) tdOptions = "";
String thOptions = request.getParameter("thOptions");
if (thOptions == null) thOptions = "";
// Now, get the name of the object that contains the data to display.
String data = request.getParameter("data");
if (data == null)
{
getServletContext().log("No data available");
throw new ServletException(
"No data parameter available");
}
// Get the actual data object.
Object dataOb = request.getAttribute(data);
if (dataOb == null)
{
getServletContext().log("No data object found");
throw new ServletException(
"Can't locate the data object named "+
data);
}
// Get the list of method/field names to display in each column.
String[] columns = request.getParameterValues("column");
// Get the types of each column field.
String[] columnType = request.getParameterValues("columnType");
// Get the headers for each column
String[] columnHeaders = request.getParameterValues(
"columnHeader");
// Create a table of column names and Fields/Methods for fetching data.
Hashtable columnAccessors =
getAccessors(dataOb, columns);
// First print the table header.
PrintWriter out = response.getWriter();
out.println("<table "+ tableOptions+">");
// If there are any headers, print them out.
if (columnHeaders != null)
{
out.println("<tr "+trOptions+">");
for (int i=0; i < columnHeaders.length; i++)
{
out.print("<th "+thOptions+">");
out.println(columnHeaders[i]);
out.println("</th>");
}
out.println("</tr>");
}
// If the object is a vector, loop through the elements.
if (dataOb instanceof Vector)
{
Vector v = (Vector) dataOb;
Enumeration e = v.elements();
while (e.hasMoreElements())
{
// For each row, print out the <tr> tag plus any options.
out.println("<tr "+trOptions+">");
// Print out the column values for the row.
printRow(out, e.nextElement(),
columns, columnType,
columnAccessors, tdOptions);
out.println("</tr>");
}
}
// If the object is an array, loop through the objects.
else if (dataOb instanceof Object[])
{
Object[] obs = (Object[]) dataOb;
for (int i=0; i < obs.length; i++)
{
// For each row, print out the <tr> tag plus any options.
out.println("<tr "+trOptions+">");
// Print out the column values for the row.
printRow(out, obs[i],
columns, columnType,
columnAccessors, tdOptions);
out.println("</tr>");
}
}
out.println("</table>");
}
protected void printRow(PrintWriter out, Object ob,
String[] columns, String[] columnTypes,
Hashtable columnAccessors, String tdOptions)
throws ServletException
{
// Loop through all the column names.
for (int i=0; i < columns.length; i++)
{
// Get the value for this column out of the object.
Object value = getColumnValue(ob, columns[i],
columnAccessors);
// Print the <td> tag.
out.print("<td "+tdOptions+">");
// If the column type is data, just print the data.
if (columnTypes[i].equalsIgnoreCase("data"))
{
out.print(value);
}
// If the column type is "image", print out an <img> tag.
else if (columnTypes[i].equalsIgnoreCase("image"))
{
out.print("<img src=\""+value+"\">");
}
out.print("</td>");
}
}
/**Use either a Field or a Method object to fetch a value from an object.*/
protected Object getColumnValue(Object ob, String columnName,
Hashtable columnAccessors)
throws ServletException
{
// Get the object used to fetch this column's value.
Object accessor = columnAccessors.get(columnName);
// If the column is a field...
if (accessor instanceof Field)
{
// ... use the field's get method to fetch the value.
try
{
Field f = (Field) accessor;
return f.get(ob);
}
// Log and then return the IllegalAccessException.
catch (IllegalAccessException exc)
{
getServletContext().log(
"Error getting column "+
columnName, exc);
throw new ServletException(
"Illegal access exception for column "+
columnName);
}
}
// If the column is a Method...
else if (accessor instanceof Method)
{
// ... invoke the method.
try
{
Method m = (Method) accessor;
// The NO_PARAMS value is an empty array of Class defined at the top
// of this class.
return m.invoke(ob, NO_PARAMS);
}
// Log and then return any exceptions that come up while invoking the method.
catch (IllegalAccessException exc)
{
getServletContext().log(
"Error getting column "+
columnName, exc);
throw new ServletException(
"Illegal access exception for column "+
columnName);
}
catch (InvocationTargetException exc)
{
getServletContext().log(
"Error getting column "+
columnName, exc);
throw new ServletException(
"Invocation target exception "+
"for column "+columnName);
}
}
// If the column is neither a Field nor a Method, return null. You should
// never get to this point.
return null;
}
/** Creates a table mapping column name to Field/Method */
protected Hashtable getAccessors(Object ob, String[] columns)
throws ServletException
{
Hashtable result = new Hashtable();
// First, get the Class for the kind of object being displayed.
Class obClass = null;
if (ob instanceof Object[])
{
// If the objects are in an array, get the first object in the array.
Object[] obs = (Object[]) ob;
// If there are no objects, don't bother filling the table
// because it won't be needed.
if (obs.length == 0) return result;
obClass = obs[0].getClass();
}
else if (ob instanceof Vector)
{
// If the objects are in a vector, get the first element of the vector.
Vector v = (Vector) ob;
// If there are no objects, don't bother filling the table
// because it won't be needed.
if (v.size() == 0) return result;
obClass = v.elementAt(0).getClass();
}
// For each column, look for a field and then a method with the column name.
for (int i=0; i < columns.length; i++)
{
// First see whether there is a field that matches the column name.
try
{
Field f = obClass.getField(columns[i]);
// If so, put it in the table and go to the next column name.
result.put(columns[i], f);
continue;
}
catch (Exception ignore)
{
}
// Now see whether there is a method that matches this column name.
try
{
// The NO_PARAMS value is an empty array of Class defined at the top
// of this class.
Method m = obClass.getMethod(columns[i],
NO_PARAMS);
// If so, put it in the table.
result.put(columns[i], m);
}
catch (Exception exc)
{
getServletContext().log(
"Exception location field "+
columns[i], exc);
throw new ServletException(
"Can't locate field/method for "+
columns[i]);
}
}
return result;
}
}
Although the TableServlet servlet is pretty large compared with the amount of code needed to write out a single table, it begins to pay off as you start using it in several JSPs. Listing 10.11 shows a JavaServer Page that invokes TableServlet. Listing 10.11 Source Code for ShowTable.jsp
<html>
<body>
<%
// Initialize an array with some data to display.
Person[] people = new Person[]
{ new Person("Samantha Tippin", 9, "770-123-4567"),
new Person("Kaitlyn Tippin", 6, "770-123-4567"),
new Person("Edward Alexander", 3, "No phone"),
new Person("Star Alexander", 12, "Phone off hook"),
new Person("Norton Alexander", 12, "No phone")
};
// Stick the array in the request where the servlet can get to it.
request.setAttribute("people", people);
%>
<%-- Invoke the Table servlet, tell it the name of the attribute
where the data is stored (data=people), set the border size to 4
on the <table> tag, and describe each column to display --%>
<jsp:include page="/TableServlet" flush="true">
<jsp:param name="data" value="people"/>
<jsp:param name="tableOptions" value="BORDER=4"/>
<jsp:param name="column" value="name"/>
<jsp:param name="columnType" value="data"/>
<jsp:param name="columnHeader" value="Name"/>
<jsp:param name="column" value="age"/>
<jsp:param name="columnType" value="data"/>
<jsp:param name="columnHeader" value="Age"/>
<jsp:param name="column" value="getPhoneNumber"/>
<jsp:param name="columnType" value="data"/>
<jsp:param name="columnHeader" value="Phone #"/>
</jsp:include>
</body>
</html>
<%!
// Define a class to contain information. This would normally not
// be defined within a JSP.
public class Person
{
public String name;
public int age;
protected String phoneNumber;
public Person(String aName, int anAge,
String aPhoneNumber)
{
name = aName;
age = anAge;
phoneNumber = aPhoneNumber;
}
// Just to show that methods work as well as fields, allow the
// phone number to be accessed only via a method call.
public String getPhoneNumber()
{
return phoneNumber;
}
}
%>
Figure 10.5 shows the output from ShowTable.jsp. Figure 10.5. You can create a reusable servlet to generate HTML elements such as tables.
 |
| [ Team LiB ] |
|
