When writing JSP programs, programmers may miss some bugs, and these bugs may appear anywhere in the program. There are usually the following types of exceptions in JSP code:
Checked exception: A checked exception is a typical user error or an error that the programmer cannot foresee. For example, if a file is about to be opened, but the file cannot be found, an exception is thrown. These exceptions cannot simply be ignored at compile time.
Run-time exception: A run-time exception may have been avoided by the programmer. This exception will be ignored at compile time.
Error: There is no exception here, but the problem is that it is beyond the control of the user or programmer. Errors are often ignored in the code and there is little you can do about it. For example, stack overflow error. These errors will be ignored at compile time.
This section will give you several simple and elegant ways to handle runtime exceptions and errors.
The exception object is an instance of a Throwable subclass and is only available in error pages. The following table lists some important methods in the Throwable class:
| serial number | Method & Description |
|---|---|
| 1 | public String getMessage() returns exception information. This information is initialized in the Throwable constructor |
| 2 | public ThrowablegetCause() returns the cause of the exception, type is Throwable object |
| 3 | public String toString() returns the class name |
| 4 | public void printStackTrace() outputs the exception stack trace to System.err |
| 5 | public StackTraceElement [] getStackTrace() returns the exception stack trace in the form of an array of stack trace elements |
| 6 | public ThrowablefillInStackTrace() uses the current stack trace to fill the Throwable object |
JSP provides the option to specify error pages for each JSP page. Whenever a page throws an exception, the JSP container automatically calls the error page.
The following example specifies an error page for main.jsp. Use the <%@page errorPage="XXXXX"%> directive to specify an error page.
<%@ page errorPage="ShowError.jsp" %><html><head> <title>Error Handling Example</title></head><body><% // Throw an exception to invoke the error page int x = 1; if (x == 1) { throw new RuntimeException("Error condition!!!"); } %></body></html>Now, write the ShowError.jsp file as follows:
<%@ page isErrorPage="true" %><html><head><title>Show Error Page</title></head><body><h1>Opps...</h1><p>Sorry, an error occurred.</p><p>Here is the exception stack trace: </p><pre><% exception.printStackTrace(response.getWriter()); %>
Notice that the ShowError.jsp file uses the <%@page isErrorPage="true"%> directive, which tells the JSP compiler that it needs to generate an exception instance variable.
Now try to access the main.jsp page, it will produce the following results:
java.lang.RuntimeException: Error condition!!!......Opps...Sorry, an error occurred.Here is the exception stack trace:
You can use JSTL tags to write the error page ShowError.jsp. The code in this example has almost the same logic as the code in the previous example, but the code in this example is better structured and provides more information:
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %><%@page isErrorPage="true" %><html><head><title> Show Error Page</title></head><body><h1>Opps...</h1><table><tr valign="top"><td><b>Error:</b></td><td>${pageContext.exception}</td></tr><tr valign="top"><td> <b>URI:</b></td><td>${pageContext.errorData.requestURI}</td></tr><tr valign="top"><td><b>Status code:</b></td><td>${pageContext.errorData.statusCode}</td></tr><tr valign="top"><td><b>Stack trace:</b> </td><td><c:forEach var="trace" items="${pageContext.exception.stackTrace}"><p>${trace}</p></c:forEach></td></tr></table></body></html>The running results are as follows:
If you want to put exception handling in a page and handle different exceptions differently, then you need to use try...catch block.
The following example shows how to use a try...catch block by placing this code in main.jsp:
<html><head> <title>Try...Catch Example</title></head><body><% try{ int i = 1; i = i / 0; out.println("The answer is " + i); } catch (Exception e){ out.println("An exception occurred: " + e.getMessage()); } %></body></html>Try accessing main.jsp, it will produce the following results:
An exception occurred: / by zero