Debugging from JSP and Java Applications
This tutorial gives you an overview of how
to use the debugging feature of eclipse to debug your web or Java
projects.
General
Author:
Sascha
Wolski
Sebastian
Hennebrueder
http://www.laliluna.de/tutorial.html
? Tutorials for Struts, JSF, EJB, Hibernate, xdoclet, eclipse and
more.
Datum:
January, 25 2005
Development
Tools
Eclipse
3.x
MyEclipse
plugin 3.8
(A
cheap and quite powerful Extension to Eclipse to develop Web
Applications and EJB (J2EE) Applications. I think that there is a
test version availalable at MyEclipse.)
PDF
Version des Tutorials
http://www.laliluna.de/assets/tutorials/debugging-jsp-java-tutorial-en.pdf
Introduction
The Eclipse Platform features a Java
debugger that provides all standard debugging functionality,
including the ability to perform step execution, setting of
breakpoints and values, inspecting variables and values, and to
suspend and resume threads. With the extension MyEclipse you are
able to debug JSP and included files too.
The debug perspective
First we will explain the debug perspective.
You can activate the perspective under Windows > Open
Perspective > Other...
Alternatively, you can click on the debug
icon of the perspective tool bar.
Debug view
The Debug view displays the stack trace for
the suspended threads for each target you are debugging. Each entry
is the variable state of a method right when it called the next
method. The view allows you to manage the debugging of a program in
the workbench.
Variables view
The view displays information about the
variables in the selected class. You can get information about the
variable like value, size and more from this view.
Breakpoints
view
The view lists all breakpoints you have set
in the workbench project. In this view, you can enable or disable
breakpoints, remove them, or add a new ones. You can also
double-click a breakpoint to display its location in the editor.
This view also lists Java exception breakpoints,
which suspend execution at the point where the exception is thrown.
You can add or remove exceptions.
Expressions view
You can inspect data from each class of a
suspended thread, and other places in this view. It opens
automatically when an item is added to the view.
Debug a web project
In the next steps I will explain how you can
debug a project. I will refer to the project JSP + Servlet you can
download on our site.
http://www.laliluna.de/assets/tutorials/java-servlets-jsp-tutorial.zip
Open the class BookEdit.java and set
a breakpoint in the method doGet(..) at line 72. You can set a
breakpoint by double-click on the info bar on the left edge of the
editor, or right click Toggle Breakpoint.
Set the second breakpoint in the
bookEdit.jsp file on the HTML start tag at line 14.
Note: If you don't have installed
MyEclipse, you can't set a breakpoint in jsp and included files.
To see all breakpoints you can use the
Breakpoints view in the debug perspective.
Breakpoint Properties
You can apply some properties to each
breakpoint, for example how many times a breakpoint can hit before it
suspends the thread or on which condition it suspends the thread.
Select the breakpoint right mouse button and
choose the option Breakpoint Properties.
Hit Count
The hit count sets a number of times the
breakpoint can be executed before the thread suspends. Very helpful
in a loop expression or if you want to know the value of a expression
after some hits.
Enable Condition
There are two options to suspend a thread by
using a condition.
If you want that the condition suspends the
thread when it is true select the option condition is true on
Suspend when.
Example:
action.equals("edit");
If you want that the condition suspends the
thread when the value of the condition changes select the option
value of the condition changes
on Suspend when.
Suspend Policy
You can define if the breakpoint suspends
only the thread or the complete virtual machine.
Deploy and Debug
Now we want to debug the project, so deploy
it and call it in your web browser.
Choose Edit on the
list of books.
After you clicked the Edit link,
eclipse shows you the following confirm window. You can choose if
you want to switch to the debug perspective. Yes, you want it ;- )
The first
entry in the debug view represents the state of the method where the
breakpoint was set. You can preview the state of a entry simply by
clicking on it, also the file will be open where the method is
placed.
You can see the values of all variables of
the selected entry in the Variables view.
A marked line and an arrow at the breakpoint
shows that the debugger is suspended on this line.
If you've decided that you missed an
important place in the execution process, perhaps the breakpoint was
in the wrong place, or maybe you accidentally stepped over some code
you wanted to inspect earlier in the process. Eclipse has a feature
called Drop to frame, that
essentially lets you 'rewind' the execution to the beginning of any
method in the stack. This is especially useful when you perform
variable modification or code hot swapping.
Right click on the
frame and choose the option Drop to Frame.
Inspect
expressions
To get informations about expression, you
can inspect a expression. Right click on the marked expression, you
want to inspect and choose Inspect
or press Ctrl + Shift + I.
A pop-up window appears that holds the informations about the
expression.
Watch expressions
Watch is similar to inspect an expression.
Watch means that the expression will be added to the Expressions
view, where you can watch the informations about the expression. Add
two expressions to the Expressions view.
Mark the expressions, right click and choose
Watch.
Now the two expression are in the
Expressions view.
Display
expressions
If you want to display the type and the
value of an expression, mark the expression and choose the Display
option from the context menu (right mouse button) or press Ctrl +
Shift + D.
Run
to Line
If you set a breakpoint and somewhere under
the breakpoint is a line you want to go, you can use the option Run
to Line. The code between the breakpoint and the selected line
will be executed. Select the line you want to go, press the right
mouse button and choose the option Run to Line
or use the key binding Ctrl + R.
Debugging
JSP files (supported by MyEclipse)
Debugging
a JSP file is the same like debugging a Java class, but the most
features (Watch, Inspect, etc) are not implemented. The only way to
get the values of variables is the Variables view.
That's it. You will only need to debug, when
you make mistakes. Avoid them and forget the tutorial.
文章来源：http://www.laliluna.de/debugging-jsp-java-tutorial.html