Short introduction to Debugging in general and small insights on how to use NetBeans interactive debugger with Java Code.

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Outline Introduction Debugging Common Acronymes
Debugging
Debugging with NetBeans IDE
Andreas Ruppen
andreas.ruppen@unifr.ch
University of Fribourg
Department of Informatics
Software Engineering Group
April 18, 2014

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Outline Introduction Debugging Common Acronymes
1 Introduction
2 Debugging
3 Common Acronymes

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Generalities I
Definition (Debugging)
Debugging is the process of identifying and eliminating bugs in
a software system. Debugging can be a quite complex process
especially when systems are distributed, deployed or tightly
coupled.
Generalities
In general debugging is done with the aid of some software.
This is what we call interactive debugging. This software can
control the program flow, watch log files or memory dumps and
much more.

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Generalities II
Origin (Wikipedia)
The terms "bug" and "debugging" are both popularly attributed
to Admiral Grace Hopper in the 1940s. While she was working
on a Mark II Computer at Harvard University, her associates
discovered a moth stuck in a relay and thereby impeding
operation, whereupon she remarked that they were "debugging"
the system. However the term "bug" in the meaning of technical
error dates back at least to 1878 and Thomas Edison.

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Generalities III
Figure : First Bug (from wikipedia.org)

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Difficulties I
The debugging skills of the programmer are a major factor
when it comes to eliminating bugs.
Software tends to become more and more complex which
makes debugging harder.
Mobile devices also introduces new challenges for
debugging. The code is run and developed on different
machines.
High-level languages such as Java are easier to debug
than low level languages like C.
Bugs may occur through memory corruption which is
difficult to track. This comes from the fact that the thrown
error is not necessary the origin of the problem.

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Types of Debugging
Interactive Debugging Software
Debugging software monitors the flow of the program. It can
also modify this flow, inspect and change attribute values,
pausing the execution at predefined locations, influence the
outcome of branches and switch statements etc. . .
Static Debugging Software
Instead of running the software and closely observe its
execution, some tools look statically at the code to identify
potential problems. These static code analysis tools come
handy to identify problems which are hard to reproduce like
memory leaks. It is most useful to pinpoint erroneous pointers,
infinite recursive structures.

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Debugging techniques I
for interactive debugging
A first step in debugging is to reproduce the problem. This
sounds trivial but can become quite complex in the case of
memory leaks, distributed systems, multithreaded systems
etc. . . A good bug report always includes the precise steps
to reproduce the problem.
Once the problem reproduced, to make debugging easier,
the input is simplified (the bug could come from a too large
input file for example, if the smaller file passes the problem
is found). These simplifications are the most time made
manually (divide-and-conquer). Smaller input also allows
faster debugging (think of a large for-loop iterating over
each line of a file).

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Debugging techniques II
for interactive debugging
From this point on, a developer has several choices to
precisely identify the problem:
He can use a debugging tool (like gdb or one integrated in
the IDE) to examine the memory and the local variables at
some interesting points of the program.
Another method is to use tracing (think about last weeks
logging).
Post-mortem debugging, which consists of memory dump
analysis
Remote debugging for deployed applications.

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Debugging in NetBeans IDE
Use Case
What is debugging?
Run through the code with the interpreter.
Allows to see whether the execution path is as expected.
Using the Debugger
We can use the debugger:
to verify if a programm behaves as we except.
to identify the nature of a runtime-error.
to force the programm entering a given state.

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Debugging in NetBeans IDE
Howto
Fist steps
Open a project in NetBeans IDE.
Define the Breakpoints at some interesting points.
Start the Debugger
Debugging mode
Runs the code with the Java virtual machine.
Everything that is possible in normal mode, should also be
possible in debugging mode.
Execution stops for user action at the defined breakpoints.

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Breakpoints I
Defining Breakpoints
Definition (Breakpoint)
A Breakpoint is an indication for the Java Debugger. They are
ignored by the Java virtual machine when running in normal
mode. However when running in debug mode, the execution
stops at every breakpoint and waits for a user action before it
continues to the next break points.

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Breakpoints II
Defining Breakpoints
Actions
When the execution of the programm stops at a breakpoint
following actions are possible:
Inspection or modification of dynamic variables.
Step-by-step execution.
Go to the next breakpoint (or to the end of the application if
there are no more breakpoints).

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Breakpoints in NetBeans IDE
Defining breakpoints
Example (Usage)
Define a Breakpoint by
clicking on the
corresponding line.
The line number is
replaced by a red
square.
It’s not possible to define
breakpoints on non
executable lines (i.e.
commentary lines)

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Running the Debugger
The ANT way
Debug Target
The ANT file which comes with the exercises provides a
debug target.
This target can be launched manually (by right clicking on
the build.xml file in NetBeans IDE).
It is possible to launch the debugger from command line
with the following arguments
1 −Xrunjdwp : transport =dt_socket , server=y , suspend=n , address=5432
which makes it listening on port 5432 for a debugging tool
like the one integrated in NetBeans IDE.

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Running the Debugger
The NetBeans IDE way
Debugging with and IDE
It is possible (and even recommended) to do the
debugging tasks directly from the IDE.
NetBeans IDE offers a great debugging support.
To launch the debugger, either click the debug icon in the
launch bar (the 6th one), or select the debug target from
the ANT script or select debug from the right click on the
project.
Once started the program starts as normal but the
execution stops at the first breakpoint.

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The debugging console
NetBeans IDE layout
When the debugger is
started in NetBeans IDE
a new tab becomes
visible.
This tab is updated on
each step of the
debugger.
It shows the value of all fields of the current class.
Besides inspecting the value of each field, it is also
possible to change them.

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Using the NetBeans IDE Debugger
Associated GUI Elements
When the debugger stops
on a line, this line is
highlighted green.
All other debugging GUI elements relate to this line or the
class containing this line.

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Outline Introduction Debugging Common Acronymes
Using the NetBeans IDE Debugger
Associated GUI Elements
When everyhting seems ok to continue, NetBeans IDE offers
several buttons for going on:
Stop (the debugger)
Pause
Continue (to next
Breakpoint)
Step Over
Step over Expression
Step into
Step out
Run to cursor
Apply Code Change
Take GUI Snapshot

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Used acronyms
IDE Integrated Development Environment
gdb GNU Debugger
GNU GNU is not Unix