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Introduction
Debugging is the routine process of locating
and removing bugs, errors or abnormalities from programs. It’s a must have
skill for any Java developer because it helps to find subtle bug that are not
visible during code reviews or that only happens when a specific condition
occurs. The Eclipse Java IDE provides many debugging tools and views grouped in
the Debug Perspective to help the you as a developer debug effectively and
efficiently.
There are many improvements included in the
latest Eclipse Java Development Tools (JDT) release included in the Eclipse
Oxygen Simultaneous Release. This article will start with a beginner’s guide to
start you with debugging.
Guide to Quick Start Debugging
Here are some quick tips and tools that will help you get
started quickly with debugging your Java project.
Launching and Debugging a Java program
A Java program can be debugged simply by
right clicking on the Java editor class file from Package explorer. Select
Debug As → Java Application or use the shortcut Alt + Shift + D, J instead.
Either actions mentioned above creates a new
Debug Launch Configuration and uses it to start the Java application.
In most cases, users can edit and save the
code while debugging without restarting the program.This works with the support
of HCR (Hot Code Replacement), which has been specifically added as a standard
Java technique to facilitate experimental development and to foster iterative
trial-and-error coding.
Breakpoints
A breakpoint is a signal that tells the
debugger to temporarily suspend execution of your program at a certain point in
the code.
To define a breakpoint in your source code,
right-click in the left margin in the Java editor and select Toggle Breakpoint.
Alternatively, you can double-click on this position.
The Breakpoints view allows you to delete
and deactivate Breakpoints and modify their properties.
All breakpoints can be enabled/disabled
using Skip All Breakpoints. Breakpoints can also be imported/exported to and
from a workspace.
Debug Perspective
The debug perspective offers additional
views that can be used to troubleshoot an application like Breakpoints,
Variables, Debug, Console etc. When a Java program is started in the debug
mode, users are prompted to switch to the debug perspective.
Ø
Debug view – Visualizes call stack
and provides operations on that.
Ø
Breakpoints view – Shows all the
breakpoints.
Ø
Variables/Expression view – Shows the
declared variables and their values. Press Ctrl+Shift+d or Ctrl+Shift+i on a
selected variable or expression to show its value. You can also add a permanent
watch on an expression/variable that will then be shown in the Expressions view
when debugging is on.
Ø
Display view – Allows to Inspect the
value of a variable, expression or selected text during debugging.
Ø Console view – Program output is shown here.
Stepping commands
The Eclipse Platform helps developers debug
by providing buttons in the toolbar and key binding shortcuts to control
program execution.
For more information about debugging visit: Eclipse Stepping
Commands Help
Advanced Tools to Debug Complex
Scenarios
This section will give you more advanced
tips and tricks to help you debug your Java project. The Eclipse Oxygen release
includes many great improvements for Java debugging. Here’s a quick overview.
Watchpoints, Exception Breakpoints, Conditional Breakpoints
Watchpoints - A watchpoint is a special
breakpoint that stops the execution of an application whenever the value of a
given expression/field changes, without specifying where it might occur. User
can specify by Breakpoint Properties… if they want the execution to stop when
the watch expression is Accessed, Modified or both.
Exception Breakpoints – An exception breakpoint is
specified for thrown exception using Add Java Exception Breakpoint.
Breakpoint for NullPointerException will halt whenever/wherever this exception is
thrown.
Condition Breakpoints – Eclipse users can create
conditions to restrict the activation of breakpoints.
Breakpoint will be activated only if value
of Boolean b is true. Hit count can be provided to halt the execution at nth
hit of the breakpoint. The breakpoint is disabled until either it is re-enabled
or its hit count is changed or the program ends.
Remote Debugging
The Eclipse IDE allows you to debug
applications that runs on another Java Virtual Machine (JVM) or even on another
machine. You can create a new debug configuration of the Remote Java
Application type. To enable remote debugging you need to start your Java
application with certain flags. Connection Type can be specified as a Socket
Attach or Socket Listen. Socket Listen supports multiple incoming connections.
New Debug Features in Eclipse Oxygen
Here are the new features that have been
added to the latest Eclipse Java IDE release.
Tracepoints
A new feature in the Eclipse Platform that
allows users to creates conditional breakpoints to print out messages without
halting at the breakpoints and cluttering the code base.
The Eclipse Platform created tracepoint with
systrace template.
Trigger Point
Now users can activate Trigger Point. A set
of trigger points can be defined for the breakpoints in a workspace.
All the other breakpoints that are initially
suppressed by triggers will be hit only after any of the all Trigger Points
have been hit. All the triggers are disabled after a Trigger Point is hit and
will be re-enabled after the run.
Any breakpoint can be set as a trigger point
by using Breakpoint Properties via the dialog or the detail pane of the
Breakpoints view.
Triggers are rendered with an overlay of
"T" and the breakpoints suppressed by the triggers are rendered with
an overlay of "T" with a cut.
Logical Structures
In the Variables view, collection objects
directly show their contained elements instead of their internal structure.
Logical structures are now activated by default in the Oxygen release. Show Logical
Structure can be turned off to show the internal structure.
The Show Logical Structure context menu lets
you create, choose or edit the representation.
Method Result After Step Operation
Also new to the Oxygen release are Method
Results. During debugging, the last method result (per return or throw) that
was observed during Step Into, Step Over or Step Return, is shown as first line
in the Variables view.
Launch Group
Also new in Oxygen, Launch Group launch
configuration type allows you to launch multiple other launch configurations
sequentially, with configurable actions after launching each group member. New
launch groups can be created via the Run → Run Configurations... or Run → Debug
Configurations... dialogs.
Breakpoints Grouping and Sorting
Eclipse users can now group the breakpoints
by different categories in Eclipse Oxygen. Breakpoint Working Sets defines a
group of breakpoints. User can perform actions like enable/disable on a working
set.
Breakpoints are sorted by Name by default,
sorting order can be changed to Creation Time.
Conclusion
This article covered some of the important
debugging tools and some new tools provided by the Eclipse Java IDE in the
Oxygen Release. Eclipse Oxygen is now available for download here.
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