The Debugger

1
The Debugger

• Unit 06

2
Unit Objectives

• To learn how to
• Use Eclipses debugger
• Set breakpoints
• Use inspectors

3
The Application Developer Debugger

• The text-based debugger (jdb) supplied with the
  JDK does not have to be invoked
• Eclipses debugger is a visual debugger
• It is opened under the following conditions
• Execution hits a breakpoint that you inserted
• An uncaught exception occurs
• You select the debug button ( ) on any tool
  bar

4
Debugger Capabilities

• Modify
• Code while in the debugger
• Data values while stepping through code
• While in the debugger
• Set breakpoints
• No unnecessary editions are created
• Multithreading support
• To debug your code you do not have to
• Recompile in the Java editor, or
• Add source code, such as System.out.println(...)
  into the code

5
Debugger Perspective
Execution Stack Frame

• Debugger can
• be used to inspect,
• change, and step
• through code
• The source can
• be changed,saved,
• and resumed from
• within the debugger

Variables Pane
Source
6
Stack Frames

• The debugger displays lists of stack frames
  before exception or breakpoint occur
• Stack frames
• Correspond to a called method
• Are in reverse chronological order -- Last one
  executed is first
• Any stack frame can be selected and dropped to.
  The code is then
• re-executed
• Choose a frame and
• select the Relaunch
• option defined in the
• pop-up
• Variable values are
• not reset when a
• stack frame is
• dropped

7
Stepping Through Code

• Method source for the current stack frame can be
  executed line by line using the debuggers
  navigation buttons

Step Into
Step Return
Step Over
Terminate
Suspend
Resume
8
Stepping Through Code Commands

• Resume - Continues execution until breakpoint or
  thread ends
• Suspend - Interrupts a running thread
• Terminate - Ends the execution of the selected
  thread
• Step Into Steps into a method call and executes
  the first line of code in that method
• Step Over - Executes the next line of code in the
  current method
• Step Return - Continues execution until the end
  of the current method (until a return)
• Step with Filters Continues execution until the
  next line of code which is not filtered out

1 2 3 4 5 6 7
9
Breakpoints

• Setting a breakpoint in a method causes execution
  to be suspended when the method appears on the
  stack frame
• Source can
• then be
• stepped
• through
• and objects
• inspected
• Breakpoints
• remain set
• until they
• are explicitly
• removed
• from a
• method

breakpoints
10
Breakpoint Properties

• Right-click on a breakpoint to get to its
  properties sheet
• Properties include
• Enabled and disabled
• Hit count
• Doesnt break until n occurrences
• Suspend Policy
• Thread vs. VM
• Enabling condition
• Can break when a specified condition is true, or
  whenever the value of a specified expression
  changes
• Restriction to certain threads

11
Exception Breakpoints

• Exception can be enabled as breakpoints in the
  Breakpoint view
• Execution will suspend wherever the exception is
  raised
• Can specify caught, uncaught or both.

12
Evaluating Expressions

• The Display and Expressions views can be used to
  examine the value of an expression.

13
The Variables View

• The Variables view allows you to examine the
  contents of an object
• A '' next to a variable name indicates that it
  is an object
• An object can be opened up to reveal the values
  of its attributes

14
What You Have Learned

• How to
• Start the debugger
• Add breakpoints to your code
• Use the debugger to step through code and inspect
  objects