1. Debugging Techniques

2. Overview Visibility into VM state VM state is also mutable
Lots of tools for altering execution

3. Hot Swap Replace the code for a given class within a running VM
Swap out the body of a method
Can’t change class hierarchy or class “shape”
Can only replace loaded classes, not load new ones
Must wait for class file to be loaded
Lost when VM is restarted
Methods currently on the stack will run with their original code

4. Breakpoint Types Line Method Standard, most commonly used
Stop on method entry or exit
Can be set at the interface or superclass level
Excruciatingly slow in IntelliJ

5. Breakpoint Types Watchpoint Exception
Stop on member variable access or modification
Very useful when you don’t know where a value is being set
Access option is rarely used
Exception
Stop when an exception is thrown
Stops for subclasses being thrown as well

6. Breakpoint Conditions
Expression that must evaluate to true or the breakpoint is skipped
Much faster than stopping, inspecting, and continuing
But not really fast enough for code that’s executed thousands of times
Can include method calls

7. Breakpoint Options Instance filter Pass count Suspend policy
Only stop if a particular object is the “this” pointer
Pass count
Skip the first n instances of the breakpoint, then enable
Suspend policy
Stop all threads
Stop single thread
Don’t stop any threads
Log any expression to the console instead

8. Watch Window Supports essentially any Java statement/expression
Use it aggressively to call methods instead of poking through data structures
To invoke methods, must be on a breakpoint or step (not paused)

9. Watch Window Tricks Runaway thread (infinite loop, etc)
Set a breakpoint in that thread
Set a local or member variable to null
Continue
NPE stops thread
Test development
Set breakpoint on current last line of test code
Write the next line of code in the watch window
Once you get it right, move into the test itself
Repeat

10. Watch Windows Tricks Cache busting Exception logging
Set a breakpoint on the line of code that checks for a cached value
Set a watch that clears the entire cache, or removes that item
Step through code that reloads the cache
Exception logging
Sometimes exceptions are completely swallowed
Set a breakpoint with exception in scope
Call e.printStackTrace(), view the stack in the console

11. Misc Selective Step-Into Pop stack frame
Shift-F7 lets you choose target method on the current line
Avoid the step into/out repeats to get to your desired destination
Pop stack frame
Immediately exit the current method
VM will re-invoke the same method
Any side effects have already happened, but local variables are reset