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Debugging Unveiled
Johannes Bechberger
mostlynerdless.de
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If debugging is the
process of removing
software bugs, then
programming must be
the process of putting
them in.
— Edsger Dijkstra
“
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➜ java LineCounter.java \
lines LineCounter.java
36
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➜ java LineCounter.java lines LineCounter.java
Exception in thread "main" java.nio.file.NoSuchFileException: lines
at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
at java.base/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:261
at java.base/java.nio.file.Files.newByteChannel(Files.java:379)
at java.base/java.nio.file.Files.newByteChannel(Files.java:431)
at java.base/java.nio.file.spi.FileSystemProvider.newInputStream(FileSystemProvider.java:420)
at java.base/java.nio.file.Files.newInputStream(Files.java:159)
at java.base/java.nio.file.Files.newBufferedReader(Files.java:2897)
at java.base/java.nio.file.Files.readAllLines(Files.java:3392)
at java.base/java.nio.file.Files.readAllLines(Files.java:3433)
at LineCounter.countLines(LineCounter.java:24)
at LineCounter.main(LineCounter.java:10)
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Let’s look at the code
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public static void main(String[] args) throws IOException {
Path file = Path.of(args[0]);
int count = switch (args[0]) {
case "lines" -> countLines(file);
case "code_lines" -> countCodeLines(file);
default -> {
System.err.println("Usage: java ... " +
" ");
System.exit(1);
yield 0;
}
};
System.out.println(count);
}
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public static int countLines(Path file) {
return Files.readAllLines(file).size();
}
public static int countCodeLines(Path file) {
int count = 0;
for (String line : Files.readAllLines(file)) {
if (!line.isBlank() && !line.startsWith("//")) {
count++;
}
}
return count;
}
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Any ideas?
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Let’s use a debugger
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... and more
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Slight focus on IntelliJ
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Why debug?
•Find and fix bugs!
•Analyze the code
•Add more logging on the fly
•Change behavior on the fly
•Analyze memory issues
•And much more
— Egor Ushakov
“
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Question time
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Any OpenJDK
developers here?
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Guessed so
But still feel free to correct me.
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Who of you used
a debugger before?
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Who of you remotely
debugged before?
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Good to know my audience
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Debugging
Testing
Profiling
Toolbox
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Introduction
Remote
JDWP
On Demand
Architecture
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General Architecture
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IntelliJ
JDI
JDWP
Agent
JVM
Java
JDWP JVMTI
VSCode
Java Debug
Server
DAP Java
JPDA
eclipse debugger
based
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Introduction
Remote
JDWP
On Demand
Architecture
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Remote Debugging
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-agentlib:jdwp=
transport=dt_socket,
server=y,
suspend=y,
address=*:5005
Any issues?
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-agentlib:jdwp=
transport=dt_socket,
server=y,
suspend=y,
address=*:5005
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Differences between
remote and non-remote?
None
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Little Known Gems
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Introduction
Remote
JDWP
On Demand
Architecture
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On Demand Debugging
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➜ java
-agentlib:jdwp=transport=dt_socket,server=y,
suspend=y,address=*:5005,onjcmd=y
src/test/java/OnThrowAndJCmd.java &
➜ echo $! # get pid
# wait some time and then start debugging on demand
➜ jcmd $! VM.start_java_debugging
jcmd 97145 VM.start_java_debugging
97145:
Debugging has been started.
Transport : dt_socket
Address : *:5005
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On Exception Debugging
throw new Ex("");
start debugging
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Firewalls
only dt_socket
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Let's go on a short journey
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I went to
Devoxx
Belgium
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Writing a blog
post on the
train
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What a
train
station
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Meeting some people loosely related to Java
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Looking into
a JDWP agent
bug at the
Coretto booth
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That has
been there
for ages
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A mysterious Bug
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Introduction
Remote
JDWP
On Demand
Architecture
only 120 more slides
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JDWP
From IDE to JVM calls
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IntelliJ
JDI
JDWP
Agent
JVM
Java
JDWP JVMTI
VSCode
Java Debug
Server
DAP Java
JPDA
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Packets
length id flags (= 0) command set command data
4 bytes 4 bytes 1 byte 1 bytes 1 bytes variable
Request/Event
length id flags (= 0x80) error code data
4 bytes 4 bytes 1 byte 1 bytes variable
Reply
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And lot's of these
From connection start to first suspend
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Demo
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JDWP
From connection start to first suspend
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Debugger JVM
"JDWP-Handshake"
"JDWP-Handshake"
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Debugger JVM
VM Start Event
Initial thread
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Debugger JVM
ID sizes
fieldID, methodID,
objectID, refTypeID,
frameID size
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Debugger JVM
Enable class prep events
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Debugger JVM
Request version
VM version and name
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Debugger JVM
Request capabilities
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Debugger JVM
Request new capabilities
since JDWP 1.4
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Debugger JVM
Request all classes
[kind, id, signature, status]
Reason for slow start of session
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Debugger JVM
Request all threads
[thread id]
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Debugger JVM
Enable thread start events
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Debugger JVM
Main and notify threads started
thread ids
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Debugger JVM
Prepared URLClassPath.FileLoader
class
thread id, kind, type id, signature, status
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Debugger JVM
Prepared LineCounter class
thread id, kind, type id, signature, status
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Debugger JVM
Get line table of methods
Obtain information on class
[code index, line]
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Debugger JVM
Get source file of class
Obtain information on class
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Debugger JVM
... and more
Obtain information on class
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Debugger JVM
Set breakpoint
JVM supports breakpoints
Do you know any runtimes that don't?
Python < 3.12
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Debugger JVM
Resume
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Debugger JVM
Hit breakpoint
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Debugger JVM
Hit breakpoint
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Debugger JVM
Get current frames, ...
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Debugger JVM
Resume
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Debugger JVM
...
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Debugger JVM
VM Death
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Many commands
are available
https://docs.oracle.com/en/java/javase/17/docs/specs/jdwp/jd
wp-protocol.html
* 95 to be precise with JDK 20
*
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Debugger JVM
VM Death
But does the JVM have to die?
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Who killed the JVM?
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Debugger JVM
It dies on its own or via
Exit
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So we can reattach
a debugger?
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Terminate
Disconnect
Eclipse
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Close tab
Stop
(Re)connect
IntelliJ
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IntelliJ
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Conditional Breakpoints
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Any performance
problems?
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Do conditional breakpoints
have JVM support?
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defined nowhere
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Many packages
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No support
Emulated via breakpoints and evaluation
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Debugger JVM
Handshake, ...
line.contains("//")
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Debugger JVM
Set breakpoint at count++
line.contains("//")
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Debugger JVM
Suspend
line.contains("//")
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Debugger JVM
Get value of line
line.contains("//")
import java.io.IOException;
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Debugger JVM
Create String "//"
line.contains("//")
string reference
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Debugger JVM
Get methods of String
line.contains("//")
method references
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Debugger JVM
Invoke contains method
line.contains("//")
false
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Debugger JVM
Resume
line.contains("//")
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Debugger JVM
Invoke contains method
line.contains("//")
false
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Debugger JVM
Suspend event
line.contains("//")
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Debugger JVM
Get value of line
line.contains("//")
import java.nio.file.Files;
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Debugger JVM
...
line.contains("//")
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Debugger JVM
Resume
line.contains("//")
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Debugger JVM
...
line.contains("//")
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Debugger JVM
Suspend event
line.contains("//")
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Debugger JVM
Get value of line
line.contains("//")
// Count lines in a file
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Debugger JVM
Create String "//"
line.contains("//")
string reference
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Debugger JVM
Invoke contains method
line.contains("//")
true
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Debugger JVM
Get frame and stack info
line.contains("//")
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Debugger JVM
line.contains("//")
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This explains
the breakpoint options
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Any guess how
watches, eval expression, ...
are implemented?
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Same...
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Reasons for this
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JVMTI is a terrible API,
we should get rid of it
as much as possible.
— Anonymous Java Architect
“
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Conditional Breakpoints
are complex
... and slow, especially with large conditions
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Any ideas of
improvements?
if you could control both IDE and JVM
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I came up with one
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Implement a Caching and Prefetching tunnel
Cloud
Application
Developer
Debugger Internet
get x
x
get x
x
get y
y
get y
y
JDWP
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Implement a Caching and Prefetching tunnel
Cloud
Application
Developer
Debugger Internet
get x
x
JDWP
get y
y
C
o
n
get x
x
JDWP
get y
y
C
o
n
get x,y
x, y
get x,y
x, y
HTTP
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Is DAP better?
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But...
IntelliJ
JDI
JDWP
Agent
JVM
Java
JDWP JVMTI
VSCode
Java Debug
Server
DAP Java
JPDA
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Extract methods
... and potentially use hotswap
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Recompile class and hot swap
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Add condition to code
... and potentially use hotswap
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Question time
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Are methods with
breakpoints interpreted?
Yes, they are deoptimized.
https://github.com/openjdk/jdk/blob/edcc559f09364da3692862e1f3d0636aa8eec1d4/
src/hotspot/share/code/codeCache.cpp#L1425
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Do you know safepoints?
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What do they have in
common with stepping?
Stepping is also implemented
with safepoints
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Are methods interpreted
during single stepping?
Yes, they are deoptimized.
https://github.com/openjdk/jdk/blob/d31391597433cf275fc615e0148c48c34acf6e11/
src/hotspot/share/prims/jvmtiEventController.cpp#L205
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# of resources on this topic?
None, just one from
Apache Harmony
https://harmony.apache.org/subcomponents/drlvm/breakpoints_and_ss.html
2011
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More IDE features
and how they are implemented using JDWP
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Hot Swap
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Debugger JVM
Redefine classes request
[class id, byte code bytes]
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Debugger JVM
Get methods and fields
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Set Specific Value
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Debugger JVM
Get visible classes
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Debugger JVM
Get value of args[1]
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Debugger JVM
Get methods of Path
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Debugger JVM
Invoke of method
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Debugger JVM
Set stack frame value request
thread, frame, slot + slot value
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Force Return
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Debugger JVM
Force early return request
thread, return value
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Does forced return,
execute finalize blocks?
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Does forced return,
execute finalize blocks?
No No further instructions are executed
in the called method. Specifically,
finally blocks are not executed.
“
— JDWP documentation for JDK 17
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Drop/Reset Frame
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Debugger JVM
Pop frames request
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Debugger JVM
Get frames
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Throw Exception
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Debugger JVM
Create RuntimeEx instance
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Debugger JVM
Stop request
thread, throwable
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It's arcane
But better than Python and fast enough
PyData 16th
November
DIY Python
Debugger
and really versatile
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Debugging
Testing
Profiling
Toolbox
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@parttimen3rd on Twitter
parttimenerd on GitHub
mostlynerdless.de
@SweetSapMachine
sapmachine.io