Debugging Unveiled: Exploring Debugger Internals to Make You a Better Developer

​​Debuggers are indispensable tools for Java developers, empowering them to conquer bugs and unravel complex systems. But have you ever wondered how they work? Curious about the implementation of features like conditional breakpoints and remote debugging? Wondering about all the cool features in modern IDEs and how they are implemented and how IDEs even communicate with your JVM?

Join me for a deep dive into debuggers, unlocking their secrets to maximize their potential. Gain invaluable insights and elevate your debugging game in this illuminating talk.

Transcript

1. Public Debugging Unveiled Johannes Bechberger mostlynerdless.de
2. Public
3. Public If debugging is the process of removing software bugs, then programming must be the process of putting them in. — Edsger Dijkstra “
4. Public ➜ java LineCounter.java \ lines LineCounter.java 36
5. Public ➜ 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)
6. Public Let’s look at the code
7. Public 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 ... " + "<lines|code_lines> <file>"); System.exit(1); yield 0; } }; System.out.println(count); }
8. Public 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; }
9. Public Any ideas?
10. Public Let’s use a debugger
11. Public
12. Public
13. Public
14. Public
15. Public
16. Public ... and more
17. Public Slight focus on IntelliJ...
18. Public Breakpoint properties in IntelliJ
19. Public Breakpoint properties in NetBeans
20. Public Disabling line numbers is harder in IntelliJ
21. Public Eclipse
22. Public 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 “
23. Public Question time
24. Public Any OpenJDK developers here?
25. Public Guessed so But still feel free to correct me.
26. Public Who of you used a debugger before?
27. Public
28. Public Who of you remotely debugged before?
29. Public Good to know my audience
30. Public Debugging Testing Profiling Toolbox
31. Public
32. Public
33. Public Introduction Remote JDWP On Demand Architecture
34. Public General Architecture
35. Public IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java Debug Server DAP Java JPDA eclipse debugger based
36. Public Introduction Remote JDWP On Demand Architecture
37. Public Remote Debugging
38. Public
39. Public -agentlib:jdwp= transport=dt_socket, server=y, suspend=y, address=*:5005 Any issues?
40. Public -agentlib:jdwp= transport=dt_socket, server=y, suspend=y, address=*:5005
41. Public Differences between remote and non-remote? None
42. Public
43. Public Little Known Gems
44. Public Introduction Remote JDWP On Demand Architecture
45. Public On Demand Debugging
46. Public ➜ 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
47. Public
48. Public On Exception Debugging throw new Ex(""); start debugging
49. Public ➜ java -agentlib:jdwp=transport=dt_socket,server=y, suspend=y,address=*:5005,onthrow=Ex, launch=exit src/test/java/OnThrowAndJCmd.java # or ➜ java -agentlib:jdwp=transport=dt_socket,server=y, suspend=y,address=*:5005,onuncaught=y, launch=exit src/test/java/OnThrowAndJCmd.java
50. Public What about the launch option?
51. Public tmux_jdb.sh #!/bin/sh tmux new-session -d -s jdb -- jdb -attach $2
52. Public ➜ java “-agentlib:jdwp=transport=dt_socket,server=y, suspend=y,address=*:5005, onthrow=Ex,launch=sh tmux_jdb.sh” src/test/java/OnThrowAndJCmd.java # in another console after the exception is # thrown ➜ tmux attach -t jdb
53. Public
54. Public
55. Public Firewalls only dt_socket
56. Public
57. Public JDB + onthrow Demo
58. Public java "-agentlib:jdwp=transport=dt_socket,server=y, suspend=y,address=*:5005,onthrow=Ex,launch=exit" src/test/java/OnThrowAndJCmd.java # in other terminal jdb -attach 5005
59. Public Let's go on a short journey
60. Public I went to Devoxx Belgium
61. Public Writing a blog post on the train
62. Public What a train station
63. Public Meeting some people loosely related to Java
64. Public Looking into a JDWP agent bug at the Coretto booth
65. Public That has been there for ages
66. Public A mysterious Bug
67. Public
68. Public
69. Public Exception in thread "event-handler" java.lang.NullPointerException: Cannot invoke "com.sun.jdi.ObjectReference.referenceType()" because the return value of "com.sun.jdi.event.ExceptionEvent.exception()" is null at jdk.jdi/com.sun.tools.example.debug.tty.TTY.exceptionEvent(TT Y.java:171) at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.exceptio nEvent(EventHandler.java:295) at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.handleEv ent(EventHandler.java:133) at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.run(Even tHandler.java:78) at java.base/java.lang.Thread.run(Thread.java:1583)
70. Public cbEarlyException handler ex.class == "Ex"? initialize(env, thread, EI_EXCEPTION) yes start debugging session debugger attached set standard Exception handler send Ex exception event ignore no send Exception event 2nd Ex throw 1st Ex throw null pointers
71. Public
72. Public Introduction Remote JDWP On Demand Architecture only 120 more slides
73. Public JDWP From IDE to JVM calls
74. Public 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 Request/Event
75. Public JDWP From connection start to first suspend
76. Public Debugger JVM "JDWP-Handshake" "JDWP-Handshake"
77. Public Debugger JVM VM Start Event Initial thread
78. Public Debugger JVM ID sizes fieldID, methodID, objectID, refTypeID, frameID size
79. Public Debugger JVM Enable class prep events
80. Public Debugger JVM Request version VM version and name
81. Public Debugger JVM Request capabilities
82. Public Debugger JVM Request new capabilities since JDWP 1.4
83. Public Debugger JVM Request all classes [kind, id, signature, status] Reason for slow start of session
84. Public Debugger JVM Request all threads [thread id]
85. Public Debugger JVM Enable thread start events
86. Public Debugger JVM Main and notify threads started thread ids
87. Public Debugger JVM Prepared URLClassPath.FileLoader class thread id, kind, type id, signature, status
88. Public Debugger JVM Prepared LineCounter class thread id, kind, type id, signature, status
89. Public Debugger JVM Get line table of methods Obtain information on class [code index, line]
90. Public Debugger JVM Get source file of class Obtain information on class
91. Public Debugger JVM ... and more Obtain information on class
92. Public Debugger JVM Set breakpoint JVM supports breakpoints Do you know any runtimes that don't? Python < 3.12
93. Public Debugger JVM Resume
94. Public Debugger JVM Hit breakpoint
95. Public Debugger JVM Hit breakpoint
96. Public Debugger JVM Get current frames, ...
97. Public Debugger JVM Resume
98. Public Debugger JVM ...
99. Public Debugger JVM VM Death
100. Public 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 *
101. Public Conditional Breakpoints
102. Public Any performance problems?
103. Public Do conditional breakpoints have JVM support?
104. Public defined nowhere
105. Public No support Emulated via breakpoints and evaluation
106. Public Debugger JVM Handshake, ... line.contains("//")
107. Public Debugger JVM Set breakpoint at count++ line.contains("//")
108. Public Debugger JVM Suspend line.contains("//")
109. Public Debugger JVM Get value of line line.contains("//") import java.io.IOException;
110. Public Debugger JVM Create String "//" line.contains("//") string reference
111. Public Debugger JVM Get methods of String line.contains("//") method references
112. Public Debugger JVM Invoke contains method line.contains("//") false
113. Public Debugger JVM Resume line.contains("//")
114. Public Debugger JVM Invoke contains method line.contains("//") false
115. Public Debugger JVM Suspend event line.contains("//")
116. Public Debugger JVM Get value of line line.contains("//") import java.nio.file.Files;
117. Public Debugger JVM ... line.contains("//")
118. Public Debugger JVM Resume line.contains("//")
119. Public Debugger JVM ... line.contains("//")
120. Public Debugger JVM Suspend event line.contains("//")
121. Public Debugger JVM Get value of line line.contains("//") // Count lines in a file
122. Public Debugger JVM Create String "//" line.contains("//") string reference
123. Public Debugger JVM Invoke contains method line.contains("//") true
124. Public Debugger JVM Get frame and stack info line.contains("//")
125. Public Debugger JVM line.contains("//")
126. Public This explains the breakpoint options
127. Public
128. Public
129. Public
130. Public
131. Public Any guess how watches, eval expression, ... are implemented?
132. Public Same...
133. Public Reasons for this
134. Public JVMTI is a terrible API, we should get rid of it as much as possible. — Anonymous Java Architect “
135. Public Conditional Breakpoints are complex ... and slow, especially with large conditions
136. Public Any ideas of improvements? if you could control both IDE and JVM
137. Public I came up with one
138. Public Implement a Caching and Prefetching tunnel Cloud Application Developer Debugger Internet get x x get x x get y y get y y JDWP
139. Public 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
140. Public
141. Public Is DAP better?
142. Public
143. Public
144. Public But... IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java Debug Server DAP Java JPDA
145. Public Extract methods ... and potentially use hotswap
146. Public
147. Public
148. Public Recompile class and hot swap
149. Public Add condition to code ... and potentially use hotswap
150. Public
151. Public Question time
152. Public Are methods with breakpoints interpreted? Yes, they are deoptimized. https://github.com/openjdk/jdk/blob/edcc559f09364da3692862e1f3d0636aa8eec1d4/ src/hotspot/share/code/codeCache.cpp#L1425
153. Public Do you know safepoints?
154. Public What do they have in common with stepping? Stepping is also implemented with safepoints
155. Public Are methods interpreted during single stepping? Yes, they are deoptimized. https://github.com/openjdk/jdk/blob/d31391597433cf275fc615e0148c48c34acf6e11/ src/hotspot/share/prims/jvmtiEventController.cpp#L205
156. Public # of resources on this topic? None, just one from Apache Harmony https://harmony.apache.org/subcomponents/drlvm/breakpoints_and_ss.html 2011
157. Public More IDE features and how they are implemented using JDWP
158. Public Hot Swap
159. Public
160. Public
161. Public
162. Public Debugger JVM Redefine classes request [class id, byte code bytes]
163. Public Debugger JVM Get methods and fields
164. Public Set Specific Value
165. Public
166. Public
167. Public
168. Public
169. Public Debugger JVM Get visible classes
170. Public Debugger JVM Get value of args[1]
171. Public Debugger JVM Get methods of Path
172. Public Debugger JVM Invoke of method
173. Public Debugger JVM Set stack frame value request thread, frame, slot + slot value
174. Public https://github.com/openjdk/jdk/blo b/af8973dc509c1f326223e3ffd1773 c9e930141d8/src/hotspot/share/pri ms/jvmtiImpl.cpp#L671
175. Public Force Return
176. Public
177. Public
178. Public
179. Public
180. Public Debugger JVM Force early return request thread, return value
181. Public Does forced return, execute finalize blocks?
182. Public 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
183. Public Drop/Reset Frame
184. Public Debugger JVM Pop frames request
185. Public Debugger JVM Get frames
186. Public Throw Exception
187. Public
188. Public
189. Public Debugger JVM Create RuntimeEx instance
190. Public Debugger JVM Stop request thread, throwable
191. Public Method and field breakpoints
192. Public Debugging Testing Profiling Toolbox
193. Public
194. Public @parttimen3rd on Twitter parttimenerd on GitHub mostlynerdless.de @SweetSapMachine sapmachine.io