Debugging Unveiled

​​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, how they are implemented and how IDEs even communicate with your JVM?

Join me for a deep dive into debuggers, unlocking their secrets and hidden gems to maximize their potential.

Transcript

1. Debugging Unveiled Johannes Bechberger mostlynerdless.de
3. If debugging is the process of removing software bugs, then programming must be the process of putting them in. — Edsger Dijkstra “
4. ➜ java LineCounter.java \ lines LineCounter.java 36
5. ➜ 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. Let’s look at the code
7. 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 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. Any ideas?
10. Let’s use a debugger
16. ... and more
17. Slight focus on IntelliJ
18. 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 “
19. Question time
20. Any OpenJDK developers here?
21. Guessed so But still feel free to correct me.
22. Who of you used a debugger before?
24. Who of you remotely debugged before?
25. Good to know my audience
26. Debugging Testing Profiling Toolbox
29. Introduction Remote JDWP On Demand Architecture
30. General Architecture
31. IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java Debug Server DAP Java JPDA eclipse debugger based
32. Introduction Remote JDWP On Demand Architecture
33. Remote Debugging
35. -agentlib:jdwp= transport=dt_socket, server=y, suspend=y, address=*:5005 Any issues?
36. -agentlib:jdwp= transport=dt_socket, server=y, suspend=y, address=*:5005
37. Differences between remote and non-remote? None
39. Little Known Gems
40. Introduction Remote JDWP On Demand Architecture
41. On Demand Debugging
42. ➜ 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
43. On Exception Debugging throw new Ex(""); start debugging
44. Firewalls only dt_socket
46. Let's go on a short journey
47. I went to Devoxx Belgium
48. Writing a blog post on the train
49. What a train station
50. Meeting some people loosely related to Java
51. Looking into a JDWP agent bug at the Coretto booth
52. That has been there for ages
53. A mysterious Bug
58. Introduction Remote JDWP On Demand Architecture only 120 more slides
59. JDWP From IDE to JVM calls
60. IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java Debug Server DAP Java JPDA
61. 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
62. And lot's of these From connection start to first suspend
63. Demo
64. JDWP From connection start to first suspend
65. Debugger JVM "JDWP-Handshake" "JDWP-Handshake"
66. Debugger JVM VM Start Event Initial thread
67. Debugger JVM ID sizes fieldID, methodID, objectID, refTypeID, frameID size
68. Debugger JVM Enable class prep events
69. Debugger JVM Request version VM version and name
70. Debugger JVM Request capabilities
71. Debugger JVM Request new capabilities since JDWP 1.4
72. Debugger JVM Request all classes [kind, id, signature, status] Reason for slow start of session
73. Debugger JVM Request all threads [thread id]
74. Debugger JVM Enable thread start events
75. Debugger JVM Main and notify threads started thread ids
76. Debugger JVM Prepared URLClassPath.FileLoader class thread id, kind, type id, signature, status
77. Debugger JVM Prepared LineCounter class thread id, kind, type id, signature, status
78. Debugger JVM Get line table of methods Obtain information on class [code index, line]
79. Debugger JVM Get source file of class Obtain information on class
80. Debugger JVM ... and more Obtain information on class
81. Debugger JVM Set breakpoint JVM supports breakpoints Do you know any runtimes that don't? Python < 3.12
82. Debugger JVM Resume
83. Debugger JVM Hit breakpoint
84. Debugger JVM Hit breakpoint
85. Debugger JVM Get current frames, ...
86. Debugger JVM Resume
87. Debugger JVM ...
88. Debugger JVM VM Death
89. 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 *
90. Debugger JVM VM Death But does the JVM have to die?
91. Who killed the JVM?
92. Debugger JVM It dies on its own or via Exit
93. So we can reattach a debugger?
94. Terminate Disconnect Eclipse
95. Close tab Stop (Re)connect IntelliJ
96. IntelliJ
97. Conditional Breakpoints
98. Any performance problems?
99. Do conditional breakpoints have JVM support?
100. defined nowhere
101. Many packages
102. No support Emulated via breakpoints and evaluation
103. Debugger JVM Handshake, ... line.contains("//")
104. Debugger JVM Set breakpoint at count++ line.contains("//")
105. Debugger JVM Suspend line.contains("//")
106. Debugger JVM Get value of line line.contains("//") import java.io.IOException;
107. Debugger JVM Create String "//" line.contains("//") string reference
108. Debugger JVM Get methods of String line.contains("//") method references
109. Debugger JVM Invoke contains method line.contains("//") false
110. Debugger JVM Resume line.contains("//")
111. Debugger JVM Invoke contains method line.contains("//") false
112. Debugger JVM Suspend event line.contains("//")
113. Debugger JVM Get value of line line.contains("//") import java.nio.file.Files;
114. Debugger JVM ... line.contains("//")
115. Debugger JVM Resume line.contains("//")
116. Debugger JVM ... line.contains("//")
117. Debugger JVM Suspend event line.contains("//")
118. Debugger JVM Get value of line line.contains("//") // Count lines in a file
119. Debugger JVM Create String "//" line.contains("//") string reference
120. Debugger JVM Invoke contains method line.contains("//") true
121. Debugger JVM Get frame and stack info line.contains("//")
122. Debugger JVM line.contains("//")
123. This explains the breakpoint options
128. Any guess how watches, eval expression, ... are implemented?
129. Same...
130. Reasons for this
131. JVMTI is a terrible API, we should get rid of it as much as possible. — Anonymous Java Architect “
132. Conditional Breakpoints are complex ... and slow, especially with large conditions
133. Any ideas of improvements? if you could control both IDE and JVM
134. I came up with one
135. Implement a Caching and Prefetching tunnel Cloud Application Developer Debugger Internet get x x get x x get y y get y y JDWP
136. 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
137. Is DAP better?
140. But... IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java Debug Server DAP Java JPDA
141. Extract methods ... and potentially use hotswap
144. Recompile class and hot swap
145. Add condition to code ... and potentially use hotswap
147. Question time
148. Are methods with breakpoints interpreted? Yes, they are deoptimized. https://github.com/openjdk/jdk/blob/edcc559f09364da3692862e1f3d0636aa8eec1d4/ src/hotspot/share/code/codeCache.cpp#L1425
149. Do you know safepoints?
150. What do they have in common with stepping? Stepping is also implemented with safepoints
151. Are methods interpreted during single stepping? Yes, they are deoptimized. https://github.com/openjdk/jdk/blob/d31391597433cf275fc615e0148c48c34acf6e11/ src/hotspot/share/prims/jvmtiEventController.cpp#L205
152. # of resources on this topic? None, just one from Apache Harmony https://harmony.apache.org/subcomponents/drlvm/breakpoints_and_ss.html 2011
153. More IDE features and how they are implemented using JDWP
154. Hot Swap
155. Debugger JVM Redefine classes request [class id, byte code bytes]
156. Debugger JVM Get methods and fields
157. Set Specific Value
162. Debugger JVM Get visible classes
163. Debugger JVM Get value of args[1]
164. Debugger JVM Get methods of Path
165. Debugger JVM Invoke of method
166. Debugger JVM Set stack frame value request thread, frame, slot + slot value
167. Force Return
172. Debugger JVM Force early return request thread, return value
173. Does forced return, execute finalize blocks?
174. 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
175. Drop/Reset Frame
176. Debugger JVM Pop frames request
177. Debugger JVM Get frames
178. Throw Exception
179. Debugger JVM Create RuntimeEx instance
180. Debugger JVM Stop request thread, throwable
181. It's arcane But better than Python and fast enough PyData 16th November DIY Python Debugger and really versatile
182. Debugging Testing Profiling Toolbox
184. @parttimen3rd on Twitter parttimenerd on GitHub mostlynerdless.de @SweetSapMachine sapmachine.io