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Debugging Unveiled

Johannes Bechberger
November 08, 2023
4.5k

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.

Johannes Bechberger

November 08, 2023
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  1. If debugging is the process of removing software bugs, then

    programming must be the process of putting them in. — Edsger Dijkstra “
  2. ➜ 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)
  3. 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); }
  4. 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; }
  5. 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 “
  6. IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java

    Debug Server DAP Java JPDA eclipse debugger based
  7. ➜ 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
  8. 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
  9. JVMTI is a terrible API, we should get rid of

    it as much as possible. — Anonymous Java Architect “
  10. 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
  11. Are methods interpreted during single stepping? Yes, they are deoptimized.

    https://github.com/openjdk/jdk/blob/d31391597433cf275fc615e0148c48c34acf6e11/ src/hotspot/share/prims/jvmtiEventController.cpp#L205
  12. # of resources on this topic? None, just one from

    Apache Harmony https://harmony.apache.org/subcomponents/drlvm/breakpoints_and_ss.html 2011
  13. 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
  14. It's arcane But better than Python and fast enough PyData

    16th November DIY Python Debugger and really versatile