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

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Public Debugging Unveiled Johannes Bechberger mostlynerdless.de

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Public

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Public 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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Public ➜ java LineCounter.java \ lines LineCounter.java 36

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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)

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Public Let’s look at the code

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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 ... " + " "); System.exit(1); yield 0; } }; System.out.println(count); }

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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; }

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Public Any ideas?

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Public Let’s use a debugger

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Public

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Public ... and more

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Public Slight focus on IntelliJ...

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Public Breakpoint properties in IntelliJ

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Public Breakpoint properties in NetBeans

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Public Disabling line numbers is harder in IntelliJ

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Public Eclipse

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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 “

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Public Question time

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Public Any OpenJDK developers here?

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Public Guessed so But still feel free to correct me.

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Public Who of you used a debugger before?

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Public

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Public Who of you remotely debugged before?

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Public Good to know my audience

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Public Debugging Testing Profiling Toolbox

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Public

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Public

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Public Introduction Remote JDWP On Demand Architecture

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Public General Architecture

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Public IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java Debug Server DAP Java JPDA eclipse debugger based

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Public Introduction Remote JDWP On Demand Architecture

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Public Remote Debugging

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Public

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Public -agentlib:jdwp= transport=dt_socket, server=y, suspend=y, address=*:5005 Any issues?

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Public -agentlib:jdwp= transport=dt_socket, server=y, suspend=y, address=*:5005

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Public Differences between remote and non-remote? None

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Public

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Public Little Known Gems

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Public Introduction Remote JDWP On Demand Architecture

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Public On Demand Debugging

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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

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Public

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Public On Exception Debugging throw new Ex(""); start debugging

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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

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Public What about the launch option?

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Public tmux_jdb.sh #!/bin/sh tmux new-session -d -s jdb -- jdb -attach $2

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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

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Public

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Public

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Public Firewalls only dt_socket

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Public

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Public JDB + onthrow Demo

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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

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Public Let's go on a short journey

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Public I went to Devoxx Belgium

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Public Writing a blog post on the train

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Public What a train station

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Public Meeting some people loosely related to Java

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Public Looking into a JDWP agent bug at the Coretto booth

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Public That has been there for ages

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Public A mysterious Bug

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Public

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Public

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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)

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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

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Public

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Public Introduction Remote JDWP On Demand Architecture only 120 more slides

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Public JDWP From IDE to JVM calls

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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

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Public JDWP From connection start to first suspend

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Public Debugger JVM "JDWP-Handshake" "JDWP-Handshake"

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Public Debugger JVM VM Start Event Initial thread

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Public Debugger JVM ID sizes fieldID, methodID, objectID, refTypeID, frameID size

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Public Debugger JVM Enable class prep events

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Public Debugger JVM Request version VM version and name

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Public Debugger JVM Request capabilities

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Public Debugger JVM Request new capabilities since JDWP 1.4

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Public Debugger JVM Request all classes [kind, id, signature, status] Reason for slow start of session

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Public Debugger JVM Request all threads [thread id]

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Public Debugger JVM Enable thread start events

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Public Debugger JVM Main and notify threads started thread ids

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Public Debugger JVM Prepared URLClassPath.FileLoader class thread id, kind, type id, signature, status

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Public Debugger JVM Prepared LineCounter class thread id, kind, type id, signature, status

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Public Debugger JVM Get line table of methods Obtain information on class [code index, line]

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Public Debugger JVM Get source file of class Obtain information on class

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Public Debugger JVM ... and more Obtain information on class

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Public Debugger JVM Set breakpoint JVM supports breakpoints Do you know any runtimes that don't? Python < 3.12

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Public Debugger JVM Resume

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Public Debugger JVM Hit breakpoint

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Public Debugger JVM Hit breakpoint

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Public Debugger JVM Get current frames, ...

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Public Debugger JVM Resume

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Public Debugger JVM ...

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Public Debugger JVM VM Death

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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 *

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Public Conditional Breakpoints

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Public Any performance problems?

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Public Do conditional breakpoints have JVM support?

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Public defined nowhere

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Public No support Emulated via breakpoints and evaluation

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Public Debugger JVM Handshake, ... line.contains("//")

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Public Debugger JVM Set breakpoint at count++ line.contains("//")

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Public Debugger JVM Suspend line.contains("//")

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Public Debugger JVM Get value of line line.contains("//") import java.io.IOException;

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Public Debugger JVM Create String "//" line.contains("//") string reference

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Public Debugger JVM Get methods of String line.contains("//") method references

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Public Debugger JVM Invoke contains method line.contains("//") false

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Public Debugger JVM Resume line.contains("//")

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Public Debugger JVM Invoke contains method line.contains("//") false

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Public Debugger JVM Suspend event line.contains("//")

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Public Debugger JVM Get value of line line.contains("//") import java.nio.file.Files;

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Public Debugger JVM ... line.contains("//")

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Public Debugger JVM Resume line.contains("//")

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Public Debugger JVM ... line.contains("//")

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Public Debugger JVM Suspend event line.contains("//")

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Public Debugger JVM Get value of line line.contains("//") // Count lines in a file

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Public Debugger JVM Create String "//" line.contains("//") string reference

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Public Debugger JVM Invoke contains method line.contains("//") true

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Public Debugger JVM Get frame and stack info line.contains("//")

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Public Debugger JVM line.contains("//")

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Public This explains the breakpoint options

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Public

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Public Any guess how watches, eval expression, ... are implemented?

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Public Same...

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Public Reasons for this

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Public JVMTI is a terrible API, we should get rid of it as much as possible. — Anonymous Java Architect “

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Public Conditional Breakpoints are complex ... and slow, especially with large conditions

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Public Any ideas of improvements? if you could control both IDE and JVM

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Public I came up with one

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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

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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

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Public

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Public Is DAP better?

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Public

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Public

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Public But... IntelliJ JDI JDWP Agent JVM Java JDWP JVMTI VSCode Java Debug Server DAP Java JPDA

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Public Extract methods ... and potentially use hotswap

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Public

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Public

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Public Recompile class and hot swap

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Public Add condition to code ... and potentially use hotswap

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Public

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Public Question time

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Public 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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Public Do you know safepoints?

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Public What do they have in common with stepping? Stepping is also implemented with safepoints

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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

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Public # 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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Public More IDE features and how they are implemented using JDWP

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Public Hot Swap

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Public

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Public

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Public

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Public Debugger JVM Redefine classes request [class id, byte code bytes]

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Public Debugger JVM Get methods and fields

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Public Set Specific Value

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Public

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Public Debugger JVM Get visible classes

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Public Debugger JVM Get value of args[1]

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Public Debugger JVM Get methods of Path

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Public Debugger JVM Invoke of method

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Public Debugger JVM Set stack frame value request thread, frame, slot + slot value

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Public https://github.com/openjdk/jdk/blo b/af8973dc509c1f326223e3ffd1773 c9e930141d8/src/hotspot/share/pri ms/jvmtiImpl.cpp#L671

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Public Force Return

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Public

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Public Debugger JVM Force early return request thread, return value

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Public Does forced return, execute finalize blocks?

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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

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Public Drop/Reset Frame

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Public Debugger JVM Pop frames request

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Public Debugger JVM Get frames

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Public Throw Exception

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Public

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Public

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Public Debugger JVM Create RuntimeEx instance

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Public Debugger JVM Stop request thread, throwable

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Public Method and field breakpoints

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Public Debugging Testing Profiling Toolbox

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Public

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Public @parttimen3rd on Twitter parttimenerd on GitHub mostlynerdless.de @SweetSapMachine sapmachine.io