This is draft post, work in progress:
jdb Utility
Professional java developers use some IDE for debugging/troubleshooting applications. Intellij Idea, Eclipse and NetBeans are some popular IDEs. Do you know JDK also provides command line debugger jdb?
If you don’t have access to any professional IDE, and you have access to JDK, you can use jdb. The jdb command line utility is included in the JDK. We can connect local or remote JVM using this for inspection or debugging.
jdb is available in jdk/bin directory. It uses the Java Debug Interface (JDI) to launch or connect to the target JVM. The Java Debug Interface (JDI) provides a Java programming language interface for debugging Java programming language applications. JDI is a part of the Java Platform Debugger Architecture.
Troubleshoot java application with the jdb Utility
This section we will see how to attach jdb to java application and start debugging and monitoring.
jdb command
This is format of the jdb command:
jdb [options] [classname] [arguments]
options: This represents the jdb command-line options (e.g. attach, launch).
classname: This represents the name of the main class to debug.
arguments: This represents the arguments that are passed to the main() method of the class.
Java program we will be debugging in this post
Following is sample class we are going to debug and try to understand different features available. It is important to compile this class with -g option (javac -g Test.java) in order to see local variables while debugging.
public class Test
{
public static void main(String[] args)
{
System.out.println("First Line of main function");
System.out.println("Second Line of main function");
System.out.println("Third Line of main function");
int i=0;
System.out.println("i: " + i);
i = 2;
System.out.println("i: " + i);
while(true)
{
}
}
}
Attach jdb to Java application
Below command is the most common way to start application with jdb debugger, here we are not passing any jdb option, only class name Test is passed, and class Test doesn’t require any argument.
jdb Test
In this way we start executing main class Test in the similar way we start in professional IDE eclipse or intellij. jdb stops the JVM before executing that class’s first instruction.
Another way to use the jdb command is by attaching it to a JVM that’s already running. Syntax for starting JVM with debugger port is:
java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:5005 Test
To attach jdb with this remote jvm use below syntax:
jdb -attach 5005
debugging and monitoring
For this post we are not using remote debugging, using following command to attach with jvm:
/jdk/bin/jdb Test
Initializing jdb ...
Setting a break point at line number 5 using stop:
> stop at Test:5
Deferring breakpoint Test:5.
It will be set after the class is loaded.
Start execution of application’s main class using run:
> run Test
run Test
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
>
VM Started: Set deferred breakpoint Test:5
Breakpoint hit: "thread=main", Test.main(), line=5 bci=0
5 System.out.println("First Line of main function");
execute current line using step:
main[1] step
> First Line of main function
Step completed: "thread=main", Test.main(), line=6 bci=8
6 System.out.println("Second Line of main function");
execute current line using step:
main[1] step
> Second Line of main function
Step completed: "thread=main", Test.main(), line=7 bci=16
7 System.out.println("Thrid Line of main function");
execute current line using step:, this is also an example of messed up console print by 2 threads.
main[1] step
> Third Line of main fu
nStep completed: ction
"thread=main", Test.main(), line=9 bci=24
9 int i=0;
execute current line using step:
main[1] step
>
Step completed: "thread=main", Test.main(), line=10 bci=26
10 System.out.println("i: " + i);
printing local variable i using print:
main[1] print i
i = 0
printing all local variables in current stack frame using locals:
main[1] locals
Method arguments:
args = instance of java.lang.String[0] (id=841)
Local variables:
i = 0
dump a thread’s stack using where:
main[1] where
[1] Test.main (Test.java:10)
list threads in running application using threads:
main[1] threads
Group system:
(java.lang.ref.Reference$ReferenceHandler)804 Reference Handler running
(java.lang.ref.Finalizer$FinalizerThread)805 Finalizer cond. waiting
(java.lang.Thread)806 Signal Dispatcher running
(java.lang.Thread)803 Notification Thread running
Group main:
(java.lang.Thread)1 main running
Group InnocuousThreadGroup:
(jdk.internal.misc.InnocuousThread)807 Common-Cleaner cond. waiting
continue execution from the breakpoint using cont:
main[1] cont
> i: 0
i: 2
coming out of jdb:
quit
Conclusion
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