validating - Debugging with JDB

The Java Debugger (JDB) is a command-line tool that ships with the JDK. JDB is at least conceptually a descendant of the Gnu Debugger (GDB, which was inspired by the original Unix DB), in terms of the instructions for debugging and its command-line interface. JDB is useful for learning about debugging and performing simple debugging tasks, and it's helpful to know it's always available wherever the JDK is installed.

// validating/SimpleDebugging.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// {ThrowsException}

public class SimpleDebugging {
  private static void foo1() {
    System.out.println("In foo1");
    foo2();
  }

  private static void foo2() {
    System.out.println("In foo2");
    foo3();
  }

  private static void foo3() {
    System.out.println("In foo3");
    int j = 1;
    j--;
    int i = 5 / j;
  }

  public static void main(String[] args) {
    foo1();
  }
}
/*My Output:
In foo1
In foo2
In foo3
java.lang.ArithmeticException: / by zero
        at SimpleDebugging.foo3(SimpleDebugging.java:22)
        at SimpleDebugging.foo2(SimpleDebugging.java:15)
        at SimpleDebugging.foo1(SimpleDebugging.java:10)
        at SimpleDebugging.main(SimpleDebugging.java:26)
*/

Run JDB

$ jdb SimpleDebugging
Initializing jdb ...
> catch Exception
Deferring all Exception.
It will be set after the class is loaded.
> run
run SimpleDebugging
VM start exception: VM initialization failed for: /Library/Java/JavaVirtualMachines/jdk1.8.0_192.jdk/Contents/Home/jre/bin/java -Xdebug -Xrunjdwp:transport=dt_soc
ket,address=*.local:52281,suspend=y SimpleDebugging


ERROR: transport error 202: gethostbyname: unknown host
ERROR: JDWP Transport dt_socket failed to initialize, TRANSPORT_INIT(510)
JDWP exit error AGENT_ERROR_TRANSPORT_INIT(197): No transports initialized [debugInit.c:750]

The solution is change your hosts file, add 127.0.0.1 *(* in the address=*.local)

$ jdb SimpleDebugging
Initializing jdb ...
> catch Exception
Deferring all Exception.

It will be set after the class is loaded.
> run
run SimpleDebugging
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
>
VM Started: In foo1
In foo2
In foo3

Exception occurred: java.lang.ArithmeticException (uncaught)"thread=main", SimpleDebugging.foo3(), line=22 bci=15
22        int i = 5 / j;

main[1]
main[1] list // we can list the point when it hits the excecution stopped in the program source using the list command
18      private static void foo3() {
19        System.out.println("In foo3");
20        int j = 1;
21        j--;
22 =>     int i = 5 / j;
23      }
24
25      public static void main(String[] args) {
26        foo1();
27      }
main[1]

main[1] locals
Local variable information not available.  Compile with -g to generate variable information
main[1] wherei // prints the stack frames pushed in the method stack of the current thread
  [1] SimpleDebugging.foo3 (SimpleDebugging.java:22), pc = 15
  [2] SimpleDebugging.foo2 (SimpleDebugging.java:15), pc = 8
  [3] SimpleDebugging.foo1 (SimpleDebugging.java:10), pc = 8
  [4] SimpleDebugging.main (SimpleDebugging.java:26), pc = 0

when run with -g (Generate all debugging info)

$ Javac SimpleDebugging.java -g

main[1] locals
Method arguments:
Local variables:
j = 0

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/validating/SimpleDebugging.java