You may be able to use [jsadebugd][1] ([JDK][2]) to attach a debug server to the process (available on Windows with the [Debugging Tools for Windows](http://msdn.microsoft.com/en-US/windows/hardware/gg463009)). It is marked as experimental, so you may want to try it out on a test machine first.

Usage:

    jsadebugd &lt;pid&gt;
    jdb -connect sun.jvm.hotspot.jdi.SADebugServerAttachingConnector:debugServerName=localhost

The connector name withe arg can be found using `jdb -listconnectors`.


  [1]: http://docs.oracle.com/javase/7/docs/technotes/tools/share/jsadebugd.html
  [2]: http://docs.oracle.com/javase/7/docs/technotes/tools/

&gt; Just to clarify it is not possible to use tools like jdb to attach to already running JVMs &gt; &gt; unless they were started in debug mode

in soviet russia source reads you

    jdb -connect sun.jvm.hotspot.jdi.SAPIDAttachingConnector:pid=9426

[VisualVM][1] isn&#39;t a debugger, but you can get thread dumps and heap dumps from it that can be useful in diagnosing some problems. The most useful features require JVM 5 or 6.



  [1]: https://visualvm.github.io/

using jstack (useful in case of deadlocks) or the [btrace][1] VisualVM plugin could also do the trick


  [1]: https://kenai.com/projects/btrace

You can always use jdb and debug by hand :P 

I am trying to create a WPF application that takes command line arguments. If no arguments are given, the main window should pop up. In cases of some specific command line arguments, code should be run with no GUI and exit when finished. Any suggestions on how this should properly be done would be appreciated.

WPF Command Line

I ask because I am sending a byte stream from a C process to Java. On the C side the 32 bit integer has the LSB is the first byte and MSB is the 4th byte.

So my question is: On the Java side when we read the byte as it was sent from the C process, what is [endian][1] on the Java side?

A follow-up question: If the endian on the Java side is not the same as the one sent, how can I convert between them?


  [1]: https://en.wikipedia.org/wiki/Endianness

Does Java read integers in little endian or big endian?

Normally to attach a debuger to a running jvm you would need start the jvm with arguments such as the following:

    &gt; java -Xdebug -Xrunjdwp:transport=dt_socket,address=1000,server=y,suspend=n

Now if I want to debug a process that wasn&#39;t started in debug mode, what can I do?

This situation arrises when a production system (i.e. started without debug args) exhibits a &#39;random&#39; (I use the term loosely) bug. So I can&#39;t restart the jvm with the appropriate arguments, because nobody knows how to reproduce the bug again. Is it impossible to attach to the JVM in this situation?

Just to clarify it is not possible to use tools like jdb to attach to already running JVMs unless they were started in debug mode

from the JVM man page


&gt; **Another way to use jdb is by attaching it to a Java VM that is
&gt; already running. A VM that is to be
&gt; debugged with jdb must be started with
&gt; the following options:**



Debug a java application without starting the JVM with debug arguments

Normally to attach a debuger to a running jvm you would need start the jvm with arguments such as the following:
<pre><code class="hljs language-ini">> java -Xdebug -Xrunjdwp:transport=dt_socket,address=1000,server=y,suspend=n
</code></pre>
Now if I want to debug a process that wasn't started in debug mode, what can I do?
This situation arrises when a production system (i.e. started without debug args) exhibits a 'random' (I use the term loosely) bug. So I can't restart the jvm with the appropriate arguments, because nobody knows how to reproduce the bug again. Is it impossible to attach to the JVM in this situation?
Just to clarify it is not possible to use tools like jdb to attach to already running JVMs unless they were started in debug mode
from the JVM man page
> Another way to use jdb is by attaching it to a Java VM that is
> already running. A VM that is to be
> debugged with jdb must be started with
> the following options:

Is there any performance penalty for the following code snippet?

    for (int i=0; i&lt;someValue; i++)
    {
        Object o = someList.get(i);
        o.doSomething;
    }

Or does this code actually make more sense?
    
    Object o;
    for (int i=0; i&lt;someValue; i++)
    {
        o = someList.get(i);
        o.doSomething;
    }

If in byte code these two are totally equivalent then obviously the first method looks better in terms of style, but I want to make sure this is the case.


Declare an object inside or outside a loop?

Upcasting is allowed in Java, however downcasting gives a compile error. 

The compile error can be removed by adding a cast but would anyway break at the runtime. 

In this case why Java allows downcasting if it cannot be executed at the runtime?  
Is there any practical use for this concept? 

    public class demo {
      public static void main(String a[]) {
          B b = (B) new A(); // compiles with the cast, 
                             // but runtime exception - java.lang.ClassCastException
      }
    }

    class A {
      public void draw() {
        System.out.println(&quot;1&quot;);
      }

      public void draw1() {
        System.out.println(&quot;2&quot;);
      }
    }

    class B extends A {
      public void draw() {
        System.out.println(&quot;3&quot;);
      }
      public void draw2() {
        System.out.println(&quot;4&quot;);
      }
    }

Downcasting in Java

Can any one tell the bit size of **boolean** in Java?

What is the size of a boolean variable in Java?

Is there a way to set up a second persistence.xml file in a Maven project such that it is used for testing instead of the normal one that is used for deployment?

I tried putting a persistence.xml into src/test/resources/META-INF, which gets copied into target/test-classes/META-INF, but it seems target/classes/META-INF (the copy from the src/main/resources) gets preferred, despite `mvn -X test` listing the classpath entries in the right order:

    [DEBUG] Test Classpath :
    [DEBUG]   /home/uqpbecke/dev/NetBeansProjects/UserManager/target/test-classes
    [DEBUG]   /home/uqpbecke/dev/NetBeansProjects/UserManager/target/classes
    [DEBUG]   /home/uqpbecke/.m2/repository/junit/junit/4.5/junit-4.5.jar
    ...

I would like to be able to run tests against a simple hsqldb configuration without having to change the deployment version of the JPA configuration, ideally straight after project checkout without any need for local tweaking.

How to configure JPA for testing in Maven

Is there some easy way to pad Strings in Java?

Seems like something that should be in some StringUtil-like API, but I can&#39;t find anything that does this.

How can I pad a String in Java?

When debugging in PHP, I frequently find it useful to simply stick a [var_dump()][1] in my code to show me what a variable is, what its value is, and the same for anything that it contains.

What is a good Python equivalent for this?

  [1]: http://php.net/var-dump


What is a Python equivalent of PHP&#39;s var_dump()?

I wish to print a `Stack&lt;Integer&gt;` object as nicely as the Eclipse debugger does (i.e. `[1,2,3...]`) but printing it with `out = &quot;output:&quot; + stack` doesn&#39;t return this nice result.

Just to clarify, I&#39;m talking about Java&#39;s built-in collection so I can&#39;t override its `toString()`.

How can I get a nice printable version of the stack?

Printing Java Collections Nicely (toString Doesn&#39;t Return Pretty Output)

I was looking into Valgrind to help improve my C coding/debugging when I discovered it is only for Linux - I have no other need or interest in moving my OS to Linux so I was wondering if there is a equally good program for Windows.

Is there a good Valgrind substitute for Windows?

I&#39;ve been a software developer for over twenty years, programming in C, Perl, SQL, Java,  PHP, JavaScript, and recently Python.  I&#39;ve never had a problem I could not debug using some careful thought, and well-placed debugging `print` statements.

I respect that many people say that my techniques are primitive, and using a real debugger in an IDE is much better.  Yet from my observation, IDE users don&#39;t appear to  debug faster or more successfully than I can, using my stone knives and bear skins.  I&#39;m sincerely open to learning the right tools, I&#39;ve just never been shown a compelling advantage to using visual debuggers.

Moreover, I have never read a tutorial or book that showed how to debug effectively using an IDE, beyond the basics of how to set breakpoints and display the contents of variables.

What am I missing?  What makes IDE debugging tools so much more effective than thoughtful use of diagnostic `print` statements?

Can you suggest resources (tutorials, books, screencasts) that show the finer techniques of IDE debugging?

---

Sweet answers!  Thanks much to everyone for taking the time.  Very illuminating.  I voted up many, and voted none down.  

Some notable points:

 - Debuggers can help me do *ad hoc* inspection or alteration of variables, code, or any other aspect of the runtime environment, whereas manual debugging requires me to stop, edit, and re-execute the application (possibly requiring recompilation).
 - Debuggers can attach to a running process or use a crash dump, whereas with manual debugging, &quot;steps to reproduce&quot; a defect are necessary.
 - Debuggers can display complex data structures, multi-threaded environments, or full runtime stacks easily and in a more readable manner.
 - Debuggers offer many ways to reduce the time and repetitive work to do almost any debugging tasks. 
 - Visual debuggers and console debuggers are both useful, and have many features in common.
 - A visual debugger integrated into an IDE also gives you convenient access to smart editing and all the other features of the IDE, in a single integrated development environment (hence the name).

Why is debugging better in an IDE?

A friend asked me last week how to enumerate or list all variables within a program/function/etc. for the purposes of debugging (essentially getting a snapshot of everything so you can see what variables are set to, or if they are set at all). I looked around a bit and found a relatively good way for Python:

&lt;pre&gt;
#!/usr/bin/python                                                                                                                                                                                                                           
foo1 = &quot;Hello world&quot;
foo2 = &quot;bar&quot;
foo3 = {&quot;1&quot;:&quot;a&quot;,
        &quot;2&quot;:&quot;b&quot;}
foo4 = &quot;1+1&quot;

for name in dir():
    myvalue = eval(name)
    print name, &quot;is&quot;, type(name), &quot;and is equal to &quot;, myvalue
&lt;/pre&gt;

which will output something like:

&lt;pre&gt;
__builtins__ is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  &amp;lt;module &#39;__builtin__&#39; (built-in)&amp;gt;
__doc__ is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  None
__file__ is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  ./foo.py
__name__ is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  __main__
foo1 is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  Hello world
foo2 is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  bar
foo3 is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  {&#39;1&#39;: &#39;a&#39;, &#39;2&#39;: &#39;b&#39;}
foo4 is &amp;lt;type &#39;str&#39;&amp;gt; and is equal to  1+1
&lt;/pre&gt;

I have so far found a partial way in PHP (courtesy of [link text][1]) but it only lists all variables and their types, not the contents:

&lt;pre&gt;
&amp;lt;?php
// create a few variables
$bar = &#39;foo&#39;;
$foo =&#39;bar&#39;;
// create a new array object
$arrayObj = new ArrayObject(get_defined_vars());
// loop over the array object and echo variables and values
for($iterator = $arrayObj-&amp;gt;getIterator(); $iterator-&amp;gt;valid(); $iterator-&amp;gt;next())
        {
        echo $iterator-&amp;gt;key() . &#39; =&amp;gt; &#39; . $iterator-&amp;gt;current() . &#39;&amp;lt;br /&amp;gt;&#39;;
        }
?&amp;gt;
&lt;/pre&gt;

**So I put it to you: how do you list all variables and their contents in your favorite language?**

-------

Edit by [VonC][2]: I propose this question follows the spirit of a little &quot;[code-challenge][3]&quot;.  
If you do not agree, just edit and remove the tag and the link.


  [1]: http://www.phpro.org/examples/List-all-variables.html
  [2]: https://stackoverflow.com/users/6309/vonc
  [3]: https://stackoverflow.com/questions/172184

Enumerate or list all variables in a program of [your favorite language here]

I have a Java application that I run from a console which in turn executes an another Java process. I want to get a thread/heap dump of that child process.

On Unix, I could do a `kill -3 &lt;pid&gt;` but on Windows AFAIK the only way to get a thread dump is Ctrl-Break in the console. But that only gives me the dump of the parent process, not the child.

Is there another way to get that heap dump?

How to get a thread and heap dump of a Java process on Windows that&#39;s not running in a console

It seems rare to read of a Python &quot;virtual machine&quot; while in Java &quot;virtual machine&quot; is used all the time.

Both interpret byte codes; why call one a virtual machine and the other an interpreter?


Java &quot;Virtual Machine&quot; vs. Python &quot;Interpreter&quot; parlance?

I dont understand the difference between LLVM and the java (bytecode), what are they?

-edit- by &#39;what are they&#39; i mean the differences between LLVM and java (bytecode) not what are LLVM and java.


What are the differences between LLVM and java bytecode?

How do you write (and run) a correct micro-benchmark in Java?

I&#39;m looking for some code samples and comments illustrating various things to think about.

Example: Should the benchmark measure time/iteration or iterations/time, and why?

Related: https://stackoverflow.com/questions/410437/is-stopwatch-benchmarking-acceptable

How do I write a correct micro-benchmark in Java?

Is anyone attempting to implement C# for the JVM? As a Java developer, I&#39;ve been eyeing C# with envy, but am unwilling to give up the portability and maturity of the JVM, not to mention the diverse range of tools for it.

I know there are some important differences between the JVM and CLR but is there anything that is a showstopper? 



Implementing C# for the JVM

Is there a way that I can set the default heap size for the jvm on my own computer?  I want to set it to 1g, because I&#39;m always running custom programs that always hit the overage point in the default jvm size.

I just dont want to have to remember to type -XmX1g everytime I run my java app from the command line...  

There has to be an admin way to do this right?

increase the java heap size permanently?

Given these two commands

A:

    $ java -Xms10G -Xmx10G myjavacode input.txt

B:

    $ java -Xms5G -Xmx5G myjavacode input.txt

I have two questions:

 1. Since command A reserves more memory with its parameters, will A run faster than B? 
 2. How do `-Xmx` and `-Xms` affect the running process and the output of my program?

Speed tradeoff of Java&#39;s -Xms and -Xmx options

After reading already asked question on the subject and a lot of googling I am still not able to have a clear view of **-Xms** option

My question is: what&#39;s the difference between `java -Xms=512m -Xmx=512m` and `java -Xms=64m -Xmx=512m`?

For now I have the following answer:

The only difference is in the number of garbage collections that will be run during my application&#39;s run and the number of memory allocations. Am I right ?

Here are my reasons for this answer:

Setting the `-Xms` option to `512m` doesn&#39;t result in my application using really `512M` of physical memory after startup. I guess this is related to modern OS virtual memory management and lazy pages allocations. (I noticed that setting `-Xms` to `512M` or to `64M` doesn&#39;t change at all the initial used memory reported either by top on Linux or by the task manager on windows)

Can someone help me to understand the impact of this `Xms` option or point me to links that will help me to understand it?

Thanks in advance

Manu


JVM heap parameters

I need to check if some option that can be passed to JVM is explicitly set or has its default value.

To be more specific:
I need to create one specific thread with higher native stack size than the default one, but in case the user wants to take care of such things by himself by specifying the `-Xss` option I want to create all threads with default stack size (which will be specified by user in -Xss option).

I&#39;ve checked classes like `java.lang.System` and `java.lang.Runtime`, but these aren&#39;t giving me any useful information about VM arguments.

Is there any way to get the information I need?


How to get VM arguments from inside of Java application?

In a shell script, I have set the JAVA_OPTS environment variable (to enable remote debugging and increase memory), and then I execute the jar file as follows:

    export JAVA_OPTS=&quot;-Xdebug -Xrunjdwp:transport=dt_socket,address=8001,server=y,suspend=n -Xms512m -Xmx512m&quot;
    java -jar analyse.jar $*

But it seems there is no effect of the JAVA_OPTS env variable as I cannot connect to remote-debugging and I see no change in memory for the JVM.

What could be the problem?

PS: I cannot use those settings in the `java -jar analyse.jar $*` command because I process command line arguments in the application. 

Content Type	Original Author	Original Content on Stackoverflow
Question	hhafez	View Question on Stackoverflow
Solution 1 - Java	McDowell	View Answer on Stackoverflow
Solution 2 - Java	potted plant	View Answer on Stackoverflow
Solution 3 - Java	sk.	View Answer on Stackoverflow
Solution 4 - Java	Vijay	View Answer on Stackoverflow
Solution 5 - Java	OscarRyz	View Answer on Stackoverflow

Debug a java application without starting the JVM with debug arguments

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Does Java read integers in little endian or big endian?

WPF Command Line

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