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React native, Debugging a native library?

Suppose there's a RN library which has native (java/swift) code in it.
I'd like to debug this java code, (it seems it has a bug)
How can I set a breakpoint in this native code? I can't find this file in the android-studio
For RN .60+ where there's no explicit link step..

You can use LogCat and the Debug Console in Android Studio and Xcode respectively.
As this is only aimed at Android, this answer will focus on LogCat only.
Getting Logs:
If you navigate to the native library and find a Java file which specifies a package name, you can use this to filter LogCat.
For example, you can use com.reactnativecommunity.netinfo if you were debugging react-native-device-info
Ensuring that your device and debuggable process is set within LogCat, you can filter out Logs by type, if your library is throwing an Error, filter it by Error and apply the package name to filter out any unwanted logs.
From here you should receive a detailed stack trace which includes the problematic files, from there you can start to debug.
Debugging:
Once you've located the problematic files, open it within Android Studio.
Click on the right side, next line number to add a breakpoint.
With the app running, click on the Debug icon and select your app/running process.
When your breakpoints are hit, Android Studio will pause and allow you to debug.

Use java tracing facilities with a desktop application

Is there a way to turn tracing and logging on for a java application which is neither a Java Web Start nor applet type of java application? I'm talking about an application that would be executed by either double clicking on an executable jar file or launched from the command line by typing java -jar nameofjarfile.jar. I have enabled logging and tracing in the Java Control Panel but this seems to have no effect. The only trace logs that I see are trace logs generated for execution of the java control panel. As far as I can discern from the documentation the options in the java control panel to enable logging and tracing are specific to Web Start and applet style applications. When I launch my desktop java application no .trace file is generated.
Thanks in advance.

It appears there is no equivalent to the trace option offered in the Java Control Panel for apps that are launched using the regular virtual machine. The options in the Java Control Panel are specifically for Java Web Start Apps and Java Applets. It has no effect on the Java Desktop Applications launched by double clicking an executable jar file or by typing java -jar javaapp.jar at the command line. While the documentation states that tracing is output from the java console to a .trace file the console to which they speak is the Java Console that is only available for Web Start and Applets. They are not speaking of just standard out and standard error. While both standard out and standard error does get output to the Java Console the Java Console also includes boot strap information of the JVM itself such as the java version, the exact path of the java executable file, proxy information and much more. I'm sure there may be a way to generate equivalent data it can not be done through the Java Control Panel's trace and log options or with Deployment Property options such as -DDeployment.trace = true. You can see the information I'm speaking of by going to the java tutorials and launching one of the many web start apps they link to in their tutorials. Make sure to go to the Java Control Panel and tick the Show Console option under the advanced tab. When you launch a Java Web Start App with this option selected the Java Console will open. The output to that console is what is dumped to the .trace file when Enable Tracing is selected in the same Advanced tab of the Java Control Panel. If you also enable logging it appears that console output is output to a .log file but in an XML format.

Eclipse debug mode not opening

I am building a web application with Google Web Toolkit (GWT ) framework.
When i try to run the project in debug mode , it runs normally as in "run" mode itself. I dont get the debug perspective also.
To see if it was problem with my eclipse , i created a simple HelloWorld java application and the debug mode works perfectly .
In a seperate thread with similar issue it was advised to update JDK version. Which i did , but still didnt help
Why when starting GWT in debug mode, my break points don't break
Please help

Try Changing the perspective to Debug.
Windows -> Perspective ->Open Perspective ->Debug

I was able to make the breakpoints work. Basically i was trying to put breakpoints in the onmoduleload method which is in the client side packages. Looks like debug for client side is not provided in eclipse.
I was able to put breakpoints in Serverside classes and the breakpoints were hit.
For client side apparently we can use chrome and view source and see the breakpoints.I am yet to try it

Launching Eclipse as a debugger

What’s the equivalent of
System.Diagnostics.Debugger.Break();
in the Java world?
Purpose: I have a tomcat based webapp launched by a custom build tool and need to debug the application in eclipse. In the .net world the above statement when encountered will prompt the OS to attach a debugger and I can attach Visual Studio to debug. I am trying to achieve the equivalent in java with eclipse

Here's an excellent article on remote debugging using Eclipse. They even have a section discussing Tomcat.

Here's a link that I used to debug web apps on Tomcat. It goes through installing Eclipse, Tomcat and Java and then setting up Tomcat to run in Eclipse. Towards the bottom explains how to debug a servlet in Eclipse.
http://www.windofkeltia.com/j2ee/wtp-tutorial.html

In general the Java program cannot tell if the JVM runs in debug mode or not, and there is no way to from your program to say that you always want to start a debugger HERE.
You can, however, tell the DEBUGGER that you want to have a breakpoint at a given location, and you will then enter the debugger when the program reaches that spot.
EDIT: You will need to investigate your launcher to see how you can trick it to contain the options needed to enable debugging in the JVM. You may also see if jvisualvm can give you the information you need as it can attach to an unprepared Sun JVM.

How do you debug a Rails application?

Is it possible to debug a Rails application in a similar way to a Java application - setting breakpoints and stepping into the code?
What are the best tools for this?
I have a hybrid Java/Ruby on Rails application which I can run in Eclipse or Netbeans.
I would like to step into some code in this app and try to figure out the cause of a problem I'm having.
In Eclipse if I set a breakpoint in my blog_controller and then choose the 'Debug' button, it seems to use the ruby-debug-ide gem to execute the code but I get this unhelpful output and no option to step into any source:
Fast Debugger (ruby-debug-ide 0.4.5) listens on localhost:56726
./war/WEB-INF/app/controllers/blog_controller.rb:1
C:/Ruby18/lib/ruby/gems/1.8/gems/ruby-debug-ide-0.4.5/lib/ruby-debug.rb:101:in `debug_load'
C:/Ruby18/lib/ruby/gems/1.8/gems/ruby-debug-ide-0.4.5/lib/ruby-debug.rb:101:in `debug_program'
C:/Ruby18/lib/ruby/gems/1.8/gems/ruby-debug-ide-0.4.5/bin/rdebug-ide:82
C:/Ruby18/bin/rdebug-ide:19:in `load'
C:/Ruby18/bin/rdebug-ide:19
Uncaught exception: uninitialized constant ApplicationController
I'm not sure if I'm doing something wrong or if this is all I can expect.

The debugger I use the most is the ruby-debug gem, which is a gdb-esque command line debugger. Once you learn a few commands it is very quick and effective, and provides you with some handy features like being able to fire up irb in the context of your program and make on-the-fly changes.
And being command line based it comes in handy when you need to debug a on a remote server.

You can expect more. I have used Aptana's RadRails version of Eclipse to debug a Rails app as you describe--setting breakpoints and stepping through the code.
You may be doing something wrong. It looks as if it is trying to debug an individual controller file, rather than debugging the Rails app. When I try to execute a controller file from the command line, I get a similar message:
C:\workspace\myapp\app\controllers>ruby users_controller.rb
users_controller.rb:1: uninitialized constant ApplicationController (NameError)
In Aptana RadRails, I choose Run > Debug As > Ruby Application to debug the app.

For the vim users I strongly recomend looking into the vim-ruby-debugger, which fits in great with Tim Pope's rails.vim scripts.
It gives you a handy :Rdebugger command, allows you to set breakpoints and open a split window to display variable values.

maybe not relevant, but I wanted to post somewhere: got the error: "undefined method `run_init_script' for Debugger:Module" running the debugger in rails 2.3.2. Did a sudo gem install ruby-debug and the problem went away.

I'd recommend just setting up breakpoints (I actually just puts to console) for 99% of debugging with RoR - this method is simple and usable across any IDE, so you never need to learn how a new debugger works.

Actually, I had the same problem with Aptana. Run > Debug As > Ruby Application just doesn't work. I finally made the debugger work by going to the Servers tab, and then start the server in debug mode. After that, set some breakpoints and trigger the corresponding action. Hope this helps.

Debugging? That's just knowing where to look in the case of Ruby (and by extension, Rails) most of the time.
The problem in this case is that you probably still have your ApplicationController called application.rb where it should be renamed to application_controller.rb.

Debuggin in rails is simple if you know how to read the error stacktrace!! But if you need to explicitly watch out the values during the runtime then u can use the rails breakpointer.Below is the link to how-to on breakpointer ..hope this helps!!!
http://destiney.com/blog/rails-breakpointer

I can't speak for Eclipse (never worked well for me) or Aptana (not tried) but from experience I can say that both NetBeans and RubyMine will do what you want. I both cases you should probably make sure that the ruby-debug-base and ruby-debug-ide gems are up-to-date: RubyMine in particular didn't work for me until that was done.