debug hover item in chrome devtools

Chrome devtools is our friend, always.

Today when I was developing an angular 4.x app with primeng library, i have to check the class set on the tooltip component. As we know the tooltip is hover event based, so if we hover on it to make it showup and then shift our focus to the dev tool element tab, the tooptip would disappear.

Chrome tool has a feature to activate the hover stuff(:hover) on specific element for CSS sake. It is quite handy but obviously it does not apply in this use case since this tooltip is js based.

Search around and finally find a solution: using F8 or CMD + \ which is pause the script execution.

Steps are quite straightforward:

Mouse over the tooltip, and press F8 while it is displayed.

Now you can now use the inspector to look at the CSS.

How Java Debug works

You can just attach your IDE to a running application (which has been runned for debug as we’ll see later), or you can even debug it from command line. And the application you debug can even be be in a different machine.

The magic lies in where the debug information actually resides. Apparently people normally think that is the IDE that knows how to debug your programs, but the truth is that is the program who knows how to debug itself, and makes that information available to whoever wants to use it.

The way it works is basically the following. When you compile a program, the .class files get debug information within them, like line numbers or local variables that are made accessible to others who want to access this information. You can then run the program in debug mode passing the following options to your java program execution(you can of course run any java program like this, including mvn goals, appllication servers, etc)

User Jdb

There are many ways to start a jdb session. The most frequently used way is to have jdb launch a new Java Virtual Machine (VM) with the main class of the application to be debugged. This is done by substituting the command jdb for java in the command line. For example, if your application’s main class is MyClass, you use the following command to debug it under JDB:

C:\> jdb MyClass 

When started this way, jdb invokes a second Java VM with any specified parameters, loads the specified class, and stops the VM before executing that class’s first instruction.

Another way(used by ide)

java -agentlib:jdwp=transport=dt_shmem,address=jdbconn,server=y,suspend=n Sum 3 4

You can then attach jdb to the VM with the following commmand:

C:\> jdb -attach jdbconn

This line basically says: Run this program in debug mode, use the jdwp protocol, with a socket that listens to port 4000 and waits for connections to continue.

The jdwp protocol is a communication protocol used by the Java application to receive and issue commands and reply to them.

For example, you can connect to this port when the application is running an issue commands like “print variablex” to know the value of a variable, “stop at x” to set a breakpoint, etc. The application issues notification commands like “breakpoint reached”.
The truth is that the protocol is a little more complex than this, but this is enough to know to illustriate the point.

With the previous said, we can see that it would be even possible to debug an application with the use of Telnet! (we’ll see later)

Well, enough theory. Let’s see an example Any simple example will do. We’ll make a simple program that takes two parameters from command line and prints the sum. The program won’t be well designed (In the sense that will include some useless temp variables, no validations, etc) but will do to illustrate the example.

class Sum{
    public static void main(String[] args){
        int sum1 = Integer.parseInt(args[0]);
        int sum2 = Integer.parseInt(args[1]);
        int suma= sum1+sum2;
        System.out.println("La suma es "+suma);

So we compile it: javac -g (the g option adds extra debug info to the class. Like local variable names)
And we run it in debug mode:

C:\> jdb Sum

you’ll get some output like
Initializing jdb …

That’s it, you have a debug session started. Now the interesting. Execute the following in your jdb session:

stop at Sum:6

You now have a breakpoint on line 6. execute run on the session, and the program will run until that breakpoint. you’ll get the output

Deferring breakpoint Sum:6.
It will be set after the class is loaded.

We can then type ‘run‘ to start jvm.

Now let’s see the value of our variables: run the following commands (one at a time) on the jdb session and see the results.

print sum1
print sum2
print suma
set suma = 10

This is pretty cool stuff. You can debug your program from command line.


From here and here

debug nodejs with nodemon and intellij

Noticed that if I run nodemon within intellij, I would not be able to debug any more inside intellij.
Not sure what reason it is. If just run the nodejs, no problem. However once I add the ‘/usr/local/bin/nodemon’ into the Node parameters of the run config, the breakpoint would not work.


To solve this problem, i have to use the Node.js remote debug.
Steps are

1. add a debug port to the Node parameters

–debug=3001. Note It is double dash. this should be the same as the port in the following remote debug config.

in the intellij doc, another option is to use –debug-brk which would enable the debug to run from the start. I tried this param, it works if I just change some front end html/js, however it would not work when backend js changed. Might have other workaround if you have to debug some init processes.

debug config intellij nodejs

debug config intellij nodejs

2. add a new Node.js remote debug config in Intellij

remote debug intellij node

remote debug intellij node

Now you should be able to use run RUN to start the server.js application and use DEBUG to start the ‘remote server debug’ and then set break point in your node app. 🙂