Debugging Java Programms ?

Hello,

I'm not a professional Java Programmer, just some experience in it.

Due sometimes a Program through it's complexity can behave strange (sudden crash, System freezes, or false output etc.). It might e caused through System incompatibility, Bugs or faulty Code, I don't know precisely.

However sometimes rudementary Tests, like System.out.println(), if-else etc can help a bit, by having less Information about Debugging.

On Internet I found some Links which explains it, through using some helpful Tools, like Online Debugger, JUnit, IDE built-in Debugging Tools etc

For example I programmed a primitive Media Player in JavaFX. At first glance it runs fine. But after an hour or so, the Desktop freezes, with a Critical Kernel Power Event (Event Manager).
Without going through the Windows Debug Image with a Debugger. How could I create a Test Environment, to start like a Pro ?

I thought a VM basically to Test separately from the Host, but which Tests with which Tools ?

Thanks.

Regards,
John

Hi John,

On a code level I would recommend a unit testing tool such as jUnit to write automated tests that make assertions on the functionality of your "units" (you define what a unit is. It could be a method, a class, or a collection of classes that represent some unit of function in your system)

But it sounds like you have some kind of system resource over-consumption problem that take an hour or so to completely overwhelm your operating system. For that I would recommend a JVM profiling tool such as VisualVM to monitor CPU and memory usage. If you have a memory leak, for instance, that tool will show you what data constructs in your system are using all the memory. For example a list of something that grows forever. I've gotten good results with VisualVM so it's worth a look.

You can also configure your JVM to emit a heap dump on Out Of Memory with a command option
java -XX:+HeapDumpOnOutOfMemoryError
Then use VisualVM to inspect the heap dump. The heap is basically the memory storage so you can use the heap dump to see what has consumed it all to the point of failure.

Good luck!

Agree with Tim. JUnit isn't a debugger, but a testing tool. Test all the “units” in your program, starting small, i.e. every method. Give the methods multiple inputs, normal and abnormal, and see what happens. Make sure your methods are small enough to count as “units” by themselves. Consider throwing exceptions for abnormal input. public Grade calculateGrade(Exam e, mark i) {   if (ex == null)   {      throw new NullPointerException(            "null not permitted in calculateGrade method.");   }   Exam ex = new Exam(e); // copy before validation   if (somethingAboutTheExamIsWrong)   {      throw new IllegalArgumentException("something");   }   if (mark < 0 || mark > 100)   {      throw new IllegalArgumentException(            "mark must be in range 0...100inc.");   }   Grade grade = ???   // etc   return grade; }If you want anybody else you use that method, make sure all those tests are recorded in the documentation comments. You can get JUnit to test for the correct exception being thrown, as well as the correct grades. Most people would replace lines 3‑7 with a call to this method or its overloaded version. Copying from a null will probably cause an exception to be thrown, but you will find programming easier if you use informative exception messages.

Unit testing can help a lot, as can other tests.

For ongoing work, though, don't use System.out.println(), use a logger such as Log4J or the JDK-standard (¨JULI¨) logger. A logger allows you to fine-tune output so that you can turn on more detailed levels of print when you need to analyse a problem, then turn them back off when you don't need that info as part of your regular output.

Loggers also can do things like timestamp messages and can be routed to one or more output channels. Not just to the console, but to log files, send emails. even route messages to a pager/SMS service for emergency notifications.

Something to note: The Java debugger is built into the JVM. It's always present, although you can only activate it as a JVM startup option, not switch it on and off at will. The debugger itself is a simple command interface operated via a designated debug port, but most Java IDEs can drive a more intelligent means of working with it using a "remote debug" program run mode. In this case, "remote" actually means a separate JVM from the one that the IDE runs in, so you can use the same method both on actual remote machines or the localhost interface on the same machine. The only issue is that the application source code that the IDE is pointing to should match the compiled class code of the JVM being debugged or weird things will happen.

In a professional IT shop, developers generally code and test either on their own desktops or a designated alpha-testing machine (often both). One place I worked did development under Windows, had an alpha-test machine and final production servers that both ran the Solaris OS. The advantage of Java's ẅrite once/run anywhere design!

IDE's such as Eclipse or IntelliJ IDEA can help a lot in debugging.

You can set breakpoints or watches and either run unit or other tests or the application as such.

Debugging and running unit tests: these are no alternatives rather than each other completing activities.

Test cases should ensure that the application runs ok against specifications and debugging is needed to figure out what went wrong when test cases fail. Only debugging an application or Java classes is far too laborious.

I have used JUnit a few times, but usually I do my testcases simply in the "public static void main"- method, using breakpoints, single steppers and System.outs. I am just a hobby programmer, so that is good enough for me.

Thanks for all the detailed Answers, very helpful.

Now I have a Rule of Thumb for doing  these Tests on a regular basis.

Honestly I must admit that the case of the culprit App that caused to freeze the System, must not be at all the Java Program.
I noticed that the System freezed randomly in other cases too.
I installed all the Drivers and the System with it's third-party apps is up-to-date, and clean(Garbage, Cache, Windows Defender etc.).

I suspect there i something with the Mainboard Components, or just too weak Power. It's just an Mini-ITX Form, so CPU, Chipset and Cooling-System etc. might suffer.
Set Up :
CPU 11th Gen Intel(R) Core(TM) i5-11600K @ 3.90GHz, 3912 MHz, 6 Cores, 12 logical Processors
BIOS-Mode UEFI
BaseBoard-Manufacturer ASRock
Mainboard Z590M-ITX/ax
Physical Memory (RAM) 32.0 GB
Kernel-DMA-Security ON

Just struggling sometimes when a couple of Apps(VM's, Audio-Video, Browser, IDE's etc.) are running at the same Time. Or just 5 min ago, I was running a VM copying from a shared Folder -> freeze after 7 min.
The most Time the Performance looks good. Maybe the throughput of data bus and their Interfaces could be somehow a bottleneck, which push up the Performance to the limit.

Looking for an ATX and more powerful Components could be better.
Or at least making a good Benchmark of the System.

Or any Ideas of a Rule of Thumb for Computers for Programming ?

So yeah, sorry about. Nontheless the interesting Topic of Testing and Debugging of Software.

Many Thanks,

Please don´t quote entire messages if you can. It takes up space and we already read the original (usually!)

Java apps should not, as a rule, lock up the system. If they do, then chances are that your system has too little RAM and it has gone into page-thrashing. You can overload the CPU, but that's less likely to nuke the whole system, since application CPU usages are tuned by the system task manager. There are tuning tools that can help with those problems.

I've been swapping out my tower ATX servers for micro-boxes. From what I can tell, yes, the ATX's are more powerful, despite running on 2011-era motherboards, but not so much that I care. With current system load requirements, I figure that the lower electrical power needs and more convenient physical size more than make up for things. I get them used for cheap off EBay and add RAM, as needed.

The machine I am typing this on is a Raspberry Pi 4, incidentally. For most needs it is all the desktop power I require, though I have something bigger for CAD and IDE work. The Pi can run Java, though the older models were noticeably slow doing so. The main fault with the Pi is that even with 8GB RAM, it can end up page-thrashing. which can require a reboot.

Thanks,

Sorry, I was amazed from the Answers,

Yes ATX Format should have bigger Data Bus, Chipset, CPU etc. so better Performance.

RaspBerry PI is also nice for an All in One PC, for medium load use. I tended to use it as Sensors for any Usage.

I choosed Mini-ITX because I didn't have much space, at that time. By Time I added new Software in the learning Progress.

Regards,

JT: please don't edit posts after people have read them and replied. That causes confusion about what they knew when they replied. Please write any changes in new posts.

Campbell Ritchie wrote:JT: please don't edit posts after people have read them and replied. That causes confusion about what they knew when they replied. Please write any changes in new posts.

Yes Sir,

Sorry in future I will take care about.

Reading the Rules, think twice before posting and be good.

Our Lord Jesus Christ bless you.

Debugging professionally usually involves three main practices:

1️⃣ Unit Testing (JUnit) – Write automated tests for small methods or modules and verify behavior using different inputs.

2️⃣ IDE Debugger – Use breakpoints and step-by-step execution in tools like IntelliJ IDEA or Eclipse instead of relying only on System.out.println().

3️⃣ Profiling Tools – Use tools such as VisualVM or JConsole to monitor CPU and memory usage. Long-running freezes are often caused by memory leaks or resource overuse.

Additional tips:
✔ Use a proper logging framework (Log4J or java.util.logging)
✔ Enable JVM option -XX:+HeapDumpOnOutOfMemoryError for memory debugging
✔ Test in a separate environment or virtual machine for safer experimentation