The key to Java debugging is to master the use of VisualVM and JConsole tools. 1. VisualVM is a graphical troubleshooting tool that integrates multi-JDK tools. It can view the JVM running status in real time, analyze hotspot methods and frequent objects through "Sampler", and supports remote monitoring configuration and plug-in extensions. 2. JConsole is suitable for quickly viewing memory, threads, and class loading, and can detect deadlocks and observe GC frequency. 3. It is better to use the two in combination: first use JConsole to observe the exception, then deeply analyze the performance bottlenecks through VisualVM, and it is recommended to grab the heap dump for further diagnosis.

Java debugging is an indispensable skill for developers. Especially when you are facing complex performance issues, memory leaks, or thread deadlocks, printing logs may be far from enough. At this time, it is particularly important to use tools like VisualVM and JConsole.

1. VisualVM: an intuitive and powerful performance analysis tool

VisualVM is a graphical monitoring and troubleshooting tool that integrates multiple JDK tools. It allows you to see the running status of the JVM at a glance, such as heap memory usage, thread status, class loading information, etc.

Basic usage:

• After starting the Java application, open VisualVM directly (usually in the JDK/bin directory).
• Click your program process in the application list on the left to see the real-time CPU and memory chart.
• Use the "Sampler" tag to perform CPU and memory sampling, helping you discover hotspot methods or frequently created objects.

Practical Tips:

• If you want to remotely monitor Java programs on the server, remember to add JMX parameters when starting:

   -Dcom.sun.management.jmxremote
  -Dcom.sun.management.jmxremote.port=8089
  -Dcom.sun.management.jmxremote.ssl=false
  -Dcom.sun.management.jmxremote.authenticate=false

• Installing plug-ins can extend functions, such as "Threads Inspector" can help analyze thread blocking and other problems.

2. JConsole: A classic monitoring tool that comes with JDK

Although the interface is not as modern as VisualVM, JConsole is still a very practical tool, especially suitable for quickly viewing memory, threading, class loading and MBean situations.

Recommended usage:

• Run the jconsole command directly to start, select local or remote connection.
• The “Memory” tag can see the memory changes in each area (Eden, Survivor, Old Gen), which is very helpful in judging GC frequency.
• The "Threads" page can view the current status of all threads and supports detection of deadlocks.

Analysis of common phenomena:

• If you find that Old Gen memory continues to grow, it cannot be recycled after GC, which is likely to be a memory leak.
• The number of threads is abnormally increasing. Combined with the thread state diagram, we can determine whether the thread pool is not released or thread blocking has occurred.

3. Combination of use: complementary advantages and more comprehensive investigation of problems

Although both are monitoring tools, the focus is slightly different. VisualVM is more inclined to performance analysis, while JConsole is more suitable for real-time monitoring and basic diagnosis.

You can use it in this way:

• First use JConsole to quickly observe whether there are obvious exceptions, such as frequent Full GC or thread blocking.
• Then use VisualVM for in-depth analysis, such as seeing which methods take up the most CPU time, or which objects have not been recycled.

Tips:

• Crawling heap dump is a good habit. In VisualVM, you can right-click to apply → Heap Dump. After exporting the hprof file, you can further analyze it using the MAT tool.
• If you do performance tuning frequently, it is recommended to organize commonly used JVM parameters into scripts to facilitate and quickly start applications with monitoring parameters.

Basically that's it. Mastering the basic operations of these two tools can already deal with most common Java debugging scenarios. You don't need to use the full process every time, but at critical moments, they can really save you a lot of trouble.

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