Guided: Profiling and Diagnosing Java SE Applications

Table of Contents

1. Challenge

   Introduction and Setup

   Welcome to the Guided Code Lab on Profiling and Diagnosing Java SE Applications!

   In this lab, you'll act as a performance engineer tasked with fixing a slow and memory-hungry Java application. You will use standard, powerful tools that come with the JDK to find and fix the root causes of these problems.

   The Scenario

   The DataProcessor application continuously loads and processes batches of customer data. However, it contains two common but critical bugs:

   1. A CPU bottleneck that makes the application unnecessarily slow.
   2. A memory leak that causes its memory footprint to grow indefinitely, leading to an eventual crash.

   Your Goal

   Your goal is to use profiling tools to understand the application's behavior, identify the faulty code, and implement fixes to make the application both fast and stable. You will be guided on how to use tools like Java VisualVM to analyze CPU, memory, and threads.

   info> This lab experience was developed by the Pluralsight team using Forge, an internally developed AI tool utilizing Gemini technology. All sections were verified by human experts for accuracy prior to publication. For issue reporting, please contact us.

2. Challenge

   Step 2: Baseline Profiling and Initial Analysis

   Before you can fix anything, you need to understand the problem. The first step is to compile and run the application to observe its current, problematic behavior. You will also add some basic logging to get a rough idea of what's happening with the application's memory.

   After compiling, you would typically run the application and connect a profiler like VisualVM. You'd notice two things right away:

   1. The CPU usage is very high.
   2. The memory usage, as reported by your logging, steadily increases and never goes down.
3. Challenge

   Step 3: Analyze and Fix the CPU Bottleneck

   Running a CPU profiler (like the Sampler in VisualVM) on the application would quickly reveal that a significant amount of time is spent in the DataProcessor.generateReport method. This is what's known as a 'hotspot'.

   Looking at the code, the cause is the use of the += operator to concatenate strings in a loop. In Java, strings are immutable. This means that each time you use +=, a new string object is created in memory, and the contents of the old and new parts are copied over. Doing this repeatedly in a loop is extremely inefficient.

   The standard solution is to use StringBuilder, which is a mutable sequence of characters designed specifically for this scenario.

4. Challenge

   Step 4: Diagnose and Fix the Memory Leak

   Now that the CPU is no longer maxed out, the memory issue becomes more prominent. If you were to watch the application's heap in VisualVM's Monitor tab, you'd see the 'Used heap' size grow in a sawtooth pattern, but the bottom of the 'teeth' would constantly rise. This indicates that objects are surviving garbage collection—a classic sign of a memory leak.

   A heap dump analysis would show that millions of com.example.Customer objects are being kept alive by a static list: CUSTOMER_CACHE. The application adds customers to this cache in every cycle but never removes them. The garbage collector cannot reclaim the memory from these objects because a static variable holds a permanent reference to them.

   Your task is to fix this by clearing the cache after the objects inside it are no longer needed.

5. Challenge

   Step 5: Investigate Thread Behavior

   The final area of diagnostics is thread analysis. Unresponsive applications are often caused by threads that are deadlocked or stuck waiting for a resource indefinitely. You can investigate this by taking a thread dump.

   A thread dump is a snapshot of the state of all threads in the JVM. Each thread will be in a specific state, such as:

   • RUNNABLE: The thread is actively executing code.
   • BLOCKED: The thread is waiting to acquire a lock.
   • WAITING / TIMED_WAITING: The thread is waiting for another thread or a specific amount of time.

   To practice this, you will add a method that simulates a blocking operation, which will cause the main thread to enter a TIMED_WAITING state. You could then use a tool like jcmd <pid> Thread.print or VisualVM's Threads tab to see this in action.

6. Challenge

   Conclusion

   Congratulations on completing the lab!

   You have successfully diagnosed and fixed major performance and stability issues in a Java application. You have learned how to:

   1. Capture CPU, memory, and thread metrics: You added logging for memory and learned where to look in profiling tools for key metrics.
   2. Diagnose memory leaks and performance bottlenecks: You used conceptual profiling data to identify a CPU hotspot (String concatenation) and a memory leak (a static collection that is never cleared).
   3. Apply profiling data to improve application health: You refactored the inefficient code and patched the leak, resulting in a fast and stable application.

   These skills are fundamental for any professional Java developer responsible for building and maintaining robust, high-performance systems.