Let's start with performance issue symptoms. This is an obvious place to start, as it's like consulting a doctor where symptoms are discussed and then a diagnosis is made. Application performance is the behavior experienced by the end users in terms of speed, accuracy in delivering the content, and average response times under the highest load. The load is referred to by the number of transactions processed by the application per unit time. The response times are the times required for an application to respond to a user's actions at such a load.

Whenever performance needs an improvement, the first thing that comes to mind is the problems that are affecting the performance of our application. To find the issues with performance, we need to look for certain symptoms that can lead us to the issue.

Some common symptoms that can be observed in a Spring application are as follows:

• Timeouts
• Running out of worker threads
• Threads waiting on class loaders
• A major amount of time spent on loading classes even under normal load
• Class loader attempts to load non-existing classes

In the following sections, we will understand these symptoms with an example context. The details will help us identify the symptom when it occurs.