To begin the debugging session, you will need the runtime/debug configuration defined for the project. We described this process in detail in Chapter 5, Make It Happen – Running Your Project. This time, though, instead of running the defined configuration, use the Shift + F9 (PC) or Ctrl + D (Mac) keyboard shortcut; or click on the Debug icon on the toolbar, as shown here:

When you start the debug configuration, two things will happen. First, IntelliJ IDEA will analyze the breakpoints you set up and, if they are valid, will mark them with the valid breakpoint icon, as shown here:

Otherwise, if the breakpoint is invalid, the IDE will mark it with the invalid breakpoint icon and thus ignore it. The breakpoint can be invalid because it is placed on the line with the comment or in a block with an opening or closing brace, for example, as shown here:

Hover over the invalid breakpoint to see the reason why IntelliJ IDEA decided not to take it into account: