Code coverage is a measurement of how many lines/blocks/arcs of your code are executed while the automated tests are running. Code coverage is collected by using a specialized tool to instrument the binaries to add tracing calls and run a full set of automated tests against the instrumented product.

Jacoco
Code Coverage
Jacoco

Java code coverage tools are of two types: first, tools that add statements to the Java source code and require its recompilation. Second, tools that instrument the bytecode, either before or during execution.