JaCoCo - Coverage Counter

# Coverage Counters

JaCoCo uses a set of different counters to calculate coverage metrics. All these counters are derived from information contained in Java class files which basically are Java byte code instructions and debug information optionally embedded in class files. This approach allows efficient on-the-fly instrumentation and analysis of applications even when no source code is available. In most cases the collected information can be mapped back to source code and visualized down to line level granularity. Anyhow there are limitations to this approach. The class files have to be compiled with debug information to calculate line level coverage and provide source highlighting. Not all Java language constructs can be directly compiled to corresponding byte code. In such cases the Java compiler creates so called synthetic code which sometimes results in unexpected code coverage results.

## Instructions (C0 Coverage)

The smallest unit JaCoCo counts are single Java byte code instructions. Instruction coverage provides information about the amount of code that has been executed or missed. This metric is completely independent from source formatting and always available, even in absence of debug information in the class files.

## Branches (C1 Coverage)

JaCoCo also calculates branch coverage for all `if` and `switch` statements. This metric counts the total number of such branches in a method and determines the number of executed or missed branches. Branch coverage is always available, even in absence of debug information in the class files. Note that exception handling is not considered as branches in the context of this counter definition.

If the class files haven been compiled with debug information decision points can be mapped to source lines and highlighted accordingly:

- No coverage: No branches in the line has been executed (red diamond)
- Partial coverage: Only a part of the branches in the line have been executed (yellow diamond)
- Full coverage: All branches in the line have been executed (green diamond)

## Cyclomatic Complexity

JaCoCo also calculates cyclomatic complexity for each non-abstract method and summarizes complexity for classes, packages and groups. According to its definition by McCabe1996 cyclomatic complexity is the minimum number of paths that can, in (linear) combination, generate all possible paths through a method. Thus the complexity value can serve as an indication for the number of unit test cases to fully cover a certain piece of software. Complexity figures can always be calculated, even in absence of debug information in the class files.

The formal definition of the cyclomatic complexity v(G) is based on the representation of a method's control flow graph as a directed graph:

> v(G) = E - N + 2

Where E is the number of edges and N the number of nodes. JaCoCo calculates cyclomatic complexity of a method with the following equivalent equation based on the number of branches (B) and the number of decision points (D):

> v(G) = B - D + 1

Based on the coverage status of each branch JaCoCo also calculates covered and missed complexity for each method. Missed complexity again is an indication for the number of test cases missing to fully cover a module. Note that as JaCoCo does not consider exception handling as branches try/catch blocks will also not increase complexity.

## Lines

For all class files that have been compiled with debug information, coverage information for individual lines can be calculated. A source line is considered executed when at least one instruction that is assigned to this line has been executed.

Due to the fact that a single line typically compiles to multiple byte code instructions the source code highlighting shows three different status for each line containing source code:

- No coverage: No instruction in the line has been executed (red background)
- Partial coverage: Only a part of the instruction in the line have been executed (yellow background)
- Full coverage: All instructions in the line have been executed (green background)

Depending on source formatting a single line of a source code may refer to multiple methods or multiple classes. Therefore the line count of methods cannot be simply added to obtain the total number for the containing class. The same holds true for the lines of multiple classes within a single source file. JaCoCo calculates line coverage for classes and source file based on the actual source lines covered.

## Methods

Each non-abstract method contains at least one instruction. A method is considered as executed when at least one instruction has been executed. As JaCoCo works on byte code level also constructors and static initializers are counted as methods. Some of these methods may not have a direct correspondence in Java source code, like implicit and thus generated default constructors or initializers for constants.

## Classes

A class is considered as executed when at least one of its methods has been executed. Note that JaCoCo considers constructors as well as static initializers as methods. As Java interface types may contain static initializers such interfaces are also considered as executable classes.