概念概述
分支覆盖率（Branch Coverage）指的是：分支覆盖率衡量每个判定结果是否被执行，能发现语句覆盖遗漏的 else 与短路路径，是判断逻辑测试充分性的指标。
从更严格的定义看，Branch coverage is a code coverage criterion that counts whether each control-flow branch, such as both sides of an if condition, has been executed by the test suite. 该概念在工程实践中的价值，在于它能帮助团队用统一的语言描述一个具体的质量属性或设计约束，从而在评审与协作中减少歧义。
为什么需要它
It is used to judge how much decision logic tests exercise, especially around validation and error handling. 它与 Stress Testing Beyond Design Limits、Load Testing to Target Concurrency、Approval Testing 等概念经常一起出现，用于解决相近的工程问题。理解它的适用场景，有助于在多种备选方案之间做出有依据的取舍，而不是凭直觉选择技术方案。
常见误解
- branch coverage is identical to line coverage
- high branch coverage proves the assertions are meaningful
需要避免的反模式
- chasing branch coverage with tests that assert nothing
- treating low coverage as automatically acceptable for legacy modules
实践要点
- A single statement can contain multiple branches, so a line can be executed while one of its decision outcomes remains untested, which is exactly what branch coverage reveals.
- Short-circuit operators and ternary expressions create implicit branches that are easy to miss when writing tests.
- Coverage counts execution only; it says nothing about whether the assertions would detect a regression, so pair it with mutation testing for real confidence.