Smartphone vendors are using multiple methods to kill processes of Android apps to reduce the battery consumption. This motivates developers to find ways to extend the liveness time of their apps, hence the name diehard apps in this paper. Although there are blogs and articles illustrating methods to achieve this purpose, there is no systematic research about them. What's more important, little is known about the prevalence of diehard apps in the wild. In this paper, we take a first step to systematically investigate diehard apps by answering the following research questions. First, why and how can they circumvent the resource-saving mechanisms of Android? Second, how prevalent are they in the wild? In particular, we conduct a semi -automated analysis to illustrate insights why existing methods to kill app processes could be evaded, and then systematically present 12 diehard methods. After that, we develop a system named DiehardDetector to detect diehard apps in a large scale. The experimental result of applying DiehardDetector to more than 80k Android apps downloaded from Google Play showed that around 21 % of apps adopt various diehard methods. Moreover, our system can achieve high precision and recall.