At ordinary times, our program can be installed and run on the simulator, because in the process of developing applications, ADT will be automatically signed with the default key and certificate because it is compiled in Debug interview, while when compiled in release mode, apk files will not be automatically signed, so manual signing is required. Signing apk can bring the following benefits: 1. Application upgrade: If you want users to seamlessly upgrade to the new version, you must sign with the same certificate. This is because if the upgraded application is signed with the same certificate, only the upgraded application will be allowed to be installed. If you use a different certificate, your application will be required to use a different package name, which is equivalent to installing a brand new application. If you want to upgrade the application, the signing certificate should be the same and the package name should be the same! 2. Modularization of applications: Android system can allow multiple applications signed by the same certificate to run in one process, and the system actually treats them as one application. At this point, our application can be deployed in a modular way, and users can upgrade one of the modules independently. 3. Both code and data can be enjoyed. Android provides a signature-based permission mechanism, so one application can be used by another application signed by the same certificate. By signing multiple applications with the same certificate, code and data can be safely shared between applications by using signature-based permission check. 4. If different applications want to share data or code, they should run in the same process and sign with the same certificate.