App Engine offers you a choice between two environments for Java applications:
standard environment and flexible environment. Both environments have the same
code-centric developer workflow, scale quickly and efficiently to handle
increasing demand, and enable you to use Google’s proven serving technology to
build your web, mobile, and IoT applications quickly and with minimal
operational overhead. While the available environments have a lot in common,
they differ in a few important ways.
Choose your preferred environment
Optimized to scale nearly instantaneously to handle huge traffic spikes.
Most cost-effective for applications that have significant periods where they
are not serving traffic.
OpenJDK based Java runtime supports the standard public Java library.
Doesn't impose a security manager--your code won't be restricted by Java