why is bazel faster than gradle

Question

originally, I use gradle to build my android project, but recently, I migrate it to bazel, and I find that bazel is truly fast than gradle, so I want to know why, but the doc of bazel doesn't give much idea about this, can anyone help me？

Thanks very much!

Answer 1

Full disclosure: I work on Bazel.

That's not an easy question to answer for two reasons. First, performance is highly dependent on the scenario. For example, we'd generally expect a clean build to be slower than a build where only a single file has changed. Second, I don't know how Gradle works internally, and they've done a lot of work recently to improve Gradle performance.

But I can talk about Bazel and what we're doing to make it fast. We've been working on build performance for ~10 years, starting long before we made it public.

The key feature is that we require all dependencies to be declared, and we track them explicitly. If you use a header file in C++, or depend on a Java library, you must declare this dependency in your BUILD file (and we enforce that these are declared by sandboxing individual actions). There are three effects from this:

First, we can heavily parallelize the build, because we know which things depend on which other things.

Second, we can make incremental builds very fast, because we can tell what parts of the build have to be re-done when you change a specific file (BUILD file, header file, source file, ...).

Third, we almost never have to do clean builds. Other build tools often require 'make clean' to get into a predictable state - since Bazel knows all the dependencies, it can get to a predictable state on every single build.

Another effect is that we can cache remotely (i.e., across users), and even execute on another machine, although neither of these are fully supported at the time of this writing.