Reducing CI Build Times with the Gradle Build Cache

The Cost of the 'Clean' Build

In many CI/CD pipelines, the default behavior is to start with a fresh environment and run ./gradlew clean build. While this ensures a pristine state, it forces Gradle to re-execute every single task—compilation, testing, and linting—even if only a single line of a README file changed. As projects grow, these builds transition from a few minutes to an hour, creating a bottleneck that slows down the entire development cycle.

The solution is the Gradle Build Cache. Unlike the standard 'up-to-date' check, which only works if the local build folder is preserved, the Build Cache allows Gradle to reuse task outputs from any previous build, regardless of whether the local directory was deleted or if the build is running on a completely different agent in a cloud cluster.

How the Build Cache Differs from Up-to-Date Checks

It is common to confuse Up-to-Date checks with the Build Cache, but they operate on different levels of persistence.

• Up-to-Date Checks: Gradle looks at the build folder on the current machine. If the inputs haven't changed since the last run, it skips the task. This is lost the moment you run clean.
• Build Cache: Gradle creates a unique fingerprint (a hash) of all task inputs—source files, compiler versions, and properties. It stores the resulting output in a cache directory. If a future build produces the same fingerprint, Gradle pulls the output from the cache, even if the local build folder is empty.

Implementing a Shared Remote Cache

A local cache helps a single developer, but a Remote Build Cache helps the whole team. When a CI server completes a build, it pushes the results to a central HTTP cache. When a developer pulls the latest code, their local Gradle instance downloads the pre-compiled classes from the server instead of compiling them locally.

Configuration Example

To enable the build cache, add the following to your gradle.properties file (located in the project root or ~/.gradle/gradle.properties):

org.gradle.caching=true

To connect to a remote cache server (such as a Gradle Enterprise instance or a custom HTTP cache), configure your settings.gradle (or settings.gradle.kts) file. This configuration should be run with permissions to access the network and the specific cache URL.

buildCache {
    local {
        enabled = true
    }
    remote("http-cache") {
        url = "https://gradle-cache.yourcompany.com/cache"
        // Use credentials for push access in CI, read-only for developers
        credentials { 
            username = System.getenv("GRADLE_CACHE_USER")
            password = System.getenv("GRADLE_CACHE_PASSWORD")
        }
        push = true // Set to false for local dev machines to prevent cache pollution
    }
}

Avoiding Cache Misses

The build cache is only effective if the task fingerprints remain stable. A common failure point is non-determinism. If a task includes a timestamp in a generated file or uses absolute file paths (e.g., /Users/username/project/src) instead of relative paths, the fingerprint will change every time the build runs on a different machine.

To diagnose why a task is not being cached, you can use a Build Scan. This tool provides a detailed comparison of two builds, highlighting exactly which input property changed—such as a different JDK version or a modified environment variable—causing the cache miss.

Trade-offs and Limitations

The build cache is not a silver bullet. There are two primary risks to consider:

• Network Latency: If your remote cache is hosted in a different region than your CI runners, the time spent downloading a large artifact may exceed the time it would take to simply re-compile it locally.
• Cache Pollution: If a non-deterministic task is allowed to push to the remote cache, it can overwrite valid entries with unstable ones, forcing other team members to re-run tasks.

Verifying the Result

To verify that the cache is working, run your build with the --build-cache flag (if not already enabled in properties):

./gradlew assemble --build-cache

Look for the FROM-CACHE label next to the tasks in the console output. If you see UP-TO-DATE, it means the local folder was preserved. If you see FROM-CACHE, Gradle successfully retrieved the output from the cache storage. To test the remote aspect, run a build on a clean agent and verify that tasks are marked FROM-CACHE without having run them locally first.