Build performance & isolation

Two engine capabilities that need no change to your project: isolation puts the whole build - or just the program it launches - inside a throwaway container, so untrusted code cannot reach your host; and the build cache hands a step the output of an earlier build instead of re-running it. Both switch on from the command line, so any project gains them for free.

Why isolate a build

A build runs untrusted code even when you customise nothing. The stock pipeline compiles and runs your tests - and everything the test dependencies drag in - and the artifact it produces has a main that runs later, all with the full rights of whoever started the build. A single compromised dependency (a malicious release, a hijacked account, a typo-squatted coordinate) executes with those same rights.

Pinning guarantees you get the exact bytes you vetted rather than a silently swapped artifact - but it guarantees what runs, not that what runs is safe. Docker addresses the other half: it confines what that code can reach when it executes, so even a malicious dependency cannot read your host secrets or write outside the sandbox.

Running the build in a container

Set -Djenesis.project.docker=true to run the entire build inside a throwaway container instead of directly on the host JVM:

java -Djenesis.project.docker=true build/jenesis/Project.java

A minimal image is built on demand the first time and cached for later runs. Inside the container neither your home directory nor the host environment is present, so a test or dependency that reaches for ~/.aws/credentials or a CI secret finds nothing.

To target a different image, add -Djenesis.project.docker.image=<reference>. The implicit image runs with --cap-drop ALL and --security-opt no-new-privileges; a named image is run as you named it, without those two flags, so harden it in the image itself if you swap it.

What is mounted automatically

Every location the project is configured with is represented inside the container, almost all of them at their host path so that paths resolve identically:

• the project root - writable;
• the JDK - read-only, at /opt/java-home, so the same Java runs inside;
• the local Maven and module repositories (~/.m2, ~/.jenesis) - read-only, with MAVEN_REPOSITORY_LOCAL / JENESIS_REPOSITORY_LOCAL forwarded so the in-container JVM finds them despite its different home;
• out-of-root target / artifacts locations - writable;
• out-of-root configuration, BOM, and jenesis.project.metadata folders - read-only;
• an out-of-root jenesis.project.cache or file:// jenesis.cache.uri cache - writable, created on the host first so it is not left root-owned.

Anything else the build needs from outside the root is invisible inside the container.

Adding mounts and environment

A build/jenesis symlinked to a shared engine checkout, a sibling source tree, or a generated-sources directory lives outside the root and so is not present. Add such paths with -Djenesis.project.docker.mount=<host>[:<container>],...:

• a bare host is mounted at the same path inside the container (host:host) - what a symlink or absolute reference needs to resolve; host:container remaps it instead;
• these mounts are read-only - the build should not write outside its own tree;
• relative host paths resolve against the project root, and several mounts are comma-separated.

For the rare case that the build must write to a host path outside the project root, use -Djenesis.project.docker.mountWritable=<host>[:<container>],.... Reach for it sparingly: every writable mount is a hole in the confinement.

By default no host environment is forwarded into the container. Pass selected variables with -Djenesis.project.docker.env=<name>[=<value>],...: a bare name forwards the host's current value, while name=value sets it explicitly. This is the channel for a build input that legitimately lives in the environment - a private-repository token, a proxy setting - and is opt-in so ambient host secrets do not leak in by default.

Running the launched program in a container

Isolating the build does not isolate the program it produces, whose main runs later with the same host rights. Execute.java (see Building & running) can launch that program inside a container too, independently of whether the build itself was dockerised:

java -Djenesis.execute.docker=true build/jenesis/Execute.java

The container does not receive the host environment and its home is not the host's, so the artifact runs but the secrets are out of reach. -Djenesis.execute.docker.image=<reference> overrides the image, and -Djenesis.execute.docker.mount (read-only), -Djenesis.execute.docker.mountWritable (read-write), and -Djenesis.execute.docker.env=<name>[=<value>],... behave exactly like their jenesis.project.docker.* counterparts. Because the build runs as usual and only the launch crosses the container boundary, the build image and the runtime image can differ.

The build cache

Every build already has an incremental cache: Jenesis content-hashes each step's inputs and outputs under target/, so a warm rebuild only re-runs the steps whose inputs changed (see Core concepts). The build cache adds a second tier outside target/ that can hand a step the output of an earlier build - a different checkout, machine, or CI job - instead of re-running it at all. It lives in two places that compose: a project-local folder and a shared location you name.

A project-local cache

The simplest form needs only a flag. Jenesis keeps a content-addressed cache under .jenesis/cache, rooted at the project root:

java -Djenesis.project.cache build/jenesis/Project.java

The value is a filesystem path (never a URI): an empty value, as above, resolves to .jenesis/cache under the project root, and a value relocates it. Each entry lives at .jenesis/cache/<step-hash>/<inputs-hash>/, where the step hash identifies the step by its serialised form and the inputs hash folds every input file's content hash. On a miss the build runs the step and stores the result. On a hit it materialises the cached output - hard-linked, so near free - and the step body never runs. Because it sits outside target/, it survives a target/ wipe.

That survival is the point. -Djenesis.executor.rebuild=true deletes target/ first, so the incremental cache is gone and every step is a forced miss that would normally re-run from scratch. The build cache serves them anyway:

java -Djenesis.project.cache \
     -Djenesis.executor.rebuild=true \
     build/jenesis/Project.java

The steps still print [EXECUTED] - their output was produced - but it came from the cache, not from javac, so each returns almost instantly. On a real module a compile that took seconds returns at once. Add -Djenesis.print.cache to make it explicit: each step served from the cache prints a [LOADED] line and each written to it a [STORED] line. Delete .jenesis/cache to start over.

A shared cache

.jenesis/cache is private to one checkout. To share results across checkouts, machines, or CI, name an explicit location with -Djenesis.cache.uri=. The value is a URI:

-Djenesis.cache.uri=https://cache.example.com       # a cache server
-Djenesis.cache.uri=file:///mnt/team/jenesis-cache  # a shared (or local) folder

A file:// URI resolves the same on-disk format as the local cache. An http(s):// URL selects an HTTP backend that GETs and PUTs the same entries to a cache server, naming the project with -Djenesis.cache.project=<project> and authenticating with -Djenesis.cache.key=<key>. Both are sent as headers, never in the URL, and both fall back to the JENESIS_CACHE_PROJECT / JENESIS_CACHE_KEY environment variables. -Djenesis.cache.connect and -Djenesis.cache.read set the HTTP timeouts as ISO-8601 durations (PT1S and PT10S by default). A non-URI value is rejected; use file:// for an on-disk location.

The shared cache can be used two ways:

• As a replacement - the shared cache only, no local tier. Fitting for an ephemeral CI runner whose disk is thrown away anyway: pass jenesis.cache.uri alone.
• Layered behind the local cache - set both -Djenesis.project.cache and -Djenesis.cache.uri=.... Every read tries .jenesis/cache first and falls through to the shared cache only on a miss. A shared hit is copied into the local cache on the way past, so the next read is local, and a store writes through to both.

java -Djenesis.project.cache \
     -Djenesis.cache.uri=https://cache.example.com \
     -Djenesis.cache.project=acme -Djenesis.cache.key=alice \
     build/jenesis/Project.java

Tuning with cache.properties

Drop an optional cache.properties at the cache root - .jenesis/cache/cache.properties for the project-local cache, or <folder>/cache.properties for a file-system shared one - to tune the writes. Every key has a default, so the file may be omitted entirely:

Eviction runs on write and goes by file timestamp, which touch keeps fresh on every read - so the three caps (steps, versions, size) approximate an LRU, and ttl adds an age dimension on top of them.