Mímir

Requirements:

• Java 17+
• Maven 3.9+ (tested with 3.9.11 and 4.0.0-rc-5)

Used by Maven CI among others.

Goal: A system-wide cache for Maven. Will make you to love to nuke your local repository. It adds a layer of cache between resolver and transport. This implies it works irrelevant of location of your local repository and its kind (enhanced, split, whatever). You have one single "local" system-wide cache (by def in ~/.mimir/local) and it is consulted before Maven would go remote. Out of the box Mimir caches release artifacts from Maven Central, but it is not limited to it: you can tell Mimir which remote repositories to cache (see Which repositories are cached below). This local cache can be used by all Maven builds running on same workstation.

Another goal is to introduce "local cache sharing" across LAN, so make Mimir able to share caches across LAN from several workstations: basically if one workstation has the cache content, share it to neighbors. Daemon does this by default.

Concept:

• "wraps" the actual connector being used by Resolver, provides "extra cache layer" between connector and transport
• supports "local" (writeable) and "remote" (read only) Nodes, discovered on LAN or other means
• assumes getting content from Node (LAN neighbor) is faster than getting it from real remote (also saves outbound bandwidth)
• may later provide "interfacing" to real MRMs as well (ie a Node may go for real MRM)
• the node could offer locally and also remotely cached contents
• on local caching, "hard linking" should be used whenever possible (otherwise fallback to plain "copy") to avoid content duplication
• is irrelevant is project using it sits on "classic" or "split" or whatever local repository (as it is cache layer "above" local repository)
• RemoteRepository handling is configurable: you pick which repositories (by id, with optional filters) get cached; there are still TBDs here, especially around real MRM integration.

To use it

With Maven 3 create project-wide, or with Maven 4-rc-5+ create user-wide ~/.m2/extensions.xml like this:

<?xml version="1.0" encoding="UTF-8"?>
<extensions>
    <extension>
        <groupId>eu.maveniverse.maven.mimir</groupId>
        <artifactId>extension3</artifactId>
        <version>${version.mimir}</version>
    </extension>
</extensions>

IF you have docker, Tailscale (or just non-trivial networking setup), help Mimir components (like JGroups and publishers are) to select your LAN interface: create ~/.mimir/daemon.properties file with following content:

mimir.localHostHint=match-address\:192.168.1.*

(use yor LAN IP address).

To make Mimir stop you nagging to auto-update, set mimir.checkupdates in ~/.mimir/session.properties.

More on site documentation.

And just build with Maven...

Build requirements:

• Java 21
• Maven 3.9.9+

Which repositories are cached

By default Mimir caches release artifacts coming from Maven Central only. But you are not stuck with that: the mimir.session.repositories setting lets you list exactly which remote repositories Mimir should cache. Put it in ~/.mimir/session.properties (or pass it as -Dmimir.session.repositories=...):

mimir.session.repositories=central,foo,bar

The value is a comma-separated list of repository specs, and each spec is matched against the remote repositories your build actually uses. The matching predicates are OR-ed together, so an artifact is cached if it comes from any listed repository.

A spec can be:

• a repository id (e.g. foo) — matches the remote repository whose id is foo;
• the wildcard * — matches any release repository Mimir encounters;
• an id or * followed by modifiers in parentheses, comma-separated and without spaces, for example foo(releaseOnly,httpsOnly) or *(httpsOnly).

Available modifiers:

Modifier	Meaning
directOnly	Repository is used directly (not a mirrorOf target and not an MRM).
releaseOnly	Repository has only its release policy enabled (no snapshots).
httpsOnly	Repository is served over HTTPS.

A couple of things worth knowing:

• Mimir only ever caches release artifacts — snapshots are always ignored, because caching mutable content would be unsafe. So do not use Mimir if your workflow relies on mutable release artifacts.
• When you leave the setting unset, the effective default is central(directOnly,releaseOnly,httpsOnly) — i.e. Maven Central, accessed directly over HTTPS.
• If you list central as a plain id (as in the central,foo,bar example above), it no longer carries those strict modifiers. If you want to keep Central strict while adding more repositories, spell it out: central(directOnly,releaseOnly,httpsOnly),foo,bar.

Resolving Log

Mimir can log every artifact resolution, giving you a full record of what was resolved during a build — including groupId, artifactId, version, classifier, extension, the repository it came from, the canonical artifact URL, and whether the file was served from cache or downloaded fresh.

Enabling

Add -Dmimir.resolvingLog.globalPath=/some/path or -Dmimir.resolvingLog.projectPath=/some/path to your Maven invocation:

mvn verify -Dmimir.resolvingLog.projectPath=/some/path

Or set it permanently in ~/.mimir/session.properties:

mimir.resolvingLog.globalPath=/some/path

Output files

When enabled, the configured files are written out to configured paths. Global file accumulates across builds — each build appends to it. It persists until you delete it manually (log rotation is your responsibility if the file grows large).

The mimir.resolvingLog.globalPath provided path is resolved against Mimir basedir (default ~/.mimir).

To also write a per-project log that gets wiped by mvn clean, configure a project-relative path:

mimir.resolvingLog.projectPath=target/mimir-resolving-log.csv

The mimir.resolvingLog.projectPath is resolved against project basedir.

Both files are written simultaneously when both are configured.

To override the global file location:

mimir.resolvingLog.globalPath=/path/to/my-resolving-log.csv

Format

The default format is CSV with a header row:

seq,groupId,artifactId,version,classifier,extension,repositoryId,repositoryUrl,artifactUrl,status,context,scope
2026-05-16T14:46:29.899608696Z@1,org.slf4j,slf4j-api,2.0.17,,pom,central,https://repo.maven.apache.org/maven2,https://repo.maven.apache.org/maven2/org/slf4j/slf4j-api/2.0.17/slf4j-api-2.0.17.pom,cache,project,(model)
2026-05-16T14:46:29.899608696Z@2,org.slf4j,slf4j-parent,2.0.17,,pom,central,https://repo.maven.apache.org/maven2,https://repo.maven.apache.org/maven2/org/slf4j/slf4j-parent/2.0.17/slf4j-parent-2.0.17.pom,cache,project,(model)
2026-05-16T14:46:29.899608696Z@3,org.slf4j,slf4j-bom,2.0.17,,pom,central,https://repo.maven.apache.org/maven2,https://repo.maven.apache.org/maven2/org/slf4j/slf4j-bom/2.0.17/slf4j-bom-2.0.17.pom,cache,project,(model)
2026-05-16T14:46:29.899608696Z@4,org.slf4j,slf4j-api,2.0.17,,jar,central,https://repo.maven.apache.org/maven2,https://repo.maven.apache.org/maven2/org/slf4j/slf4j-api/2.0.17/slf4j-api-2.0.17.jar,cache,project/compile,compile

To use JSON Lines instead:

mimir.resolverLog.format=jsonl

{"seq":"2026-05-16T14:46:29.899608696Z@1","groupId":"com.google.guava","artifactId":"guava","version":"33.6.0-jre","classifier":"","extension":"jar","repositoryId":"central","repositoryUrl":"https://repo.maven.apache.org/maven2","artifactUrl":"https://repo.maven.apache.org/maven2/com/google/guava/guava/33.6.0-jre/guava-33.6.0-jre.jar","status":"cache"}

The seq field is a unique identifier for each resolution event, combining a timestamp with a sequence number to ensure uniqueness even for multiple resolutions occurring at the same time. The timestamp is created at the very first artifact resolution of a Maven session and counter is incremented for each resolution.

Status values

Value	Meaning
cache	Served from the Mimir local cache — no network request was made
remote	Downloaded from the remote repository and stored in cache
failed	Download was attempted but failed (network error, 404, etc.)

All artifact types are logged: .jar, .pom, .asc signatures, .war, .aar, etc. Internal existence-check probes are not logged.

Configuration reference

Property	Default	Description
mimir.resolvingLog.enabled	false	Enable resolving logging
mimir.resolvingLog.path	~/.mimir/mimir-resolving-log.csv	Path to the global log file
mimir.resolvingLog.projectPath	(unset)	Optional second log file (relative to execution root)
mimir.resolvingLog.format	csv	Output format: csv or jsonl