Java Build Tools: AI-Optimized Technical Reference

Executive Summary

Problem: Java build tools suffer from critical performance and reliability issues that significantly impact development productivity.

Core Issues:

• Maven: 3-20 minute build times for single line changes, unreliable dependency resolution
• Gradle: Memory consumption (14GB for hello world), daemon crashes, cache corruption
• Mill: 10x faster builds but team adoption barriers due to Scala syntax
• Bazel: Requires dedicated build engineering teams (20+ people at Google/Uber scale)
• SBT: Slower than Maven for Java projects, complex configuration

Build Tool Performance & Resource Requirements

Maven

Performance:

• Build times: 3-20 minutes for incremental changes
• Memory usage: Single-threaded execution
• Failure rate: Random dependency resolution failures

Resource Requirements:

• Expertise: Low - XML configuration, copy-paste friendly
• Migration cost: N/A (baseline)
• Team training: Minimal

Critical Warnings:

• <scope>provided</scope> behaves differently in Docker containers
• Downloads entire internet repeatedly despite local cache
• Surefire plugin breaks with JUnit version mismatches
• Single-threaded execution wastes multi-core systems

Gradle

Performance:

• Build times: 2-8 minutes when daemon working
• Memory usage: 8-14GB RAM for simple projects
• Failure rate: Daemon crashes weekly, cache corruption

Resource Requirements:

• Expertise: Medium - Groovy/Kotlin DSL knowledge required
• Migration cost: 2-4 weeks for medium projects
• Team training: 1-2 weeks for basic competency

Critical Warnings:

• Build cache corrupts weekly requiring manual deletion
• Daemon dies randomly (especially Tuesdays - timing related to maintenance windows)
• Memory leaks cause GC overhead limit exceeded errors
• Plugin version conflicts create cryptic error messages

Mill

Performance:

• Build times: 30 seconds to 3 minutes (3-10x faster than Maven)
• Memory usage: Efficient multi-core utilization
• Failure rate: Watch mode stops working monthly

Resource Requirements:

• Expertise: High - Scala syntax knowledge required
• Migration cost: 4-8 weeks including CI/CD rewrites
• Team training: 2-4 weeks, junior developers may quit

Critical Warnings:

• Scala syntax creates team adoption barriers
• Error messages require Scala type system knowledge
• IntelliJ plugin inconsistent import behavior
• CI scripts require complete rewrite (different output directories)

Bazel

Performance:

• Build times: 3 seconds for incremental builds when properly configured
• Memory usage: Efficient with remote caching
• Failure rate: Low when maintained by dedicated team

Resource Requirements:

• Expertise: Very High - requires dedicated build engineers
• Migration cost: 2+ years for large codebases
• Team training: Months, requires specialized knowledge

Critical Warnings:

• Requires 20+ dedicated build engineers (Uber/Google scale)
• BUILD files needed for every directory
• Error messages incomprehensible without distributed systems knowledge
• Migration only justified for 1000+ module monorepos

SBT

Performance:

• Build times: Slower than Maven for Java projects
• Memory usage: Loads Scala compiler machinery unnecessarily
• Failure rate: High classpath resolution issues

Resource Requirements:

• Expertise: High - Scala ecosystem knowledge required
• Migration cost: 3+ months of debugging
• Team training: Significant Scala learning curve

Critical Warnings:

• Assembly plugin packages JARs differently than Maven
• Binary compatibility hell with Scala libraries
• Command syntax differs from standard Java tooling
• Designed for Scala, not optimized for Java-only projects

Migration Decision Framework

When to Migrate

Definitely migrate if:

• Build times >20 minutes consistently
• Spending more time waiting than coding
• Maven downloading internet daily breaks development flow

Consider migration if:

• Build times 10-20 minutes
• Team has strong Scala/functional programming background (for Mill)
• Have dedicated build engineering resources (for Bazel)

Don't migrate if:

• Build times <10 minutes
• Team lacks time for 4-8 week migration
• DevOps team already stressed
• No tolerance for months of broken CI/CD

Migration Cost Analysis

Tool	Migration Time	Team Training	Ongoing Maintenance	ROI Threshold
Mill	4-8 weeks	2-4 weeks	Low (when working)	>15 min builds
Gradle	2-4 weeks	1-2 weeks	Medium (daemon issues)	>8 min builds
Bazel	2+ years	3+ months	High (dedicated team)	>1000 modules
SBT	3+ months	2+ months	High (Scala expertise)	Never for Java

Critical Failure Modes & Solutions

Maven Failures

"Could not resolve dependencies":

• Root cause: Maven Central connectivity or local cache corruption
• Solution: Delete .m2/repository, retry build
• Frequency: Weekly

"GC overhead limit exceeded":

• Root cause: Insufficient heap space for large dependency trees
• Solution: export MAVEN_OPTS="-Xmx4g"
• Prevention: Regular cache cleanup

Docker build differences:

• Root cause: <scope>provided</scope> resolution differences
• Solution: Explicit dependency scoping in Docker profiles
• Impact: Production deployment failures

Gradle Failures

Daemon crashes:

• Root cause: Memory leaks in long-running daemon process
• Solution: ./gradlew --stop && rm -rf ~/.gradle/caches
• Frequency: 2-3 times per week
• Prevention: Add --no-daemon flag for CI builds

Build cache corruption:

• Root cause: Concurrent cache access or interrupted builds
• Solution: Delete cache directory, restart daemon
• Impact: Forces full rebuilds, lost productivity

Memory consumption:

• Root cause: Global gradle.properties with excessive heap settings
• Solution: Project-specific memory tuning
• Monitoring: Check for org.gradle.jvmargs=-Xmx8g in global config

Mill Failures

Watch mode stops working:

• Root cause: File system event handling issues
• Solution: Restart Mill process
• Frequency: Monthly
• Workaround: Manual rebuild commands

IntelliJ import issues:

• Root cause: Plugin incompatibility with Mill versions
• Solution: Delete .idea directory, reimport project
• Impact: Lost IDE configuration

Scala compilation errors:

• Root cause: Type system mismatches in build configuration
• Solution: Requires Scala knowledge for debugging
• Team impact: Blocks junior developers

Bazel Failures

Target build failures:

• Root cause: Complex dependency graph resolution
• Solution: Use Bazel query language for debugging
• Expertise required: Distributed systems knowledge

BUILD file maintenance:

• Root cause: Manual dependency tracking across large codebase
• Solution: Dedicated build engineering team
• Cost: 20+ full-time engineers at scale

Production Configuration Recommendations

Maven Production Settings

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-surefire-plugin</artifactId>
  <version>3.0.0-M9</version> <!-- Avoid 5.10.1 compatibility issues -->
</plugin>

Critical JVM settings:

export MAVEN_OPTS="-Xmx4g -XX:+UseG1GC"

Gradle Production Settings

gradle.properties:

org.gradle.jvmargs=-Xmx2g
org.gradle.daemon=false  # For CI environments
org.gradle.parallel=true
org.gradle.caching=true

Critical warnings:

• Never set global heap >4GB without monitoring
• Disable daemon in CI to prevent memory leaks
• Monitor cache directory size (can grow to 10GB+)

Mill Production Settings

build.sc:

def ivyDeps = Agg(
  ivy"com.fasterxml.jackson.core:jackson-core:2.15.2"
)

Team onboarding requirements:

• Scala syntax training (2 weeks minimum)
• Functional programming concepts
• Mill-specific configuration patterns

Security & Compliance Considerations

Maven Security Issues

• Maven 3.8.1+ blocks HTTP repositories (breaks legacy Nexus configs)
• Solution: Migrate internal repositories to HTTPS
• Impact: 1-week IT infrastructure update required

Gradle Security Issues

• Build cache can contain sensitive information
• Solution: Configure cache exclusions for sensitive modules
• Monitoring: Regular cache directory audits

Mill Security Issues

• Scala code execution in build files
• Solution: Code review all build.sc changes
• Risk: Build-time code injection if not properly reviewed

Team Impact Assessment

Developer Productivity Impact

Positive impacts:

• Mill: 10x faster builds = more coding time
• Gradle: Better IDE integration when working
• Bazel: Near-instant incremental builds at scale

Negative impacts:

• Mill: 30% of junior developers quit during migration
• Gradle: 20% productivity loss debugging daemon issues
• Bazel: 6-month learning curve for basic competency

DevOps Impact

Migration costs:

• Complete CI/CD pipeline rewrite (4-8 weeks)
• Docker image updates (1-2 weeks)
• Monitoring/alerting reconfiguration (1 week)
• Security scanning tool updates (2 weeks)

Ongoing operational burden:

• Mill: Low maintenance once stable
• Gradle: Weekly daemon management
• Bazel: Requires dedicated build platform team

Resource Links by Category

Immediate Problem Solving

• Maven failures: stackoverflow.com/questions/tagged/maven+build-failure
• Gradle daemon issues: stackoverflow.com/questions/tagged/gradle+daemon
• Mill build errors: stackoverflow.com/questions/tagged/mill-build-tool

Migration Planning

• Mill Java tutorial: mill-build.org/mill/javalib/intro.html
• Uber Bazel migration: uber.com/blog/go-monorepo-bazel/ (2-year timeline)
• Performance comparisons: gradle.org/gradle-and-maven-performance/

Team Training

• Mill getting started: mill-build.org/mill/index.html
• Bazel Java tutorial: bazel.build/start/java
• SBT documentation: scala-sbt.org/1.x/docs/Getting-Started.html

Emergency Support

• Mill creator direct access: gitter.im/lihaoyi/mill
• Bazel community: slack.bazel.build/
• General build issues: devops.stackexchange.com/questions/tagged/build-tools

Decision Trees

Build Tool Selection

1. Build time <10 minutes: Stay with Maven
2. Build time 10-20 minutes: Consider Gradle if team has capacity
3. Build time >20 minutes: Evaluate Mill if team can handle Scala syntax
4. >1000 modules: Consider Bazel if you can hire dedicated team
5. Java-only project: Never choose SBT

Migration Go/No-Go Criteria

Go if ALL true:

• Build times consistently >15 minutes
• Team has 4-8 weeks for migration
• DevOps team has bandwidth for CI/CD rewrites
• Management tolerates 2-3 months of reduced productivity

No-go if ANY true:

• Build times <10 minutes
• Team lacks Scala knowledge (for Mill)
• Critical project deadlines within 3 months
• DevOps team already overloaded

This technical reference provides structured decision-making data for automated build tool selection and migration planning based on real-world operational experience and quantified impacts.