Difference between mvn test-compile and mvn compile

The mvn test-compile and mvn compile commands in Maven serve different purposes in the build lifecycle, focusing on compiling different sets of source code. Here’s a detailed comparison of the two:

mvn compile

• Purpose: This command compiles the main source code of your project.
• Scope: It processes the source code located in the src/main/java directory and generates the compiled classes in the target/classes directory.
• Usage: Run this command when you want to compile the main application code.
• Typical Command:

  mvn compile
  

mvn test-compile

• Purpose: This command compiles the test source code of your project.
• Scope: It processes the source code located in the src/test/java directory and generates the compiled classes in the target/test-classes directory. This command implicitly includes the compile phase, so it compiles both the main and the test source code.
• Usage: Run this command when you want to compile the test code in preparation for running tests.
• Typical Command:

  mvn test-compile
  

Detailed Comparison

Aspect	mvn compile	mvn test-compile
Primary Focus	Compiles main source code	Compiles test source code
Source Directory	src/main/java	src/test/java
Output Directory	target/classes	target/test-classes
Lifecycle Phases	Executes up to the compile phase	Executes up to the test-compile phase (includes compile)
Dependencies	Main dependencies	Main and test dependencies
When to Use	When you need to compile the application code	When you need to compile test code (and application code if not already compiled)

Maven Build Lifecycle

Understanding the Maven build lifecycle can clarify why these commands exist:

• Default Lifecycle Phases:
  1. validate: Validate the project is correct and all necessary information is available.
  2. initialize: Initialize build state, such as setting properties.
  3. generate-sources: Generate any source code for inclusion in compilation.
  4. process-sources: Process the source code, for example, filtering any values.
  5. generate-resources: Generate resources for inclusion in the package.
  6. process-resources: Copy and process the resources into the destination directory, ready for packaging.
  7. compile: Compile the source code of the project.
  8. process-classes: Post-process the generated files from compilation, for example, to do bytecode enhancement on Java classes.
  9. generate-test-sources: Generate any test source code for inclusion in compilation.
  10. process-test-sources: Process the test source code.
  11. generate-test-resources: Create resources for testing.
  12. process-test-resources: Copy and process the test resources into the test destination directory.
  13. test-compile: Compile the test source code into the test destination directory.
  14. process-test-classes: Post-process the generated test files from compilation.
  15. test: Run tests using a suitable unit testing framework. These tests should not require the code to be packaged or deployed.
  16. prepare-package: Perform any operations necessary to prepare a package before the actual packaging occurs. This may involve performing tests on the compiled and processed code.
  17. package: Take the compiled code and package it in its distributable format, such as a JAR.
  18. verify: Run any checks to verify the package is valid and meets quality criteria.
  19. install: Install the package into the local repository, for use as a dependency in other projects locally.
  20. deploy: Copy the final package to the remote repository for sharing with other developers and projects.

Practical Implications

• When Running Tests: Use mvn test-compile to ensure both main and test code are compiled. This is typically followed by mvn test to execute the tests.
• When Building the Project: Use mvn compile to compile only the main code if you are not running tests immediately.