使用.PHONY规则触发“nothing to be done for”错误的技术咨询

Let's break down why your current setup is failing, then fix it with a clean, reliable Makefile that behaves exactly as you expect.

What's Going Wrong? #

Your issues stem from two core mistakes:

1. You're targeting actual test files: When you use tests/%.java as a target, Make sees these as existing, unmodified files. It assumes they're already "up-to-date" and skips running the test rule, hence the "Nothing to be done" message.
2. Static wildcard expansion: The $(wildcard tests/*.java) variable expands once when Make first loads the file. New test files added later won't be picked up unless you force Make to re-parse the file (like running make clean).

Fixed Makefile #

Here's the corrected version that resolves both problems:

PROG = Test
# Generate virtual test targets (not actual files) from test Java files
TEST_TARGETS = $(patsubst tests/%.java, tests/%.test, $(wildcard tests/*.java))
.SUFFIXES: .java .class

JCC = javac
JAR = jar

all: $(PROG).jar

# Mark test targets as PHONY to ensure they always run
.PHONY: test clean $(TEST_TARGETS)

test: $(PROG).jar $(TEST_TARGETS)

# Rule for virtual test targets - runs the corresponding Java test file
tests/%.test:
	@echo "#### testing tests/$*.java ####"
	@java -jar $(PROG).jar tests/$*.java
	@echo "> Passed"

$(PROG).jar: Main.class
	$(JAR) cmf $(PROG)MainClass.txt $(PROG).jar $^

Main.class: Main.java
	$(JCC) $^

clean:
	rm -f *.class $(PROG).jar

Key Fixes Explained #

1. Virtual Test Targets:
   We use $(patsubst) to convert each tests/%.java file into a non-existent tests/%.test target. Since these targets don't correspond to real files, Make will always try to run their rules, no matter the state of your test files.

2. Proper PHONY Usage:
   Only mark test and the virtual $(TEST_TARGETS) as PHONY. Marking actual test files as PHONY breaks Make's dependency checking logic, which was part of your original problem.

3. Dynamic Test Detection:
   If you add new test files and want Make to pick them up without make clean, run:

   make test -B
   

   The -B flag (short for --always-make) forces Make to re-parse the wildcard variable and run all tests, even if it thinks things are up-to-date.

How to Verify #

• Run make test: It will execute all tests in the tests/ directory every time, no "Nothing to be done" messages.
• Add a new .java file to tests/, then run make test -B: The new test will be picked up and executed automatically.

内容的提问来源于stack exchange，提问作者collin.clark