Today I have spent hours trying to get wrangle Maven into letting me set up a sane Java project. The hardest parts were enforcing warnings-as-errors, and getting Maven to shut up a bit.
Some code that warns
My first problem was writing some Java code that causes the compiler to emit warnings. For some reason I can’t understand, my Java 1.8 compiler was not emitting warnings (even with -Xlint:all) for unused variables, use of deprecated methods, or unknown @SuppressWarnings types (suggestions from SO 1752607).
Instead, I had to use an unnecessary cast:
$ cat src/tast/java/ExampleTest.java
public class ExampleTest {
public void warn() {
String fixmePlease = (String)"Hello";
}
}
Now, finally, I got a warning:
$ javac -Xlint:all src/test/ExampleTest.java
src/test/ExampleTest.java:3: warning: [cast] redundant cast to String
String s = (String) "Hello!";
^
1 warning
Maven compiler settings for warnings-as-errors
I tried a vast set of combinations of properties like maven.compiler.failOnWarning and maven.compiler.fork (as specified in the maven compiler plugin docs) before giving up on properties. Making a property called maven.compiler.failOnWarning seems to have no effect whatsoever, ever.
So I decided I must try the (very verbose) plugin tag containing a configuration tag, as also specified in the maven compiler plugin docs. After a lot of messing about with flags that seemed incompatible, and Maven silently ignoring things it didn’t understand, I came to a working config.
On the way, I discovered that setting the “fork” property to true is a non-starter, because Maven simply drops the compiler output in that case, meaning you can’t see what is going wrong when it does.
Finally, I had a pom file like this:
cat pom.xml
<?xml version="1.0"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.example</groupId>
<artifactId>Example</artifactId>
<packaging>jar</packaging>
<version>0.0.1</version>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.6.0</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
<failOnWarning>true</failOnWarning>
<showWarnings>true</showWarnings>
<compilerArgs>
<arg>-Xlint:all</arg>
</compilerArgs>
</configuration>
</plugin>
</plugins>
</build>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
</dependencies>
</project>
Which manages successfully to tell the compiler to show all warnings, and to fail when it sees one. (Thanks to SO 9192613, among others.)
I dare not change anything, for fear that it will stop working again without any explanation.
Quieting Maven
If you tell Maven to be quiet with -q or -e it will merrily fail the build because of a warning, but not tell you what the warning was.
Maven does not appear to have a command line option to set the log level to display warnings and errors only, but you can force it to do so by setting the environment variable MAVEN_OPTS like this:
MAVEN_OPTS=MAVEN_OPTS=-Dorg.slf4j.simpleLogger.defaultLogLevel=warn mvn clean test-compile
(Thanks to SO 4782089.)
And, with some guesswork (partly based on Configuring Maven) I found that if I put something similar in .mvn/jvm.config I didn’t have to type it every time:
$ cat .mvn/jvm.config
-Dorg.slf4j.simpleLogger.defaultLogLevel=warn
Failing on warnings, and seeing them!
I don’t know whether to feel triumphant or defeated, but, it works!
$ mvn clean test-compile
[WARNING] COMPILATION WARNING :
[WARNING] src/test/java/ExampleTest.java:[3,20] redundant cast to java.lang.String
[ERROR] COMPILATION ERROR :
[ERROR] src/test/java/ExampleTest.java: warnings found and -Werror specified
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.6.0:testCompile (default-testCompile) on project unmatcheddrrepair: Compilation failure
[ERROR] src/test/java/ExampleTest.java: warnings found and -Werror specified
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
I wish I could suppress double-reporting of the error, and the extra rubbish at the end (and the super-long absolute paths of each file that push the actual errors off the side of the screen), but at this point, I must try and do what I was trying to do in the first place.
Guess how much I am liking Maven at this point.
