Hi all,
thank you for your helpful answers. I grasped a lot about how Maven
internally works and learned some tricks (e.g., dependency:analyze).
This is a great mailing list indeed.
@Laird: that is quite interesting. From all the other posts I really
gained the impression that my original question was actually unsolvable
(e.g., Stephen explicitly stated the goals are "incompatible").
But this also led me to a new question.
Always eager to learn, I tried - just to see what would happen -
specifying in my POM at the same time <maven.compiler.{source,target}>
in the <properties> section, *as well as* specifying <{source,target}>
in the <configuration> section of the maven-compiler-plugin. It turns
out that the latter takes precedence over the former (i.e., when
different Java versions are specified in these two places, the
<configuration> of the maven-compiler-plugin is used).
Therefore, from my understanding up to this point, I suspected that
there must be something in the "uber"-pom akin to this:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>SOME_DEFAULT_VERSION</version>
<configuration>
<source>${maven.compiler.source}</source>
<target>${maven.compiler.target}</target>
</configuration>
</plugin>
where the maven.compiler.{source,target} would be specified in the
<properties> section of the "uber"-pom. That would have explained both
why Maven complains when I reconfigure the maven-compiler-plugin without
specifying the plugin version, as well as why redefining
maven.compiler.{source,target} in the <properties> section of my own pom
is a working solution for my original question.
So I finally decided to take a look at this "uber"-pom :)
I was suprised to find there is *no mention* of the
maven-compiler-plugin *at all* in this super-pom, that is,
$ export M2_HOME=/usr/share/maven
$ unzip -p $M2_HOME/lib/maven-model-builder-3.0.4.jar
org/apache/maven/model/pom-4.0.0.xml | grep compiler
$
...the latter command produces no output (I'm using Maven 3.0.4, as can
be seen from the path).
(Alternatively, one might search for "compiler" at
http://maven.apache.org/ref/3.0.4/maven-model-builder/super-pom.html,
but I wanted to make sure that indeed the same uber-pom is used on my
machine).
This is quite confusing. I firmly expected to see some kind of
configuration of the maven-compiler-plugin in the uber-pom. That is what
many of you led me to believe:
Stephen:
the "core" plugins all have a version specified in the über-pom that is
embedded within Maven.
Randal:
As was pointed out, core components such as the compiler plugin have a stable
default version via the 'uber-pom' as of a particular version of Maven
Mark:
generated POMs *do* specify the version of
m-compiler-p, by *not* specifying it. There's a built-in set of
defaults(1) buried inside Maven, which specify much of the
not-otherwise-specified.
So... where is this stable maven-compiler-plugin version specified,
precisely?
I did find some mentions of a stable maven-compiler-plugin version in
$M2_HOME/lib/maven-core-3.0.4.jar:META-INF/plexus/components.xml as well
as
$M2_HOME/lib/maven-core-3.0.4.jar:META-INF/plexus/artifact-handlers.xml,
but I'm not sure whether that's the actual place where the "default
version" of the maven-compiler-plugin is configured, and certainly none
of them is the uber-pom.
I'd just like to see it; I like to understand how stuff works ;-)
Thanks,
Malte
On 11/26/2013 04:26 PM, Stephen Connolly wrote:
On 26 November 2013 14:59, Malte Skoruppa <skoru...@cs.uni-saarland.de>wrote:
Hi,
I'm new to Maven and I'm currently going through the "Getting Started"
guide.
While reading, the following question came to me.
As far as I understand, Maven defaults to compiling all Java source files
with compatibility for JDK 1.3 (i.e., -source 1.3).
Well actually with newer versions of the compiler plugin the default has
been upped to 1.5
The guide explains how to change that behaviour, by configuring the
maven-compiler-plugin in the pom.xml:
http://maven.apache.org/guides/getting-started/index.
html#How_do_I_use_plug-ins
So far so good. However, if I do this, it appears that I also have to
specify the version of maven-compiler-plugin to be used (e.g., 2.5.1 in the
code snippet shown under the above link).
Generally, I don't want to do that. On the one hand, I would like for
Maven to simply use the latest version of the maven-compiler-plugin that is
available (the default behaviour).
Not the default behaviour any more.... at least since 2.0.9 IIRC... the
"core" plugins all have a version specified in the über-pom that is
embedded within Maven. Thus if you do not specify a version, you get the
version specified in the über-pom... for non-core plugins you will get the
latest, but your build is now irreproducible. Best practice is to specify
the version for all plugins.
On the other hand, I would like to use Java features above 1.3 (for
instance, generics).
So my first attempt was to simply remove the "<version>2.5.1</version>"
part from the pom.xml under the maven-compiler-plugin configuration. While
this did in principle work, Maven was not happy at all and complained with
this message:
[WARNING] Some problems were encountered while building the effective
model for com.mycompany.app:my-app:jar:1.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for
org.apache.maven.plugins:maven-compiler-plugin
is missing. @ line 19, column 15
[WARNING]
[WARNING] It is highly recommended to fix these problems because they
threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support
building such malformed projects.
So my question boils down to this: is there some way for me to achieve the
following three things at the same time:
(1) have Maven compile my source files with -source 1.4 (or anything
higher than 1.3)
(2) *not* specify the maven-compiler-plugin version to use; instead, have
Maven dynamically use the latest one available;
(3) *not* have Maven bitch about my project being "malformed" ;-)
I would imagine that this is not such a rare scenario:
(1) I do want to use Java features higher than those available in Java 1.3;
Then set source and target to 1.3... and hope you never upgrade to Java 8
(2) I do *not* want having to monitor the maven-compiler-plugin by myself
all the time in order to check for updates and keep my pom.xml referring to
the currently latest version;
Incompatible goal. If you have a version of the compiler plugin that is
working. Leave it as is. If it isn't broken, don't fix it. If you decide
that it is broken (e.g. you think the version you have is not compiling
fast enough) then try upgrading... this way you have control over your build
(3) yet I do *not* want Maven complaining about my project.xml being
malformed either.
Specify the version, or live with Maven complaining.
The Maven way is to make doing "the right thing" easy... doing the "not so
right thing" should encounter some friction... but unless we have good
reason we should not prevent you from shooting your foot off... we can make
it difficult to aim the gun at your foot though!
So what's the "clean" way to do this? :-)
For clarity, I have written down the steps to reproduce my problem in the
Appendix, below.
Thanks,
Malte
APPENDIX: Steps to reproduce:
1. Generate a simple Maven project using the very command stated on
http://maven.apache.org/guides/getting-started/index.
html#How_do_I_make_my_first_Maven_project :
mvn archetype:generate \
-DarchetypeGroupId=org.apache.maven.archetypes \
-DgroupId=com.mycompany.app \
-DartifactId=my-app
2. Replace the generated src/main/java/com/mycompany/app/App.java with
something that uses generics (not available in Java 1.3), e.g.:
package com.mycompany.app;
import java.util.*;
/**
* Hello world!
*
*/
public class App {
public static void main( String[] args) {
List<String> myList = new ArrayList<String>();
myList.add( "Hello");
myList.add( "world");
for( Iterator<String> i = myList.iterator(); i.hasNext();)
System.out.println( i.next());
}
}
3. Try to compile using the command 'mvn compile'. Maven fails:
$ mvn compile
[INFO] Scanning for projects...
...
[INFO] ------------------------------------------------------------
------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------
------------
[INFO] Total time: 0.995s
[INFO] Finished at: Tue Nov 26 15:26:00 CET 2013
[INFO] Final Memory: 6M/117M
[INFO] ------------------------------------------------------------
------------
[ERROR] Failed to execute goal org.apache.maven.plugins:
maven-compiler-plugin:2.0.2:compile (default-compile) on project my-app:
Compilation failure
[ERROR] /home/malte/dev/java/testMaven/my-app/src/main/
java/com/mycompany/app/App.java:[13,5] error: generics are not supported
in -source 1.3
...
4. Ok, so let's add the following to our pom.xml. This is copied&pasted
from http://maven.apache.org/guides/getting-started/index.
html#How_do_I_use_plug-ins :
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.5.1</version>
<configuration>
<source>1.5</source>
<target>1.5</target>
</configuration>
</plugin>
</plugins>
</build>
Running 'mvn compile' again, everything works fine now.
5. Yet, I would like to avoid having to specify the version of
maven-compiler-plugin to be used. I simply want Maven to use the latest one
available. So, let's remove the line
<version>2.5.1</version>
from the pom.xml again.
Running 'mvn compile' again, this works too, but Maven complains:
$ mvn compile
[INFO] Scanning for projects...
[WARNING]
[WARNING] Some problems were encountered while building the effective
model for com.mycompany.app:my-app:jar:1.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for
org.apache.maven.plugins:maven-compiler-plugin
is missing. @ line 19, column 15
[WARNING]
[WARNING] It is highly recommended to fix these problems because they
threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support
building such malformed projects.
[WARNING]
...
[INFO] ------------------------------------------------------------
------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------
------------
[INFO] Total time: 0.877s
[INFO] Finished at: Tue Nov 26 15:37:17 CET 2013
[INFO] Final Memory: 5M/117M
[INFO] ------------------------------------------------------------
------------
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On 11/27/2013 01:47 AM, Laird Nelson wrote:
On Tuesday, November 26, 2013, Malte Skoruppa wrote:
is there some way for me to achieve the following three things at the same
time:
(1) have Maven compile my source files with -source 1.4 (or anything
higher than 1.3)
(2) *not* specify the maven-compiler-plugin version to use; instead, have
Maven dynamically use the latest one available;
(3) *not* have Maven bitch about my project being "malformed" ;-)
Yes. In your pom.xml's <properties> section add this:
<maven.compiler.source>1.6</maven.compiler.source>
<maven.compiler.target>1.6</maven.compiler.target>
No other configuration or verbiage needed.
All of the other posters' words are good. But no one answered your question.
Best,
Laird
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