未插入 Maven 属性

Question

Maven 属性有时用于插件配置中的插值，有时则不存在。为什么是这样？

例如，在下面的片段中，antrun 插件可以访问 dependencies 属性并按预期返回 jar 的本地路径。 exec 插件失败，因为 ${...} 的计算结果为 null。

<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/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.example</groupId>
<artifactId>eg</artifactId>
<version>0.0.1-SNAPSHOT</version>
<dependencies>
    <dependency>
        <groupId>commons-io</groupId>
        <artifactId>commons-io</artifactId>
        <version>2.0</version>
    </dependency>
</dependencies>
<build>
    <plugins>

        <plugin>
            <artifactId>maven-antrun-plugin</artifactId>
            <executions>
                <execution>
                    <phase>process-resources</phase>
                    <configuration>
                        <tasks>
                            <echo>Path in antrun: </echo>
                            <echo>${maven.dependency.commons-io.commons-io.jar.path}</echo>
                        </tasks>
                    </configuration>
                    <goals>
                        <goal>run</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>exec-maven-plugin</artifactId>
            <version>1.2</version>
            <executions>
                <execution>
                    <goals>
                        <goal>exec</goal>
                    </goals>
                    <phase>process-resources</phase>
                    <configuration>
                        <executable>echo</executable>
                        <arguments>
                            <argument>Path in mvn exec</argument>
                            <argument>${maven.dependency.commons-io.commons-io.jar.path}</argument>
                        </arguments>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

输出：

$ mvn process-resources
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building eg 0.0.1-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.5:resources (default-resources) @ eg ---
[debug] execute contextualize
[WARNING] Using platform encoding (Cp1252 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] Copying 0 resource
[INFO]
[INFO] --- maven-antrun-plugin:1.3:run (default) @ eg ---
[INFO] Executing tasks
     [echo] Path in antrun:
     [echo] C:\Users\Jim\.m2\repository\commons-io\commons-io\2.0\commons-io-2.0.jar
[INFO] Executed tasks
[INFO]
[INFO] --- exec-maven-plugin:1.2:exec (default) @ eg ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.288s
[INFO] Finished at: Mon Feb 11 14:24:03 GMT 2013
[INFO] Final Memory: 4M/121M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2:exec (default) on project eg: Misconfigured argument, value is null. Set the argument to an empty value if this is the required behaviour. -> [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/MojoExecutionException

Answer 1

类依赖属性仅对maven-antrun-plugin 可用。我不知道你是否必须在实际的 Ant 脚本中使用 Maven 依赖项，但如果你玩过它，你就会知道 Ant 有：

<artifact:dependencies ...>
    <pom file="mypom.xml"/>
</artifact:dependencies>

这基本上是让您使用 Maven 依赖项的路径的原因。

在您的情况下，我相信这会从您的视线中隐藏起来，并且您已经将依赖项的路径作为属性。我想说的是插件正在为您自己处理这个问题。因此，其他插件看不到这些属性，因为maven-antrun-plugin 没有导出实际的 Maven 属性，而是 -- Ant 本身范围内的属性。

我希望这更有意义。