versions：use-releases对特殊的dependencyManagement条目抛出nullpointer异常

Question

如果我们在pom上调用versions:use-releases，其中dependencyManagement条目没有版本标记（例如因为它只设置范围或排除），我们会得到一个nullpointer异常。采用以下示例pom：

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<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>de.continentale.spu</groupId>
  <artifactId>testversionsuse</artifactId>
  <version>1.0.0-SNAPSHOT</version>

  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <scope>compile</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>

  <dependencies>
    <dependency>
      <groupId>log4j</groupId>
      <artifactId>log4j</artifactId>
      <version>1.2.17</version>
    </dependency>
  </dependencies>

</project>

然后调用versions:use-releases会导致异常：

[ERROR] Failed to execute goal org.codehaus.mojo:versions-maven-plugin:2.3:use-releases (default-cli) on project testversionsuse: Execution default-cli of goal org.codehaus.mojo:versions-maven-plugin:2.3:use-releases failed. NullPointerException -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.codehaus.mojo:versions-maven-plugin:2.3:use-releases (default-cli) on project testversionsuse: Execution default-cli of goal org.codehaus.mojo:versions-maven-plugin:2.3:use-releases failed.
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:224)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
    at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:307)
    at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:193)
    at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:106)
    at org.apache.maven.cli.MavenCli.execute(MavenCli.java:862)
    at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:286)
    at org.apache.maven.cli.MavenCli.main(MavenCli.java:197)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
Caused by: org.apache.maven.plugin.PluginExecutionException: Execution default-cli of goal org.codehaus.mojo:versions-maven-plugin:2.3:use-releases failed.
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:145)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
    ... 20 more
Caused by: java.lang.NullPointerException
    at java.util.regex.Matcher.getTextLength(Matcher.java:1283)
    at java.util.regex.Matcher.reset(Matcher.java:309)
    at java.util.regex.Matcher.<init>(Matcher.java:229)
    at java.util.regex.Pattern.matcher(Pattern.java:1093)
    at org.codehaus.mojo.versions.UseReleasesMojo.useReleases(UseReleasesMojo.java:118)
    at org.codehaus.mojo.versions.UseReleasesMojo.update(UseReleasesMojo.java:89)
    at org.codehaus.mojo.versions.AbstractVersionsUpdaterMojo.process(AbstractVersionsUpdaterMojo.java:324)
    at org.codehaus.mojo.versions.AbstractVersionsUpdaterMojo.execute(AbstractVersionsUpdaterMojo.java:260)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
    ... 21 more
[ERROR] 
[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/PluginExecutionException

知道为什么会这样吗？