maven-resources-plugin error using copy-resources goal: 'resources', 'outputDirectory' missing or invalid - maven

Question

I'm trying to use the maven-resources-plugin to do some filtering using the copy-resources goal, and ran into the following error:
Failed to execute goal org.apache.maven.plugins:maven-resources-plugin:2.5:copy-resources (default-cli) on project bar: The parameters 'resources', 'outputDirectory' for goal org.apache.maven.plugins:maven-resources-plugin:2.5:copy-resources are missing or invalid
To isolate the problem, I created a very simple pom.xml, copied pretty nearly verbatim from http://maven.apache.org/plugins/maven-resources-plugin/examples/copy-resources.html, ran it, and got the same error.
I'm invoking it with
mvn resources:copy-resources
Any ideas? Here's the test pom.xml.
<?xml version="1.0" encoding="UTF-8"?>
<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>foo</groupId>
<artifactId>bar</artifactId>
<version>1.0-SNAPSHOT</version>
<build>
<plugins>
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>2.5</version>
<executions>
<execution>
<id>copy-resources</id>
<!-- here the phase you need -->
<phase>validate</phase>
<goals>
<goal>copy-resources</goal>
</goals>
<configuration>
<outputDirectory>${basedir}/target/extra-resources</outputDirectory>
<resources>
<resource>
<directory>src/non-packaged-resources</directory>
<filtering>true</filtering>
</resource>
</resources>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>

Answer 1

The main problem you had is that you are invoking the plugin goal directly using
mvn resources:copy-resources
which does not necessarily create the output directory. Instead call the correct Maven lifecycle phase.
mvn process-resources
For a complete list of the lifecycle phases just run the mvn command without anything..
In general it almost always better to invoke a lifecycle phase rather than a goal directly since it guarantees that any preconditions are met (e.g. cant compile test classes before the classes to be tested..).

Answer 2

Check if #bmargulies answer works for you. You can refer to these examples.
In any case, you do not need to use <pluginManagement> to achieve this. <pluginManagement> is used in multi-module scenarios to facilitate inheritance of plugin configurations.
You need to move the configuration out of execution element. The following snippet works.
<?xml version="1.0" encoding="UTF-8"?>
<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>foo</groupId>
<artifactId>bar</artifactId>
<version>1.0-SNAPSHOT</version>
<build>
<plugins>
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>2.5</version>
<configuration>
<outputDirectory>${basedir}/target/extra-resources</outputDirectory>
<resources>
<resource>
<directory>src/non-packaged-resources</directory>
<filtering>true</filtering>
</resource>
</resources>
</configuration>
<executions>
<execution>
<id>copy-resources</id>
<!-- here the phase you need -->
<phase>validate</phase>
<goals>
<goal>copy-resources</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>

Answer 3

Just remove the executions and their configuration. Normally you might want to define resources in <build> > <resources> respectively <build> > <testResources> (see http://maven.apache.org/plugins/maven-resources-plugin/examples/resource-directory.html) directly not within the plugin config using the default lifecycle process-(test-)resources which is automatically hooked by copy-(test-)resources..
Yes, it's a bad example on their page!