我正在关注Maven by Example中的简单天气教程。当我执行程序时，我得到以下异常。这需要任何特定的类路径设置吗？
聚甲醛
<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.maven.weather</groupId>
<artifactId>simple-weather</artifactId>
<version>1.0</version>
<packaging>jar</packaging>
<name>simple-weather</name>
<url>http://maven.apache.org</url>
<licenses>
<license>
<name>Apache 2</name>
<url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
<distribution>repo</distribution>
<comments>A business-friendly OSS license</comments>
</license>
</licenses>
<organization>
<name>Sonatype</name>
<url>http://www.sonatype.com</url>
</organization>
<developers>
<developer>
<id>jason</id>
<name>Jason Van Zyl</name>
<email>jason@maven.org</email>
<url>http://www.sonatype.com</url>
<organization>Sonatype</organization>
<organizationUrl>http://www.sonatype.com</organizationUrl>
<roles>
<role>developer</role>
</roles>
<timezone>-6</timezone>
</developer>
</developers>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.14</version>
</dependency>
<dependency>
<groupId>dom4j</groupId>
<artifactId>dom4j</artifactId>
<version>1.6.1</version>
</dependency>
<dependency>
<groupId>jaxen</groupId>
<artifactId>jaxen</artifactId>
<version>1.1.1</version>
</dependency>
<dependency>
<groupId>velocity</groupId>
<artifactId>velocity</artifactId>
<version>1.5</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
主.java
package com.example.maven.weather;
import java.io.InputStream;
import org.apache.log4j.PropertyConfigurator;
public class Main {
public static void main(String[] args) throws Exception {
// Configure Log4J
PropertyConfigurator.configure(Main.class.getClassLoader()
.getResource("log4j.properties"));
// Read the Zip Code from the Command-line (if none supplied, use 60202)
String zipcode = "60202";
try {
zipcode = args[0];
} catch( Exception e ) {}
// Start the program
new Main(zipcode).start();
}
private String zip;
public Main(String zip) {
this.zip = zip;
}
public void start() throws Exception {
// Retrieve Data
InputStream dataIn = new YahooRetriever().retrieve( zip );
// Parse Data
Weather weather = new YahooParser().parse( dataIn );
// Format (Print) Data
System.out.print( new WeatherFormatter().format( weather ) );
}
}
跑：
mvn exec:java -Dexec.mainClass=com.exa
mple.maven.weather.Main
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building simple-weather 1.0
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] >>> exec-maven-plugin:1.2.1:java (default-cli) @ simple-weather >>>
[INFO]
[INFO] <<< exec-maven-plugin:1.2.1:java (default-cli) @ simple-weather <<<
[INFO]
[INFO] --- exec-maven-plugin:1.2.1:java (default-cli) @ simple-weather ---
[WARNING]
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:297)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.NullPointerException
at org.apache.log4j.PropertyConfigurator.doConfigure(PropertyConfigurato
r.java:433)
at org.apache.log4j.PropertyConfigurator.configure(PropertyConfigurator.
java:336)
at com.example.maven.weather.Main.main(Main.java:12)
... 6 more
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 0.902s
[INFO] Finished at: Thu May 30 14:23:06 IST 2013
[INFO] Final Memory: 4M/15M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:java (d
efault-cli) on project simple-weather: An exception occured while executing the
Java class. null: InvocationTargetException: NullPointerException -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e swit
ch.
[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 rea
d the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionE
xception