Java在导入maven包时给出NoClassDefFoundError

Question

我有一个简单的java代码App.java。我正试图从apachecommonsmath3库导入apachecommons Fraction类。所以，作为一个示例代码，我把它放在我的App.java:中

package myApp;
import org.apache.commons.math3.fraction.Fraction;
public class App 
{
public static void main( String[] args )
{
Fraction f = new Fraction(2, 3);
System.out.println( "Hello World!" );
System.out.println( f );
}
}

使用以下POM:

<?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/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>myApp</groupId>
<artifactId>Proj1</artifactId>
<version>1.0-SNAPSHOT</version>
<name>Proj1</name>
<url>http://www.example.com</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>1.7</maven.compiler.source>
<maven.compiler.target>1.7</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-math3 -->
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-math3</artifactId>
<version>3.6.1</version>
</dependency>
</dependencies>
<build>
<pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
<plugins>
<!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<version>3.1.0</version>
</plugin>
<!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>3.0.2</version>
</plugin>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.0</version>
</plugin>
<plugin>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.22.1</version>
</plugin>
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>3.0.2</version>
</plugin>
<plugin>
<artifactId>maven-install-plugin</artifactId>
<version>2.5.2</version>
</plugin>
<plugin>
<artifactId>maven-deploy-plugin</artifactId>
<version>2.8.2</version>
</plugin>
<!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
<plugin>
<artifactId>maven-site-plugin</artifactId>
<version>3.7.1</version>
</plugin>
<plugin>
<artifactId>maven-project-info-reports-plugin</artifactId>
<version>3.0.0</version>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>3.2.4</version>
<configuration>
<!-- put your configurations here -->
</configuration>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>shade</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</pluginManagement>
</build>
</project>

我试着把utdemir建议的插件放在他对类似问题的回答中，但在我的情况下，当我运行时：

mvn package
java -cp target/Proj1-1.0-SNAPSHOT.jar myApp.App

虽然它显然编译正确，但我得到了以下错误：

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/math3/fraction/Fraction
at myApp.App.main(App.java:8)
Caused by: java.lang.ClassNotFoundException: org.apache.commons.math3.fraction.Fraction
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:636)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:182)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:519)
... 1 more

我该怎么办？

Answer 1

使用mvn运行应用程序比直接调用java要容易得多。这是因为在以后的情况下，您需要使用-cp标志手动将类路径设置为正确的值。这可能会导致错误，就像您的情况一样。

以下是如何使用mvn运行应用程序

mvn exec:java -Dexec.mainClass="myApp.App"

更多详细信息可以在这里找到