使用Java ServiceLoader会导致NoSuchElementException错误

我的类DoFindMiddleService扩展了抽象类DoFindMiddle。这是DoFindMiddle类：

public abstract class DoFindMiddle<T extends Comparable<T>> {
   public abstract ListItem<T> findMiddle(ListItem<T> paramListItem);
   public static <D extends Comparable<D>> DoFindMiddle<D> newInstance() {
     return (DoFindMiddle)ServiceLoader.load(DoFindMiddle.class).iterator().next();
   }
}

这是DoFindMiddleService类的第一部分：

package com.nng.interview.homework.applicant.kovacskalman;

import com.nng.interview.homework.exercise.DoFindMiddle;
import java.util.Iterator;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;

public class DoFindMiddleService<T extends Comparable<T>> extends DoFindMiddle<T> {

public static DoFindMiddle service;
public volatile ServiceLoader<DoFindMiddleService> loader;
public DoFindMiddleService() {
    loader = ServiceLoader.load(DoFindMiddleService.class);
}
public static synchronized DoFindMiddle getInstance() {
    if (service == null) {
        service = new DoFindMiddleService();
    }
    return service;
}
public String getDefinition(String word) {
    String definition = null;

    try {
        Iterator<DoFindMiddleService> dictionaries = loader.iterator();
        while (definition == null && dictionaries.hasNext()) {
            DoFindMiddleService d = dictionaries.next();
            definition = d.getDefinition(word);
        }
    } catch (ServiceConfigurationError serviceError) {
        definition = null;
        serviceError.printStackTrace();

    }
    return definition;
}

.....
}

如果我在单元测试中尝试以下操作：

final DoFindMiddle<Integer> instance = new DoFindMiddleService();

然后就可以了。但是当我尝试单元测试时：

final DoFindMiddle<Integer> instance = DoFindMiddle.newInstance();

然后，它导致NoSuchElementException错误。我的错是什么？

我的pom.xml是以下内容：

<?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>com.nng.interview.homework.applicant</groupId>
    <name>Kovács Kálmán</name>
    <artifactId>kovacskalman</artifactId>
    <version>1.0</version>
    <packaging>jar</packaging>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.3.2</version>
                <configuration>
                    <source>1.7</source>
                    <target>1.7</target>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>
    <dependencies>
        <dependency>
            <groupId>com.nng.interview.homework</groupId>
            <artifactId>comnnginterviewhomework</artifactId>
            <version>1.0.2</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.10</version>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.hamcrest</groupId>
                    <artifactId>hamcrest-core</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>
    <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>
</project>

当我尝试执行时，出现以下错误：

d:\kovacskalman>mvn exec:java -Dexec.mainClass="com.nng.interview.homework.appli
cant.kovacskalman.App"
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building Kovacs Kalman 1.0
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- exec-maven-plugin:1.3.2:java (default-cli) @ kovacskalman ---
[WARNING] Warning: killAfter is now deprecated. Do you need it ? Please comment
on MEXEC-6.
[WARNING]
java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:293)
        at java.lang.Thread.run(Thread.java:744)
Caused by: java.util.NoSuchElementException
        at java.util.ServiceLoader$LazyIterator.next(ServiceLoader.java:357)
        at java.util.ServiceLoader$1.next(ServiceLoader.java:445)
        at com.nng.interview.homework.exercise.DoFindMiddle.newInstance(DoFindMi
ddle.java:52)
        at com.nng.interview.homework.applicant.kovacskalman.App.<init>(App.java
:9)
        at com.nng.interview.homework.applicant.kovacskalman.App.main(App.java:1
3)
        ... 6 more
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.076 s
[INFO] Finished at: 2014-11-01T15:19:51+01:00
[INFO] Final Memory: 9M/96M