mrunit目前有一个已知的问题，没有很好的文档记录，那就是测试 MultipleOutputs 需要使用运行测试 PowerMockRunner 和一个 PrepareForTest 应用于模拟reducer类的注解。jira问题mrunit-13和mrunit-213对此进行了详细讨论。mrunit-213仍未解决/未修复。
将powermock添加到项目中之后，在排列mockito和powermock的正确兼容版本时会引发一些进一步的挑战。有关将powermock与mockito结合使用的文档介绍了哪些版本是兼容的。
我试过对你的样品做些改动。那已经过去了 NullPointerException ，但后来我遇到了最后一个问题。测试中声明的预期路径输出与 "filename" 减速器代码使用的路径。我更改了预期的路径输出，以使测试完全通过。
这是我的最终结果：一个完全工作的项目与您的样本测试。好好享受！

pom.xml文件

<?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>test</groupId>
    <artifactId>test-mrunit</artifactId>
    <packaging>jar</packaging>
    <version>0.0.1-SNAPSHOT</version>
    <name>Test MRUnit</name>
    <description>Test MRUnit</description>
    <properties>
        <hadoop.version>2.7.1</hadoop.version>
        <powermock.version>1.6.4</powermock.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-common</artifactId>
            <version>${hadoop.version}</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-mapreduce-client-core</artifactId>
            <version>${hadoop.version}</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-all</artifactId>
            <version>1.10.19</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-core</artifactId>
            <version>${powermock.version}</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-module-junit4</artifactId>
            <version>${powermock.version}</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-api-mockito</artifactId>
            <version>${powermock.version}</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.mrunit</groupId>
            <artifactId>mrunit</artifactId>
            <version>1.1.0</version>
            <classifier>hadoop2</classifier>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

src/main/java/areducer.java

import java.io.IOException;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.output.MultipleOutputs;

public class AReducer extends Reducer<Text, Text, Text, NullWritable>{
    private MultipleOutputs<Text, NullWritable> mos = null;

    @Override
    public void setup(Context context) throws IOException {
        mos = new MultipleOutputs<Text, NullWritable>(context);
    }

    @Override
    public void reduce(Text key, Iterable<Text> values, Context context)
            throws IOException, InterruptedException {
        mos.write(key, NullWritable.get(), "filename");
    }

    @Override
    public void cleanup(Context context) throws IOException, InterruptedException {
        mos.close();
    }
}

src/test/java/testaducer.java

import java.io.IOException;
import java.util.ArrayList;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mrunit.mapreduce.ReduceDriver;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest(AReducer.class)
public class TestAReducer {

    @Test
    public void reducerMRUnit() throws IOException{
        ReduceDriver reduceDriver = new ReduceDriver(new AReducer());
        String output = "";
        ArrayList<Text> list = new ArrayList<Text>(0);
        list.add(new Text(""));
        reduceDriver.withInput(new Text(""), list);
        reduceDriver.withPathOutput(new Text(output), NullWritable.get(), "filename");
        reduceDriver.runTest();
    }
}