Log4j 2でログ出力をテストするサンプルソース ref: http://qiita.com/kazurof/items/abbd42f11bfc125f3190

build.gradle

apply plugin: 'java'

sourceCompatibility = '1.8'
targetCompatibility = '1.8'

def defaultEncoding = 'UTF-8'

tasks.withType(AbstractCompile) each { it.options.encoding = defaultEncoding }

test {
  testLogging.showStandardStreams = true
}

repositories {
  jcenter()
}

dependencies {
  testCompile 'junit:junit:4.12'
  compile group: 'org.apache.logging.log4j', name: 'log4j-api', version: '2.5'
  compile group: 'org.apache.logging.log4j', name: 'log4j-core', version: '2.5'
}

file4.txt

C:\home\idea\log4j2>gradle test
:compileJava
:processResources
:classes
:compileTestJava
:processTestResources UP-TO-DATE
:testClasses
:test

org.example.SampleClassTest > testDoSomething STANDARD_OUT
    2016-02-22 01:40:58,092 INFO  [Test worker] - message ->nantoka (org.example.SampleClass.java:11)
    2016-02-22 01:40:58,097 INFO  [Test worker] - message ->kantoka (org.example.SampleClass.java:11)

BUILD SUCCESSFUL

Total time: 14.312 secs
C:\home\idea\log4j2>

log4j2.xml

<?xml version="1.0" encoding="UTF-8"?>
<configuration status="OFF">
  <appenders>
    <Console name="console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d %-5p [%t] - %m (%C.java:%L)%n"/>
    </Console>
  </appenders>

  <loggers>
    <Logger name="org.example" level="debug" additivity="false">
      <appender-ref level="info" ref="console"/>
    </Logger>
    <root level="trace">
      <appender-ref ref="console"/>
    </root>
  </loggers>
</configuration>

SampleClass.java

package org.example;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class SampleClass {
  private static final Logger LOGGER = LogManager.getLogger(SampleClass.class);

  void doSomething(String message) {
    // Java8 & Log4j2として、ラムダを使うことでinfoか否かの判定をLog4j2に実施させる。
    // みんなラムダで幸せになろうよ。
    LOGGER.info(() -> "message ->" + message);
  }
}

SampleClassTest.java

package org.example;

import org.apache.logging.log4j.core.Appender;
import org.apache.logging.log4j.core.LoggerContext;
import org.apache.logging.log4j.core.appender.WriterAppender;
import org.apache.logging.log4j.core.config.Configuration;
import org.apache.logging.log4j.core.config.LoggerConfig;
import org.apache.logging.log4j.core.layout.PatternLayout;
import org.junit.Test;

import java.io.StringWriter;
import java.io.Writer;

import static org.hamcrest.core.Is.is;
import static org.junit.Assert.assertThat;
import static org.junit.Assert.assertTrue;

public class SampleClassTest {
  static final String TEST_APPENDER_NAME = "fortest";

  @Test
  public void testDoSomething() throws Exception {
    StringWriter writer = new StringWriter();
    //現在のロガーに、writerを持つappenderを追加する。
    addAppender(writer, TEST_APPENDER_NAME);

    // テスト実行して、writerに流れてきている文字列を検証する。
    SampleClass sut = new SampleClass();
    sut.doSomething("nantoka");
    assertThat(writer.toString(), is("message ->nantoka" + System.lineSeparator()));

    // 追加したappenderを削除する。writerの中身も消す。
    removeAppender(TEST_APPENDER_NAME);
    writer.getBuffer().delete(0, writer.getBuffer().length());

    // もう一度テスト実行して、writerに流れてきている文字列を検証する。今度は空である。
    sut = new SampleClass();
    sut.doSomething("kantoka");
    assertTrue(writer.toString().length() == 0);
  }

  void addAppender(Writer writer, String name) {
    final LoggerContext context = LoggerContext.getContext(false);
    final Configuration config = context.getConfiguration();
    final PatternLayout layout = PatternLayout.createDefaultLayout(config);

    Appender appender = WriterAppender.createAppender(layout, null, writer, name, false, true);
    appender.start();
    config.addAppender(appender);
    updateLoggers(appender, config);
  }

  private void updateLoggers(final Appender appender, final Configuration config) {
    for (final LoggerConfig loggerConfig : config.getLoggers().values()) {
      loggerConfig.addAppender(appender, null, null);
    }
    config.getRootLogger().addAppender(appender, null, null);
  }

  private void removeAppender(String name) {
    final LoggerContext context = LoggerContext.getContext(false);
    final Configuration config = context.getConfiguration();
    for (final LoggerConfig loggerConfig : config.getLoggers().values()) {
      loggerConfig.removeAppender(name);
    }
    config.getRootLogger().removeAppender(name);
  }
}