这在log4j 2中的工作方式与在log4j-1.2中完全不同。
在log4j 2中，你可以创建一个插件，手册中有一个自定义附加器的例子：http://logging.apache.org/log4j/2.x/manual/extending.html#Appenders
扩展org.apache.logging.log4j.core.appender.AbstractAppender可能会很方便，但这不是必需的。
当您使用@Plugin(name="MyCustomAppender", ....注解自定义Appender类时，插件名称将成为配置元素名称，因此具有自定义Appender的配置将如下所示：

<Configuration packages="com.yourcompany.yourcustomappenderpackage">
  <Appenders>
    <MyCustomAppender name="ABC" otherAttribute="...">
    ...
  </Appenders>
  <Loggers><Root><AppenderRef ref="ABC" /></Root></Loggers>
</Configuration>

请注意，配置上的packages属性是一个逗号分隔的列表，其中包含所有带有自定义log4j 2插件的包。Log4j 2将在类路径中搜索这些包，以查找带有@Plugin注解的类。
下面是一个打印到控制台的自定义附加器示例：

package com.yourcompany.yourcustomappenderpackage;

import java.io.Serializable;
import java.util.concurrent.locks.*;
import org.apache.logging.log4j.core.*;
import org.apache.logging.log4j.core.config.plugins.*;
import org.apache.logging.log4j.core.layout.PatternLayout;

// note: class name need not match the @Plugin name.
@Plugin(name="MyCustomAppender", category="Core", elementType="appender", printObject=true)
public final class MyCustomAppenderImpl extends AbstractAppender {

    private final ReadWriteLock rwLock = new ReentrantReadWriteLock();
    private final Lock readLock = rwLock.readLock();

    protected MyCustomAppenderImpl(String name, Filter filter,
            Layout<? extends Serializable> layout, final boolean ignoreExceptions) {
        super(name, filter, layout, ignoreExceptions);
    }

    // The append method is where the appender does the work.
    // Given a log event, you are free to do with it what you want.
    // This example demonstrates:
    // 1. Concurrency: this method may be called by multiple threads concurrently
    // 2. How to use layouts
    // 3. Error handling
    @Override
    public void append(LogEvent event) {
        readLock.lock();
        try {
            final byte[] bytes = getLayout().toByteArray(event);
            System.out.write(bytes);
        } catch (Exception ex) {
            if (!ignoreExceptions()) {
                throw new AppenderLoggingException(ex);
            }
        } finally {
            readLock.unlock();
        }
    }

    // Your custom appender needs to declare a factory method
    // annotated with `@PluginFactory`. Log4j will parse the configuration
    // and call this factory method to construct an appender instance with
    // the configured attributes.
    @PluginFactory
    public static MyCustomAppenderImpl createAppender(
            @PluginAttribute("name") String name,
            @PluginElement("Layout") Layout<? extends Serializable> layout,
            @PluginElement("Filter") final Filter filter,
            @PluginAttribute("otherAttribute") String otherAttribute) {
        if (name == null) {
            LOGGER.error("No name provided for MyCustomAppenderImpl");
            return null;
        }
        if (layout == null) {
            layout = PatternLayout.createDefaultLayout();
        }
        return new MyCustomAppenderImpl(name, filter, layout, true);
    }
}

有关插件的更多详细信息：http://logging.apache.org/log4j/2.x/manual/plugins.html
如果手册还不够，那么查看log4j-core中内置的appender的源代码可能会很有用。

正如您所指出的，AppenderSkeleton不再可用，因此How to create my own Appender in log4j?中的解决方案将不起作用。
如果您需要多个日志消息，则使用Mockito或类似库创建带有ArgumentCaptor的Appender将不起作用，因为MutableLogEvent将在多个日志消息中重用。
我为log4j2找到的最通用的解决方案是提供一个记录所有消息的mock实现，它不需要任何额外的库，如Mockito或JMockit。

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.Logger;
import org.apache.logging.log4j.core.appender.AbstractAppender;    

private static MockedAppender mockedAppender;
private static Logger logger;

@Before
public void setup() {
    mockedAppender.message.clear();
}

/**
 * For some reason mvn test will not work if this is @Before, but in eclipse it works! As a
 * result, we use @BeforeClass.
 */
@BeforeClass
public static void setupClass() {
    mockedAppender = new MockedAppender();
    logger = (Logger)LogManager.getLogger(ClassWithLoggingToTest.class);
    logger.addAppender(mockedAppender);
    logger.setLevel(Level.INFO);
}

@AfterClass
public static void teardown() {
    logger.removeAppender(mockedAppender);
}

@Test
public void test() {
    // do something that causes logs
    for (String e : mockedAppender.message) {
        // add asserts for the log messages
    }
}

private static class MockedAppender extends AbstractAppender {

    List<String> message = new ArrayList<>();

    protected MockedAppender() {
        super("MockedAppender", null, null);
    }

    @Override
    public void append(LogEvent event) {
        message.add(event.getMessage().getFormattedMessage());
    }
}

看起来插件附加器在启动时被扫描，不能在运行时添加。是真的吗？
要在运行时添加新的附加器，您可以使用monitorInterval属性更新日志配置，即每60秒更新一次：

<Configuration monitorInterval="60">