什么是MDC机制

MDC（Mapped Diagnostic Contexts）映射诊断上下文，主要用在做日志链路跟踪时，动态配置用户自定义的一些信息，比如requestId、sessionId等等。MDC使用的容器支持多线程操作，满足线程安全。

MDC的使用

• pom.xml依赖

<!-- 日志log4j2 -->
<dependency>
    <groupId>com.lmax</groupId>
    <artifactId>disruptor</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>

此处日志具体实现采用的是log4j2，依赖slf4j-api 版本1.7.25

• log4j2配置日志输出

<!-- 控制台 -->
<Console name="Console" target="SYSTEM_OUT">
    <PatternLayout
            pattern="%d{yyyy-MM-dd HH:mm:ss.SSS} - %X{requestId} %-5p %t [%c:%L]-%m%n"/>
</Console>

在patternLayout中 %X{requestId} 显示我们定义的变量
日志配置详解可以戳这里：【系统日志笔记一】——丰富Log4j2配置

• MainLogAspect.java

package com.lucas.device.aop;

import javax.servlet.http.HttpServletRequest;

import com.phlico.common.framework.tool.unique.IdWorkerUtil;
import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.Signature;
import org.aspectj.lang.annotation.*;
import org.slf4j.MDC;
import org.springframework.stereotype.Component;
import org.springframework.web.context.request.RequestContextHolder;
import org.springframework.web.context.request.ServletRequestAttributes;

import lombok.extern.slf4j.Slf4j;

/**
 * <Description> 系统日志切面<br>
 *
 * @author xubin<br>
 * @version 1.0<br>
 * @taskId <br>
 * @CreateDate 2019/4/12 <br>
 */

@Aspect
@Slf4j
@Component
public class MainLogAspect {

    /**
     * Description: 自定义切点<br>
     * 
     * @author xubin <br>
     * @taskId <br>
     */
    @Pointcut("@annotation(com.lucas.device.aop.Logc)")
    public void pointCut() {
    }

    /**
     * Description: 前置通知-记录请求信息<br>
     * 
     * @author xubin <br>
     * @taskId <br>
     * @param joinPoint <br>
     */
    @Before("pointCut()")
    public void doBeforeAdvice(JoinPoint joinPoint) {
        Signature signature = joinPoint.getSignature();
        ServletRequestAttributes sra = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = sra.getRequest();
        // 获取目标方法的参数信息
        Object[] obj = joinPoint.getArgs();
        // MDC容器增加requestId
        MDC.put("requestId", IdWorkerUtil.getFlowIdWorkerInstance().nextId());
        log.info("[MainLogAspect]-request url:{}, class: {}, method: {}, param: {}", request.getRequestURI(), signature
                .getDeclaringTypeName(), signature.getName(), obj[0].toString());

    }

    /**
     * Description: 后置通知-记录返回信息<br>
     * 
     * @author xubin <br>
     * @taskId <br>
     * @param joinPoint <br>
     * @param result <br>
     */
    @AfterReturning(returning = "result", pointcut = "pointCut()")
    public void doAfterReturningAdvice(JoinPoint joinPoint, Object result) {
        Signature signature = joinPoint.getSignature();
        ServletRequestAttributes sra = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = sra.getRequest();
        log.info("[MainLogAspect]-response url:{}, class: {}, method: {}, param: {}", request.getRequestURI(), signature
                .getDeclaringTypeName(), signature.getName(), result.toString());
        // MDC容器移除requestId
        MDC.remove("requestId");
    }

    /**
     * Description: 后置异常通知-记录返回出现异常<br>
     * 
     * @author xubin <br>
     * @taskId <br>
     * @param joinPoint <br>
     * @param exception <br>
     */
    @AfterThrowing(value = "pointCut()", throwing = "exception")
    public void doAfterThrowingAdvice(JoinPoint joinPoint, Throwable exception) {
        Signature signature = joinPoint.getSignature();
        ServletRequestAttributes sra = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = sra.getRequest();
        log.info("[MainLogAspect]-response exception url:{}, class: {}, method: {}", request.getRequestURI(), signature
                .getDeclaringTypeName(), signature.getName());
        // MDC容器移除requestId
        MDC.remove("requestId");
    }
}

MainLogAspect.java是自定义切面，拦截@Logc的请求，输出请求的入参出参，详细介绍可以戳：【系统日志笔记二】——撸起袖子写个自定义日志注解
在方法中合适的位置添加MDC.put和MDC.remove

• 运行结果
  
  可以看到，日志中同一线程内的用户操作都会打印出一个requestId，如果线程中没有requestId则会输出一个空字符串。红框内第三行是在当前线程中启用了子线程，此时子线程并未集成父线程中MDC容器，其中原因可以在下面的源码跟踪里找到。

MDC源码与原理

MDC的源码在slf4j-api包的org.slf4j路径下，源码：

package org.slf4j;

import java.io.Closeable;
import java.util.Map;

import org.slf4j.helpers.NOPMDCAdapter;
import org.slf4j.helpers.BasicMDCAdapter;
import org.slf4j.helpers.Util;
import org.slf4j.impl.StaticMDCBinder;
import org.slf4j.spi.MDCAdapter;

public class MDC {

    static final String NULL_MDCA_URL = "http://www.slf4j.org/codes.html#null_MDCA";
    static final String NO_STATIC_MDC_BINDER_URL = "http://www.slf4j.org/codes.html#no_static_mdc_binder";
    static MDCAdapter mdcAdapter;

    /**
     * An adapter to remove the key when done.
     */
    public static class MDCCloseable implements Closeable {
        private final String key;

        private MDCCloseable(String key) {
            this.key = key;
        }

        public void close() {
            MDC.remove(this.key);
        }
    }

    private MDC() {
    }

    /**
     * As of SLF4J version 1.7.14, StaticMDCBinder classes shipping in various bindings
     * come with a getSingleton() method. Previously only a public field called SINGLETON 
     * was available.
     * 
     * @return MDCAdapter
     * @throws NoClassDefFoundError in case no binding is available
     * @since 1.7.14
     */
    private static MDCAdapter bwCompatibleGetMDCAdapterFromBinder() throws NoClassDefFoundError {
        try {
            return StaticMDCBinder.getSingleton().getMDCA();
        } catch (NoSuchMethodError nsme) {
            // binding is probably a version of SLF4J older than 1.7.14
            return StaticMDCBinder.SINGLETON.getMDCA();
        }
    }

    static {
        try {
            mdcAdapter = bwCompatibleGetMDCAdapterFromBinder();
        } catch (NoClassDefFoundError ncde) {
            mdcAdapter = new NOPMDCAdapter();
            String msg = ncde.getMessage();
            if (msg != null && msg.contains("StaticMDCBinder")) {
                Util.report("Failed to load class \"org.slf4j.impl.StaticMDCBinder\".");
                Util.report("Defaulting to no-operation MDCAdapter implementation.");
                Util.report("See " + NO_STATIC_MDC_BINDER_URL + " for further details.");
            } else {
                throw ncde;
            }
        } catch (Exception e) {
            // we should never get here
            Util.report("MDC binding unsuccessful.", e);
        }
    }

    /**
     * Put a diagnostic context value (the <code>val</code> parameter) as identified with the
     * <code>key</code> parameter into the current thread's diagnostic context map. The
     * <code>key</code> parameter cannot be null. The <code>val</code> parameter
     * can be null only if the underlying implementation supports it.
     * 
     * <p>
     * This method delegates all work to the MDC of the underlying logging system.
     *
     * @param key non-null key 
     * @param val value to put in the map
     * 
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static void put(String key, String val) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key parameter cannot be null");
        }
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.put(key, val);
    }

    /**
     * Put a diagnostic context value (the <code>val</code> parameter) as identified with the
     * <code>key</code> parameter into the current thread's diagnostic context map. The
     * <code>key</code> parameter cannot be null. The <code>val</code> parameter
     * can be null only if the underlying implementation supports it.
     *
     * <p>
     * This method delegates all work to the MDC of the underlying logging system.
     * <p>
     * This method return a <code>Closeable</code> object who can remove <code>key</code> when
     * <code>close</code> is called.
     *
     * <p>
     * Useful with Java 7 for example :
     * <code>
     *   try(MDC.MDCCloseable closeable = MDC.putCloseable(key, value)) {
     *     ....
     *   }
     * </code>
     *
     * @param key non-null key
     * @param val value to put in the map
     * @return a <code>Closeable</code> who can remove <code>key</code> when <code>close</code>
     * is called.
     *
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static MDCCloseable putCloseable(String key, String val) throws IllegalArgumentException {
        put(key, val);
        return new MDCCloseable(key);
    }

    /**
     * Get the diagnostic context identified by the <code>key</code> parameter. The
     * <code>key</code> parameter cannot be null.
     * 
     * <p>
     * This method delegates all work to the MDC of the underlying logging system.
     *
     * @param key  
     * @return the string value identified by the <code>key</code> parameter.
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static String get(String key) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key parameter cannot be null");
        }

        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        return mdcAdapter.get(key);
    }

    /**
     * Remove the diagnostic context identified by the <code>key</code> parameter using
     * the underlying system's MDC implementation. The <code>key</code> parameter
     * cannot be null. This method does nothing if there is no previous value
     * associated with <code>key</code>.
     *
     * @param key  
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static void remove(String key) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key parameter cannot be null");
        }

        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.remove(key);
    }

    /**
     * Clear all entries in the MDC of the underlying implementation.
     */
    public static void clear() {
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.clear();
    }

    /**
     * Return a copy of the current thread's context map, with keys and values of
     * type String. Returned value may be null.
     * 
     * @return A copy of the current thread's context map. May be null.
     * @since 1.5.1
     */
    public static Map<String, String> getCopyOfContextMap() {
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        return mdcAdapter.getCopyOfContextMap();
    }

    /**
     * Set the current thread's context map by first clearing any existing map and
     * then copying the map passed as parameter. The context map passed as
     * parameter must only contain keys and values of type String.
     * 
     * @param contextMap
     *          must contain only keys and values of type String
     * @since 1.5.1
     */
    public static void setContextMap(Map<String, String> contextMap) {
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.setContextMap(contextMap);
    }

    /**
     * Returns the MDCAdapter instance currently in use.
     * 
     * @return the MDcAdapter instance currently in use.
     * @since 1.4.2
     */
    public static MDCAdapter getMDCAdapter() {
        return mdcAdapter;
    }

}

可以看到MDC具体操作都是通过MDCAdapter.java接口来操作，静态块中初始化得到MDCAdapter实现类的实例。

• MDCAdapter.java

public class NOPMDCAdapter implements MDCAdapter {

    public void clear() {
    }

    public String get(String key) {
        return null;
    }

    public void put(String key, String val) {
    }

    public void remove(String key) {
    }

    public Map<String, String> getCopyOfContextMap() {
        return null;
    }

    public void setContextMap(Map<String, String> contextMap) {
        // NOP
    }

}

slf4j-api包中对MDCAdapter接口提供了两个实现类BasicMDCAdapter和NOPMDCAdapter。在对slf4j提供具体实现的依赖包时也会提供其他MDCAdapter的实现，比如logback中提供了LogbackMDCAdapter，log4j2中提供了Log4jMDCAdapter。上述例子中集成了log4j2，下面看下Log4jMDCAdapter（log4j-slf4j-impl.jar）的具体实现：

public class Log4jMDCAdapter implements MDCAdapter {

    @Override
    public void put(final String key, final String val) {
        ThreadContext.put(key, val);
    }

    @Override
    public String get(final String key) {
        return ThreadContext.get(key);
    }

    @Override
    public void remove(final String key) {
        ThreadContext.remove(key);
    }

    @Override
    public void clear() {
        ThreadContext.clearMap();
    }

    @Override
    public Map<String, String> getCopyOfContextMap() {
        return ThreadContext.getContext();
    }

    @Override
    @SuppressWarnings("unchecked") // nothing we can do about this, restricted by SLF4J API
    public void setContextMap(@SuppressWarnings("rawtypes") final Map map) {
        ThreadContext.clearMap();
        for (final Map.Entry<String, String> entry : ((Map<String, String>) map).entrySet()) {
            ThreadContext.put(entry.getKey(), entry.getValue());
        }
    }

Log4jMDCAdapter内容很简洁，对容器的存取使用了一个封装类ThreadContext。
ThreadContext源码片段：

...//省略代码若干
private static ThreadContextMap contextMap;
private static ThreadContextStack contextStack;
...//省略代码若干
static void init() {
    contextMap = null;
    final PropertiesUtil managerProps = PropertiesUtil.getProperties();
    disableAll = managerProps.getBooleanProperty(DISABLE_ALL);
    useStack = !(managerProps.getBooleanProperty(DISABLE_STACK) || disableAll);
    useMap = !(managerProps.getBooleanProperty(DISABLE_MAP) || disableAll);

    contextStack = new DefaultThreadContextStack(useStack);
    if (!useMap) {
        contextMap = new NoOpThreadContextMap();
    } else {
        contextMap = ThreadContextMapFactory.createThreadContextMap();
    }
}
...//省略代码若干
public static void put(final String key, final String value) {
        contextMap.put(key, value);
    }
public static String get(final String key) {
        return contextMap.get(key);
    }
...//省略代码若干

ThreadContext类中存取数据使用了另一个封装ThreadContextMap，而默认ThreadContextMap由ThreadContextMapFactory.createThreadContextMap初始化：

public static ThreadContextMap createThreadContextMap() {
        final PropertiesUtil managerProps = PropertiesUtil.getProperties();
        final String threadContextMapName = managerProps.getStringProperty(THREAD_CONTEXT_KEY);
        final ClassLoader cl = ProviderUtil.findClassLoader();
        ThreadContextMap result = null;
        if (threadContextMapName != null) {
            try {
                final Class<?> clazz = cl.loadClass(threadContextMapName);
                if (ThreadContextMap.class.isAssignableFrom(clazz)) {
                    result = (ThreadContextMap) clazz.newInstance();
                }
            } catch (final ClassNotFoundException cnfe) {
                LOGGER.error("Unable to locate configured ThreadContextMap {}", threadContextMapName);
            } catch (final Exception ex) {
                LOGGER.error("Unable to create configured ThreadContextMap {}", threadContextMapName, ex);
            }
        }
        if (result == null && ProviderUtil.hasProviders()) {
            final String factoryClassName = LogManager.getFactory().getClass().getName();
            for (final Provider provider : ProviderUtil.getProviders()) {
                if (factoryClassName.equals(provider.getClassName())) {
                    final Class<? extends ThreadContextMap> clazz = provider.loadThreadContextMap();
                    if (clazz != null) {
                        try {
                            result = clazz.newInstance();
                            break;
                        } catch (final Exception e) {
                            LOGGER.error("Unable to locate or load configured ThreadContextMap {}",
                                    provider.getThreadContextMap(), e);
                            result = createDefaultThreadContextMap();
                        }
                    }
                }
            }
        }
        if (result == null) {
            result = createDefaultThreadContextMap();
        }
        return result;
    }
    private static ThreadContextMap createDefaultThreadContextMap() {
        if (Constants.ENABLE_THREADLOCALS) {
            if (PropertiesUtil.getProperties().getBooleanProperty(GC_FREE_THREAD_CONTEXT_KEY)) {
                return new GarbageFreeSortedArrayThreadContextMap();
            }
            return new CopyOnWriteSortedArrayThreadContextMap();
        }
        return new DefaultThreadContextMap(true);
    }

最后默认使用DefaultThreadContextMap。DefaultThreadContextMap片段：

public DefaultThreadContextMap(final boolean useMap) {
        this.useMap = useMap;
        this.localMap = createThreadLocalMap(useMap);
    }

    // LOG4J2-479: by default, use a plain ThreadLocal, only use InheritableThreadLocal if configured.
    // (This method is package protected for JUnit tests.)
    static ThreadLocal<Map<String, String>> createThreadLocalMap(final boolean isMapEnabled) {
        final PropertiesUtil managerProps = PropertiesUtil.getProperties();
        final boolean inheritable = managerProps.getBooleanProperty(INHERITABLE_MAP);
        if (inheritable) {
            return new InheritableThreadLocal<Map<String, String>>() {
                @Override
                protected Map<String, String> childValue(final Map<String, String> parentValue) {
                    return parentValue != null && isMapEnabled //
                    ? Collections.unmodifiableMap(new HashMap<>(parentValue)) //
                            : null;
                }
            };
        }
        // if not inheritable, return plain ThreadLocal with null as initial value
        return new ThreadLocal<>();
    }

到这里已经很清楚的看到容器默认使用的是ThreadLocal<Map<String, String>>存储。所以在新建子线程时候需要通过MDC.getCopyOfContextMap()和MDC.setContextMap()将父线程MDC容器内容传递到子线程中。当然源码中注释提到，可以通过配置使用InheritableThreadLocal，这样子线程可以直接继承父线程中MDC容器内容，但是因为性能问题，默认是不启用InheritableThreadLocal的。
子线程示例：


可以看到子线程也打印出了父线程中设置的requestId值。
跟踪上面贴出来的源码，启用InheritableThreadLocal需要我们在工程里增加配置文件log4j2.component.properties，log4j-api.jar在加载时会通过PropertiesUtil读取该文件内配置 的属性。
log4j2.component.properties

# 启用InheritableThreadLocal
isThreadContextMapInheritable = false

至此代码内无需在手动操作MDC.getCopyOfContextMap()和MDC.setContextMap()了，子线程会继承父线程内已经设置的MDC内容。

对于想要自定义ThreadContextMap的需要，也可以在配置文件log4j2.component.propertie中指定自己的实现类。
ThreadContextMapFactory.java

public static ThreadContextMap createThreadContextMap() {
        final PropertiesUtil managerProps = PropertiesUtil.getProperties();
        final String threadContextMapName = managerProps.getStringProperty(THREAD_CONTEXT_KEY);
        final ClassLoader cl = ProviderUtil.findClassLoader();
        ThreadContextMap result = null;
        // 在log4j2.component.propertie文件配置log4j2.threadContextMap = xx.xxx.xxx.yourThreadContextMap,会使用自定义ThreadContextMap实体
        if (threadContextMapName != null) {
            try {
                final Class<?> clazz = cl.loadClass(threadContextMapName);
                if (ThreadContextMap.class.isAssignableFrom(clazz)) {
                    result = (ThreadContextMap) clazz.newInstance();
                }
            } catch (final ClassNotFoundException cnfe) {
                LOGGER.error("Unable to locate configured ThreadContextMap {}", threadContextMapName);
            } catch (final Exception ex) {
                LOGGER.error("Unable to create configured ThreadContextMap {}", threadContextMapName, ex);
            }
        }
        // 此处会检查log4j-core.jar内META-INF/log4j-provider.properties文件是否指定了ThreadContextMap实现类
        if (result == null && ProviderUtil.hasProviders()) {
            final String factoryClassName = LogManager.getFactory().getClass().getName();
            for (final Provider provider : ProviderUtil.getProviders()) {
                if (factoryClassName.equals(provider.getClassName())) {
                    final Class<? extends ThreadContextMap> clazz = provider.loadThreadContextMap();
                    if (clazz != null) {
                        try {
                            result = clazz.newInstance();
                            break;
                        } catch (final Exception e) {
                            LOGGER.error("Unable to locate or load configured ThreadContextMap {}",
                                    provider.getThreadContextMap(), e);
                            result = createDefaultThreadContextMap();
                        }
                    }
                }
            }
        }
        // 使用默认ThreadContextMap实体
        if (result == null) {
            result = createDefaultThreadContextMap();
        }
        return result;
    }