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。
<!-- 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
<!-- 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
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
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。
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;
}