本文主要是介绍slf4j打印traceid，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
我们在调试代码时会打很多日志，这些错综复杂的日志往往混杂在一起，很难筛选出某个请求链路的日志。我们就希望在每个请求到来时生成一个唯一的traceid，可以set到请求头信息中，打日志时带上就可以快速筛选请求链路的日志了。slf4j提供了这样的功能（MDC），slf4j用ThreadLocal来存储traceid。可以用拦截器、aop等方式使用，以下是拦截器的示例：
- 首先注册一个拦截器，在preHandle方法中调用MDC.put()方法，保存随机生成的id。在afterCompletion方法中删除。
public class TraceInterceptor extends HandlerInterceptorAdapter {@Overridepublic boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {MDC.put("trace", UUID.randomUUID().toString());return super.preHandle(request, response, handler);}@Overridepublic void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)throws Exception {MDC.remove("trace");super.afterCompletion(request, response, handler, ex);}
}<mvc:interceptors><bean class="com.xxx.interceptor.TraceInterceptor"/>
</mvc:interceptors>
- 在logback.xml中添加%X{name}
<property name="defaultPattern"value="%d{yyyy-MM-dd HH:mm:ss.SSS} %-5p %X{trace} %t \\(%F:%L\\) - %msg%n"/>
然后就可以在日志中看到每个请求的traceid了,可以通过cat grep查看请求链路的日志
2020-09-04 15:41:59.386 INFO 159920597.704 b5491258-6b41-4cf9-8980-3bf336713afc 10.58.75.46:60454 1440771206 http-nio-8085-exec-10 (XXX.java:432) - [onf] config INITIALIZE, key:dot, version:-1, stage:null
源码分析：
MDC通过mdcAdapter变量进行操作
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;public static void put(String key, String val) throws IllegalArgumentException {if (key == null) {throw new IllegalArgumentException("key parameter cannot be null");} else if (mdcAdapter == null) {throw new IllegalStateException("MDCAdapter cannot be null. See also http://www.slf4j.org/codes.html#null_MDCA");} else {mdcAdapter.put(key, val);}}static {try {mdcAdapter = bwCompatibleGetMDCAdapterFromBinder();} catch (NoClassDefFoundError var2) {mdcAdapter = new NOPMDCAdapter();String msg = var2.getMessage();if (msg == null || !msg.contains("StaticMDCBinder")) {throw var2;}Util.report("Failed to load class \"org.slf4j.impl.StaticMDCBinder\".");Util.report("Defaulting to no-operation MDCAdapter implementation.");Util.report("See http://www.slf4j.org/codes.html#no_static_mdc_binder for further details.");} catch (Exception var3) {Util.report("MDC binding unsuccessful.", var3);}}。。。}
可以看到mdcAdapter = bwCompatibleGetMDCAdapterFromBinder();
private static MDCAdapter bwCompatibleGetMDCAdapterFromBinder() throws NoClassDefFoundError {try {return StaticMDCBinder.getSingleton().getMDCA();} catch (NoSuchMethodError var1) {return StaticMDCBinder.SINGLETON.getMDCA();}}StaticMDCBinder类
public class StaticMDCBinder {public static final StaticMDCBinder SINGLETON = new StaticMDCBinder();private StaticMDCBinder() {}public MDCAdapter getMDCA() {return new LogbackMDCAdapter();}public String getMDCAdapterClassStr() {return LogbackMDCAdapter.class.getName();}
}LogbackMDCAdapter类及put方法，可以看到有一个ThreadLocal变量
public class LogbackMDCAdapter implements MDCAdapter {final ThreadLocal<Map<String, String>> copyOnThreadLocal = new ThreadLocal();private static final int WRITE_OPERATION = 1;private static final int MAP_COPY_OPERATION = 2;final ThreadLocal<Integer> lastOperation = new ThreadLocal();public void put(String key, String val) throws IllegalArgumentException {if (key == null) {throw new IllegalArgumentException("key cannot be null");} else {Map<String, String> oldMap = (Map)this.copyOnThreadLocal.get();Integer lastOp = this.getAndSetLastOperation(1);if (!this.wasLastOpReadOrNull(lastOp) && oldMap != null) {oldMap.put(key, val);} else {Map<String, String> newMap = this.duplicateAndInsertNewMap(oldMap);newMap.put(key, val);}}}。。。}
还一个BasicMDCAdapter类暂时不知道怎么使用的，内部维护了一个InheritableThreadLocal的变量。只知道是为了解决这个问题：ThreadLocal只保证在同一个线程间共享变量，也就是说如果这个线程起了一个新线程，那么新线程是不会得到父线程的变量信息的。
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