为每个请求分配traceId的两种方式及父子线程本地变量传递
需求背景
有时候我们需要某个请求下的所有的traceId都是一致的，以获得统一解析的日志文件。便于排查问题。
为每一个请求分配同一个traceId据我所知有两种方式：MDC和ThreadLocal，MDC的内部实现也是ThreadLocal，下面分别介绍这两种方式。
一、MDC
MDC(Mapped Diagnostic Contexts)，翻译过来就是：映射的诊断上下文。意思是：在日志中（映射的）请求ID（requestId），可以作为我们定位（诊断）问题的关键字（上下文）。
有了MDC工具，只要在接口或切面植入 put 和 remove 代码，就可以在定位问题时，根据映射的唯一 requestID 快速过滤出某次请求的所有日志。
另外，当客户端请求到来时，可以将客户端id，ip，请求参数等信息保存在MDC中，同时在logback.xml中配置，那么会自动在每个日志条目中包含这些信息。
slf4j的MDC机制其内部基于ThreadLocal实现，可参见ThreadLocal这篇博客：https://www.cnblogs.com/yangyongjie/p/10574591.html
MDC类基本原理其实非常简单，其内部持有一个ThreadLocal实例，用于保存context数据，MDC提供了put/get/clear等几个核心接口，用于操作ThreadLocal中的数据；ThreadLocal中的K-V，可以在logback.xml中声明，即在layout中通过声明“%X{Key}”来打印MDC中保存的此key对应的value在日志中。
在使用MDC时需要注意一些问题，这些问题通常也是ThreadLocal引起的，比如我们需要在线程退出之前清除（clear）MDC中的数据；在线程池中使用MDC时，那么需要在子线程退出之前清除数据；可以调用MDC.clear()方法。
MDC部分源码：
package org.slf4j; public class MDC { // 将上下文的值作为 MDC 的 key 放到线程上下的 map 中 public static void put(String key, String val); // 通过 key 获取上下文标识 public static String get(String key); // 通过 key 移除上下文标识 public static void remove(String key); // 清除 MDC 中所有的 entry public static void clear(); }
1、请求没有子线程的情况下代码实现：
1）使用Aop拦截请求
/** * 为每一个的HTTP请求添加线程号 * * @author yangyongjie * @date 2019/9/2 * @desc */ @Aspect @Component public class LogAspect { private static final String STR_THREAD_ID = "threadId"; @Pointcut(value = "@annotation(org.springframework.web.bind.annotation.RequestMapping)") private void webPointcut() { // doNothing } /** * 为所有的HTTP请求添加线程号 * * @param joinPoint * @throws Throwable */ @Around(value = "webPointcut()") public void around(ProceedingJoinPoint joinPoint) throws Throwable { // 方法执行前加上线程号 MDC.put(STR_THREAD_ID, UUID.randomUUID().toString().replaceAll("-", "")); // 执行拦截的方法 joinPoint.proceed(); // 方法执行结束移除线程号 MDC.remove(STR_THREAD_ID); } }
2）log4j日志配置
log4j.appender.stdout.layout.ConversionPattern=[%-5p]%d{yyyy-MM-dd HH:mm:ss.SSS}[%t]%X{threadId}[%c:%L] - %m%n
需要注意日志红色中字符串 threadId 需要和 日志拦截中MDC put的key是一样的。
2、请求有子线程的情况
slf4j的MDC机制其内部基于ThreadLocal实现，可参见Java基础下的 ThreadLocal这篇博客，https://www.cnblogs.com/yangyongjie/p/10574591.html。所以我们调用 MDC.put()方法传入
的请求ID只在当前线程有效。所以，主线程中设置的MDC数据，在其子线程（线程池）中是无法获取的。那么主线程如何将MDC数据传递给子线程？
官方建议
1）在父线程新建子线程之前调用MDC.getCopyOfContextMap()方法将MDC内容取出来传给子线程
2）子线程在执行操作前先调用MDC.setContextMap()方法将父线程的MDC内容设置到子线程
代码实现
1）使用Aop拦截请求，与上面相同
2）log4j日志配置与上面相同
3）装饰器模式装饰子线程，有两种方式：
方式一：使用装饰器模式，对Runnable接口进行一层装饰，在创建MDCRunnable类对Runnable接口进行一层装饰。
在创建MDCRunnable类时保存当前线程的MDC值，再执行run()方法
装饰器MDCRunnable装饰Runnable：
import org.slf4j.MDC; import java.util.Map; /** * 装饰器模式装饰Runnable，传递父线程的线程号 * * @author yangyongjie * @date 2020/3/9 * @desc */ public class MDCRunnable implements Runnable { private Runnable runnable; /** * 保存当前主线程的MDC值 */ private final Map<String, String> mainMdcMap; public MDCRunnable(Runnable runnable) { this.runnable = runnable; this.mainMdcMap = MDC.getCopyOfContextMap(); } @Override public void run() { // 将父线程的MDC值赋给子线程 for (Map.Entry<String, String> entry : mainMdcMap.entrySet()) { MDC.put(entry.getKey(), entry.getValue()); } // 执行被装饰的线程run方法 runnable.run(); // 执行结束移除MDC值 for (Map.Entry<String, String> entry : mainMdcMap.entrySet()) { MDC.put(entry.getKey(), entry.getValue()); } } }
使用MDCRunnable代替Runnable：
// 异步线程打印日志，用MDCRunnable装饰Runnable new Thread(new MDCRunnable(new Runnable() { @Override public void run() { logger.debug("log in other thread"); } })).start(); // 异步线程池打印日志，用MDCRunnable装饰Runnable EXECUTOR.execute(new MDCRunnable(new Runnable() { @Override public void run() { logger.debug("log in other thread pool"); } })); EXECUTOR.shutdown();
方式二：装饰线程池
/** * 装饰ThreadPoolExecutor，将父线程的MDC内容传给子线程 * @author yangyongjie * @date 2020/3/19 * @desc */ public class MDCThreadPoolExecutor extends ThreadPoolExecutor { private static final Logger LOGGER= LoggerFactory.getLogger(MDCThreadPoolExecutor.class); public MDCThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler); } @Override public void execute(final Runnable runnable) { // 获取父线程MDC中的内容，必须在run方法之前，否则等异步线程执行的时候有可能MDC里面的值已经被清空了，这个时候就会返回null final Map<String, String> context = MDC.getCopyOfContextMap(); super.execute(new Runnable() { @Override public void run() { // 将父线程的MDC内容传给子线程 MDC.setContextMap(context); try { // 执行异步操作 runnable.run(); } finally { // 清空MDC内容 MDC.clear(); } } }); } }
用MDCThreadPoolExecutor 代替ThreadPoolExecutor ：
private static final MDCThreadPoolExecutor MDCEXECUTORS=new MDCThreadPoolExecutor(1,10,60,TimeUnit.SECONDS,new LinkedBlockingQueue<Runnable>(600), new CustomThreadFactory("mdcThreadPoolTest"), new RejectedExecutionHandler() { @Override public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) { // 打印日志，并且重启一个线程执行被拒绝的任务 LOGGER.error("Task：{},rejected from：{}", r.toString(), executor.toString()); // 直接执行被拒绝的任务，JVM另起线程执行 r.run(); } }); LOGGER.info("父线程日志"); MDCEXECUTORS.execute(new Runnable() { @Override public void run() { LOGGER.info("子线程日志"); } });
二、ThreadLocal方式
ThreadLocal可以用于在同一个线程内，跨类、跨方法传递数据。因此可以用来透传全局上下文
1、没有子线程的情况
1）创建线程的请求上下文
/** * 线程上下文，一个线程内所需的上下文变量参数，使用ThreadLocal保存副本 * * @author yangyongjie * @date 2019/9/12 * @desc */ public class ThreadContext { /** * 每个线程的私有变量，每个线程都有独立的变量副本，所以使用private static final修饰，因为都需要复制进入本地线程 */ private static final ThreadLocal<ThreadContext> THREAD_LOCAL = new ThreadLocal<ThreadContext>() { @Override protected ThreadContext initialValue() { return new ThreadContext(); } }; public static ThreadContext currentThreadContext() { /*ThreadContext threadContext = THREAD_LOCAL.get(); if (threadContext == null) { THREAD_LOCAL.set(new ThreadContext()); threadContext = THREAD_LOCAL.get(); } return threadContext;*/ return THREAD_LOCAL.get(); } public static void remove() { THREAD_LOCAL.remove(); } private String threadId; public String getThreadId() { return threadId; } public void setThreadId(String threadId) { this.threadId = threadId; } @Override public String toString() { return JacksonJsonUtil.toString(this); } }
2）使用Aop拦截请求，给每个请求线程ThreadLocalMap添加本地变量ThreadContext 对象并为对象的threadId属性赋值。
/** * 为每一个的HTTP请求添加线程号 * * @author yangyongjie * @date 2019/9/2 * @desc */ @Aspect @Component public class LogAspect { private static final Logger LOGGER = LoggerFactory.getLogger(LogAspect.class); @Pointcut(value = "@annotation(org.springframework.web.bind.annotation.RequestMapping)") private void webPointcut() { // doNothing } /** * 为所有的HTTP请求添加线程号 * * @param joinPoint * @throws Throwable */ @Around(value = "webPointcut()") public Object around(ProceedingJoinPoint joinPoint) throws Throwable { // 方法执行前加上线程号，并将线程号放到线程本地变量中 ThreadContext.currentThreadContext().setThreadId(StringUtil.uuid()); // 执行拦截的方法 Object result; try { result = joinPoint.proceed(); } finally { // 方法执行结束移除线程号，并移除线程本地变量,防止内存泄漏 ThreadContext.remove(); } return result; } }
3）获取线程号
String threadId = ThreadContext.currentThreadContext().getThreadId();
2、请求有子线程的情况
首先了解一下InheritableThreadLocal<T>，它是对ThreadLocal<T> 的扩展和继承，它的数据ThreadLocal.ThreadLocalMap保存在Thread的inheritableThreadLocals变量中，同时如果我们在当前线程开启一个新线程，而且当前线程存在inheritableThreadLocals变量，那么子线程会copy一份当前线程（父线程）中的这个变量持有的值。
源码：
Thread类的inheritableThreadLocals 属性：
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
Thread的构造方法会调用init方法，而init方法的inheritableThreadLocals参数默认为true：
public Thread() { init(null, null, "Thread-" + nextThreadNum(), 0); } public Thread(Runnable target) { init(null, target, "Thread-" + nextThreadNum(), 0); } private void init(ThreadGroup g, Runnable target, String name, long stackSize) { init(g, target, name, stackSize, null, true); }
那么如果当前线程的inheritableThreadLocals变量不为空，就可以将当前线程的变量继续往下传递给它创建的子线程：
private void init(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc, boolean inheritThreadLocals) { if (name == null) { throw new NullPointerException("name cannot be null"); } this.name = name; Thread parent = currentThread(); SecurityManager security = System.getSecurityManager(); if (g == null) { /* Determine if it's an applet or not */ /* If there is a security manager, ask the security manager what to do. */ if (security != null) { g = security.getThreadGroup(); } /* If the security doesn't have a strong opinion of the matter use the parent thread group. */ if (g == null) { g = parent.getThreadGroup(); } } /* checkAccess regardless of whether or not threadgroup is explicitly passed in. */ g.checkAccess(); /* * Do we have the required permissions? */ if (security != null) { if (isCCLOverridden(getClass())) { security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION); } } g.addUnstarted(); this.group = g; this.daemon = parent.isDaemon(); this.priority = parent.getPriority(); if (security == null || isCCLOverridden(parent.getClass())) this.contextClassLoader = parent.getContextClassLoader(); else this.contextClassLoader = parent.contextClassLoader; this.inheritedAccessControlContext = acc != null ? acc : AccessController.getContext(); this.target = target; setPriority(priority); if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals); /* Stash the specified stack size in case the VM cares */ this.stackSize = stackSize; /* Set thread ID */ tid = nextThreadID(); }
这里是重点，当inheritThreadLocals 参数为true，且创建此线程的线程即parent（父线程）的inheritableThreadLocals不为空的时候，就会在创建当前线程的时候将父线程的 inheritableThreadLocals 复制到 此新建的子线程。
if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
createInheritedMap()方法就是调用ThreadLocalMap的私有构造方法来产生一个实例对象，把父线程的不为null的线程变量都拷贝过来：
/** * Factory method to create map of inherited thread locals. * Designed to be called only from Thread constructor. * * @param parentMap the map associated with parent thread * @return a map containing the parent's inheritable bindings */ static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) { return new ThreadLocalMap(parentMap); } /** * Construct a new map including all Inheritable ThreadLocals * from given parent map. Called only by createInheritedMap. * * @param parentMap the map associated with parent thread. */ private ThreadLocalMap(ThreadLocalMap parentMap) { Entry[] parentTable = parentMap.table; int len = parentTable.length; setThreshold(len); table = new Entry[len]; for (int j = 0; j < len; j++) { Entry e = parentTable[j]; if (e != null) { @SuppressWarnings("unchecked") ThreadLocal<Object> key = (ThreadLocal<Object>) e.get(); if (key != null) { Object value = key.childValue(e.value); Entry c = new Entry(key, value); int h = key.threadLocalHashCode & (len - 1); while (table[h] != null) h = nextIndex(h, len); table[h] = c; size++; } } } }
实际应用代码实现：
/** * 线程上下文，一个线程内所需的上下文变量参数，使用InheritableThreadLocal保存副本，可以将副本传递给子线程 * @author yangyongjie * @date 2020/3/20 * @desc */ public class InheritableThreadContext { private static final InheritableThreadLocal<ThreadContext> INHERITABLE_THREAD_LOCAL = new InheritableThreadLocal<ThreadContext>() { @Override protected ThreadContext initialValue() { return new ThreadContext(); } }; public static ThreadContext currentThreadContext() { /*ThreadContext threadContext = INHERITABLE_THREAD_LOCAL.get(); if (threadContext == null) { INHERITABLE_THREAD_LOCAL.set(new ThreadContext()); threadContext = INHERITABLE_THREAD_LOCAL.get(); } return threadContext;*/ return INHERITABLE_THREAD_LOCAL.get(); } public static void remove() { INHERITABLE_THREAD_LOCAL.remove(); } private String threadId; public String getThreadId() { return threadId; } public void setThreadId(String threadId) { this.threadId = threadId; } @Override public String toString() { return JacksonJsonUtil.toString(this); } }
使用示例：
String uuid = UUID.randomUUID().toString().replaceAll("-", "");
// 当前线程的本地变量（为当前线程的inheritableThreadLocals属性赋值）
InheritableThreadContext.currentThreadContext().setThreadId(uuid);
LOGGER.info("[{}]父线程日志",InheritableThreadContext.currentThreadContext().getThreadId());
// 创建子线程
new Thread(new Runnable() {
@Override
public void run() {
LOGGER.info("[{}]子线程日志",InheritableThreadContext.currentThreadContext().getThreadId());
// 移除子线程本地变量
InheritableThreadContext.remove();
}
}).start();
// 线程池中的线程也是线程工厂通过new Thread创建的
EXECUTORS.execute(new Runnable() {
@Override
public void run() {
LOGGER.info("[{}]线程池子线程日志",InheritableThreadContext.currentThreadContext().getThreadId());
// 移除子线程本地变量
InheritableThreadContext.remove();
}
});
// 移除父线程本地变量
InheritableThreadContext.remove();
运行结果：
[INFO ]2020-03-20 13:53:24.056[main] - [4eb5063b1dd84448807cb94f7d4df87a]父线程日志 [INFO ]2020-03-20 13:53:26.133[Thread-8] - [4eb5063b1dd84448807cb94f7d4df87a]子线程日志 [INFO ]2020-03-20 13:53:26.137[From CustomThreadFactory-inheritable-worker-1] - [4eb5063b1dd84448807cb94f7d4df87a]线程池子线程日志
不使用InheritableThreadLocal而使用ThreadLocal的结果（将上面使用示例中的InheritableThreadContext替换为ThreadContext）：
[INFO ]2020-03-20 13:57:54.592[main] - [926f164b97914d29a3638c945c125d44]父线程日志 [INFO ]2020-03-20 13:57:56.214[Thread-8] - [null]子线程日志 [INFO ]2020-03-20 13:57:56.215[From CustomThreadFactory-worker-1] - [null]线程池子线程日志
注意：使用ThreadLocal和InheritableThreadLocal透传上下文时，需要注意线程间切换、异常传输时的处理，避免在传输过程中因处理不当而导致的上下文丢失。
Spring中对InheritableThreadLocal的使用：
Spring中对一个request范围的数据进行传递时，如当有请求到来需要将 HttpServletRequest的值进行传递，用的就是InheritableThreadLocal。
Spring的RequestContextHolder部分源码：
public abstract class RequestContextHolder { private static final ThreadLocal<RequestAttributes> requestAttributesHolder = new NamedThreadLocal<RequestAttributes>("Request attributes"); private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder = new NamedInheritableThreadLocal<RequestAttributes>("Request context"); /** * Reset the RequestAttributes for the current thread. */ public static void resetRequestAttributes() { requestAttributesHolder.remove(); inheritableRequestAttributesHolder.remove(); } /** * Bind the given RequestAttributes to the current thread, * <i>not</i> exposing it as inheritable for child threads. * @param attributes the RequestAttributes to expose * @see #setRequestAttributes(RequestAttributes, boolean) */ public static void setRequestAttributes(RequestAttributes attributes) { setRequestAttributes(attributes, false); } /** * Bind the given RequestAttributes to the current thread. * @param attributes the RequestAttributes to expose, * or {@code null} to reset the thread-bound context * @param inheritable whether to expose the RequestAttributes as inheritable * for child threads (using an {@link InheritableThreadLocal}) */ public static void setRequestAttributes(RequestAttributes attributes, boolean inheritable) { if (attributes == null) { resetRequestAttributes(); } else { if (inheritable) { inheritableRequestAttributesHolder.set(attributes); requestAttributesHolder.remove(); } else { requestAttributesHolder.set(attributes); inheritableRequestAttributesHolder.remove(); } } } /** * Return the RequestAttributes currently bound to the thread. * @return the RequestAttributes currently bound to the thread, * or {@code null} if none bound */ public static RequestAttributes getRequestAttributes() { RequestAttributes attributes = requestAttributesHolder.get(); if (attributes == null) { attributes = inheritableRequestAttributesHolder.get(); } return attributes; } }
NamedInheritableThreadLocal继承了InheritableThreadLocal ，添加了一个name属性：
public class NamedInheritableThreadLocal<T> extends InheritableThreadLocal<T> { private final String name; /** * Create a new NamedInheritableThreadLocal with the given name. * @param name a descriptive name for this ThreadLocal */ public NamedInheritableThreadLocal(String name) { Assert.hasText(name, "Name must not be empty"); this.name = name; } @Override public String toString() { return this.name; } }
从RequestContextListener可以看到当请求事件到来时，需要将HttpServletRequest的值通过RequestContextHolder进行传递：
public class RequestContextListener implements ServletRequestListener { ... public void requestInitialized(ServletRequestEvent requestEvent) { if (!(requestEvent.getServletRequest() instanceof HttpServletRequest)) { throw new IllegalArgumentException( "Request is not an HttpServletRequest: " + requestEvent.getServletRequest()); } HttpServletRequest request = (HttpServletRequest) requestEvent.getServletRequest(); ServletRequestAttributes attributes = new ServletRequestAttributes(request); request.setAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE, attributes); LocaleContextHolder.setLocale(request.getLocale()); RequestContextHolder.setRequestAttributes(attributes); } ... }
总结
1）若是日志线程号需要通过线程上下文传递时，使用MDC加日志文件配置的方式；有子线程时，使用MDCRunnable装饰Runnable
2）若是用户信息需要随着请求上下文传递时，使用ThreadLocal的方式；有子线程时，使用InheritableThreadLocal
3）若是上面两个信息都需要传递，那就结合一起使用。日志使用MDC，其他信息使用ThreadLocal。
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