在做日志链路追踪的场景中，我们需要将traceId从父线程传递到子线程，我们无法直接通过ThreadLocal进行值传递：
public class Main {
private static ThreadLocal<String> threadLocal = new ThreadLocal<>();
public static void main(String[] args) {
threadLocal.set("mainThread");
System.out.println("value:"+threadLocal.get());
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
String value = threadLocal.get();
System.out.println("value:"+value);
}
});
thread.start();
}
}
输出：
value:mainThread
value:null
InheritableThreadLocal继承自ThreadLocal，在Thread的init方法中有这样的逻辑：
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
即，线程的inheritThreadLocals是会从父线程传递到子线程的：
public class Main {
private static InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
public static void main(String[] args) {
threadLocal.set("mainThread");
System.out.println("value:"+threadLocal.get());
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
String value = threadLocal.get();
System.out.println("value:"+value);
}
});
thread.start();
}
}
输出：
value:mainThread
value:mainThread
我们在过滤器中设置traceId：
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain chain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) servletRequest;
// 从请求头中获取traceId
String traceId = request.getHeader("traceId");
// 不存在就生成一个
if (traceId == null || "".equals(traceId)) {
traceId = UUID.randomUUID().toString();
}
// 放入MDC中
MDC.put("traceId", traceId);
chain.doFilter(servletRequest, servletResponse);
}
SpringBoot中默认使用的日志框架是logback，Slf4j提供了MDC ，具体的装饰者实现类是LogbackMDCAdapter
在早期的版本中子线程可以直接自动继承父线程的MDC容器中的内容，因为MDC在早期版本中使用的是InheritableThreadLocal来作为底层实现。但是由于性能问题被取消了，最后还是使用的是ThreadLocal来作为底层实现。这样子线程就不能直接继承父线程的MDC容器。
package ch.qos.logback.classic.util;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import org.slf4j.spi.MDCAdapter;
public class LogbackMDCAdapter implements MDCAdapter {
final ThreadLocal<Map<String, String>> copyOnThreadLocal = new ThreadLocal<Map<String, String>>();
private static final int WRITE_OPERATION = 1;
private static final int MAP_COPY_OPERATION = 2;
// keeps track of the last operation performed
final ThreadLocal<Integer> lastOperation = new ThreadLocal<Integer>();
private Integer getAndSetLastOperation(int op) {
Integer lastOp = lastOperation.get();
lastOperation.set(op);
return lastOp;
}
所以，Logback官方建议我们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，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。
@Override
public void execute(Runnable command) {
// 提交者的本地变量
Map<String, String> contextMap = MDC.getCopyOfContextMap();
super.execute(()->{
if (contextMap != null) {
// 如果提交者有本地变量，任务执行之前放入当前任务所在的线程的本地变量中
MDC.setContextMap(contextMap);
}
try {
command.run();
} finally {
// 任务执行完，清除本地变量，以防对后续任务有影响
MDC.clear();
}
});
}
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