在 springboot 项目中，如何将主线程中的 MDC 对 ThreadPoolTaskExecutor 管理的子线程中可见

在 Spring Boot 3.x 项目中，如果你想将主线程中的 MDC（Mapped Diagnostic Context）数据传递到 ThreadPoolTaskExecutor 管理的子线程中，可以使用 TaskDecorator 接口。

TaskDecorator 允许你在每个任务被执行之前进行一些处理，例如将 MDC 数据从主线程复制到子线程。以下是一个实现示例：

1. 定义一个 MDC 传递的 `TaskDecorator`

import org.slf4j.MDC;
import org.springframework.core.task.TaskDecorator;

import java.util.Map;

public class MdcTaskDecorator implements TaskDecorator { 

    @Override
    public Runnable decorate(Runnable runnable) { 
        // Capture the context of the current thread (the main thread)
        Map<String, String> contextMap = MDC.getCopyOfContextMap();
        
        return () -> { 
            try { 
                // Set the context map for the child thread
                if (contextMap != null) { 
                    MDC.setContextMap(contextMap);
                }
                runnable.run();
            } finally { 
                // Clear the context map after the child thread has finished
                MDC.clear();
            }
        };
    }
}

2. 配置 `ThreadPoolTaskExecutor` 并使用 `TaskDecorator`

在你的 Spring 配置类中，配置 ThreadPoolTaskExecutor 并设置 TaskDecorator：

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;

import java.util.concurrent.Executor;

@Configuration
public class ThreadPoolConfig { 

    @Bean(name = "taskExecutor")
    public Executor taskExecutor() { 
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(5);
        executor.setMaxPoolSize(10);
        executor.setQueueCapacity(25);
        executor.setThreadNamePrefix("MyExecutor-");

        // Set the TaskDecorator
        executor.setTaskDecorator(new MdcTaskDecorator());
        executor.initialize();

        return executor;
    }
}

3. 在主线程中设置 MDC 并使用 `ThreadPoolTaskExecutor`

在主线程中，可以像平常一样使用 MDC，然后提交任务到线程池：

import org.slf4j.MDC;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
import org.springframework.stereotype.Service;

@Service
public class MyService { 

    @Autowired
    private ThreadPoolTaskExecutor taskExecutor;

    public void executeTask() { 
        // Set some MDC context in the main thread
        MDC.put("traceId", "12345");

        // Submit task to the thread pool
        taskExecutor.execute(() -> { 
            // The MDC context is now available here
            System.out.println("Trace ID: " + MDC.get("traceId"));
        });

        // Clear MDC context after task submission if necessary
        MDC.clear();
    }
}