线程池和 InheritedThreadLocal

Question

我看过下面的问题。答案是改用信号量。这没有回答我面临的问题中所述的其他问题之一。

Using InheritableThreadLocal with ThreadPoolExecutor -- or -- a ThreadPoolExecutor that doesn't reuse threads

我有一个父线程，它为 InhertiedThreadLocal 中的每个新请求设置一些唯一标识符，并向 ThreadPool 提交 2 个可运行任务，即 2 个线程。 对于初始请求，在父线程中为 InheritedThreadLocal 设置的值会正确传播到 ChildThread。 对于接下来的请求，子线程没有接收到父线程设置的最新的 InheritedThreadLocal，而是使用了 ChildThread 中的旧值。

这是因为线程池重用线程，而 InheritedThreadLocal 仅在创建新线程时才被复制。

现在如何在线程池场景中将 InheritedThreadLocal 的最新值从父线程传播到子线程。 有没有办法解决这个问题？

Answer 1

我根据需要编写了这些方法。

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.util.concurrent.ThreadPoolExecutor;

public class EnvUtils {
    /**
     * Extract the current inheritableThreadLocals map from the current thread.
     * Typical usage is in a threadpool, where:
     * <ul>
     * <li>You run {@link EnvUtils#extract()} in the running thread, to store
     * the information somewhere
     * <li>You create a method {@link ThreadPoolExecutor#beforeExecute()} in which
     * you run {@link EnvUtils#copy(Object)} with the above-stored information.
     * </ul>
     *
     * @return The current inheritableThreadLocals of the current thread
     */
    public static Object extract() {
        Object toreturn = null;
        try {
            // get field descriptor
            Field inthlocalsField = Thread.class.getDeclaredField("inheritableThreadLocals");
            inthlocalsField.setAccessible(true);
            //the object stored there
            Object inthlocalsMap = inthlocalsField.get(Thread.currentThread());
            // no need to copy, it will be done by the copy() method below
            toreturn = inthlocalsMap;
        } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
            // This may happen in a different Java implementation
            throw new RuntimeException(e);
        }
        return toreturn;
    }

    /**
     * Replaces the field inheritableThreadLocals of the current thread with the values provided.<br/>
     * It is the same as if the current thread was just created from the thread whose <code>stored</code>
     * values come from.<br/>
     * Must be called in the thread which want to inherit from given {@link inheritableThreadLocals} map.<br/>
     * <b>Note 1:</b> This does not modify non-inheritable thread locals<br/>
     * <b>Note 2:</b> This delete all previous values of {@link inheritableThreadLocals} in the current thread<br/>
     *
     * @param stored
     *            The stored inheritableThreadLocals value, coming from the extract() method
     */
    public static void copy(final Object stored) {
        try {
            // find ThreadLocalMap class
            String threadLocalClassName = ThreadLocal.class.getName();
            Class<?> threadLocaLMapClass = Class.forName(threadLocalClassName + "$ThreadLocalMap");
            // check that given object is an instance of the class
            if (stored == null || !threadLocaLMapClass.isInstance(stored)) {
                throw new IllegalArgumentException("Given object is not a ThreadLocalMap: " + stored);
            }
            // get constructor of ThreadLocalMap
            Constructor<?> creator = threadLocaLMapClass.getDeclaredConstructor(threadLocaLMapClass);
            creator.setAccessible(true);
            // get field descriptor of the thread
            Field inthlocalsField = Thread.class.getDeclaredField("inheritableThreadLocals");
            inthlocalsField.setAccessible(true);
            // create new inherited map
            Object newObj = creator.newInstance(stored);
            // set it to the current thread
            inthlocalsField.set(Thread.currentThread(), newObj);

        } catch (ClassNotFoundException | NoSuchMethodException | SecurityException | NoSuchFieldException
                | IllegalAccessException | IllegalArgumentException | InvocationTargetException
                | InstantiationException e) {
            // This may happen in a different Java implementation
            throw new RuntimeException(e);
        }

    }

}

EnvUtils.extract() 返回对当前线程的inheritableThreadLocals 映射的引用。

现在，当您创建要在ThreadPool 中调用的Runnable 时，只需将其存储在inheritableThreadInfo = EnvUtils.extract() 字段中，并在其run() 方法中调用EnvUtils.copy(inheritableThreadInfo)。

注意：此解决方案使用了大量反射，因此它受制于 Java 实现。我在 Oracle Java 1.8 上进行了测试。

Answer 2

如果我们这样配置 ThreadPoolExecutor

new ThreadPoolExecutor(0, 2, 0L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>());

它每次都会创建一个新的线程，因此会继承InheritableThreadLocals，但它几乎不能称为ThreadPoolExecutor