TransmittableThreadLocal在使用线程池等会缓存线程的组件情况下传递ThreadLocal

/**
  * 该函数在父线程创建子线程，向子线程复制InheritableThreadLocal变量时使用
*/
protected T childValue(T parentValue) {
    return parentValue;
}

/**
  * 由于重写了getMap，操作InheritableThreadLocal时，
  * 将只影响Thread类中的inheritableThreadLocals变量，
  * 与threadLocals变量不再有关系
*/
ThreadLocalMap getMap(Thread t) {
     return t.inheritableThreadLocals;
}

/**
 * 类似于getMap，操作InheritableThreadLocal时，
 * 将只影响Thread类中的inheritableThreadLocals变量，
 * 与threadLocals变量不再有关系
 */
void createMap(Thread t, T firstValue) {
    t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}

public class Thread implements Runnable {
   ......(其他源码)
    /* 
     * 当前线程的ThreadLocalMap，主要存储该线程自身的ThreadLocal
     */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal，自父线程集成而来的ThreadLocalMap，
     * 主要用于父子线程间ThreadLocal变量的传递
     * 本文主要讨论的就是这个ThreadLocalMap
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
    ......(其他源码)
}

hread thread = new Thread();

**
 * Allocates a new {@code Thread} object. This constructor has the same
 * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
 * {@code (null, null, gname)}, where {@code gname} is a newly generated
 * name. Automatically generated names are of the form
 * {@code "Thread-"+}<i>n</i>, where <i>n</i> is an integer.
 */
public Thread() {
    init(null, null, "Thread-" + nextThreadNum(), 0);
}

/**
 * 默认情况下，设置inheritThreadLocals可传递
 */
private void init(ThreadGroup g, Runnable target, String name,
                  long stackSize) {
    init(g, target, name, stackSize, null, true);
}

/**
 * 初始化一个线程.
 * 此函数有两处调用，
 * 1、上面的 init()，不传AccessControlContext，inheritThreadLocals=true
 * 2、传递AccessControlContext，inheritThreadLocals=false
 */
private void init(ThreadGroup g, Runnable target, String name,
                  long stackSize, AccessControlContext acc,
                  boolean inheritThreadLocals) {
    ......（其他代码）

    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

    ......（其他代码）
}

static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
    return new ThreadLocalMap(parentMap);
}


/**
 * 构建一个包含所有parentMap中Inheritable ThreadLocals的ThreadLocalMap
 * 该函数只被 createInheritedMap() 调用.
 */
private ThreadLocalMap(ThreadLocalMap parentMap) {
    Entry[] parentTable = parentMap.table;
    int len = parentTable.length;
    setThreshold(len);
    // ThreadLocalMap 使用 Entry[] table 存储ThreadLocal
    table = new Entry[len];

    // 逐一复制 parentMap 的记录
    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) {
                // 可能会有同学好奇此处为何使用childValue，而不是直接赋值，
                // 毕竟childValue内部也是直接将e.value返回；
                // 个人理解，主要为了减轻阅读代码的难度
                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++;
            }
        }
    }
}

// 在父线程中设置
TransmittableThreadLocal<String> parent = new TransmittableThreadLocal<String>();
parent.set("value-set-in-parent");

// =====================================================

// 在子线程中可以读取，值是"value-set-in-parent"
String value = parent.get();

TransmittableThreadLocal<String> parent = new TransmittableThreadLocal<String>();
parent.set("value-set-in-parent");

Runnable task = new Task("1");
// 额外的处理，生成修饰了的对象ttlRunnable
Runnable ttlRunnable = TtlRunnable.get(task);
executorService.submit(ttlRunnable);

// =====================================================

// Task中可以读取，值是"value-set-in-parent"
String value = parent.get();

TransmittableThreadLocal<String> parent = new TransmittableThreadLocal<String>();
parent.set("value-set-in-parent");

Callable call = new Call("1");
// 额外的处理，生成修饰了的对象ttlCallable
Callable ttlCallable = TtlCallable.get(call);
executorService.submit(ttlCallable);

// =====================================================

// Call中可以读取，值是"value-set-in-parent"
String value = parent.get();

ExecutorService executorService = ...
// 额外的处理，生成修饰了的对象executorService
executorService = TtlExecutors.getTtlExecutorService(executorService);

TransmittableThreadLocal<String> parent = new TransmittableThreadLocal<String>();
parent.set("value-set-in-parent");

Runnable task = new Task("1");
Callable call = new Call("2");
executorService.submit(task);
executorService.submit(call);

// =====================================================

// Task或是Call中可以读取，值是"value-set-in-parent"
String value = parent.get();

// ## 1. 框架上层逻辑，后续流程框架调用业务 ##
TransmittableThreadLocal<String> context = new TransmittableThreadLocal<String>();
context.set("value-set-in-parent");

// ## 2. 应用逻辑，后续流程业务调用框架下层逻辑 ##
ExecutorService executorService = Executors.newFixedThreadPool(3);

Runnable task = new Task("1");
Callable call = new Call("2");
executorService.submit(task);
executorService.submit(call);

// ## 3. 框架下层逻辑 ##
// Task或是Call中可以读取，值是"value-set-in-parent"
String value = context.get();

java -javaagent:path/to/transmittable-thread-local-2.x.x.jar \
    -cp classes \
    com.alibaba.ttl.threadpool.agent.demo.AgentDemo

java -javaagent:path/to/ttl-foo-name-changed.jar \
    -Xbootclasspath/a:path/to/ttl-foo-name-changed.jar \
    -cp classes \
    com.alibaba.ttl.threadpool.agent.demo.AgentDemo

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>transmittable-thread-local</artifactId>
    <version>2.10.2</version>
</dependency>