InheritableThreadLocal详解

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1、简介

在上一篇 ThreadLocal详解 中,我们详细介绍了ThreadLocal原理及设计,从源码层面上分析了ThreadLocal。但由于ThreadLocal设计之初就是为了绑定当前线程,如果希望当前线程的ThreadLocal能够被子线程使用,实现方式就会相当困难(需要用户自己在代码中传递)。在此背景下,InheritableThreadLocal应运而生。

Inheritable thread-local variables are used in preference to ordinary thread-local variables when the per-thread-attribute being maintained in the variable (e.g., User ID, Transaction ID) must be automatically transmitted to any child threads that are created.

2、应用

调用链追踪:在调用链系统设计中,为了优化系统运行速度,会使用多线程编程,为了保证调用链ID能够自然的在多线程间传递,需要考虑ThreadLocal传递问题(大多数系统会使用线程池技术,这已经不仅仅是InheritableThreadLocal能够解决的了,我会在另外一篇文章中介绍相关技术实现)。

3、InheritableThreadLocal类

InheritableThreadLocal类重写了ThreadLocal的3个函数:

    /**
     * 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);
    }

注意:由于重写了getMap()和createMap()两个函数,所以当

4、线程间传值实现原理

说到InheritableThreadLocal,还要从Thread类说起:

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)
}

Thread类中包含 threadLocalsinheritableThreadLocals 两个变量,其中 inheritableThreadLocals 即主要存储可自动向子线程中传递的ThreadLocal.ThreadLocalMap。
接下来看一下父线程创建子线程的流程,我们从最简单的方式说起:

4.1、用户创建Thread

Thread thread = new Thread();

4.2、Thread创建

    /**
     * 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);
    }

4.3、Thread初始化

    /**
     * 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)
    }

可以看到,采用默认方式产生子线程时,inheritThreadLocals=true;若此时父线程inheritableThreadLocals不为空,则将父线程inheritableThreadLocals传递至子线程。

4.4、ThreadLocal.createInheritedMap

让我们继续追踪createInheritedMap:

    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++;
                    }
                }
            }
        }

从ThreadLocalMap可知,子线程将parentMap中的所有记录逐一复制至自身线程。

5、总结

InheritableThreadLocal主要用于子线程创建时,需要自动继承父线程的ThreadLocal变量,方便必要信息的进一步传递。

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