详解InheritableThreadLocal类原理

在Java并发编程中,InheritableThreadLocal 与 ThreadLocal 都可以用于线程间通信,不同的是 InheritableThreadLocal 继承了 ThreadLocal,并且扩展了 ThreadLocal。使用类 InheritableThreadLocal 可使子线程继承父线程的值。相反,类 ThreadLocal 不能实现值继承。

使用示例:

public class LocalThread extends Thread {
    private static InheritableThreadLocal local = new InheritableThreadLocal();

    @Override
    public void run() {
        System.out.println("thread线程:"+ local.get());
    }

    public static void main(String[] args) throws InterruptedException {
        local.set("main的值");
        LocalThread t = new LocalThread();
        t.start();
        System.out.println("main线程:"+ local.get());
    }

}

分析下 InheritableThreadLocal 类源码:

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    /**
     * Computes the child's initial value for this inheritable thread-local
     * variable as a function of the parent's value at the time the child
     * thread is created.  This method is called from within the parent
     * thread before the child is started.
     * <p>
     * This method merely returns its input argument, and should be overridden
     * if a different behavior is desired.
     *
     * @param parentValue the parent thread's value
     * @return the child thread's initial value
     */
    protected T childValue(T parentValue) {
        return parentValue;
    }
 
    /**
     * Get the map associated with a ThreadLocal.
     *
     * @param t the current thread
     */
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }
 
    /**
     * Create the map associated with a ThreadLocal.
     *
     * @param t the current thread
     * @param firstValue value for the initial entry of the table.
     */
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

可以看到,getMap() 方法和 creatMap() 方法都是重写的 ThreadLocal 类方法,区别在于把 ThreadLocal 中的 threadLocals 换成了 inheritableThreadLocals,这两个变量都是ThreadLocalMap类型,并且都是Thread类的属性,源码如下:

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

inheritableThreadLocal 如何实现值继承的呢?继续看下面的代码:

        /**
         * Construct a new map including all Inheritable ThreadLocals
         * from given parent map. Called only by createInheritedMap.
         *
         * @param parentMap the map associated with parent thread.
         */
        private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];

            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) {
                        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++;
                    }
                }
            }
        }

在构造方法的完整源代码算法中可以发现,子线程将父线程中的 table 对象以复制的方式赋值给子线程的 table 数组,这个过程是在创建 Thread 类对象时发生的,也就说明当子线程对象创建完毕后,子线程中的数据就是主线程中旧的数据,主线程使用新的数据时,子线程还是使用旧的数据,因为主子线程使用两个 Entry[] 对象数组各自存储自己的值。

这部分涉及到 Java 的值传递。对于对象来说,值的内容其实是对象的引用。当在父线程中修改对象的某一属性,子线程由于引用着相同对象,所以可以感知到,本质上是在操作同一块内存地址。

对于基本数据类型(int、long)来说,由于传递的是值,在父线程改变了数据后,子线程依旧使用的是旧的数据。这里尤其要提 String 字符串,String 虽然不是基本数据类型,但是由于内部字符数组被 final 修饰带来的不可变型,当父线程修改其 String 类型数据时,等于替换掉该 String 对象,而并不是修改原 String 对象的值,所以子线程依旧不会发生变化。

另外,重写类 InheritableThreadLocal 的 childValue() 方法可以对继承的值进行加工,比如通过调用clone() 方法返回 parentValue 的浅拷贝,以达到子线程无法影响父线程的目的。

代码如下:

public class Local extends InheritableThreadLocal {

    @Override
    protected Object initialValue() {
        return new Date();
    }

    @Override
    protected Object childValue(Object parentValue) {
        return parentValue+"[子线程增强版]";  // parentValue.clone();
    }
}
posted @ 2019-08-16 15:14 薛勤 阅读(...) 评论(...) 编辑 收藏