ThreadLocal.withInitial

原创已于 2022-01-21 16:29:42 修改 · 979 阅读

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于 2022-01-21 16:28:20 首次发布

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本文详细介绍了ThreadLocal的作用及其实现原理，特别是初始化过程和如何获取当前线程的值。通过示例代码验证了即使调用了remove方法，再次获取时依然能返回初始值而非null的特点。

作用

初始化ThreadLocal值

源码

    /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

    /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

取不到值的时候会执行setInitialValue()，因此ThreadLocal设置了初始值，哪怕执行了ThreadLocal的remove方法后，执行get()，获取的也是初始值，而不是null。

验证

验证代码

    @Test
    public void testThreadLocal () throws InterruptedException {
        ThreadLocal<Integer> threadLocal = ThreadLocal.withInitial(() -> 123);
        System.out.println(threadLocal.get());
        threadLocal.set(234);
        threadLocal.remove();
        System.out.println(threadLocal.get());
    }

执行结果：