ThreadLocal

ThreadLocal

threadlocal 线程变量，每个线程都有单独的变量。用于线程之间的隔离。

ThreadLocal 使用

public class TestThread {
    static ThreadLocal<Integer> threadLocal = ThreadLocal.withInitial(()->0);
    static List<Thread> ths = new ArrayList<>();
    public static void main(String[] args) {
        for (int i = 0; i < 50; i++) {
            ths.add(new Thread(new Runnable() {
                @Override
                public void run() {
                    threadLocal.set(threadLocal.get()+1);   // 设置当前线程值
                    System.out.println(Thread.currentThread().getName()+":"+threadLocal.get()); //获取线程值

                }
            }));
        }

        for (Thread th : ths) {
            th.start();
        }
    }

}

ThreadLocal 中 变量

ThreadLocal.ThreadLocalMap threadLocals = null;

ThreadLocal 中 set 方法

public void set(T value) {
    Thread t = Thread.currentThread(); //获取当前线程引用
    ThreadLocalMap map = getMap(t);   // 获取当前线程的 ThreadLocalMap
    if (map != null)
        map.set(this, value);        //当前存在，设置值
    else
        createMap(t, value);         //当前不存在，创建并设置值
}

ThreadLocal 中 get 方法

public T get() {
    Thread t = Thread.currentThread();  //获取当前线程引用
    ThreadLocalMap map = getMap(t);    // 获取当前线程的 ThreadLocalMap
    if (map != null) {  //取值返回
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();  //初始化 ThreadLocalMap
}

//初始化 当前线程的 ThreadLocalMap
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;
}

void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

ThreadLocalMap 为 ThreadLocal的内部类

static class ThreadLocalMap {
    
    private Entry[] table;//数据数组
    
    private int size = 0;//数组大小
    
    private int threshold; //临界值
    
    private static final int INITIAL_CAPACITY = 16; //默认大小
    
    static class Entry extends WeakReference<ThreadLocal<?>> {
        Object value; //存储线程值
        Entry(ThreadLocal<?> k, Object v) {
            super(k);
            value = v;
        }
    }
    
    /**
    * 初始化
    */
    ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
        table = new Entry[INITIAL_CAPACITY];
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        setThreshold(INITIAL_CAPACITY);
    }
}