对ThreadLocal的理解

对ThreadLocal的理解

一个线程可以有多个threadLocal对象，在每个线程中都有多个独特的threadLocal对象，不与其他线程共享
参考链接
https://www.cnblogs.com/dolphin0520/p/3920407.html

demo代码

public class Test {

ThreadLocal<Long> longLocal = new ThreadLocal<Long>(){
        protected Long initialValue() {
            return Thread.currentThread().getId();
        }
    };
    ThreadLocal<String> stringLocal = new ThreadLocal<String>(){;
        protected String initialValue() {
            return Thread.currentThread().getName();
        }
    };


    public void set() {
        longLocal.set(Thread.currentThread().getId());
        stringLocal.set(Thread.currentThread().getName());
    }

    public long getLong() {
        return longLocal.get();
    }

    public String getString() {
        return stringLocal.get();
    }

    public static void main(String[] args) throws InterruptedException {
        final Test test = new Test();

        System.out.println(test.getLong());
        System.out.println(test.getString());

        Thread thread1 = new Thread(){
            public void run() {
                test.set();
                System.out.println(test.getLong());
                System.out.println(test.getString());
            }
        };
        thread1.start();
        thread1.join();

        System.out.println(test.getLong());
        System.out.println(test.getString());
    }
    

get方法

public T get() {
    Thread t = Thread.currentThread();//获取当前线程
    ThreadLocalMap map = getMap(t);//获取map对象
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    //如果e为空的话，初始化
    return setInitialValue();
}

getMap方法

ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

ThreadLocalMap为ThreadLocal的内部类，k为当前对象，v为要副本值

     static class Entry extends WeakReference<ThreadLocal<?>> {
        /** The value associated with this ThreadLocal. */
        Object value;

        Entry(ThreadLocal<?> k, Object v) {
            super(k);
            value = v;
        }
    }

setInitialValue方法

private T setInitialValue() {
        T value = initialValue();//调用initialvalue方法初始化值
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

initialValue方法，默认返回null，可以重写

  protected T initialValue() {
        return null;
    }