ThreadLocal学习02-ThreadLocal原理

1、set()方法：

    public void set(T value) {
//获取当前线程
        Thread t = Thread.currentThread();
//得到当前线程的ThreadLocalMap
        ThreadLocalMap map = getMap(t);
        if (map != null)//存在赋值
            map.set(this, value);
        else//不存在创建
            createMap(t, value);
    }
//返回当前线程的ThreadLocalMap
   ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

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

从代码中可以看到ThreadLocalMap是线程Thread的一个属性。ThreadLocalMap类似于map,保证key的唯一性，key就是ThreadLocal得hash值，所以同一个线程多次set值会覆盖前面得值。

2、get()方法

    public T get() {
//获取当前线程
        Thread t = Thread.currentThread();
//获取线程得ThreadLocalMap属性
        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;
            }
        }
//如果没有set值，第一次查询得时候获取默认初始化值
        return setInitialValue();
    }

//给ThreadLocal赋初始值
    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;
    }

ThreadLocal得初始值是null，可以根据一下方法修改初始值：

//通过静态方法获取有初始值得threadlocal的子类
    public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
        return new SuppliedThreadLocal<>(supplier);
    }

//子类
    static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {

        private final Supplier<? extends T> supplier;

        SuppliedThreadLocal(Supplier<? extends T> supplier) {
            this.supplier = Objects.requireNonNull(supplier);
        }

        @Override
        protected T initialValue() {
            return supplier.get();
        }
    }

3、内存泄漏

可以看看这条知乎：https://zhuanlan.zhihu.com/p/102571059