Java程序员从笨鸟到菜鸟（四十五） ThreadLocal 的理解

本文主要是介绍Java程序员从笨鸟到菜鸟（四十五） ThreadLocal 的理解，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

一、定义

ThreadLocal 提供一个线程（Thread）局部变量，使每一个线程都拥有自己的局部变量，ThreadLocal 就是在多线程的环境下取保证成员变量的安全

二、深入理解 ThreadLocal 类

常用方法：

public T get() {};
public void set(T value) {};
public void remove() {};
protected T initialValue() {};

get() 方法用来获取 ThreadLocal 在当前线程中保存的变量副本，set() 用来设置当前线程变量中的副本，remove() 用来移除线程中变量的副本，initialValue() 是一个 protected 方法，一般用来在使用时进行重写，是一个延迟加载方法

get() 方法：

/*** 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();
}

先获取当前线程，然后通过 getMap(t) 方法取到一个 map,然后再获取

 /*** Get the map associated with a ThreadLocal. Overridden in* InheritableThreadLocal.** @param  t the current thread* @return the map*/ThreadLocalMap getMap(Thread t) {return t.threadLocals;}

调用当前线程 t，返回当前线程 t 的一个成员变量 threadLocals,实际上就是一个 ThreadLocalMap

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

ThreadLocalMap实现：

/*** The entries in this hash map extend WeakReference, using* its main ref field as the key (which is always a* ThreadLocal object).  Note that null keys (i.e. entry.get()* == null) mean that the key is no longer referenced, so the* entry can be expunged from table.  Such entries are referred to* as "stale entries" in the code that follows.*/static class Entry extends WeakReference<ThreadLocal<?>> {/** The value associated with this ThreadLocal. */Object value;Entry(ThreadLocal<?> k, Object v) {super(k);value = v;}}

ThreadLocalMap 的 Entry 继承了 WeakReference，并且使用 ThreadLocal 作为键值

setInitialValue()方法实现

/*** 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);elsecreateMap(t, value);return value;}

如果 map 不为空，就设置键值对，为空，再创建 Map

createMap() 实现

/*** Create the map associated with a ThreadLocal. Overridden in* InheritableThreadLocal.** @param t the current thread* @param firstValue value for the initial entry of the map*/void createMap(Thread t, T firstValue) {t.threadLocals = new ThreadLocalMap(this, firstValue);}

ThreadLocal 如何为每个线程创建变量的副本：
首先每个线程 Thread 内部都有一个 ThreadLocal.ThreadLocalMap 类型的成员变量 ThreadLocals,这个 threadLocals 就是存储实际的变量副本，键值为当前 ThreadLocal 变量，value 为变量副本(即 T 类型的变量)

初始时，在 Thread 里面，threadLocals 为空，并通过 ThreadLocal 变量调用 get() 方法或则 set() 方法，就会对 Thread 的 threadLocals 进行初始化，并且以当前 ThreadLocal 变量为键值，以 ThreadLocal 要保存的副本变量为 value,存到 threadLocals

总结：