关于jdk1.7中的ThreadLocal

最新推荐文章于 2024-06-03 19:13:37 发布

原创最新推荐文章于 2024-06-03 19:13:37 发布 · 729 阅读

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CC 4.0 BY-SA版权

本文详细解析了JDK1.7中ThreadLocal的get()方法，包括获取当前线程、访问ThreadLocalMap、弱引用来管理Entry等关键步骤，探讨了ThreadLocal如何实现线程局部变量及其内存管理机制。

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1.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)
return (T)e.value;
}
return setInitialValue();
}

第一步先获取到当前的线程

第二步通过getmap获取到当前线程的threadLocals变量。

/**
* 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;
}

在Thread中是这么定义的

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

返回的是一个 ThreadLocal中的ThreadLocalMap对象，并且没有任何修饰符，由于ThreadLocal和Thread都属于 package java.lang; 所以这两个可以互相访问

ThreadLocalMap中有这个对象，这里使用了弱引用。弱引用在没人其他变量指向它时就会释放。（这点很重要）

/**
* 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;
}
}