java中ThreadLocal入门
前言
ThreadLocal可以看做线程的本地变量，实现原理就是每个线程保存一份数据的副本，数据隔离，以空间换时间。
简单使用
public class Client3 {
public static void main(String[] args) {
ThreadLocal<String> nameThreadLocal = new ThreadLocal<>() {
@Override
protected String initialValue() {
return "test";
}
};
for (int i = 1; i <= 5; i++) {
int finalI = i;
new Thread(() -> {
String name = "lisi" + finalI;
nameThreadLocal.set(name);
System.out.println(Thread.currentThread() + " set name " + name);
try {
Thread.sleep((long) (Math.random() * 1000));
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread() + " get name " + nameThreadLocal.get());
}, "thread" + finalI).start();
}
}
}
输出为
Thread[thread3,5,main] set name lisi3
Thread[thread5,5,main] set name lisi5
Thread[thread2,5,main] set name lisi2
Thread[thread1,5,main] set name lisi1
Thread[thread4,5,main] set name lisi4
Thread[thread4,5,main] get name lisi4
Thread[thread5,5,main] get name lisi5
Thread[thread1,5,main] get name lisi1
Thread[thread2,5,main] get name lisi2
Thread[thread3,5,main] get name lisi3
每个线程向TheadLocal中设置值和取值，数据都是互不影响的。如果没设置值，取initialValue()方法的初始值。
原理分析
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
map.set(this, value);
} else {
createMap(t, value);
}
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
获取当前线程的threadLocals，类型为ThreadLocalMap，可以简单看做一个HashMap，key为ThreadLocal对象(实际上是它的弱引用)，value为要设置的值。
/**
* 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;
}
}
弱引用如果没有关联的强引用，活不过下一次GC，使用弱引用是为了方便垃圾清理。
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal<?> key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
/**
* Increment i modulo len.
*/
private static int nextIndex(int i, int len) {
return ((i + 1 < len) ? i + 1 : 0);
}
ThreadLocalMap使用线性探测法解决哈希冲突。
内存泄露问题
ThreadLocalMap 中使用的 key 为 ThreadLocal 的弱引用,而 value 是强引用。所以，如果 ThreadLocal 没有被外部强引用的情况下，在垃圾回收的时候，key 会被清理掉，而 value 不会 被清理掉。
这样一来， ThreadLocalMap 中就会出现 key 为 null 的 Entry。假如我们不做任何措施的话，value 永远无法被 GC 回收，这个时候就可能会产生内存泄露。
ThreadLocalMap 实现中已经考虑了这种情况，在调用 set() 、 get() 、 remove() 方法的时候，会清理掉 key 为 null 的记录。我们使用完 ThreadLocal 方法后，最好手动调用 remove() 方法。
