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52. ThreadLocal
原以为神秘兮兮的 ThreadLocal 是何方神圣？没想到，点开代码，竟然如此简单明了，直接上代码：
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/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
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/**
* 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;
}
/**
* 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();
}
/**
* 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);
}
}
static class ThreadLocalMap {
/**
* 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;
}
}
// ……
}
简单解释一下：
当前线程调用 ThreadLocal 类的 set 或 get 方法时，其实是调用的当前线程的 ThreadLocal.ThreadLocalMap threadLocals 变量的 set(ThreadLocal<?> key, Object value) 和 Entry getEntry(ThreadLocal<?> key)。还有一点需要稍加注意：虽然 ThreadLocal.ThreadLocalMap 名称是以 Map 结尾，但是它并没有实现 Map 接口，只是操作上有些类似。
有一点需要特别注意：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。使用完 ThreadLocal 方法后 最好手动调用 remove() 方法。
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package com.diguage.truman.concurrent;
import org.junit.jupiter.api.Test;
/**
* @author D瓜哥, https://www.diguage.com/
* @since 2020-03-09 20:20
*/
public class ThreadLocalTest {
@Test
public void test() {
new Thread(new FirstApp()).start();
}
@Test
public void testInheritableThreadLocal() {
// TODO: InheritableThreadLocal
}
private static class FirstApp implements Runnable {
private ThreadLocal<String> threadLocal
= ThreadLocal.withInitial(() -> "FirstApp-1");
private ThreadLocal<String> threadLocal2
= ThreadLocal.withInitial(() -> "FirstApp-2");
private SecondApp secondApp = new SecondApp();
private ThridApp thridApp = new ThridApp();
@Override
public void run() {
System.out.println(threadLocal.get());
System.out.println(threadLocal2.get());
new Thread(secondApp).start();
thridApp.run();
}
}
private static class SecondApp implements Runnable {
private ThreadLocal<String> threadLocal
= ThreadLocal.withInitial(() -> "SecondApp");
@Override
public void run() {
System.out.println(threadLocal.get());
}
}
private static class ThridApp implements Runnable {
private ThreadLocal<String> threadLocal
= ThreadLocal.withInitial(() -> getClass().getName());
@Override
public void run() {
threadLocal.set("new-ThridApp-value");
System.out.println(threadLocal.get());
}
}
}
关注一下： java.lang.InheritableThreadLocal。
JDK 的 InheritableThreadLocal 类可以完成父线程到子线程的值传递。但对于使用线程池等会池化复用线程的执行组件的情况，线程由线程池创建好，并且线程是池化起来反复使用的；这时父子线程关系的 ThreadLocal 值传递已经没有意义，应用需要的实际上是把任务提交给线程池时的 ThreadLocal 值传递到 任务执行时。为了解决这个问题，阿里巴巴研发了 alibaba/transmittable-thread-local 库。
哈希冲突了怎么解决？