ThreadPoolExecutor 與 ThreadLocal 配合使用中出現(xiàn)數(shù)據(jù)不一致問題
前段時間寫過一段測試代碼,發(fā)現(xiàn)使用了ThreadLocal出現(xiàn)了數(shù)據(jù)不一致的問題,之前也一直用過,沒有出現(xiàn)過.所以感到很疑惑.于是針對這個case研究了下源碼
- 單元測試代碼
/**
* <p>
* 測試ThreadLocal結(jié)合ThreadPoolExecutor是否存在數(shù)據(jù)不安全情況
* </p>
*
* @author sunla
* @version 1.0
*/
public class ThreadLocalTest {
private static final ThreadPoolExecutor EXECUTOR = new ThreadPoolExecutor(2, 2, 1000, TimeUnit.MILLISECONDS,
new LinkedBlockingDeque<Runnable>(20), new ThreadFactoryBuilder().setNameFormat("name-%s").build(),
new ThreadPoolExecutor.AbortPolicy());
private static final ThreadLocal<String> LOCAL = new ThreadLocal<String>();
@Test
public void startTest() throws Exception {
LOCAL.set("main start");
EXECUTOR.execute(()->{
System.out.println(
String.format("value is %s", LOCAL.get()));
try {
TimeUnit.MILLISECONDS.sleep(1000);
}
catch (InterruptedException e) {
e.printStackTrace();
}
});
System.out.println(
String.format("value is %s", LOCAL.get()));
}
}
- 輸出結(jié)果
NULL
main start
很奇怪.ThreadLocal 就是為了保持回話中變量共享 為什么不一致呢
我們平時使用ThreadLocal的場景 可以理解為會話中的數(shù)據(jù)共享,那怎么在這里出現(xiàn)了與期望不一致的結(jié)果呢?其實這跟ThreadLocal的內(nèi)部實現(xiàn)有關系.
ThreadLocal 內(nèi)部實現(xiàn)
/**
* 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是內(nèi)部實現(xiàn)的map
* 特點是key是ThreadLocal的引用
* 至于為什么這樣設計?我們往下看
*/
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
/**
* 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);
else
createMap(t, value);
return value;
}
/** 每個thread都有獨立的threadlocalmap */
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
- ThreadLocalMap實現(xiàn)
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;
}
}
//TO DO
}
看了threadLocal的實現(xiàn) 我們知道了,原來thread local 實例有個map 存儲的key就是線程的引用,value就是需要共享的變量.
那我們上面的代碼 難道獲取的不是同一個線程?
看過ThreadPoolExecutor實現(xiàn)的就知道 真的不是一個
ThreadPoolExecutor 實現(xiàn)
/**
* 添加工作線程
*/
private boolean addWorker(Runnable firstTask, boolean core) {
//TO DO
w = new Worker(firstTask);
final Thread t = w.thread;
//TO DO
}
/** 工作類 實現(xiàn)runnable接口 */
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
Worker(Runnable firstTask) {
setState(-1);
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** 重寫run方法 最終執(zhí)行的就是runWorker */
public void run() {
runWorker(this);
}
}
這下真相大白了,原因在線程池中會開辟新的線程執(zhí)行task.如果在主線程中(main) 放入到ThreadLocal的value在task中獲取到的就不再是main線程的ref了.而且線程池自己開辟的. 所以導致數(shù)據(jù)不一致.
這里提個問題 threadlocal有個remove方法,
如果我們不顯示調(diào)用 會怎么樣?
