我正在學(xué)習(xí)Java Concurrency In Practice并被困在8.3.1 線程創(chuàng)建和拆卸主題上。以下腳注警告保持corePoolSize零。開發(fā)人員有時會試圖將核心大小設(shè)置為零，這樣工作線程最終會被拆除，因此不會阻止 JVM 退出，但這可能會在不使用SynchronousQueue 用于他們的工作隊列（就像 newCachedThreadPool 一樣）。如果池已經(jīng)達到核心大小，只有在工作隊列已滿時，ThreadPoolExecutor 才會創(chuàng)建新線程。因此，提交到具有任何容量和核心大小為零的工作隊列的線程池的任務(wù)將在隊列填滿之前不會執(zhí)行，這通常不是所期望的。所以為了驗證這一點，我寫了這個程序，它不像上面所說的那樣工作。 final int corePoolSize = 0; ThreadPoolExecutor tp = new ThreadPoolExecutor(corePoolSize, 1, 5, TimeUnit.SECONDS, new LinkedBlockingQueue<>()); // If the pool is already at the core size if (tp.getPoolSize() == corePoolSize) { ExecutorService ex = tp; // So tasks submitted to a thread pool with a work queue that has any capacity // and a core size of zero will not execute until the queue fills up. // So, this should not execute until queue fills up. ex.execute(() -> System.out.println("Hello")); }輸出： 你好那么，程序的行為是否表明ThreadPoolExecutor如果提交了一個任務(wù)而不管corePoolSize=0. 如果是，那么教科書上的警告是什么。編輯：根據(jù)@SK 的建議測試了jdk1.5.0_22 中的代碼，并進行了以下更改：ThreadPoolExecutor tp = new ThreadPoolExecutor(corePoolSize, 1, 5, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(1));//Queue size is set to 1.但是隨著這個變化，程序終止而不打印任何輸出。那么我是否誤解了書中的這些陳述？編輯（@sjlee）：很難在評論中添加代碼，所以我將它作為編輯添加在這里...你能嘗試這個修改并針對最新的 JDK 和 JDK 1.5 運行它嗎？final int corePoolSize = 0;ThreadPoolExecutor tp = new ThreadPoolExecutor(corePoolSize, 1, 5, TimeUnit.SECONDS, new LinkedBlockingQueue<>());// If the pool is already at the core sizeif (tp.getPoolSize() == corePoolSize) { ExecutorService ex = tp; // So tasks submitted to a thread pool with a work queue that has any capacity // and a core size of zero will not execute until the queue fills up. // So, this should not execute until queue fills up. ex.execute(() -> System.out.println("Hello"));}tp.shutdown();if (tp.awaitTermination(1, TimeUnit.SECONDS)) { System.out.println("thread pool shut down. exiting.");} else { System.out.println("shutdown timed out. exiting.");}
3 回答
繁花如伊
TA貢獻2012條經(jīng)驗 獲得超12個贊
在 jdk 1.5、1.6、1.7 和 1.8 中運行這個程序時，我發(fā)現(xiàn)ThreadPoolExecutor#execute(Runnable)了 1.5、1.6 和 1.7+ 中的不同實現(xiàn)。這是我發(fā)現(xiàn)的：
JDK 1.5 實現(xiàn)
//Here poolSize is the number of core threads running.
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
for (;;) {
if (runState != RUNNING) {
reject(command);
return;
}
if (poolSize < corePoolSize && addIfUnderCorePoolSize(command))
return;
if (workQueue.offer(command))
return;
Runnable r = addIfUnderMaximumPoolSize(command);
if (r == command)
return;
if (r == null) {
reject(command);
return;
}
// else retry
}
}
當(dāng)corePoolSize為 0 時，此實現(xiàn)不會創(chuàng)建線程，因此不會執(zhí)行提供的任務(wù)。
JDK 1.6 實現(xiàn)
//Here poolSize is the number of core threads running.
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
if (runState == RUNNING && workQueue.offer(command)) {
if (runState != RUNNING || poolSize == 0)
ensureQueuedTaskHandled(command);
}
else if (!addIfUnderMaximumPoolSize(command))
reject(command); // is shutdown or saturated
}
}
即使corePoolSize為 0，JDK 1.6 也會創(chuàng)建一個新線程。
JDK 1.7+ 實現(xiàn)（類似于 JDK 1.6，但具有更好的鎖和狀態(tài)檢查）
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
即使corePoolSize是 0，JDK 1.7 也會創(chuàng)建一個新線程。
因此，corePoolSize=0在 JDK 1.5 和 JDK 1.6+ 的每個版本中，這似乎都是一個特例。
但奇怪的是，書中的解釋與任何程序結(jié)果都不符。
MMMHUHU
TA貢獻1834條經(jīng)驗 獲得超8個贊
似乎這是舊 Java 版本的錯誤，但現(xiàn)在在 Java 1.8 中不存在。
根據(jù)來自的 Java 1.8 文檔ThreadPoolExecutor.execute()：
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
* ....
*/
第二點，在加入一個worker到隊列后，再檢查一下，如果不是排隊任務(wù)，可以啟動一個新線程，而不是回滾入隊并啟動一個新線程。
這就是正在發(fā)生的事情。在第一次檢查期間，任務(wù)已排隊，但在重新檢查期間，將啟動一個新線程來執(zhí)行您的任務(wù)。
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