文章目錄

• 概述
• 關于wait set
• 示例
• 思考

概述

官方指導： https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html

或者：

https://learning.oreilly.com/library/view/the-java-language/9780133260335/ch17lev1sec2.html

---

關于wait set

Every object, in addition to having an associated monitor, has an associated wait set. A wait set is a set of threads.

A wait set is a set of threads.

When an object is first created, its wait set is empty. Elementary actions that add threads to and remove threads from wait sets are atomic. Wait sets are manipulated solely through the methods Object.wait, Object.notify, and Object.notifyAll.

Wait set manipulations can also be affected by the interruption status of a thread, and by the Thread class’s methods dealing with interruption. Additionally, the Thread class’s methods for sleeping and joining other threads have properties derived from those of wait and notification actions.

意思是說

每個對象除具有關聯的監視器外，還具有關聯wait set。

wait set是一組線程

首次創建對象時，其wait set為空。將線程添加到wait set中或從wait set中刪除線程的基本操作是原子的。wait set完全通過方法操作Object.wait，Object.notify和Object.notifyAll

wait set操作也可能受線程的中斷狀態以及Thread處理中斷的類的方法的影響。此外，Thread該類用于休眠和加入其他線程的方法具有從等待和通知操作的屬性派生的屬性。

---

示例

package com.artisan.test;import java.util.Optional; import java.util.stream.IntStream;public class WaitSet {// 顯示定義一個鎖private static final Object LOCK = new Object();public static void main(String[] args) throws InterruptedException {IntStream.rangeClosed(1, 10).forEach(i ->new Thread(String.valueOf("T_" + i)) {@Overridepublic void run() {synchronized (LOCK) {try {Optional.of(Thread.currentThread().getName() + " will come into wait set ..").ifPresent(System.out::println);LOCK.wait();Optional.of(Thread.currentThread().getName() + " will leave from wait set ...").ifPresent(System.out::println);} catch (InterruptedException e) {e.printStackTrace();}}}}.start());// 主線程休眠一秒，確保上面的10個線程都start ，不然的話 有可能 線程已經進入wait set ,10個線程還沒都啟動，就已經有線程離開 wait set了Thread.sleep(1_000);System.out.println("=====================10個線程啟動完畢=======================");// 主線程中 每次notify 1個IntStream.rangeClosed(1, 10).forEach(i -> {synchronized (LOCK) {LOCK.notify();try {// 為了方便觀察 休眠1秒Thread.sleep(1_000);} catch (InterruptedException e) {e.printStackTrace();}}});Thread.sleep(1_000);System.out.println("=====================OVER=======================");}}

運行結果：

T_1 will come into wait set .. T_8 will come into wait set .. T_7 will come into wait set .. T_6 will come into wait set .. T_5 will come into wait set .. T_4 will come into wait set .. T_3 will come into wait set .. T_2 will come into wait set .. T_10 will come into wait set .. T_9 will come into wait set .. =====================10個線程啟動完畢======================= T_1 will leave from wait set ... T_8 will leave from wait set ... T_7 will leave from wait set ... T_4 will leave from wait set ... T_5 will leave from wait set ... T_6 will leave from wait set ... T_10 will leave from wait set ... T_2 will leave from wait set ... T_3 will leave from wait set ... T_9 will leave from wait set ... =====================OVER=======================Process finished with exit code 0

根據官網和我們的驗證，總結一下

所有的對象都會有一個wait set,用來存放調用了該對象wait方法之后進入block狀態線程

線程被notify之后，不一定立即得到執行

線程從wait set中被喚醒順序不一定是FIFO,啥順序，JVM規范中并沒有給出，各個虛擬機的廠商有各自的實現

---

思考

有個方法如下：

private static void anotherThink() {synchronized (LOCK) {System.out.println("anotherThink begin to execute....");try {Optional.of(Thread.currentThread().getName() + " will come into wait set ...").ifPresent(System.out::println);LOCK.wait();} catch (InterruptedException e) {e.printStackTrace();}Optional.of(Thread.currentThread().getName() + " will leave from wait set ...").ifPresent(System.out::println);}}

Q: 拿到鎖后的第一件事情 是輸出一行日志 anotherThink begin to execute.... 當線程1，搶到鎖后 ，調用wait，放棄了執行權。 如果線程1被喚醒時，肯定要先獲取到鎖才能夠執行，那我們剛才說的 搶到鎖后打印日志會不會被執行呢？

我們來驗證下

main方法中 測試代碼如下

public static void main(String[] args) throws InterruptedException {/** new Thread("Test_Thread") {@Overridepublic void run() {anotherThink();}}.start() **/new Thread(() -> anotherThink() ,"Test Thread").start();Thread.sleep(1000);synchronized (LOCK) {LOCK.notify();}}

運行日志

根據測試結論可知，是不會打印第一行的。 而是緊接著wait后面的代碼執行，主要是源于JVM內部會記錄上次代碼的執行地址，進行地址恢復，繼續執行。

A： 所以線程被喚醒后，必須重新獲取鎖，但是JVM內部會進行地址恢復，直接繼續上次線程后續的邏輯

---

總結

以上是生活随笔為你收集整理的高并发编程-Wait Set 多线程的“休息室”的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。