{"id":30936,"date":"2023-10-23T18:04:10","date_gmt":"2023-10-23T17:04:10","guid":{"rendered":"https:\/\/www.baeldung.com\/java-wait-necessary-synchronization"},"modified":"2023-10-23T18:04:10","modified_gmt":"2023-10-23T17:04:10","slug":"why-wait-requires-synchronization","status":"publish","type":"post","link":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/2023\/10\/23\/why-wait-requires-synchronization\/","title":{"rendered":"Why wait() Requires Synchronization?"},"content":{"rendered":"

\"\"<\/p>\n

1. Introduction<\/h2>\n
<\/div>\n

In Java, we have a wait()\/notify()<\/em> API<\/a>. This API is one of the ways to synchronize between threads. In order to use the methods of this API, the current thread must own the monitor of the callee.<\/p>\n

In this tutorial, we’ll explore the reasons why this requirement makes sense.<\/p>\n

2. How wait()<\/em> Works<\/h2>\n
<\/div>\n

First, we need to briefly talk about how wait()<\/em> works in Java. In Java, in accordance with JLS, each object has a monitor<\/a>. Essentially, it means that we can synchronize on any object that we like. It was probably not a good decision, but this is what we have now.<\/p>\n

Having that, when we call wait()<\/em>, we implicitly do two things. First, we place the current thread into the JVM internal wait set for this<\/em> object monitor. The second is that once the thread is in wait set, we (or the JVM, for that matter) release the synchronization lock on this<\/em> object. Here, we need to clarify – the word this<\/em> means the object on which we call the wait()<\/em> method.<\/p>\n

And then, the current thread just waits in the set until another thread calls notify()<\/em>\/notifyAll()<\/em> on this<\/em> object.<\/p>\n

3. Why Is Monitor Ownership Required?<\/h2>\n
<\/div>\n

In the previous section, we saw that the second thing JVM does is the release of the synchronization lock on this<\/em> object. In order to release it, we obviously need to own it first. The reasoning for this is relatively simple: synchronization on wait()<\/em> comes as a requirement in order to avoid the lost wake-up problem<\/strong>. This problem essentially represents a condition where we have a waiting thread that has missed the notify signal. It mostly happens due to the race condition between threads. Let us emulate this problem with an example.<\/p>\n

Suppose we have the following Java code:<\/p>\n

private volatile Boolean jobIsDone;\r\nprivate Object lock = new Object();\r\npublic void ensureCondition() {\r\n    while (!jobIsDone) {\r\n        try {\r\n            lock.wait();\r\n        } \r\n        catch (InterruptedException e) {\r\n            \/\/ ...\r\n        }\r\n    }\r\n}\r\npublic void complete() {\r\n    jobIsDone = true;\r\n    lock.notify();\r\n}\r\n<\/code><\/pre>\n
A quick note – this code will fail in runtime with IllegalMonitorStateException<\/em>. This is because, in both methods, we do not ask for a lock <\/em>object monitor before wait()<\/em>\/notify()<\/em> calls. Thus, this code is purely for demonstration and learning purposes.<\/p>\n
Also, let’s assume we have two threads. So, thread B<\/em> is doing the useful work. Once it is done, thread B<\/em> needs to call the complete()<\/em> method to signal the completion. We also have another thread, A, <\/em>that is waiting for the job performed by B<\/em> to be completed. Thread A<\/em> makes its check for condition by calling the ensureCondition()<\/em> method. The check for the condition is happening in the loop because of the spurious wake-up problem<\/a> that occurs on the Linux kernel level, but that is another topic.<\/p>\n
4. The Problem of the Lost Wake-up<\/h2>\n
<\/div>\n
Let’s break down our example step by step. Assume thread A<\/em> called ensureCondition()<\/em> and enters the while<\/em> loop. It checked for a condition, which appeared to be false,<\/em> so it entered the try<\/em> block. Because we operate in a multithreaded environment, another thread B<\/em> can simultaneously enter the complete()<\/em> method. Therefore, B<\/em> can call set volatile flag jobIsDone<\/em> to true<\/em> and call notify()<\/em> before thread A<\/em> called wait()<\/em>.<\/p>\n
In this case, if thread B<\/em> will never enter the complete()<\/em> again, thread A<\/em> will wait forever, and therefore, all of the resources associated with it will also live forever. This leads not only to deadlocks if thread A<\/em> happens to hold another lock but to memory leaks because objects reachable from thread A<\/em> stack frames will remain alive. This is because thread A<\/em> is considered to be alive, and it can resume its execution. Therefore, GC is not allowed to garbage collect objects allocated in methods of A<\/em> stack.<\/p>\n
5. Solution<\/h2>\n
<\/div>\n
So, in order to avoid this condition, we need synchronization. Therefore, the caller must own the monitor of the callee before execution<\/strong>. So, let’s rewrite our code having synchronization concerns in mind:<\/p>\n
private volatile Boolean jobIsDone;\r\nprivate final Object lock = new Object();\r\npublic void ensureCondition() {\r\n    synchronized (lock) {\r\n        while (!jobIsDone) {\r\n            try {\r\n                lock.wait();\r\n            } \r\n            catch (InterruptedException e) { \r\n                \/\/ ...\r\n            }\r\n        }\r\n    }\r\n}\r\npublic void complete() {\r\n    synchronized (lock) {\r\n        jobIsDone = true;\r\n        lock.notify();\r\n    }\r\n}\r\n<\/code><\/pre>\n
Here, we just added a synchronized<\/em> block, where we try to acquire the lock<\/em> object monitor before invoking the wait()\/notify()<\/em> API. Now, we avoid lost wake-up if B<\/em> executes complete()<\/em> method before A<\/em> will invoke wait()<\/em>. This is because the complete()<\/em> method can be executed by B<\/em> only if A<\/em> has not acquired the lock<\/em> object monitor. Thus, A<\/em> cannot check a condition while the complete()<\/em> method is executing.<\/p>\n
6. Conclusion<\/h2>\n
<\/div>\n
In this article, we discussed why the Java wait()<\/em> method requires synchronization. We need ownership of the callee monitor in order to avoid lost wake-up anomaly. If we do not do that, the JVM will take a fail-fast approach and throw IllegalMonitorStateException<\/em>.<\/p>\n
As always, the source code for these examples can be found over on GitHub<\/a>.<\/p>\n
\"\"<\/p>\n
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\"\"<\/p>\n
Learn why the Java wait() method requires synchronization.<\/p>\n
<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/div>\n","protected":false},"author":1188,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"site-container-style":"default","site-container-layout":"default","site-sidebar-layout":"default","disable-article-header":"default","disable-site-header":"default","disable-site-footer":"default","disable-content-area-spacing":"default","footnotes":""},"categories":[22],"tags":[61,122,127,129,124,128,125,132,131,133,126,130,123,66,94,88,97,56,64,65,60,112,40,75,95,104,33,120,105,101,98,115,30,29,41,86,70,69,68,72,71,26,118,108,87,46,55,48,52,54,51,50,83,62,58,57,11465,109,35,59,63,85,79,82,96,80,27,81,114,44,42,43,45,38,39,110,117,100,111,116,73,89,90,92,91,93,84,78,37,102,34,36,77,67,74,99,113,119,28,121,32,47,49,53,103,31,76],"class_list":["post-30936","post","type-post","status-publish","format-standard","hentry","category-mobile","tag-airpods","tag-anime","tag-anime-characters","tag-anime-cosplay","tag-anime-edits","tag-anime-merchandise","tag-anime-movies","tag-anime-news","tag-anime-recommendations","tag-anime-reviews","tag-anime-series","tag-anime-streaming","tag-animes","tag-app-store","tag-app-store-samsung","tag-appgallery","tag-appgallery-oneplus","tag-apple","tag-apple-music","tag-apple-tv","tag-apple-watch","tag-bbc-sport","tag-best-mobile-games","tag-bixby","tag-bixby-xiaomi","tag-champions-league","tag-cyberpunk","tag-cyberpunk-2077","tag-fantasy-football","tag-fifa","tag-football","tag-formula-1","tag-fortnite","tag-free-fire","tag-free-mobile-games","tag-freebuds-pro","tag-galaxy-a52","tag-galaxy-note-20","tag-galaxy-s21","tag-galaxy-watch-4","tag-galaxy-z-fold-3","tag-game","tag-games","tag-golf","tag-harmonyos","tag-how-to-backup-iphone","tag-how-to-factory-reset-iphone","tag-how-to-reset-iphone","tag-how-to-restore-iphone","tag-how-to-unlock-iphone","tag-how-to-unlock-iphone-5","tag-how-to-unlock-iphone-6","tag-huawei","tag-ios","tag-ipad","tag-iphone","tag-java-concurrency","tag-live-soccer","tag-lol","tag-macbook","tag-macos","tag-mate-40-pro","tag-mi-11-lite","tag-mi-home-security-camera-basic-1080p","tag-mi-home-security-camera-basic-1080p-huawei","tag-mi-smart-band-6","tag-minecraft","tag-miui","tag-mlb-scores","tag-mobile-game-design","tag-mobile-game-development","tag-mobile-game-marketing","tag-mobile-game-monetization","tag-mobile-games","tag-mobile-gaming","tag-nba-scores","tag-nba-standings","tag-nfl","tag-nfl-scores","tag-nhl-scores","tag-one-ui","tag-oneplus","tag-oneplus-9-pro","tag-oneplus-buds-pro","tag-oneplus-nord-ce-5g","tag-oxygenos","tag-p40-pro-plus","tag-poco-x3-pro","tag-pokemon","tag-premier-league","tag-pubg","tag-pubg-mobile","tag-redmi-note-10-pro","tag-samsung","tag-samsung-pay","tag-soccer","tag-sports","tag-steam","tag-steeam","tag-top-10-anime","tag-valorant","tag-when-do-the-iphone-7-come-out","tag-when-does-the-iphone-7-come-out","tag-when-is-the-iphone-7-coming-out","tag-world-cup","tag-xbox-series-x","tag-xiaomi"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/posts\/30936","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/users\/1188"}],"replies":[{"embeddable":true,"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/comments?post=30936"}],"version-history":[{"count":1,"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/posts\/30936\/revisions"}],"predecessor-version":[{"id":30937,"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/posts\/30936\/revisions\/30937"}],"wp:attachment":[{"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/media?parent=30936"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/categories?post=30936"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/wp-json\/wp\/v2\/tags?post=30936"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}