The synchronized keyword is all about different threads reading and writing to the same variables, objects and resources. Synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than one thread, all reads or writes to that object's variables are done through synchronized methods. When you have two threads that are reading and writing to the same 'resource', say a variable named foo, you need to ensure that these threads access the variable in an atomic way. Without the synchronized keyword, your thread 1 may not see the change thread 2 made to foo, or worse, it may only be half changed. This would not be what you logically expect. Syntax-wise the synchronized keyword takes an Object as it's parameter (called a lock object), which is then followed by a { block of code }. Adding synchronized keyword to a method definition is equal to the entire method body being wrapped in a synchronized code block with the lock object being this (for instance methods) and ClassInQuestion.getClass() (for class methods)
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When execution encounters this keyword, the current thread tries to "lock/acquire/own" the lock object and execute the associated block of code after the lock has been acquired.
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Any writes to variables inside the synchronized code block are guaranteed to be visible to every other thread that similarly executes code inside a synchronized code block using the same lock object.
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Only one thread at a time can hold the lock, during which time all other threads trying to acquire the same lock object will wait (pause their execution). The lock will be released when execution exits the synchronized code block.
https://docs.oracle.com/javase/tutorial/essential/concurrency/sync.html
https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html
https://www.baeldung.com/java-synchronized