What is the use of static synchronized method in java

Question

I have one question in my mind   I read static synchronized method locked on class object  and synchronized method locks on current instance of an object So Whats the meaning of locked  on class object    Can anyone please help me on this topic

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static methods can be synchronized  But you have one lock per class  when the java class is loaded coresponding java lang class class object is there  That object s lock is needed for static synchronized methods   So when you have a static field which should be restricted to be accessed by multiple threads at once you can set those fields private and create public static synchronized setters or getters to access those fields

User · Answer

Suppose there are multiple static synchronized methods  m1  m2  m3  m4  in a class  and suppose one thread is accessing m1  then no other thread at the same time can access any other static synchronized methods

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In simple words a static synchronized method will lock the class instead of the object  and it will lock the class because the keyword static means   class instead of instance    The keyword synchronized means that only one thread can access the method at a time  And static synchronized mean       Only one thread can access the class at one time

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In general  synchronized methods are used to protect access to resources that are accessed concurrently  When a resource that is being accessed concurrently belongs to each instance of your class  you use a synchronized instance method  when the resource belongs to all instances  i e  when it is in a static variable  then you use a synchronized static method to access it   For example  you could make a static factory method that keeps a  registry  of all objects that it has produced  A natural place for such registry would be a static collection  If your factory is used from multiple threads  you need to make the factory method synchronized  or have a synchronized block inside the method  to protect access to the shared static collection   Note that using synchronized without a specific lock object is generally not the safest choice when you are building a library to be used in code written by others  This is because malicious code could synchronize on your object or a class to block your own methods from executing  To protect your code against this  create a private  lock  object  instance or static  and synchronize on that object instead

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At run time every loaded class has an instance of a Class object  That is the object that is used as the shared lock object by static synchronized methods   Any synchronized method or block has to lock on some shared object    You can also synchronize on this object manually if wanted  whether in a static method or not   These three methods behave the same  allowing only one thread at a time into the inner block   class Foo       static synchronized void methodA                               static void methodB             synchronized  Foo class                                            static void methodC             Object lock   Foo class          synchronized  lock                                           The intended purpose of static synchronized methods is when you want to allow only one thread at a time to use some mutable state stored in static variables of a class   Nowadays  Java has more powerful concurrency features  in java util concurrent and its subpackages  but the core Java 1 0 constructs such as synchronized methods are still valid and usable

User · Answer

Java VM contains a single class object per class  Each class may have some shared variables called static variables  If the critical section of the code plays with these variables in a concurrent environment  then we need to make that particular section as synchronized  When there is more than one static synchronized method only one of them will be executed at a time without preemption  That s what lock on class object does

[java] What is the use of static synchronized method in java?

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