Three usages of synchronized keywords in vernacular

synchronized Scope of keyword

synchronized There are two scopes of keywords ：
Within an object instance ,synchronized aMethod(){} It can prevent multiple threads from accessing this object at the same time synchronized Method （ If an object has more than one synchronized Method , Just one line Cheng visited one of them synchronized Method , Other threads cannot access any of this object at the same time synchronized Method ）. At this time , Of different object instances synchronized The method is non-interference . in other words , Other threads can also access... In another object instance of the same class at the same time synchronized Method ;
The scope of a class ,synchronized static aStaticMethod{} Prevent multiple threads from accessing... In this class at the same time synchronized static Method . It works on all object instances of a class .
Popular speaking , You can lock an instance object , Get the instance object to run , The other is locking a class , It is equivalent to locking all instance objects , Prevent multiple threads from accessing... In this class at the same time synchronized static Method .

synchronized The three main ways to use keywords ：

Decorated instance method : Lock the current object instance , Get the lock of the current object instance before entering the synchronization code . It's like all methods of an object vie for the right to use the object , Once a method grabs the lock , To run this method . And the other parts of this object synchronized Method cannot be run at this time .
Modified static method : That is to lock the current class , It will affect all object instances of the class , Because a static member does not belong to any instance object , It's a class member （ static Indicates that this is a static resource of this class , No matter new How many objects , Only one copy. ）. So if a thread A Calling an instance object is not static synchronized Method , And threads B You need to call the static state of the class to which this instance object belongs synchronized Method , Permissible , There will be no mutual exclusion , Because accessing static synchronized The lock occupied by the method is the lock of the current class , Access is not static synchronized The lock occupied by the method is the current instance object lock .
Decorated code block : Specify the lock object , Lock a given object , Get the lock of the given object before entering the synchronization code base . Given a lock object , Only by grabbing this lock can it run .

To sum up

synchronized Keywords added to static Static methods and synchronized(class) All the code blocks are for Class Class lock .synchronized Keyword is added to instance method to lock object instance . Try not to use synchronized(String a) because JVM in , String constant pool has caching function ！