[day 5 of JUC learning] reference atomic classes and attribute modifiers

Reference atomic class

There are three ：

• AtomicReference: Reference type atomic class . Realization principle ：CAS.

compareAndSet()： Cannot compare by anonymous object , because The content of the comparison is the address instead of Obejct.equals().

• AtomicStampedReference: Atomic class with reference version number . Realization principle ：CAS meanwhile C Version number .

compareAndSet()： Compare the content addresses and at the same time Version number Make a comparison .

• AtomicMarkableReference: Tagged reference atomic class . Realization principle ：CAS meanwhile C Mark .

compareAndSet()： Compare the content addresses and at the same time Mark Make a comparison .

public class Main {
    
    public static void main(String[] args) throws Exception {
    
        Person birdy = new Person("birdy");
        AtomicReference<Person> atomicRef = new AtomicReference<>(birdy);
        atomicRef.compareAndSet(birdy, new Person("alexander")); // success
        atomicRef.compareAndSet(new Person("birdy"), new Person("alexander")); // false

        AtomicStampedReference<Person> stampedRef = new AtomicStampedReference<>(birdy, 1);
        stampedRef.compareAndSet(birdy, new Person("alexander"), 1, 2);

        AtomicMarkableReference<Person> markRef = new AtomicMarkableReference<>(birdy, true);
        markRef.compareAndSet(birdy, new Person("alexander"), true, false);
    }
}

Problem solved

AtomicStampedReference and AtomicMarkableReference Jointly solve the data access operation under multithreading ABA Out of sync problem （ namely ： Two threads operate concurrently , Update error due to unsynchronized update . Corresponding application ：Github Submit code ）

Property modifier

You need to atomically... In a class get/set Some attribute , You can use attribute modifiers . There are three ：

• AtomicIntegerFiledUpdater: Atomic integer member modifier
• AtomicLongFieldUpdater: Atomic long integer member modifier
• AtomicReferenceFieldUpdater: Atomic reference member modifier

in the light of AtomicLongFieldUpdater analysis , Its structure is protected Of , It has an entry static method ：newUpdater()

newUpdater()

@CallerSensitive
public static <U> AtomicLongFieldUpdater<U> newUpdater(Class<U> tclass,String fieldName) {
    
    Class<?> caller = Reflection.getCallerClass();
    if (AtomicLong.VM_SUPPORTS_LONG_CAS)
      	return new CASUpdater<U>(tclass, fieldName, caller);
    else
      	return new LockedUpdater<U>(tclass, fieldName, caller);
}

Two types of updaters were found ：

• CASUpdater: adopt CAS Mechanism implementation update .
• LockedUpdater: Modify attributes by locking two long integer addresses , be used for x32 Environmental Science .

Usage method

class Person {
    
    private String name;
    private volatile long balance; //  You have to use volatile Definition Updater Members of the operation 
    private static final AtomicLongFieldUpdater<Person> UPDATER =
            AtomicLongFieldUpdater.newUpdater(Person.class, "balance");

    public long getBalance() {
    
        return UPDATER.get(this);
    }
    public void setBalance(long balance) {
    
        UPDATER.set(this, balance);
    }