Java内存模型原子性原理及实例解析

public class AtomicProblem {

  private static Logger logger = LoggerFactory.getLogger(AtomicProblem.class);
  public static final int THREAD_COUNT = 10;

  public static void main(String[] args) throws Exception {
    BankAccount sharedAccount = new BankAccount("account-csx",0.00);
    ArrayList<Thread> threads = new ArrayList<>();
    for (int i = 0; i < THREAD_COUNT; i++) {
      Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
          for (int j = 0; j < 1000 ; j++) {
            sharedAccount.deposit(10.00);
          }
        }
      });
      thread.start();
      threads.add(thread);
    }
    for (Thread thread : threads) {
      thread.join();
    }
    logger.info("the balance is:{}",sharedAccount.getBalance());
  }


  public static class BankAccount {
    private String accountName;

    public double getBalance() {
      return balance;
    }

    private double balance;

    public BankAccount(String accountName, double balance){
      this.accountName = accountName;
      this.balance =balance;
    }
    public double deposit(double amount){
      balance = balance + amount;
      return balance;
    }
    public double withdraw(double amount){
      balance = balance - amount;
      return balance;
    }
    public String getAccountName() {
      return accountName;
    }
    public void setAccountName(String accountName) {
      this.accountName = accountName;
    }
  }
}

14:40:25.981 [main] INFO com.csx.demo.spring.boot.concurrent.jmm.AtomicProblem - the balance is:98260.0

balance = balance + amount;

线程1从共享内存中加载balance的初始值0到工作内存
线程1对工作内存中的值加10

//此时线程1的CPU时间耗尽，线程2获得执行机会

线程2从共享内存中加载balance的初始值到工作内存，此时balance的值还是0
线程2对工作内存中的值加10，此时线程2工作内存中的副本值是10
线程2将balance的副本值刷新回共享内存，此时共享内存中balance的值是10

//线程2CPU时间片耗尽，线程1又获得执行机会
线程1将工作内存中的副本值刷新回共享内存，但是此时副本的值还是10，所以最后共享内存中的值也是10

a = true; //原子性
a = 5;   //原子性
a = b;   //非原子性，分两步完成，第一步加载b的值，第二步将b赋值给a
a = b + 2; //非原子性，分三步完成
a ++;   //非原子性，分三步完成

 public synchronized double deposit(double amount){
   balance = balance + amount; //1
   return balance;
 }

public double deposit(double amount) {
  readWriteLock.writeLock().lock();
  try {
    balance = balance + amount;
    return balance;
  } finally {
    readWriteLock.writeLock().unlock();
  }
}

public static class BankAccount {
  //省略其他代码
  private AtomicDouble balance;

  public double deposit(double amount) {
    return balance.addAndGet(amount);
  }
  //省略其他代码
}