Java并发编程预防死锁过程详解

class Allocator{
    //通过破坏占有且等待条件避免死锁现象的发生
    private List<Object> als = new ArrayList<>();
    //一次申请所有的资源
    synchronized boolean apply(Object from, Object to){
      if (als.contains(from) || als.contains(to)){
        //只要存在一个账户被其他的业务锁定则无法完成转账业务
        return false;
      }else {
        als.add(from);
        als.add(to);
      }
      return true;
    }
    //归还资源
    synchronized void free(Object from,Object to){
      als.remove(from);
      als.remove(to);
    }
  }
  class Account {
    //actr应该为单例
    private Allocator actr;
    private int balance;
    //转账
    void transfer(Account target, int amt){
      //一次性申请转出和转入账户，直到成功
      while (!actr.apply(this,target));
      try{
        //锁定转出账户
        synchronized (this){
          //锁定转入账户
          synchronized (target){
            if (this.balance > amt){
              this.balance -= amt;
              target.balance += amt;
            }
          }
        }
      }finally {
        actr.free(this, target);
      }
    }
  }

class Accounts{
    private int id;
    private int balance;
    //转账
    void transfer(Accounts target,int amt){
      Accounts left = this;
      Accounts right = target;
      if (this.id > target.id){
        left = target;
        right = this;
      }
      //锁定序号小的账户
      synchronized (left){
        //锁定序号大的账户
        synchronized (right){
          if (this.balance > amt){
            this.balance -= amt;
            target.balance += amt;
          }
        }
      }
    }
  }