软件构造Lab6中确保Threadsafe的方法(以注释形式描述Threadsafe的练习)

具体代码在：这里（求打星！）
在课堂上我们学到了四种确保threadsafe的方法：
　1.Confinement：限制数据共享. Don’t share the variable between threads.
　2.Immutability：共享不可变数据. Make the shared data immutable.
　3.Threadsafe data type：共享线程安全的可变数据. Encapsulate the shared data in an existing threadsafe data type that does the coordination for you.
　4.Synchronization：同步机制：通过锁的机制共享线程不安全的可变数据，变并行为串行. Use synchronization to keep the threads from accessing the variable at the same time. Synchronization is what you need to build your own threadsafedata type.

Confinement在代码中的体现：
　在Monkey类中限制数据共享.
　
Immutability在代码中的体现：
　Logger类的log作为可共享的不可变数据
　Threadsafe data type在代码中的体现：
　共享线程安全的Collections.synchronizedMap：
　
Synchronization 在代码中的体现：
　选择梯子时要对梯子上锁：
　
　对梯子内的会引发冲突的方法加锁：

/**
   * 让猴子从左岸上楼梯（此时要对该方法上锁）.
   */
  public synchronized void upfromL(Monkey monkey) {
    if (ladder.get(1) == null) {
      ladder.replace(1, monkey);
      log.info(monkey + " climb the ladder from L.");
    }
  }

  /**
   * 让猴子从右岸上楼梯（此时要对该方法上锁）.
   */
  public synchronized void upfromR(Monkey monkey) {
    if (ladder.get(ladder.size()) == null) {
      ladder.replace(ladder.size(), monkey);
      log.info(monkey + " climb the ladder from L.");
    }
  }

  /**
   * 模拟猴子在梯子上移动的方法.
   * 
   * @return 移动的终点（如果超出梯子范围返回-1）.
   */
  public synchronized int move(String direction, int from, int distance) {
    int end = from;
    if (direction.equals("L->R")) {
      for (int i = 1; i <= distance; i++) {
        if (from + i > ladder.size()) {
          end = -1;
          break;
        } else if (ladder.get(from + i) != null) {
          end = from + i - 1;
          break;
        } else {
          end = from + i;
        }
      }
    } else {
      for (int i = 1; i <= distance; i++) {
        if (from - i <= 0) {
          end = -1;
          break;
        } else if (ladder.get(from - i) != null) {
          end = from - i + 1;
          break;
        } else {
          end = from - i;
        }
      }
    }
    if (end == -1) {
      ladder.replace(from, null);
    } else if (end != from) {
      ladder.replace(end, ladder.get(from));
      ladder.replace(from, null);
    }
    return end;
  }

对用于记录的全局变量变化时要上锁：