同步块-锁定多个对象

实际上，同步是针对代码的，而不是对象或数据的。在同步块中用作参数的对象引用表示锁定。

因此，如果您有如下代码：

class Player {  // Same instance shared for all players... Don't show how we get it now.  // Use one dimensional board to simplify, doesn't matter here.  private List<Player>[] fields = Board.getBoard();  // Current position  private int x;  public synchronized int getX() {    return x;  }  public void setX(int x) {    synchronized(this) { // Same as synchronized method      fields[x].remove(this);      this.x = x;      field[y].add(this);    }  }}

然后，尽管处于同步块中，但由于锁不相同（它在不同的实例上），所以对字段的访问不受保护。因此，您的董事会的球员名单可能会变得不一致，并导致运行时异常。

相反，如果您编写下面的代码，它将起作用，因为我们只有一个共享锁可供所有玩家使用：

class Player {  // Same instance shared for all players... Don't show how we get it now.  // Use one dimensional board to simplify, doesn't matter here.  private List<Player>[] fields;  // Current position  private int x;  private static Object sharedLock = new Object(); // Any object's instance can be used as a lock.  public int getX() {    synchronized(sharedLock) {      return x;    }  }  public void setX(int x) {    synchronized(sharedLock) {      // Because of using a single shared lock,      // several players can't access fields at the same time      // and so can't create inconsistencies on fields.      fields[x].remove(this);       this.x = x;      field[y].add(this);    }  }}

请确保仅使用一个锁来访问所有播放器，否则板的状态将不一致。