Java信号量Semaphore原理及代码实例

import java.util.concurrent.Semaphore; 
class Pool { 
  private static final int MAX_AVAILABLE = 100; 
  private final Semaphore available = new Semaphore(MAX_AVAILABLE, true); 
  public Object getItem() throws InterruptedException { 
    available.acquire(); // 从此信号量获取一个许可，在提供一个许可前一直将线程阻塞，否则线程被中断 
    return getNextAvailableItem(); 
  } 
  public void putItem(Object x) { 
    if (markAsUnused(x)) 
      available.release(); // 释放一个许可，将其返回给信号量 
  } 
  // 仅作示例参考，非真实数据 
  protected Object[] items = null; 
  protected boolean[] used = new boolean[MAX_AVAILABLE]; 
  protected synchronized Object getNextAvailableItem() { 
    for (int i = 0; i < MAX_AVAILABLE; ++i) { 
      if (!used[i]) { 
        used[i] = true; 
        return items[i]; 
      } 
    } 
    return null; 
  } 
  protected synchronized boolean markAsUnused(Object item) { 
    for (int i = 0; i < MAX_AVAILABLE; ++i) { 
      if (item == items[i]) { 
        if (used[i]) { 
          used[i] = false; 
          return true; 
        } else 
          return false; 
      } 
    } 
    return false; 
  } 
} 

class SemaphoreTest { 
  private boolean signal = false; 
  public synchronized void take() { 
    this.signal = true; 
    this.notify(); 
  } 
  public synchronized void release() throws InterruptedException { 
    while (!this.signal) 
      wait(); 
    this.signal = false; 
  } 
} 

class SemaphoreTest { 
  private int signals = 0; 
  public synchronized void take() { 
    this.signals++; 
    this.notify(); 
  } 
  public synchronized void release() throws InterruptedException { 
    while (this.signals == 0) 
      wait(); 
    this.signals--; 
  } 
} 

class SemaphoreTest {
	private int signals = 0;
	private int bound = 0;
	public SemaphoreTest(int upperBound) {
		this.bound = upperBound;
	}
	public synchronized void take() throws InterruptedException {
		while (this.signals == bound)
			wait();
		this.signals++;
		this.notify();
	}
	public synchronized void release() throws InterruptedException {
		while (this.signals == 0)
			wait();
		this.signals--;
		this.notify();
	}
}