在API中是这样来描述Semaphore 的

Semaphore 通常用于限制可以访问某些资源（物理或逻辑的）的线程数目。

一个计数信号量。从概念上讲，信号量维护了一个许可集。如有必要，在许可可用前会阻塞每一个 acquire()，然后再获取该许可。每个release() 添加一个许可，从而可能释放一个正在阻塞的获取者。但是，不使用实际的许可对象，Semaphore 只对可用许可的号码进行计数，并采取相应的行动。

例如，下面的类使用信号量控制线程并发的数量

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;


public class TestSemaphore {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
         ExecutorService pool =  Executors.newCachedThreadPool();
         final Semaphore sp = new Semaphore(3,true);
         for(int i=0;i<10;i++){
             Runnable runnable = new Runnable() {
				 
				@Override
				public void run() {
                  
                	   try {
						sp.acquire();
					} catch (InterruptedException e) {
   						e.printStackTrace();
                   }
                	   System.out.println(sp.availablePermits());
                   System.out.println("线程  "+ Thread.currentThread().getName() +"进入，已有"+ (3-sp.availablePermits())+ "并发") ;
            	   try {
					Thread.sleep((long) (Math.random()*3000));
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
            	   System.out.println("线程  "+Thread.currentThread().getName() +"即将离开 " );
            	   sp.release();
            	   System.out.println("线程  "+Thread.currentThread().getName() +"离开 ，已有"+ (3-sp.availablePermits()) + "并发");
				}
			};
			pool.execute(runnable);
         }
	}

}

import java.util.concurrent.Semaphore;


public class DownloadThread {
	private static int in_index = 0;
	private static int out_index = 0;
	private static int buffer_count = 100;
	public static boolean g_downloadComplete;
	private static Semaphore g_seFull = new Semaphore(0);
	private static Semaphore g_seEmpty = new Semaphore(buffer_count);
	 public static boolean getBlockFromNet(int in_index) {
		  int i = 0;
		  while (i < 10000)
		   i++;
		  if (in_index < buffer_count - 1)
		   return false;
		  else
		   return true;
		 }
		 public static void writeBlockToDisk(int out_index) {
		  int i = 0;
		  while (i < 100000)
		   i++;
		 }

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		 g_downloadComplete = false;
		  Thread threadA = new Thread() {
		   public void run() {
		    proA();
		   }
		  };
		  Thread threadB = new Thread() {
		   public void run() {
		    proB();
		   }
		  };
		  threadB.start();
		  threadA.start();
		 }
       
	 
	
	public static void proA(){
		 while (g_seFull.availablePermits() < buffer_count) {
			   try {
			    g_seEmpty.acquire();
			   } catch (InterruptedException e1) {
			    // TODO Auto-generated catch block
			    e1.printStackTrace();
			   }
			   g_downloadComplete = getBlockFromNet(in_index);
			   in_index = (in_index + 1) % buffer_count;
			   g_seFull.release();
			   System.out.println("download a block " + in_index);
			   if (g_downloadComplete)
			    break;
			  }
	}
	
	public static void proB(){
		 while (g_seEmpty.availablePermits() > 0) {
			   try {
			    g_seFull.acquire();
			   } catch (InterruptedException e1) {
			    // TODO Auto-generated catch block
			    e1.printStackTrace();
			   }
			   writeBlockToDisk(out_index);
			   out_index = (out_index + 1) % buffer_count;
			   g_seEmpty.release();
			   System.out.println("write a block " + out_index);
			   if (g_downloadComplete && out_index == in_index)
			    break;
			  }
	}

}

Java中怎么控制线程访问资源的数量