java 单例的五种实现方式及其性能分析

package com.zgh.gof23.singleton;
/**
 * 饿汉式
 * @author yuelin
 *
 */
public class SingleDemo {
 private static SingleDemo instance = new SingleDemo();
 //私有化构造器
 private SingleDemo() {
 //防止其他通过反射调用构造方法，破解单例
 if (instance != null) {
  throw new RuntimeException();
 }
 }

 //对外提供统一的访问点
 public static SingleDemo getInstance() {
 return instance;
 }
}

package com.zgh.gof23.singleton;

/**
 * 懒汉式实现单例
 * 
 * @author zhuguohui
 *
 */
public class SingleDemo2 {
 // 此处并不初始化实例
 private static SingleDemo2 instance;

 private SingleDemo2() {
 if (instance != null) {
  throw new RuntimeException();
 }
 }

 /**
 * 当调用此方法的时候才初始化实例， 为了实现线程安全，需要使用同步方法
 * 
 * @return
 */
 public static synchronized SingleDemo2 getInstance() {
 if (instance == null) {
  instance = new SingleDemo2();
 }
 return instance;
 }
}

package com.zgh.gof23.singleton;

/**
 * 双重检查
 * 
 * @author zhuguohui
 *
 */
public class SingleDemo3 {
 private static SingleDemo3 instance;

 private SingleDemo3() {
 if (instance != null) {
  throw new RuntimeException();
 }
 }

 public static SingleDemo3 getInstance() {
 //第一重检查，提高效率
 if (instance == null) {
  synchronized (SingleDemo3.class) {
  //第二重检查保证线程安全
  if (instance == null) {
   instance = new SingleDemo3();
  }
  }
 }
 return instance;
 }
}

/**
 * 静态内部类实现单例
 * 
 * @author zhuguohui
 *
 */
public class SingleDemo4 {
 private static SingleDemo4 instance;

 private static class SingleDemo4Holder {
 private static final SingleDemo4 instance = new SingleDemo4();
 }

 private SingleDemo4() {
 if (instance != null) {
  throw new RuntimeException();
 }
 }

 /**
 * 调用这个方法的时候，JVM才加载静态内部类，才初始化静态内部类的类变量。由于由JVM初始化，保证了线程安全性，
 * 同时又实现了懒加载
 * @return
 */
 public static SingleDemo4 getInstance() {
 return SingleDemo4Holder.instance;
 }
}

/**
 * 枚举实现单例
 * 枚举由JVM实现其的单例性
 * @author zhuguohui
 *
 */
public enum SingleDemo5 {
 INSTANCE;
}

public class APP {
 public static void main(String[] args) {

 int threadCount = 100;
 long start = System.currentTimeMillis();
 final CountLock lock = new CountLock(threadCount);
 for (int i = 0; i < threadCount; i++) {
  new Thread(new Runnable() {

  @Override
  public void run() {
   for (int j = 0; j < 10000000; j++) {
   //通过更换此处，来测试不同单例实现方式在多线程环境下的性能
   SingleDemo5 demo = SingleDemo5.INSTANCE;
   }
   lock.finish();
  }
  }).start();

 }
 //等待所有线程执行完
 lock.waitForWrok();
 long end = System.currentTimeMillis();
 System.out.println("总共耗时" + (end - start));
 }
}

package com.zgh.gof23.singleton;

public class CountLock {
 //线程的总数量
 private int count;

 public CountLock(int count) {
 this.count = count;
 }

 /**
 * 当一个线程完成任务以后，调用一次这个方法
 */
 public synchronized void finish() {
 count--;
 if (count == 0) {
  notifyAll();
 }
 }

 /**
 * 需要等待其他线程执行完的线程，调用此方法。
 */
 public synchronized void waitForWrok() {
 while (count > 0) {
  try {
  wait();
  } catch (InterruptedException e) {
  // TODO Auto-generated catch block
  e.printStackTrace();
  }
 }
 }
}