java 單例的五種實現方式及其性能分析

package com.zgh.gof23.singleton;
/**
 * 餓漢式
 * @author yuelin
 *
 */
public class SingleDemo {
 private static SingleDemo instance = new SingleDemo();
 //私有化構造器
 private SingleDemo() {
 //防止其他通過反射調用構造方法，破解單例
 if (instance != null) {
  throw new RuntimeException();
 }
 }

 //對外提供統(tǒng)一的訪問點
 public static SingleDemo getInstance() {
 return instance;
 }
}

package com.zgh.gof23.singleton;

/**
 * 懶漢式實現單例
 * 
 * @author zhuguohui
 *
 */
public class SingleDemo2 {
 // 此處并不初始化實例
 private static SingleDemo2 instance;

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

 /**
 * 當調用此方法的時候才初始化實例， 為了實現線程安全，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;

/**
 * 雙重檢查
 * 
 * @author zhuguohui
 *
 */
public class SingleDemo3 {
 private static SingleDemo3 instance;

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

 public static SingleDemo3 getInstance() {
 //第一重檢查，提高效率
 if (instance == null) {
  synchronized (SingleDemo3.class) {
  //第二重檢查保證線程安全
  if (instance == null) {
   instance = new SingleDemo3();
  }
  }
 }
 return instance;
 }
}

/**
 * 靜態(tài)內部類實現單例
 * 
 * @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();
 }
 }

 /**
 * 調用這個方法的時候，JVM才加載靜態(tài)內部類，才初始化靜態(tài)內部類的類變量。由于由JVM初始化，保證了線程安全性，
 * 同時又實現了懶加載
 * @return
 */
 public static SingleDemo4 getInstance() {
 return SingleDemo4Holder.instance;
 }
}

/**
 * 枚舉實現單例
 * 枚舉由JVM實現其的單例性
 * @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++) {
   //通過更換此處，來測試不同單例實現方式在多線程環(huán)境下的性能
   SingleDemo5 demo = SingleDemo5.INSTANCE;
   }
   lock.finish();
  }
  }).start();

 }
 //等待所有線程執(zhí)行完
 lock.waitForWrok();
 long end = System.currentTimeMillis();
 System.out.println("總共耗時" + (end - start));
 }
}

package com.zgh.gof23.singleton;

public class CountLock {
 //線程的總數量
 private int count;

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

 /**
 * 當一個線程完成任務以后，調用一次這個方法
 */
 public synchronized void finish() {
 count--;
 if (count == 0) {
  notifyAll();
 }
 }

 /**
 * 需要等待其他線程執(zhí)行完的線程，調用此方法。
 */
 public synchronized void waitForWrok() {
 while (count > 0) {
  try {
  wait();
  } catch (InterruptedException e) {
  // TODO Auto-generated catch block
  e.printStackTrace();
  }
 }
 }
}