Java单例模式实现的几种方式

Java单例模式实现的几种方式

单例模式好多书上都是这么写的：

public class SingleTon1 {

  private SingleTon1(){
  }

  private static SingleTon1 instance = null;

  public static SingleTon1 getInstance(){
    if(instance == null){
      instance = new SingleTon1();
    }
    return instance;
  }
}

　　但是实际开发中是不会这么写的，因为有一个严重的问题，多线程并发访问的时候，可能会产生多个实例！！

下面列举几个常用的方法：

1.使用synchronized 关键字

package singleton;

public class SingleTon1 {

  private SingleTon1(){

  }

  private static SingleTon1 instance = null;

  //多线程问题解法一，但是效率不高！因为每次调用都会加锁！
  public static synchronized SingleTon1 getInstance(){
    if(instance == null){
      instance = new SingleTon1();
    }
    return instance;
  }
  public void print(){
    System.out.println("thread_id:"+Thread.currentThread().getId());
  }

  private static Object object = new Object();
  //很巧妙的方法，只有在null的时候加锁，之后就不加啦
  public static SingleTon1 getInstance2(){

    if(instance == null){
      synchronized (object){
        instance = new SingleTon1();
      }
    }
    return instance;
  }

}

　2.加锁

package singleton;

import java.util.concurrent.locks.ReentrantLock;

public class SingleTon2 {

  private SingleTon2(){

  }
  private static ReentrantLock lock = new ReentrantLock();
  private static SingleTon2 instance = null;

  public void print(){
    System.out.println("thread_id:"+Thread.currentThread().getId());
  }

  public static SingleTon2 getInstance2(){

    if(instance == null){
      lock.lock();
      if(instance == null){ //注意这里还要判断下！！
        instance = new SingleTon2();
      }
      lock.unlock();
    }
    return instance;
  }
}

　　3.利用静态变量：

package singleton;

public class SingleTon3 {

  public static void print(){
    System.out.println("thread_id:"+Thread.currentThread().getId());
  }

  public static Nested getNested(){

    return Nested.instance;
  }
  //这个是单例创建的类
  static class Nested{
   private Nested(){
    }
  static Nested instance = new Nested();
  }
}

以上就是常用的创建单例的模式：

Test测试代码：

package singleton;

import singleton.SingleTon3.Nested;

public class Test2 {

  public static void main(String[] args) {
    // TODO Auto-generated method stub
    Nested singleton;
    Myrunnable mm = new Myrunnable();
    Myrunnable m1 = new Myrunnable();

    Myrunnable2 m2 = new Myrunnable2();
    new Thread(m1).start();
    new Thread(m2).start();
    if(m1.singleton == m2.singleton){ //是同一个
      System.out.println("是同一个");
    }else{
      System.out.println("不是同一个");
    }
   }
}
  class Myrunnable implements Runnable{
    Nested singleton;
      @Override
      public void run() {
        // TODO Auto-generated method stub
        singleton = SingleTon3.getNested();
        SingleTon3.print();
      }
  }

  class Myrunnable2 implements Runnable{
    Nested singleton;
    @Override
    public void run() {
      // TODO Auto-generated method stub
      singleton = SingleTon3.getNested();
      SingleTon3.print();
    }
  }

输出：

是同一个

thread_id:11
thread_id:10

以上就是对Java 单例模式的资料整理，后续继续补充相关资料，谢谢大家对本站的支持！