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

　　但是實(shí)際開發(fā)中是不會(huì)這么寫的，因?yàn)橛幸粋€(gè)嚴(yán)重的問題，多線程并發(fā)訪問的時(shí)候，可能會(huì)產(chǎn)生多個(gè)實(shí)例！！

下面列舉幾個(gè)常用的方法：

1.使用synchronized 關(guān)鍵字

package singleton;
 
public class SingleTon1 {
   
   
  private SingleTon1(){
     
  }
 
  private static SingleTon1 instance = null;
   
  //多線程問題解法一，但是效率不高！因?yàn)槊看握{(diào)用都會(huì)加鎖！
  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的時(shí)候加鎖，之后就不加啦
  public static SingleTon1 getInstance2(){
     
    if(instance == null){
      synchronized (object){
        instance = new SingleTon1();
      }
    }
    return instance;
  }
 
}

　2.加鎖

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){ //注意這里還要判斷下！！
        instance = new SingleTon2();
      }
      lock.unlock();
    }
    return instance;
  }
}

　　3.利用靜態(tài)變量：

package singleton;
 
 
public class SingleTon3 {
   
  public static void print(){
    System.out.println("thread_id:"+Thread.currentThread().getId());
  }
   
  public static Nested getNested(){
   
    return Nested.instance;
  }
  //這個(gè)是單例創(chuàng)建的類
  static class Nested{
   private Nested(){
    }
  static Nested instance = new Nested();
  }
}

以上就是常用的創(chuàng)建單例的模式：

Test測(cè)試代碼：


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){ //是同一個(gè)
      System.out.println("是同一個(gè)");
    }else{
      System.out.println("不是同一個(gè)");
    }
   }
}
  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();
    }
  }

輸出：

是同一個(gè)

thread_id:11
thread_id:10

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