Material Collection for the Implementation of Singleton Pattern in Java

Implementation of the Java singleton patternThis article sorts out several implementation methods of the Java singleton pattern:

Singleton pattern is written like this in many books:

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

　　However, in actual development, it will not be written like this because there is a serious problem, when multiple threads access concurrently, it may produce multiple instances!

The following are several commonly used methods:

1.using the synchronized keyword

package singleton;
public class SingleTon1 {
  private SingleTon1{
  }
  private static SingleTon1 instance = null;
  //Solution to multi-threading problems one, but not very efficient! Because it will lock every time it is called!
  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();
  //A very clever method, only locking when it is null, and not locking after that
  public static SingleTon1 getInstance2{
    if (instance == null) {
      synchronized (object) {
        instance = new SingleTon1());
      }
    }
    return instance;
  }
}

　2.lock

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) { //Note that we still need to judge here!
        instance = new SingleTon2());
      }
      lock.unlock();
    }
    return instance;
  }
}

　　3Using static variables:

package singleton;
public class SingleTon3 {
  public static void print() {
    System.out.println("thread_id:")+Thread.currentThread().getId());
  }
  public static Nested getNested() {
    return Nested.instance;
  }
  //This is the class created by singleton pattern
  static class Nested {
   private Nested() {
    }
  static Nested instance = new Nested();
  }
}

The following are the commonly used patterns for creating singletons:

Test code:

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){2System.out.println("The same"); //is the same
      else{
    }
      System.out.println("Not the same");
    }
   }
}
  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();
    }
  }

Output:

is the same

thread_id:11
thread_id:10

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