Java类的单个实例

Question

I wanna create a single instance of a class. How can I create a single instance of a class in Java?

Answer 1

To create a truly single instance of your class (implying a singleton at the JVM level), you should make your class a Java enum .

public enum MyClass {
  INSTANCE;

  // Methods go here
}

The singleton pattern uses static state and as a result usually results in havoc when unit testing.

This is explained in Item 3 of Joshua Bloch's Effective Java.

Answer 2

Very basic singleton.

public class Singleton {
  private static Singleton instance;

  static {
    instance = new Singleton();
  }

  private Singleton() { 
    // hidden constructor
  }    

  public static Singleton getInstance() {
    return instance;
  }
}

You can also use the lazy holder pattern as well

public class Singleton {

  private Singleton() { 
    // hidden constructor
  }

  private static class Holder {
    static final Singleton INSTANCE = new Singleton();
  }

  public static Singleton getInstance() {
    return Holder.INSTANCE;
  }
}

This version will not create an instance of the singleton until you access getInstance(), but due to the way the JVM/classloader handles the creation on the inner class you are guaranteed to only have the constructor called once.

Answer 3

use the singleton pattern.

Singleton pattern

Update :

What is the singleton pattern? The singleton pattern is a design pattern that is used to restrict instantiation of a class to one object

Answer 4

In Java, how can we have one instance of the BrokerPacket class in two threads? 

So that all the threads update store the different BrokerLocation in one location array. For example:

class BrokerLocation implements Serializable {
    public String  broker_host;
    public Integer broker_port;

    /* constructor */
    public BrokerLocation(String host, Integer port) {
        this.broker_host = host;
        this.broker_port = port;
    }
}


public class BrokerPacket implements Serializable {
    public static BrokerLocation locations[];   

}