Java 单例模式

饿汉式:

构造器私有化 => 防止直接new
类内部创建对象赋予static变量
向外暴露static公共方法,返回类实例变量

package com;
 
public class Singleton {
  private String name;
  public static int n = 22;
  private static Singleton singleton = new Singleton("singleton");
 
  private Singleton(String name) {
    System.out.println("constructor was called");
    this.name = name;
  }
 
  public static Singleton getInstance() {
    return singleton;
  }
 
  @Override
  public String toString() {
    return "Singleton{" +
        "name='" + name + '\'' +
        '}';
  }
}

package com;
 
import org.jetbrains.annotations.NotNull;
 
 
public class Test {
  public static void main(String @NotNull [] args) {
    Singleton s1 = Singleton.getInstance();
    Singleton s2 = Singleton.getInstance();
    System.out.println(s1==s2);
    System.out.println(s1);
    System.out.println(s2);
  }
}

懒汉式

构造器私有化
定义static类属性保存实例
提供public static方法返回static类属性保存的实例
只有当用户使用getInstance时,才返回构造实例,再次调用,返回之前创建的实例

package com;
 
public class Singleton {
  private String name;
  public static int n = 33;
  private static Singleton singleton;  // default null
 
  private Singleton(String name) {
    System.out.println("constructor was called");
    this.name = name;
  }
 
  @Override
  public String toString() {
    return "Singleton{" +
        "name='" + name + '\'' +
        '}';
  }
 
  public static Singleton getInstance(String name) {
    if (singleton == null) {
      singleton = new Singleton(name);
    }
    return singleton;
  }
}

package com;
 
import org.jetbrains.annotations.NotNull;
 
 
public class Test {
  public static void main(String @NotNull [] args) {
    System.out.println(Singleton.n);
//    Singleton s1 = Singleton.getInstance("s1");
//    Singleton s2 = Singleton.getInstance("s2");
//    System.out.println(s1==s2);
//    System.out.println(s1);
//    System.out.println(s2);
  }
}