java 单例模式的实例详解

public class Singleton {
  private static Singleton uniqueInstance = null;

  private Singleton() {
    // Exists only to defeat instantiation.
  }

  public static Singleton getInstance() {
    if (uniqueInstance == null) {
      uniqueInstance = new Singleton();
    }
    return uniqueInstance;
  }
  // Other methods...
}

public class TestStream {
  private String name;
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  } 
  //该类只能有一个实例
  private TestStream(){}  //私有无参构造方法
  //该类必须自行创建
  //有2种方式
  /*private static final TestStream ts=new TestStream();*/
  private static TestStream ts1=null;
  //这个类必须自动向整个系统提供这个实例对象
  public static TestStream getTest(){
    if(ts1==null){
      ts1=new TestStream();
    }
    return ts1;
  }
  public void getInfo(){
    System.out.println("output message "+name);
  }
}
/**
 * 
 */
public class TestMain {
  public static void main(String [] args){
    TestStream s=TestStream.getTest();
    s.setName("张孝祥");
    System.out.println(s.getName());
    TestStream s1=TestStream.getTest();
    s1.setName("张孝祥");
    System.out.println(s1.getName());
    s.getInfo();
    s1.getInfo();
    if(s==s1){
      System.out.println("创建的是同一个实例");
    }else if(s!=s1){
      System.out.println("创建的不是同一个实例");
    }else{
      System.out.println("application error");
    }
  }
}

张孝祥
张孝祥
output message 张孝祥
output message 张孝祥
创建的是同一个实例

//饿汉式单例类.在类初始化时，已经自行实例化 
public class Singleton1 {
  //私有的默认构造子
  private Singleton1() {}
  //已经自行实例化 
  private static final Singleton1 single = new Singleton1();
  //静态工厂方法 
  public static Singleton1 getInstance() {
    return single;
  }
}

//懒汉式单例类.在第一次调用的时候实例化 
public class Singleton2 {
  //私有的默认构造子
  private Singleton2() {}
  //注意，这里没有final  
  private static Singleton2 single=null;
  //静态工厂方法 
  public synchronized static Singleton2 getInstance() {
     if (single == null) { 
       single = new Singleton2();
     } 
    return single;
  }
}

import java.util.HashMap;
import java.util.Map;
//登记式单例类.
//类似Spring里面的方法，将类名注册，下次从里面直接获取。
public class Singleton3 {
  private static Map<String,Singleton3> map = new HashMap<String,Singleton3>();
  static{
    Singleton3 single = new Singleton3();
    map.put(single.getClass().getName(), single);
  }
  //保护的默认构造子
  protected Singleton3(){}
  //静态工厂方法,返还此类惟一的实例
  public static Singleton3 getInstance(String name) {
    if(name == null) {
      name = Singleton3.class.getName();
      System.out.println("name == null"+"--->name="+name);
    }
    if(map.get(name) == null) {
      try {
        map.put(name, (Singleton3) Class.forName(name).newInstance());
      } catch (InstantiationException e) {
        e.printStackTrace();
      } catch (IllegalAccessException e) {
        e.printStackTrace();
      } catch (ClassNotFoundException e) {
        e.printStackTrace();
      }
    }
    return map.get(name);
  }
  //一个示意性的商业方法
  public String about() {  
    return "Hello, I am RegSingleton.";  
  }  
  public static void main(String[] args) {
    Singleton3 single3 = Singleton3.getInstance(null);
    System.out.println(single3.about());
  }
}