单例模式 Java 简介 学习笔记 及多种实现方式

    public class Singleton {
       //在静态初始化器中创建单例实例，这段代码保证了线程安全
        private static Singleton uniqueInstance = new Singleton();
        //Singleton类只有一个构造方法并且是被private修饰的，所以用户无法通过new方法创建该对象实例
        private Singleton(){}
        public static Singleton getInstance(){
            return uniqueInstance;
        }
    }

public class Runtime {    
    private static Runtime currentRuntime = new Runtime();    
    /**     
    * Returns the runtime object associated with the current Java application.     
    * Most of the methods of class <code>Runtime</code> are instance     
    * methods and must be invoked with respect to the current runtime object.     *     
    * @return  the <code>Runtime</code> object associated with the current    
    *          Java application.     */   
    
    public static Runtime getRuntime() {        
        return currentRuntime;    
    }    
    /** Don't let anyone else instantiate this class */   
    private Runtime() {}	//以下代码省略
}

public class Singleton {  
      private static Singleton uniqueInstance;  
      private Singleton (){
      }   
      //没有加入synchronized关键字的版本是线程不安全的
      public static Singleton getInstance() {
          //判断当前单例是否已经存在，若存在则返回，不存在则再建立单例
	      if (uniqueInstance == null) {  
	          uniqueInstance = new Singleton();  
	      }  
	      return uniqueInstance;  
      }  
 }

  public static synchronized Singleton getInstance() {  
      if (instance == null) {  
          uniqueInstance = new Singleton();  
      }  
      return uniqueInstance;  
  }  

public final class InputMethodManager {    
static InputMethodManager sInstance;         
    /**     
    * Retrieve the global InputMethodManager instance, creating it if it     
    * doesn't already exist.     
    * @hide     
    */    
    public static InputMethodManager getInstance() {
        synchronized (InputMethodManager.class) {
            if (sInstance == null) {
                IBinder b = ServiceManager.getService(Context.INPUT_METHOD_SERVICE);
                IInputMethodManager service = IInputMethodManager.Stub.asInterface(b);
                sInstance = new InputMethodManager(service, Looper.getMainLooper());
            }
            return sInstance;        
        }    
    }
}

public class Singleton {
	//volatile保证，当uniqueInstance变量被初始化成Singleton实例时，多个线程可以正确处理uniqueInstance变量
    private volatile static Singleton uniqueInstance;
    private Singleton() {
    }
    public static Singleton getInstance() {
       //检查实例，如果不存在，就进入同步代码块
        if (uniqueInstance == null) {
            //只有第一次才彻底执行这里的代码
            synchronized(Singleton.class) {
               //进入同步代码块后，再检查一次，如果仍是null，才创建实例
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}

public class Singleton {  
    private static class SingletonHolder {  
    private static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}  
    public static final Singleton getInstance() {  
    return SingletonHolder.INSTANCE;  
    }  
}   

public enum Singleton {
	 //定义一个枚举的元素，它就是 Singleton 的一个实例
    INSTANCE;  
    
public void doSomeThing() {  
     System.out.println("枚举方法实现单例");
}  

public class ESTest {

    public static void main(String[] args) {
        Singleton singleton = Singleton.INSTANCE;
        singleton.doSomeThing();//output:枚举方法实现单例

    }
}