date0412/SingleThreadSafe.java

/*单例设计模式：饿汉式和懒汉式。懒汉式多用于面试，实际开发多用饿汉式。饿汉式class Single{private Single(){}private static final Single s = new Single();public static Single getInstance(){return s;}}*///懒汉式:class Single{private Single(){}private static Single s = null;public static Single getInstance()//public static synchronized Single getInstance() //同步函数上加锁，效率会降低，因为多线程每次进来都要判断锁，不建议使用{if(s==null){synchronized(Single.class)//同步函数被静态修饰后，使用的锁是: 类名.class{if(s==null)s=new Single();}}return s;}}//用同步代码块，再加双重if判断。class SingleThreadSafe {public static void main(String[] args) {System.out.println("Hello World!");}}