- 作用：保证1个类只能创建1个实例，并且该类提供1个访问该实例的全局访问点
- 优点：减少系统性能开销
饿汉式（线程安全、调用效率高、不能延时创建）：
public class Singleton {
private static Singleton instance = new Singleton();
private Singleton() {}
public static Singleton getInstance() {
return instance;
}
}
懒汉式（线程安全、调用效率低、能延时创建）：
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
双重校验锁（double-checked locking）：
public class Singleton {
private volatile static Singleton instance;
private Singleton (){}
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
静态内部类实现方式（线程安全、调用效率高、能延时创建）：
public class Singleton {
private static class SingletonHolder {
private static final Singleton instance = new Singleton();
}
private Singleton() {}
public static Singleton getInstance() {
return SingletonHolder.instance;
}
}
枚举实现单例（不能延迟创建）：
public enum Singleton {
INSTANCE;
// 添加自己需要的操作
public void singletonOperation() {}
}
