JAVA多线程下创建单例的正确方式

一、不注重性能的情况

package singleton;public class SlowSingleton {    PRivate static SlowSingleton instance;    private SlowSingleton() {    };    public static synchronized SlowSingleton getInstance() {        if (instance == null) {            instance = new SlowSingleton();        }        return instance;    }}二、注重性能，急切创建实例的方案
package singleton;public class FastSingleton {    private static FastSingleton instance = new FastSingleton();    private FastSingleton() {    };    public static FastSingleton getInstance() {        return instance;    }}
这种方法的缺点是：不能延迟实例化
三、注重性能，double check 方案
package singleton;public class DoubleCheckSingleton {    private volatile static DoubleCheckSingleton instance;    private DoubleCheckSingleton() {    };    public static DoubleCheckSingleton getInstance() {        if (instance == null) {            synchronized (DoubleCheckSingleton.class) {                if (instance == null) {                    instance = new DoubleCheckSingleton();                }            }        }        return instance;    }}四、注重性能，静态内部类方案
package singleton;public class MagicSingleton {    private static class SingletonHolder {        private static final MagicSingleton INSTANCE = new MagicSingleton();    }    private MagicSingleton() {    }    public static final MagicSingleton getInstance() {        return SingletonHolder.INSTANCE;    }}
五、注重性能，枚举类方案
package singleton;public enum EnumSingleton {    INSTACE;         public static EnumSingleton getInstance() {        return INSTACE;    }         public void someMethod(){        System.out.println("ooo");    }         // 使用方法    public static void main(String[] args) {        EnumSingleton.INSTACE.someMethod();        // 或者        EnumSingleton.getInstance().someMethod();    }}