Java -- 單例模式的幾種實現方法

方法一

/**
 * 懶漢模式，線程不安全
 */
public class Singleton1 {
    private static Singleton1 instance;
    private Singleton1(){}
    public static Singleton1 getInstance(){
        if (instance == null){
            instance = new Singleton1();
        }
        return instance;
    }
}

方法二

/**
 * 懶漢模式，線程安全
 */
public class Singleton2 {
    private static Singleton2 instance;
    private Singleton2(){}
    public static synchronized Singleton2 getInstance(){
        if (instance == null){
            instance = new Singleton2();
        }
        return instance;
    }
}

方法三

/**
 * 惡漢模式（標版）
 */
public class Singleton3 {
    private static Singleton3 instance = new Singleton3();
    private Singleton3(){}
    public static Singleton3 getInstance(){
        return instance;
    }
}

方法四

/**
 * 惡漢（變形）
 */
public class Singleton4 {
    private static Singleton4 instance = null;
    static {
        instance = new Singleton4();
    }
    private Singleton4(){}
    public static Singleton4 getInstance(){
        return instance;
    }
}

方法五

/**
 * 靜態內部類
 */
public class Singleton5 {
    private static class SingletonHoler {
        private static final Singleton5 INSTANCE = new Singleton5();
    }
    private Singleton5(){}
    public static final Singleton5 getInstance(){
        return SingletonHoler.INSTANCE;
    }
}

方法六

/**
 * 枚舉
 */
public enum Singleton6 {
    INSTANCE;
    public void whateverMethod(){
        //TODO
    }
}

方法七

/**
 * 雙重校驗鎖
 */
public class Singleton7 {
    private volatile static Singleton7 instance;
    private Singleton7(){}
    public static Singleton7 getInstance(){
        if (instance == null){
            synchronized (Singleton7.class){
                if (instance == null){
                    instance = new Singleton7();
                }
            }
        }
        return instance;
    }
}