设计模式-单例模式

几种经典单例模式实现，参考:https://github.com/iluwatar/java-design-patterns

基础实现

public final class Singleton {

    private static final Singleton INSTANCE = new Singleton();

    private Singleton() {

    }

    public static Singleton getInstance() {
        return INSTANCE;
    }
}

利用内部类实现

public final class Singleton {

    private Singleton() {

    }

    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }

    private static class SingletonHolder {
        private final static Singleton INSTANCE = new Singleton();
    }
}

线程安全的懒加载实现

public final class Singleton {

    private static Singleton INSTANCE;

    private Singleton() {
        // 避免通过反射方式再次初始化
        if (INSTANCE == null) {
            INSTANCE = this;
        } else {
            throw new IllegalStateException("Singleton already initialized!");
        }
    }

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

线程安全的双重检查实现

public final class Singleton {

    private static volatile Singleton INSTANCE;

    private Singleton() {
        // 避免通过反射方式再次初始化
        if (INSTANCE != null) {
            throw new IllegalStateException("Singleton already initialized!");
        }
    }

    public static Singleton getInstance() {
        if (INSTANCE == null) {
            synchronized (Singleton.class) {
                if (INSTANCE == null) {
                    INSTANCE = new Singleton();
                }
            }
        }
        return INSTANCE;
    }
}