单例模式结构

单线程

// Singleton.java
public class Singleton {
    private static Singleton instance;

    private Singleton() {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

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

// Demo.java
public class Demo {
    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstance();
        Singleton singleton1 = Singleton.getInstance();
        System.out.println(singleton == singleton1);
    }
}

多线程问题（返回结构不一致）

// Singleton.java
public class Singleton {
    private static Singleton instance;
    public String value;

    private Singleton(String value) {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.value = value;
    }

    public static Singleton getInstance(String value) {
        if (instance == null) {
            instance = new Singleton(value);
        }
        return instance;
    }
}

// Demo.java
public class Demo {
    public static void main(String[] args) {
        Thread threadFoo = new Thread(new ThreadFoo());
        Thread threadBar = new Thread(new ThreadBar());
        threadFoo.start();
        threadBar.start();
    }

    static class ThreadFoo implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("Foo");
            System.out.println(singleton.value);
        }
    }

    static class ThreadBar implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("Bar");
            System.out.println(singleton.value);
        }
    }
}

多线程解决方法（采用延迟加载的线程安全单例）

// Singleton.java
public class Singleton {
    // volatile保证双端检锁正确工作
    private static volatile Singleton instance;
    public String value;

    private Singleton(String value) {
        this.value = value;
    }

    public static Singleton getInstance(String value) {
        Singleton result = null;
        if (result != null) {
            return result;
        }
        synchronized (Singleton.class) {
            if (instance == null) {
                instance = new Singleton(value);
            }
            return instance;
        }
    }
}

// Demo.java
public class Demo {
    public static void main(String[] args) {
        Thread threadFoo = new Thread(new ThreadFoo());
        Thread threadBar = new Thread(new ThreadBar());
        threadFoo.start();
        threadBar.start();
    }

    static class ThreadFoo implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("Foo");
            System.out.println(singleton.value);
        }
    }

    static class ThreadBar implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("Bar");
            System.out.println(singleton.value);
        }
    }
}