关于Java单例

public class SingletonTest implements Runnable {
    static SingletonClass1 instance = null;

    public static void main(String args[]) {
        // SingletonClass0 instance = new SingletonClass0();// The constructor
        // SingletonClass0() is not visible

        for (int i = 0; i < 15; i++) {
            new Thread(new SingletonTest()).start();
        }
        // System.out.println(instance.getClass());

    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        SingletonClass6.getInstance();
        // System.out.println(SingletonClass7.instance.hashCode());
    }
}

class SingletonClass0 {
    private static SingletonClass0 instance = null;

    private SingletonClass0() {
        System.out.println("constructor run");
    }

    public static SingletonClass0 getInstance() {// wrong 单线才单线程下没问题，多线程下仍会有多个实例
        if (instance == null) {
            instance = new SingletonClass0();
            System.out.println(instance.hashCode());
        }
        return instance;
    }
}

class SingletonClass1 {
    private static SingletonClass1 instance = null;

    private SingletonClass1() {
        System.out.println("constructor run");
    }

    public static SingletonClass1 getInstance() {// wrong 多线程下仍会有多个实例，与上一种
                                                    // 区别在于过个线程的new过程被同步了
        if (instance == null) {
            synchronized (SingletonClass1.class) {
                instance = new SingletonClass1();
                System.out.println(instance.hashCode());
            }
        }
        return instance;
    }
}

class SingletonClass2 {
    private static SingletonClass2 instance = null;

    private SingletonClass2() {
        System.out.println("constructor run");
    }

    public static SingletonClass2 getInstance() {
        synchronized (SingletonClass2.class) {// right。只会有一个线程new实例，但阻碍了后续线程读实例
            if (instance == null) {
                instance = new SingletonClass2();
                System.out.println(instance.hashCode());
            }
        }
        return instance;
    }
}

class SingletonClass3 {
    private static SingletonClass3 instance = null;

    private SingletonClass3() {
        System.out.println("constructor run");
    }

    public static SingletonClass3 getInstance() {
        synchronized (SingletonClass3.class) {// right
                                                // 只会有一个线程new实例，不会影响后续线程读实例，但instance=new
                                                // SingletonClass3()在JVM内不是原子操作，内部的几个步骤可能乱序，从而出错
            if (instance == null) {
                synchronized (SingletonClass1.class) {
                    if (instance == null) {
                        instance = new SingletonClass3();
                        System.out.println(instance.hashCode());
                    }
                }
            }
        }
        return instance;
    }
}

class SingletonClass4 {
    private static volatile SingletonClass4 instance = null;// right
                                                            // 只有一个实例，volatite保证了在JVM内部new
                                                            // instance的几个操作禁止指令重排序优化

    private SingletonClass4() {
        System.out.println("constructor run");
    }

    public static SingletonClass4 getInstance() {
        synchronized (SingletonClass4.class) {
            if (instance == null) {
                synchronized (SingletonClass1.class) {
                    if (instance == null) {
                        instance = new SingletonClass4();
                        System.out.println(instance.hashCode());
                    }
                }
            }
        }
        return instance;
    }
}

class SingletonClass5 {// right，但是由类加载器在类加载时创建实例，我们无法控制实例创建的时机以干一些事（比如某个配置文件，或是某个被其它类创建的资源）
    public volatile static SingletonClass5 instance = new SingletonClass5();

    private SingletonClass5() {
        System.out.println("constructor run");
    };

    public static SingletonClass5 getInstance() {
        System.out.println(instance.hashCode());
        return instance;
    }
}

class SingletonClass6 {// 仍然使用JVM本身机制保证了线程安全问题；由于 SingletonHolder 是私有的，除了
                        // getInstance()
                        // 之外没有办法访问它，因此它只有在getInstance()被调用时才会真正创建；同时读取实例的时候不会进行同步，没有性能缺陷；也不依赖
                        // JDK 版本
    private static class SingletonHolder {
        private static final SingletonClass6 INSTANCE = new SingletonClass6();
    }

    private SingletonClass6() {
        System.out.println("constructor run");
    }

    public static final SingletonClass6 getInstance() {
        System.out.println(SingletonHolder.INSTANCE.hashCode());
        return SingletonHolder.INSTANCE;
    }
}

class SingletonClass7 {
    public SingletonClass7() {
        System.out.println("constructor run");
    }
}

enum SingletonClass {
    instance;
}