No enclosing instance of type * is accessible. Must qualify the allocation with an enclosing instanc

今天在一位大神的博客中学习多线程的知识，其中有一段关于线程中sleep（）方法的测试代码，内容如下：

public class SleepLockTest{ private static Object obj = new Object();public static void main(String[] args){ ThreadA t1 = new ThreadA("t1"); ThreadA t2 = new ThreadA("t2"); t1.start(); t2.start();} static class ThreadA extends Thread{public ThreadA(String name){ super(name); } public void run(){ // 获取obj对象的同步锁synchronized (obj) {try {for(int i=0; i <10; i++){ System.out.printf("%s: %d\n", this.getName(), i); // i能被4整除时，休眠100毫秒if (i%4 == 0)Thread.sleep(100);}} catch (InterruptedException e) {e.printStackTrace();}}} } }

我们知道，sleep（）方法会让线程进去休眠状态，但是并不释放当前所持有的同步锁（synchronized (obj)），所以这个代码最后的结果肯定是 t1 线程先执行完，然后 t2线程继续执行。但我今天发现的一个知识点是关于内部类，因为工作中很少涉及内部类，所以这段知识有些匮乏，现在发现问题，就先将这个小问题整理一下。

当我试图将内部类ThreadA 的 static 关键字去掉的时候，编译器会在new ThreadA的时候告诉我这个错：

No enclosing instance of type SleepLockTest is accessible. Must qualify the allocation with an enclosing instance of type SleepLockTest (e.g. x.new A() where x is an instance of SleepLockTest).

这句话的大概意思是：没有可访问的内部类SleepLockTest 的实例，必须分配一个合适的内部类SleepLockTest 的实例（如x.new A()，x必须是SleepLockTest 的实例。）
这是因为我是在static void main 方法中试图创建一个非静态内部类，但是在java中，类中的静态方法不能直接调用动态方法（调用非静态内部类ThreadA的构造方法）。只有将某个内部类修饰为静态类，然后才能够在静态类中调用该类的成员变量与成员方法。所以要将static 关键字加上，或者将new ThreadA 放到main 方法外面的非静态区域中。

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