线程的状态

    public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,

        /**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link Object#wait() Object.wait}.
         */
        BLOCKED,

        /**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * <ul>
         *   <li>{@link Object#wait() Object.wait} with no timeout</li>
         *   <li>{@link #join() Thread.join} with no timeout</li>
         *   <li>{@link LockSupport#park() LockSupport.park}</li>
         * </ul>
         *
         * <p>A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called <tt>Object.wait()</tt>
         * on an object is waiting for another thread to call
         * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
         * that object. A thread that has called <tt>Thread.join()</tt>
         * is waiting for a specified thread to terminate.
         */
        WAITING,

        /**
         * Thread state for a waiting thread with a specified waiting time.
         * A thread is in the timed waiting state due to calling one of
         * the following methods with a specified positive waiting time:
         * <ul>
         *   <li>{@link #sleep Thread.sleep}</li>
         *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
         *   <li>{@link #join(long) Thread.join} with timeout</li>
         *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
         *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
         * </ul>
         */
        TIMED_WAITING,

        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }

• New 线程创建好，还未调用start
• Runnable，就绪状态+正在运行
• Blocked，等待进入synchronized方法块
• Waiting，wait方法，LockSupport.park()
• Timed_Waiting，sleep方法，wait(long)
• Terminated ，终止

如果一个线程两次调用start()，会出现什么问题? run方法是普通方法，可以多次调用。

java.lang.IllegalThreadStateException
	at java.lang.Thread.start(Thread.java:708)

sleep和wait有什么区别

1. wait方法必须要和synchronized一起使用，sleep可单独使用
2. wait属于Object类，sleep属于Thread类
3. sleep会主动唤醒，wait是被动唤醒
4. 调用sleep，线程会进入TIMED_WAITING的状态，而调用无参数的wait，是进入WAITING的状态。
5. wait方法会释放锁，sleep不会释放锁。（如果wait在synchronize的方法块中，调用wait，其他方法是可以获取到锁）

什么是守护线程

当一个程序中的用户线程都执行结束，守护线程会直接停止。默认系统创建的线程是非守护线程。