线程是比进程更加轻量级的调度执行单位，理解线程是理解并发编程的不可或缺的一部分；而生产过程中不可能永远使用裸线程，需要线程池技术，线程池是管理和调度线程的资源池。因为前不就遇到了一个关于线程状态的问题，今天就趁热打铁从源码的层面来谈一谈线程和线程池的状态及状态之间的转移。
线程JDK中，线程（Thread）定义了6种状态： NEW（新建）、RUNNABLE（可执行）、BLOCKED（阻塞）、WAITING（等待）、TIMED_WAITING（限时等待）、TERMINATED（结束）。
源码如下：
/** * A thread state. A thread can be in one of the following states: * <ul> * <li>{@link #NEW} * A thread that has not yet started is in this state. * </li> * <li>{@link #RUNNABLE} * A thread executing in the Java virtual machine is in this state. * </li> * <li>{@link #BLOCKED} * A thread that is blocked waiting for a monitor lock * is in this state. * </li> * <li>{@link #WAITING} * A thread that is waiting indefinitely for another thread to * perform a particular action is in this state. * </li> * <li>{@link #TIMED_WAITING} * A thread that is waiting for another thread to perform an action * for up to a specified waiting time is in this state. * </li> * <li>{@link #TERMINATED} * A thread that has exited is in this state. * </li> * </ul> * * <p> * A thread can be in only one state at a given point in time. * These states are virtual machine states which do not reflect * any operating system thread states. * * @since 1.5 * @see #getState */ 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; }