线程的底层分析

一、创建线程的方式

  • 继承Thread类
  • 实现Runnable接口
  • 实现Callable接口
  • 线程池创建

注：其实底层都是通过new Thread线程来创建

二、线程的生命周期

  1. public enum State {
  2.         /**
  3.          * Thread state for a thread which has not yet started.
  4.          */
  5.         NEW,
  7.         /**
  8.          * Thread state for a runnable thread.  A thread in the runnable
  9.          * state is executing in the Java virtual machine but it may
  10.          * be waiting for other resources from the operating system
  11.          * such as processor.
  12.          */
  13.         RUNNABLE,
  15.         /**
  16.          * Thread state for a thread blocked waiting for a monitor lock.
  17.          * A thread in the blocked state is waiting for a monitor lock
  18.          * to enter a synchronized block/method or
  19.          * reenter a synchronized block/method after calling
  20.          * {@link Object#wait() Object.wait}.
  21.          */
  22.         BLOCKED,
  24.         /**
  25.          * Thread state for a waiting thread.
  26.          * A thread is in the waiting state due to calling one of the
  27.          * following methods:
  28.          * <ul>
  29.          *   <li>{@link Object#wait() Object.wait} with no timeout</li>
  30.          *   <li>{@link #join() Thread.join} with no timeout</li>
  31.          *   <li>{@link LockSupport#park() LockSupport.park}</li>
  32.          * </ul>
  33.          *
  34.          * <p>A thread in the waiting state is waiting for another thread to
  35.          * perform a particular action.
  36.          *
  37.          * For example, a thread that has called <tt>Object.wait()</tt>
  38.          * on an object is waiting for another thread to call
  39.          * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
  40.          * that object. A thread that has called <tt>Thread.join()</tt>
  41.          * is waiting for a specified thread to terminate.
  42.          */
  43.         WAITING,
  45.         /**
  46.          * Thread state for a waiting thread with a specified waiting time.
  47.          * A thread is in the timed waiting state due to calling one of
  48.          * the following methods with a specified positive waiting time:
  49.          * <ul>
  50.          *   <li>{@link #sleep Thread.sleep}</li>
  51.          *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
  52.          *   <li>{@link #join(long) Thread.join} with timeout</li>
  53.          *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
  54.          *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
  55.          * </ul>
  56.          */
  57.         TIMED_WAITING,
  59.         /**
  60.          * Thread state for a terminated thread.
  61.          * The thread has completed execution.
  62.          */
  63.         TERMINATED;
  64.     }
  • NEW：创建一个线程

  • RUNNABLE：就绪状态，等待获取操作系统资源

    • 调用了Object的wait方法后，其他线程调用了notify或者notifyAll方法重新进入就绪状态
    • 其他线程调用了LockSupport的unPark方法

  • BLOCKED：阻塞状态

    • 线程进入synchronized修饰的方法或者代码块，
    • 其他线程释放锁重新进入synchronized修饰的方法或者代码块
    • 调用了Object的wait方法

  • WAITING：等待状态

    • 在没有超时的情况下调用了Object的wait方法
    • 在没有超时的情况下调用了Thread的join方法
    • 调用LockSupport的park方法

  • TIMED_WAITING：超时等待状态

    • 调用Thread的sleep方法
    • 超时调用了Object的wait()方法
    • 超时调用了Thread的join方法
    • 调用LockSupport的parkNanos方法
    • 调用LockSupport的parkUntil方法
  • TERMINATED：终止状态

三、线程的创建以及启动流程

思考：线程执行为什么不能直接调用run()方法,而要调用start()方法？

  • 使用new Thread()创建一个线程，然后调用start()方法进行java层面的线程启动
  • 接着会调用带native的本地方法start0()方法，去调用jvm中的JVM_StartThread方法进行线程创建与启动
  • 然后C++底层会根据不同操作系统平台调用创建操作系统内核线程并启动

总结：new Thread()对象后，调用start方法，会映射到jvm中创建OS thread线程，然后jvm底层会调用底层操作系统内核库的pthread_create方法创建线程