2-1 start线程开启（C源码分析）

一个线程开启都经历了什么

public class ThreadBaseDemo {
    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
        }, "t1");
        t1.start();
    }
}

• start 源码

  start 被 synchronized 修饰，必须全部执行完

  public synchronized void start() {
      /**
       * This method is not invoked for the main method thread or "system"
       * group threads created/set up by the VM. Any new functionality added
       * to this method in the future may have to also be added to the VM.
       *
       * A zero status value corresponds to state "NEW".
       */
      if (threadStatus != 0)
          throw new IllegalThreadStateException();
  
      /* Notify the group that this thread is about to be started
       * so that it can be added to the group's list of threads
       * and the group's unstarted count can be decremented. */
      group.add(this);
  
      boolean started = false;
      try {
          start0();
          started = true;  /*启动成功*/
      } finally {
          try {
              if (!started) {
                  group.threadStartFailed(this);
              }
          } catch (Throwable ignore) {
              /* do nothing. If start0 threw a Throwable then
                it will be passed up the call stack */
          }
      }
  }
  

• start0 方法

  java 线程是通过 start 的方法启动执行的，主要内容在 native 方法 start0 中，openjdk 的写 JNl^(Java Native Interface，Java本地接口)^ 一般是一一对应的，Thread.java 对应的就是 Thread.c

  start0 其实就是 JVM_StartThread。此时查看源代码可以看到在 jvm.h 中找到了声明，jvm.cpp 中有实现。

  private native void start0();
  

  start0 的执行流程一般如下所示

  

  • thread.c 部分

    

  • jvm.cpp

    
    

  • thread.cpp

    

  从上面三部分内容来看，一个线程的启动是由 jvm 配合操作系统，底层由操作系统来分配一个原生的基础线程