public Thread() {
        #这里第一个变量表示线程组，第三个变量表示线程名称。每个线程都是属于一个线程组的，每个线程也有名称。
        init(null, null, "Thread-" + nextThreadNum(), 0);
    }

     private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        init(g, target, name, stackSize, null, true);
    }
    #核心的代码在这一部分
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        #从这里说明我们可以给线程取名字，要么就是默认的名称：Thread-0 Thread-1这样的结构
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name;
        #每个线程都是有父线程的，默认的父线程就是当前创建你的线程。比如在main线程创建了一个线程Thread-0,那么Thread-0的父线程就是线程mian
        Thread parent = currentThread();
        SecurityManager security = System.getSecurityManager();
        #刚刚说到每个线程都是属于一个线程组的，这一段表示线程的线程组默认就是父线程的线程组
        if (g == null) {
            /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }

        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();

        /*
         * Do we have the required permissions?
         */
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }

        g.addUnstarted();
        #线程的线程组默认是父线程的线程组
        this.group = g;
        #线程是否是daemon也是取决于父类
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;
        setPriority(priority);
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

    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".
         */
         #同一个线程只能被start一次，多次start就报如下异常了

        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 {
            #这是native（）方法,也就是把线程启动了，就不深究下去了
            start0();
            started = true;
        } 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 */
            }
        }
    }