new一个Thread之后发生了什么

Thread的构造器都是调用了init方法，所以我们来看一下init方法

private void init(ThreadGroup g,
				  Runnable target, 
				  String name,
                  long stackSize, 
                  AccessControlContext acc,
                  boolean inheritThreadLocals) {
    // name字段不可为空，new Thread的时候会赋予“Thread-数字”的名字
    if (name == null) {
        throw new NullPointerException("name cannot be null");
    }
    this.name = name;
	// 调用该方法的线程是新线程的parent
    Thread parent = currentThread();
    // System里的SecurityManager实例是静态代码块初始化的时候调用了一个native方法里创建的
    SecurityManager security = System.getSecurityManager();
    // 线程组
    if (g == null) {
        // 首先由security决定
        if (security != null) {
            g = security.getThreadGroup();
        }
        // 如果security为空或者没有给出明确的组，则使用父线程的组
        if (g == null) {
            g = parent.getThreadGroup();
        }
    }

    /* checkAccess regardless of whether or not threadgroup is
       explicitly passed in. */
   	// MODIFY_THREADGROUP_PERMISSION权限检查，之后要对组进行操作
    g.checkAccess();

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

	// 线程组里面对未start的进程维护一个计数器
    g.addUnstarted();

    this.group = g;
    this.daemon = parent.isDaemon();
    this.priority = parent.getPriority();
    // 类加载器private ClassLoader contextClassLoader;
    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);
	
	// 是否继承父线程的ThreadLocal
    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();
}

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总结

• 配置线程组、target、name线程名、stackSize栈深、tid线程ID，priority优先级、是否是后台线程、parent父线程
• 线程组维护unstart计数器
• 配置类加载器CCL，配置inheritedAccessControlContext