java - 断线组java api

Question

我正在研究java中的ThreadGroups。根据 Javadoc，它是这样写的:

允许线程访问有关其自己线程组的信息，但是 不访问有关其线程组的父线程组的信息或 任何其他线程组。

但是当我实现以下代码时它正在工作，

public static void main(String args[]){
    //parent thread group It_Firm

    ThreadGroup It_Firm=new ThreadGroup("It_Firm");

    //Child thread group web

    ThreadGroup web=new ThreadGroup(It_Firm,"webdeveloper");

    /*
     * A thread entry in child thread group set in which i am trying to call parent's thread group activecount()   
     * method,as per the docs it will stop me to call for any information from parent's thread group or any other 
     * thread group but it  is not doing it.
    */

    Thread th=new Thread(web,new Runnable(){
        @Override
        public void run() {
            while(true){
                try {
                    Thread.sleep(500);
                    Thread ths[]=new Thread[Thread.currentThread().getThreadGroup().getParent().activeCount()];
                    Thread.currentThread().getThreadGroup().getParent().enumerate(ths);
                                    for(int i=0;i<ths.length;i++){
                        System.out.println("group name"+ths[i].getThreadGroup().getName()+" : name : "+ths[i].getName());
                        System.out.println("state"+ths[i].isAlive());
                    }

                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }

    },"prashank");

    th.start();

    //some dummy code in parent thread group 

    Thread th_pthread=new Thread(It_Firm,new Runnable(){

        @Override
        public void run() {
            boolean flag=true;
            while(flag){
                Scanner sc=new Scanner(System.in);
                char ch=sc.nextLine().charAt(0);
                if(ch=='N')
                    flag=false;
            }
        }

    },"abc pvt ltd");
    th_pthread.start();
}

现在我无法理解发生了什么，我对此很陌生，为什么我能够获取有关当前线程的线程组父级的信息。我是否遗漏了一些东西，有任何相关信息吗？

Answer 1

我相信文档的意思是，当线程 accesses it's own 时，没有安全检查。 ThreadGroup，即它不会因 SecurityException 失败:

public final ThreadGroup getThreadGroup() {
    return group;
}

但是当accessing the parent时有安全检查ThreadGroup 的，即它可能会因 SecurityException 失败:

public final ThreadGroup getParent() {
    if (parent != null)
        parent.checkAccess();
    return parent;
}

默认的SecurityManager仅在尝试访问根线程组(默认线程组)时才会检查modifyThreadGroup权限:

public void checkAccess(ThreadGroup g) {
    if (g == null) {
        throw new NullPointerException("thread group can't be null");
    }
    if (g == rootGroup) {
        checkPermission(SecurityConstants.MODIFY_THREADGROUP_PERMISSION);
    } else {
        // just return
    }
}

但是你可以install your own可以覆盖 checkAccess(ThreadGroup g) 的安全管理器方法。

参见Permissions in the Java Development Kit