ThreadGroup 线程组

在看tomcat源码时，TaskThreadFactory这个线程工厂类中用到ThreadGroup这个类，以前没有用到过，因此就特意查了一下 ThreadGroup这个属于java元老级的类，在java1.0版本中就出现了，里面还有不少已经不推荐使用线程的方法,例如suspend(),resume()这类方法。

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总的来讲，ThreadGroup里面是一个树状的结构，里面管理一些线程，能够对线程设置优先级，同时线程组中还能够有线程组。不过做用不是太大。里面的一些关键方法有如下几个：

//获取活跃的线程组的个数
activeGroupCount()
//获取活跃的线程个数
activeCount()`

在tomcat中的使用方式以下：

public class TaskThreadFactory implements ThreadFactory {

    private final ThreadGroup group;
    private final AtomicInteger threadNumber = new AtomicInteger(1);
    private final String namePrefix;
    private final boolean daemon;
    private final int threadPriority;

    public TaskThreadFactory(String namePrefix, boolean daemon, int priority) {
        SecurityManager s = System.getSecurityManager();
        group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup();
        this.namePrefix = namePrefix;
        this.daemon = daemon;
        this.threadPriority = priority;
    }

    @Override
    public Thread newThread(Runnable r) {
        TaskThread t = new TaskThread(group, r, namePrefix + threadNumber.getAndIncrement());
        t.setDaemon(daemon);
        t.setPriority(threadPriority);

        // Set the context class loader of newly created threads to be the class
        // loader that loaded this factory. This avoids retaining references to
        // web application class loaders and similar.
        if (Constants.IS_SECURITY_ENABLED) {
            PrivilegedAction<Void> pa = new PrivilegedSetTccl(
                    t, getClass().getClassLoader());
            AccessController.doPrivileged(pa);
        } else {
            t.setContextClassLoader(getClass().getClassLoader());
        }

        return t;
    }
}