Java线程组ThreadGroup

我会带着你远行 2022-05-15 08:13 413阅读 0赞

摘要：线程组ThreadGroup表示一组线程的集合,一旦一个线程归属到一个线程组之中后，就不能再更换其所在的线程组。那么为什么要使用线程组呢？个人认为有以下的好处：方便统一管理，线程组可以进行复制，快速定位到一个线程，统一进行异常设置等。ThreadGroup它其实并不属于Java并发包中的内容，它是java.lang中的内容。但是掌握对其的于理解，在实际应用中有很大的帮助。

# 一、基本方法 #

1、获取当前线程组名

Thread.currentThread().getThreadGroup().getName()

在main方法是调用输出是：main

2、将线程放入到一个线程组中去

ThreadGroup threadGroup1 = new ThreadGroup("group1");
    ThreadGroup threadGroup2 = new ThreadGroup("group2");
    Thread thread1 =new Thread(threadGroup1, "group1's member");
    Thread thread2 =new Thread(threadGroup2, "group2's member");

其中Thread中和ThreadGroup相关的构造函数：

public Thread(ThreadGroup group, Runnable target) {
    	init(group, target, "Thread-" + nextThreadNum(), 0);
        }
     
     
        public Thread(ThreadGroup group, String name) {
    	init(group, null, name, 0);
        }
     
     
        public Thread(ThreadGroup group, Runnable target, String name) {
    	init(group, target, name, 0);
        }
     
        public Thread(ThreadGroup group, Runnable target, String name,
                      long stackSize) {
    	init(group, target, name, stackSize);
        }

它们最终都是调用同一个函数：

private void init(ThreadGroup g, Runnable target, String name,
                          long stackSize) {
    	Thread parent = currentThread();
    	SecurityManager security = System.getSecurityManager();
    	if (g == null) {
             //安全检查
    	    if (security != null) {
    		g = security.getThreadGroup();
    	    }
     
            //设置线程组
    	    if (g == null) {
    		g = parent.getThreadGroup();
    	    }
    	}
     
        //检查可达性
    	g.checkAccess();
     
         //是否有权限访问
    	if (security != null) {
    	    if (isCCLOverridden(getClass())) {
    	        security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
    	    }
    	}
     
         //往线程组添加线程但未启动
         g.addUnstarted();
     
    	this.group = g;
    	this.daemon = parent.isDaemon();//是否守护线程
    	this.priority = parent.getPriority();//优先级
    	this.name = name.toCharArray();
    	if (security == null || isCCLOverridden(parent.getClass()))
    	    this.contextClassLoader = parent.getContextClassLoader();
    	else
    	    this.contextClassLoader = parent.contextClassLoader;
    	this.inheritedAccessControlContext = AccessController.getContext();
    	this.target = target;
    	setPriority(priority);
            if (parent.inheritableThreadLocals != null)
    	    this.inheritableThreadLocals =
    		ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
            this.stackSize = stackSize;
            tid = nextThreadID();
            this.me = this;
        }

3、复制线程组：

//这样可以复制group里面的thread信息
    		Thread[] threads = new Thread[threadGroup.activeCount()];
    		threadGroup.enumerate(threads);

这里的activeCount很明显就是取得活动的线程，注意。默认情况 下，连同其子线程组也会进行复制。

4、未捕获异常处理

ThreadGroup中有一个uncaughtException()方法。当线程组中某个线程发生Unchecked exception异常时，由执行环境调用此方法进行相关处理，如果有必要，可以重新定义此方法

# 二、应用实例 #

1、实例应用

package com.func.axc.threadgroup;
     
     
    import java.util.Date;
    import java.util.Random;
    import java.util.concurrent.TimeUnit;
     
    class Result {
    	
    	private String name;
     
    	public String getName() {
    		return name;
    	}
     
    	public void setName(String name) {
    		this.name = name;
    	}
    	
    }
     
    public class SearchTask implements Runnable {
     
    	public SearchTask(Result result) {
    		this.result = result;
    	}
     
    	private Result result;
     
    	@Override
    	public void run() {
    		String name = Thread.currentThread().getName();
    		System.out.println("Thread Start " + name);
    		try {
    			doTask();
    			result.setName(name);
    		} catch (InterruptedException e) {
    			System.out.printf("Thread %s: Interrupted\n", name);
    			return;
    		}
    		System.out.println("Thread end " + name);
    	}
     
    	private void doTask() throws InterruptedException {
    		Random random = new Random((new Date()).getTime());
    		int value = (int) (random.nextDouble() * 100);
    		System.out.printf("Thread %s: %d\n", Thread.currentThread().getName(),
    				value);
    		TimeUnit.SECONDS.sleep(value);
    	}
     
    	public static void main(String[] args) {
    		System.out.println("main thread start:");
    		
    		//创建5个线程，并入group里面进行管理
    		ThreadGroup threadGroup = new ThreadGroup("Searcher");
    		Result result = new Result();
    		SearchTask searchTask = new SearchTask(result);
    		for (int i = 0; i < 5; i++) {
    			Thread thred = new Thread(threadGroup, searchTask);
    			thred.start();
    			try {
    				TimeUnit.SECONDS.sleep(1);
    			} catch (InterruptedException e) {
    				e.printStackTrace();
    			}
    		}
    		//通过这种方法可以看group里面的所有信息
    		System.out.printf("Number of Threads: %d\n", threadGroup.activeCount());
    		System.out.printf("Information about the Thread Group\n");
    		threadGroup.list();
     
    		//这样可以复制group里面的thread信息
    		Thread[] threads = new Thread[threadGroup.activeCount()];
    		threadGroup.enumerate(threads);
    		for (int i = 0; i < threadGroup.activeCount(); i++) {
    			System.out.printf("Thread %s: %s\n", threads[i].getName(),
    					threads[i].getState());
    		}
    		
    		waitFinish(threadGroup);
    		//将group里面的所有线程都给interpet
    		threadGroup.interrupt();
    		
    		System.out.println("main thread end:");
    	}
     
    	private static void waitFinish(ThreadGroup threadGroup) {
    		while (threadGroup.activeCount() > 0) {
    			try {
    				TimeUnit.SECONDS.sleep(1);
    			} catch (InterruptedException e) {
    				e.printStackTrace();
    			}
    		}
    	}
     
    }

输出结果：

![20160611141316082][]

2、统一异常处理实例

package com.func.axc.threadgroup;
     
    public class ThreadGroupDemo {
     
    	public static void main(String[] args) {
    		ThreadGroup threadGroup1 =
    		// 这是匿名类写法
    		new ThreadGroup("group1") {
    			// 继承ThreadGroup并重新定义以下方法
    			// 在线程成员抛出unchecked exception
    			// 会执行此方法
    			public void uncaughtException(Thread t, Throwable e) {
    				System.out.println(t.getName() + ": " + e.getMessage());
    			}
    		};
    		// 这是匿名类写法
    		Thread thread1 =
    		// 这个线程是threadGroup1的一员
    		new Thread(threadGroup1, new Runnable() {
    			public void run() {
    				// 抛出unchecked异常
    				throw new RuntimeException("测试异常");
    			}
    		});
     
    		thread1.start();
    	}
     
    }

# 三、源码解读 #

首先看其包含的变量

public class ThreadGroup implements Thread.UncaughtExceptionHandler {
        private final ThreadGroup parent;//父亲ThreadGroup
        String name;//ThreadGroupr 的名称
        int maxPriority;//线程最大优先级
        boolean destroyed;//是否被销毁
        boolean daemon;//是否守护线程
        boolean vmAllowSuspension;//是否可以中断
     
        int nUnstartedThreads = 0;//还未启动的线程
        int nthreads;//ThreadGroup中线程数目
        Thread threads[];//ThreadGroup中的线程
     
        int ngroups;//线程组数目
        ThreadGroup groups[];//线程组数组

从源码中可以看出，其包含的变量并不是很多。这里需要注意
    (1)线程组也可以包含其他线程组。如上面的groups[].
    (2)线程组构成一棵树，在树中，除了初始线程组外，每个线程组都有一个父线程组

构造函数：

//私有构造函数
        private ThreadGroup() { 
        this.name = "system";
        this.maxPriority = Thread.MAX_PRIORITY;
            this.parent = null;
        }
     
        //默认是以当前ThreadGroup传入作为parent  ThreadGroup，新线程组的父线程组是目前正在运行线程的线程组。
        public ThreadGroup(String name) {
        this(Thread.currentThread().getThreadGroup(), name);
        }
     
        //构造函数
        public ThreadGroup(ThreadGroup parent, String name) {
            this(checkParentAccess(parent), parent, name);
        }
     
        //私有构造函数
        private ThreadGroup(Void unused, ThreadGroup parent, String name) {
        this.name = name;
        this.maxPriority = parent.maxPriority;
        this.daemon = parent.daemon;
        this.vmAllowSuspension = parent.vmAllowSuspension;
        this.parent = parent;
        parent.add(this);
        }

其终的调用构造函数只有一个，父线程组的 checkAccess 方法在checkParentAccess中会调用：

//检查parent ThreadGroup
        private static Void checkParentAccess(ThreadGroup parent) {
            parent.checkAccess();
            return null;
        }

未捕获异常设置：

public void uncaughtException(Thread t, Throwable e) {
        if (parent != null) {
            parent.uncaughtException(t, e);//父线程组不为空，设置到父线程组
        } else {
                Thread.UncaughtExceptionHandler ueh = 
                    Thread.getDefaultUncaughtExceptionHandler();
                if (ueh != null) {
                    ueh.uncaughtException(t, e);
                } else if (!(e instanceof ThreadDeath)) {
            System.err.print("Exception in thread \""
                     + t.getName() + "\" ");
                    e.printStackTrace(System.err);
                }
            }
        }

如果父线程组存在, 则调用它的uncaughtException方法.  
如果父线程组不存在, 但指定了默认处理器 (下节中的As the default handler for the application), 则调用默认的处理器  
如果默认处理器没有设置, 则写错误日志.但如果 exception是ThreadDeath实例的话, 忽略  
  
线程组复制：

//此线程组及其子组中的所有活动线程复制到指定数组中。
        public int enumerate(ThreadGroup list[]) {
            checkAccess();
        return enumerate(list, 0, true);
        }
         //此线程组及其子组中的所有活动线程复制到指定数组中。
        public int enumerate(ThreadGroup list[], boolean recurse) {
            checkAccess();
        return enumerate(list, 0, recurse);
        }
        //此线程组中的所有活动线程复制到指定数组中。如果 recurse 标志为 true，则还包括对此线程的子组中的所有活动线程的引用。如果数组太小而无法保持所有线程，则    //忽略额外的线程。
        private int enumerate(ThreadGroup list[], int n, boolean recurse) {
        int ngroupsSnapshot = 0;
        ThreadGroup[] groupsSnapshot = null;
        synchronized (this) {
            if (destroyed) {
            return 0;
            }
            int ng = ngroups;
            if (ng > list.length - n) {//防止list放不下线程数目
            ng = list.length - n;
            }
            if (ng > 0) {
            System.arraycopy(groups, 0, list, n, ng);//复制线程组
            n += ng;
            }
            if (recurse) { //取得其子组
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                        groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
            }
        }
        if (recurse) {//复制子组
            for (int i = 0 ; i < ngroupsSnapshot ; i++) {
            n = groupsSnapshot[i].enumerate(list, n, true);
            }
        }
        return n;
        }

参考文章：

http://blog.csdn.net/edward\_qing\_lee/article/details/8767612

http://www.tuicool.com/articles/3aUjea

http://www.apihome.cn/api/java/ThreadGroup.html

[20160611141316082]: /images/20220515/2640d56539af4edd813fa8af8b3caed0.png