Java7並發編程實戰（一） 守護線程的建立和運行

標籤：

Java裡有一種特殊的線程叫做守護（Daemon）線程，這種線程的優先順序很低，通常來說，當一個應用程式裡面沒有其他線程啟動並執行時候，守護線程才運行，當線程是程式中唯一啟動並執行線程時，守護線程執行結束後，JVM也就結束了這個程式。因此，守護線程通常被用來作為同一程式中普通線程的服務提供者，通常是無線迴圈的，以等待服務要求或者線程任務。

 

　　代碼實現

　　1：建立Event類，聲明兩個私人屬性

　　 

package com.packtpub.java7.concurrency.chapter1.recipe7.event;import java.util.Date;/** * Class that stores event‘s information  * */public class Event {    /**     * Date of the event     */    private Date date;        /**     * Message of the event     */    private String event;        /**     * Reads the Date of the event     * @return the Date of the event     */    public Date getDate() {        return date;    }        /**     * Writes the Date of the event     * @param date the date of the event     */    public void setDate(Date date) {        this.date = date;    }        /**     * Reads the message of the event     * @return the message of the event     */    public String getEvent() {        return event;    }        /**     * Writes the message of the event     * @param event the message of the event     */    public void setEvent(String event) {        this.event = event;    }}

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　　2：建立WirterTask類，實現Runnable介面，聲明一個存放Event對象的隊列，並實現一個帶參數的構造器，初始化這個隊列，實現線程的run()方法，執行迴圈100次，每次迴圈中都會建立一個新的Event對象，並放入隊列中，然後休眠1秒鐘

package com.packtpub.java7.concurrency.chapter1.recipe7.task;import java.util.Date;import java.util.Deque;import java.util.concurrent.TimeUnit;import com.packtpub.java7.concurrency.chapter1.recipe7.event.Event;/** * Runnable class that generates and event every second * */public class WriterTask implements Runnable {    /**     * Data structure to stores the events     */    Deque<Event> deque;        /**     * Constructor of the class     * @param deque data structure that stores the event     */    public WriterTask (Deque<Event> deque){        this.deque=deque;    }        /**     * Main class of the Runnable     */    @Override    public void run() {                // Writes 100 events        for (int i=1; i<100; i++) {            // Creates and initializes the Event objects             Event event=new Event();            event.setDate(new Date());            event.setEvent(String.format("The thread %s has generated an event",Thread.currentThread().getId()));                        // Add to the data structure            deque.addFirst(event);            try {                // Sleeps during one second                TimeUnit.SECONDS.sleep(1);            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }}

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　 3 建立CleanerTask類，並繼承Thread類，聲明存放Event對象的隊列，也實現一個帶參數的構造器，來初始化這個隊列，在這個構造器中用setDaemon()方法，把這個線程設為守護線程。實現run()方法，他將無線重複運行，並在每次運行中，取目前時間，調用clean()方法。實現clean()方法，讀取隊列的最後一個事件對象，如果這個事件是10s鐘之前建立的，將他刪除並且檢查下一個，如果有時間被刪除，clean()將列印出刪除事件的資訊，

package com.packtpub.java7.concurrency.chapter1.recipe7.task;import java.util.Date;import java.util.Deque;import com.packtpub.java7.concurrency.chapter1.recipe7.event.Event;/** * Class that review the Event data structure and delete * the events older than ten seconds * */public class CleanerTask extends Thread {    /**     * Data structure that stores events     */    private Deque<Event> deque;    /**     * Constructor of the class     * @param deque data structure that stores events     */    public CleanerTask(Deque<Event> deque) {        this.deque = deque;        // Establish that this is a Daemon Thread        setDaemon(true);    }    /**     * Main method of the class     */    @Override    public void run() {        while (true) {            Date date = new Date();            clean(date);        }    }    /**     * Method that review the Events data structure and delete     * the events older than ten seconds     * @param date     */    private void clean(Date date) {        long difference;        boolean delete;                if (deque.size()==0) {            return;        }                delete=false;        do {            Event e = deque.getLast();            difference = date.getTime() - e.getDate().getTime();            if (difference > 10000) {                System.out.printf("Cleaner: %s\n",e.getEvent());                deque.removeLast();                delete=true;            }            } while (difference > 10000);        if (delete){            System.out.printf("Cleaner: Size of the queue: %d\n",deque.size());        }    }}

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　　4：實現主類

　　

public class Main {    /**     * Main method of the example. Creates three WriterTasks and a CleanerTask     * @param args     */    public static void main(String[] args) {                // Creates the Event data structure        Deque<Event> deque=new ArrayDeque<Event>();                // Creates the three WriterTask and starts them        WriterTask writer=new WriterTask(deque);        for (int i=0; i<3; i++){            Thread thread=new Thread(writer);            thread.start();        }                // Creates a cleaner task and starts them        CleanerTask cleaner=new CleanerTask(deque);        cleaner.start();    }}

　　列印結果

Cleaner: Size of the queue: 28Cleaner: The thread 9 has generated an eventCleaner: Size of the queue: 28Cleaner: The thread 11 has generated an eventCleaner: Size of the queue: 29Cleaner: The thread 10 has generated an eventCleaner: Size of the queue: 28Cleaner: The thread 9 has generated an eventCleaner: Size of the queue: 28Cleaner: The thread 11 has generated an eventCleaner: Size of the queue: 29Cleaner: The thread 10 has generated an eventCleaner: Size of the queue: 28Cleaner: The thread 9 has generated an eventCleaner: Size of the queue: 28Cleaner: The thread 11 has generated an eventCleaner: Size of the queue: 29Cleaner: The thread 10 has generated an eventCleaner: Size of the queue: 29Cleaner: The thread 9 has generated an eventCleaner: Size of the queue: 28

我們會發現，隊列中的對象會不斷增長至30個，然後程式結束，隊列的長度維持在27-30之間，這個程式有3個WriteTask線程，每個線程向隊列寫入一個事件，然後休眠1秒鐘，在第一個10s中，隊列中有30個事件，直到3個WriterTask都結束後，CleanTask才開始執行，但是他沒有刪除任何事件，因為所有的事件都小於10秒鐘，在接下來運行中，CleanTask每秒鐘刪除3個事件，同時WriteTask會寫入3個對象，所以隊列一直在27-30之間。

 

　　

　　

　　

 

Java7並發編程實戰（一） 守護線程的建立和運行