Java中的BlockingQueue小結

BlockingQueue是java.util.concurrent下的主要用來控制線程同步的工具。

主要的方法是：put、take一對阻塞存取；add、poll一對非阻塞存取。

插入：

1) add(anObject):把anObject加到BlockingQueue里,即如果BlockingQueue可以容納,則返回true,否則拋出異常,不好

2) offer(anObject):表示如果可能的話,將anObject加到BlockingQueue里,即如果BlockingQueue可以容納,則返回true,否則返回false.

3) put(anObject):把anObject加到BlockingQueue里,如果BlockQueue沒有空間,則調用此方法的線程被阻斷直到BlockingQueue里面有空間再繼續, 有阻塞, 放不進去就等待

讀取：

4) poll(time):取走BlockingQueue里排在首位的對象,若不能立即取出,則可以等time參數規定的時間,取不到時返回null; 取不到返回null

5) take():取走BlockingQueue里排在首位的對象,若BlockingQueue為空,阻斷進入等待狀態直到Blocking有新的對象被加入為止; 阻塞, 取不到就一直等

其他：

int remainingCapacity();返回隊列剩余的容量，在隊列插入和獲取的時候，不要瞎搞，數據可能不准, 不能保證數據的准確性

boolean remove(Object o); 從隊列移除元素，如果存在，即移除一個或者更多，隊列改變了，返回true

public boolean contains(Object o); 查看隊列是否存在這個元素，存在返回true

int drainTo(Collection<? super E> c); //移除此隊列中所有可用的元素,並將它們添加到給定 collection中。取出放到集合中

int drainTo(Collection<? super E> c, int maxElements); 和上面方法的區別在於，指定了移動的數量; 取出指定個數放到集合

 

BlockingQueue有四個具體的實現類,常用的兩種實現類為：

1、ArrayBlockingQueue：一個由數組支持的有界阻塞隊列，規定大小的BlockingQueue,其構造函數必須帶一個int參數來指明其大小.其所含的對象是以FIFO(先入先出)順序排序的。

2、LinkedBlockingQueue：大小不定的BlockingQueue,若其構造函數帶一個規定大小的參數,生成的BlockingQueue有大小限制,若不帶大小參數,所生成的BlockingQueue的大小由Integer.MAX_VALUE來決定.其所含的對象是以FIFO(先入先出)順序排序的。

    LinkedBlockingQueue 可以指定容量，也可以不指定，不指定的話，默認最大是Integer.MAX_VALUE,其中主要用到put和take方法，put方法在隊列滿的時候會阻塞直到有隊列成員被消費，take方法在隊列空的時候會阻塞，直到有隊列成員被放進來。

 

LinkedBlockingQueue和ArrayBlockingQueue區別：

LinkedBlockingQueue和ArrayBlockingQueue比較起來,它們背后所用的數據結構不一樣,導致LinkedBlockingQueue的數據吞吐量要大於ArrayBlockingQueue,但在線程數量很大時其性能的可預見性低於ArrayBlockingQueue。

 

下面寫一個生產者消費者的例子：

TestBlockingQueueProducer.java:

package cn.itcast_02_blockingqueue.main;

import java.util.Random;
import java.util.concurrent.BlockingQueue;

public class TestBlockingQueueProducer implements Runnable {
    BlockingQueue<String> queue;
    Random random = new Random();

    public TestBlockingQueueProducer(BlockingQueue<String> queue) {
        this.queue = queue;
    }

    @Override
    public void run() {

        for (int i = 0; i < 10; i++) {
            try {
                Thread.sleep(random.nextInt(10));
                String task = Thread.currentThread().getName() + " made a product " + i;

                System.out.println(task);
                queue.put(task);
            } catch (InterruptedException e) {
                 
                e.printStackTrace();
            }

        }

    }

}

 

TestBlockingQueueConsumer.java:

package cn.itcast_02_blockingqueue.main;

import java.util.Random;
import java.util.concurrent.BlockingQueue;

public class TestBlockingQueueConsumer implements Runnable{  
    BlockingQueue<String> queue; 
    Random random = new Random();
    
    public TestBlockingQueueConsumer(BlockingQueue<String> queue){  
        this.queue = queue;  
    }        
    @Override  
    public void run() {  
        try {  
            Thread.sleep(random.nextInt(10));
            System.out.println(Thread.currentThread().getName()+ " trying...");
            String temp = queue.take();//如果隊列為空，會阻塞當前線程  
            System.out.println(Thread.currentThread().getName() + " get a job " +temp);  
        } catch (InterruptedException e) {  
            e.printStackTrace();  
        }  
    }  
}

 

TestBlockingQueue.java:

package cn.itcast_02_blockingqueue.main;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;

public class TestBlockingQueue {
    
    public static void main(String[] args) {
        BlockingQueue<String> queue = new LinkedBlockingQueue<String>(2);
        // BlockingQueue<String> queue = new LinkedBlockingQueue<String>();
        // 不設置的話，LinkedBlockingQueue默認大小為Integer.MAX_VALUE
        // BlockingQueue<String> queue = new ArrayBlockingQueue<String>(2);
        TestBlockingQueueConsumer consumer = new TestBlockingQueueConsumer(queue);
        TestBlockingQueueProducer producer = new TestBlockingQueueProducer(queue);
        for (int i = 0; i < 5; i++) {
            new Thread(producer, "Producer" + (i + 1)).start();
        }
        for (int i = 0; i < 5; i++) {
            new Thread(consumer, "Consumer" + (i + 1)).start();
        }
        
        //new Thread(producer, "Producer" + (5)).start();
    }
}

 

 

運行結果：

Consumer2 trying...
Producer3 made a product 0
Consumer2 get a job Producer3 made a product 0
Producer3 made a product 1
Producer4 made a product 0
Consumer5 trying...
Consumer5 get a job Producer3 made a product 1
Producer4 made a product 1
Producer4 made a product 2
Consumer3 trying...
Producer2 made a product 0
Consumer3 get a job Producer4 made a product 0
Consumer1 trying...
Producer5 made a product 0
Consumer1 get a job Producer4 made a product 1
Producer3 made a product 2
Producer1 made a product 0
Consumer4 trying...
Consumer4 get a job Producer4 made a product 2
Producer2 made a product 1
Producer4 made a product 3
Producer5 made a product 1

可以看到，只有先生產出來，才能消費，否則阻塞等待。