3.4 在一个共同点上同步任务
Java 并发API提供了一个同步实用程序，它允许两个或者多个线程在一个确定的点进行同步（synchronization）。这个类就是循环阻塞类（CyclicBarrier）。这个类与CountDownLatch类有点类似，但是也有不同之处。
CyclicBarrier类也是以一个integer数来初始化一个实例类，在一个确定点上，这个数就是所有线程必须同步的线程数。当这些线程到达确定点时，它调用await()方法来等待其它线程。当这个线程调用这用方法时，CyclicBarrier类阻塞并使之睡眠直到其它线程都到达为止。当最后的线程调用await() 方法时，它将唤醒所有等待的线程并继续执行它的工作。
CyclicBarrier类与CountDownLatch类有共同之处，但是，它们也有一些不同之处。最重要的不同之一就是一个CyclicBarrier对象能够重设置为它的初始化状态，赋值给它内部计数器。
下面来看一个例子，说明CyclicBarrier如何使用。假设我们有个大矩阵，矩阵中存放着0到10的数，现在，我们想查找值是5 的个数。为了高效的解决这个问题，我们使用了CyclicBarrier类。
定义矩阵类MatrixMock：
import java.util.Random;
/**
* This class generates a randommatrix of integer numbers between 1 and 10
*
*/
public class MatrixMock {
/**
* Bi-dimensional array with the random numbers
*/
private intdata[][];
/**
* Constructor of the class. Generates the bi-dimensional array ofnumbers.
* While generates the array, it counts thetimes that appears the number we are going
* to look for so we can check that theCiclycBarrier class does a good job
* @param size Number of rows of the array
* @param length Number of columns of the array
* @param number Number we are going to look for
*/
public MatrixMock(intsize, intlength, intnumber){
int counter=0;
data=new int[size][length];
Random random=new Random();
for (int i=0; i<size; i++) {
for (int j=0; j<length; j++){
data[i][j]=random.nextInt(10);
if (data[i][j]==number){
counter++;
}
}
}
System.out.printf("Mock: There are %d ocurrences of number in generated data.\n",counter,number);
}
/**
* This methods returns a row of the bi-dimensional array
* @param row the number of the row to return
* @return the selected row
*/
public int[]getRow(introw){
if ((row>=0)&&(row<data.length)){
return data[row];
}
return null;
}
}
定义并行执行结果类Results：
/**
* This class is used to store thenumber of occurrences of the number
* we are looking for in each row ofthe bi-dimensionalarray
*
*/
public class Results {
/**
* Array to store the number of occurrences ofthe number in each row of the array
*/
private intdata[];
/**
* Constructor of the class. Initializes itsattributes
* @param size Size of the array to store the results
*/
public Results(intsize){
data=new int[size];
}
/**
* Sets the value of one position in the arrayof results
* @param position Position in the array
* @param value Value to set in that position
*/
public void setData(int position, int value){
data[position]=value;
}
/**
* Returns the array of results
* @return the array of results
*/
public int[]getData(){
return data;
}
}
定义搜索群组类：
/**
* Group the results of eachSearcher. Sum the values stored in the Results object
* An object of this class isexecuted automatically by the CyclicBarrier when
* all the Searchers finish its job
*/
public class Grouper implements Runnable {
/**
* Results object with the occurrences of thenumber in each row
*/
private Results results;
/**
* Constructor of the class. Initializes itsattributes
* @param results Results object with the ocurrences of thenumber in each row
*/
public Grouper(Results results){
this.results=results;
}
/**
* Main method of the Grouper. Sum the valuesstored in the Results object
*/
@Override
public voidrun() {
int finalResult=0;
System.out.printf("Grouper: Processing results...\n");
int data[]=results.getData();
for (int number : data){
finalResult += number;
}
System.out.printf("Grouper: Total result: %d.\n",finalResult);
}
}
定义搜索类：
public class Searcher implements Runnable {
/**
* First row where look for
*/
private intfirstRow;
/**
* Last row where look for
*/
private intlastRow;
/**
* Bi-dimensional array with the numbers
*/
private MatrixMock mock;
/**
* Array to store the results
*/
private Results results;
/**
* Number to look for
*/
private intnumber;
/**
* CyclicBarrier to control the execution
*/
private finalCyclicBarrier barrier;
/**
* Constructor of the class. Initializes itsattributes
* @param firstRow First row where look for
* @param lastRow Last row where fook for
* @param mock Object with the array of numbers
* @param results Array to store the results
* @param number Number to look for
* @param barrier CyclicBarrier to control the execution
*/
public Searcher(intfirstRow, intlastRow, MatrixMock mock, Results results, int number, CyclicBarrier barrier){
this.firstRow=firstRow;
this.lastRow=lastRow;
this.mock=mock;
this.results=results;
this.number=number;
this.barrier=barrier;
}
/**
* Main method of the searcher. Look for thenumber in a subset of rows. For each row, saves the
* number of occurrences of the number in thearray of results
*/
@Override
public voidrun() {
int counter;
System.out.printf("%s: Processing lines from %d to %d.\n",Thread.currentThread().getName(),firstRow,lastRow);
for (int i=firstRow; i < lastRow;i++){
int row[]=mock.getRow(i);
counter=0;
for (int j=0; j<row.length; j++){
if (row[j]==number){
counter++;
}
}
results.setData(i, counter);
}
System.out.printf("%s: Lines processed.\n",Thread.currentThread().getName());
try {
barrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
/**
* Main method of the example
* @param args
*/
public staticvoidmain(String[] args) {
/*
* Initializes the bi-dimensional arrayof data
* 10000rows
* 1000numbers in each row
* Lookingfor number 5
*/
final int ROWS=10000;
final int NUMBERS=1000;
final int SEARCH=5;
final int PARTICIPANTS=5;
final int LINES_PARTICIPANT=2000;
MatrixMock mock=new MatrixMock(ROWS,NUMBERS,SEARCH);
// Initializes the object for the results
Results results=new Results(ROWS);
// Creates an Grouper object
Grouper grouper=new Grouper(results);
// Creates the CyclicBarrier object. It has 5 participants and, when
// they finish, the CyclicBarrier will execute the grouper object
CyclicBarrier barrier=newCyclicBarrier(PARTICIPANTS, grouper);
// Creates, initializes and starts 5 Searcher objects
Searcher searchers[]=new Searcher[PARTICIPANTS];
for (int i=0; i < PARTICIPANTS; i++){
searchers[i]=newSearcher(i*LINES_PARTICIPANT,
(i*LINES_PARTICIPANT)+LINES_PARTICIPANT,
mock,
results,
5,
barrier);
Thread thread=new Thread(searchers[i]);
thread.start();
}
System.out.printf("Main: The main thread has finished.\n");
}
}
执行结果：
Mock:There are 998891 ocurrences of number in generated data. Thread-0:Processing lines from 0 to 2000. Main:The main thread has finished. Thread-3:Processing lines from 6000 to 8000. Thread-4:Processing lines from 8000 to 10000. Thread-1:Processing lines from 2000 to 4000. Thread-2:Processing lines from 4000 to 6000. Thread-3:Lines processed. Thread-4:Lines processed. Thread-0:Lines processed. Thread-1:Lines processed. Thread-2:Lines processed. Grouper:Processing results... Grouper: Total result: 998891.
如果按照一般的处理操作，这个效率会大大降低。可见，CyclicBarrier的功能强大之处。