With this example we are going to demonstrate how to use a BlockingQueue. The steps of the example are described in short:
- We have created two classes that implement the Runnable interface and override its
run()API method. - The first runnable,
FileEnumerationTaskhas a BlockingQueue of Files, a File that is an empty dump file and another File. It also has a method,enumerate(File dir), where it recursively gets the array of abstract pathnames denoting the files in the directory denoted by the given abstract pathname, usinglistFile()API method of File, until it finds a File that is not a directory, usingisDirectory()API method of File. When it reaches to a file that is not a directory, it inserts it to the BlockingQueue, usingput(File e)API method of BlockingQueue. In itsrun()method it calls theenumerate(File e)method for the given file, that puts the files under the given directory to the BlockingQueue. Then it puts the dump file to the BlockingQueue. - The other runnable,
SearchTaskalso has a BlockingQueue of Files and a String keyword. It also has a method,search(File), that reads the file using a Scanner over a FileInputStream connected to the file. It iterates over the lines of the file, usinghasNextLine()andnextLine()API methods of Scanner, and if it contains the specified keyword it prints the line. In itsrun()method this runnable retrieves and removes the head of the BlockingQueue. If the file that it retrieves is equal to the dump file of theFileEnumerationTask, then it puts it in its BlockingQueue, else it uses itsrearch(File e)method for the file to search for the given keyword. - We create a BlockingQueue of Files and a new
FileEnumerationTaskfor this blocking queue and a given path to a File. - We create a new thread to run this runnable and we also create a new
SearchTaskfor a given keyword and the given blocking queue. - The threads created by the two runnables keep adding and retrieving files to the blocking queue and in this way the specified keword is searched over the files under the specified directory.
Let’s take a look at the code snippet that follows:
package com.javacodegeeks.snippets.core;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.Scanner;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
public class BlockingQueueExample {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter base directory (e.g. C:/Users/nikos7/Desktop): ");
String dir = input.nextLine();
System.out.print("Enter keyword (e.g. output): ");
String kword = input.nextLine();
final int FILE_QUEUE_SIZE = 10;
final int THREADS = 100;
BlockingQueue<File> blockingQueue = new ArrayBlockingQueue<>(FILE_QUEUE_SIZE);
FileEnumerationTask enumerator = new FileEnumerationTask(blockingQueue, new File(dir));
new Thread(enumerator).start();
for (int i = 1; i <= THREADS; i++) {
new Thread(new SearchTask(blockingQueue, kword)).start();
}
}
}
class FileEnumerationTask implements Runnable {
public static File dumFile = new File("");
private BlockingQueue<File> blockingQueue;
private File rootDir;
public FileEnumerationTask(BlockingQueue<File> blockingQueue, File rootDir) {
this.blockingQueue = blockingQueue;
this.rootDir = rootDir;
}
@Override
public void run() {
try {
enumerate(rootDir);
blockingQueue.put(dumFile);
} catch (InterruptedException ex) {
}
}
public void enumerate(File dir) throws InterruptedException {
File[] files = dir.listFiles();
for (File file : files) {
if (file.isDirectory()) {
enumerate(file);
} else {
blockingQueue.put(file);
}
}
}
}
class SearchTask implements Runnable {
public SearchTask(BlockingQueue<File> queue, String keyword) {
this.queue = queue;
this.keyword = keyword;
}
@Override
public void run() {
try {
boolean done = false;
while (!done) {
File file = queue.take();
if (file == FileEnumerationTask.dumFile) {
queue.put(file);
done = true;
} else {
search(file);
}
}
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
}
}
public void search(File file) throws IOException {
Scanner in = new Scanner(new FileInputStream(file));
int lineNumber = 0;
while (in.hasNextLine()) {
lineNumber++;
String line = in.nextLine();
if (line.contains(keyword)) {
System.out.printf("%s:%d:%s%n", file.getPath(), lineNumber,
line);
}
}
in.close();
}
private BlockingQueue<File> queue;
private String keyword;
}
Output:
Enter base directory (e.g. C:/Users/nikos7/Desktop): C:/Users/nikos7/Desktop
Enter keyword (e.g. output): output
C:Usersnikos7Desktopapache-ant-1.8.3binrunant.pl:25:# and returns the XML formatted output)
.
.
.
This was an example of how to use a BlockingQueue in Java.
