Non syncronized output even when accessing syncronized method

[Abhineet Kapil]

Hi .. Please consider following example.. where i am reading two files from c:/FileIO folder
This is a class that reads a text file ..

  public class FileRead implements Runnable{

	String url1 = null;
	String fileName1=null;
	ReadingFile reading = null;
	Thread t =null;
	static FileRead fr = null;
	
//constructor
	FileRead(String url, String fileName, ReadingFile readingFile){
		this.url1 = url;
		this.fileName1 = fileName;
		this.reading=readingFile;
		
		t = new Thread(this);
		t.start();
	}
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
			
	
		fr = new FileRead();
		fr.readFile(url1,fileName1);
		
	}
	
/*
* This method reads the input from files and is syncronized
*/
	synchronized void readFile(String url1, String fileName1){

		try {
			
			FileInputStream fis = new FileInputStream(url1+fileName1);
			BufferedInputStream bis = new BufferedInputStream(fis);
			
			Thread.sleep(5000);
			while(bis.available()>0){
				System.out.print((char)bis.read());	
			}
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

	}
		 
}
  

This is the main class

  
public class MainClass {

	/**
	 * @author Ak
	 */
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		ReadingFile rf = new ReadingFile();
		
		FileRead fr1 = new FileRead("c://FileIO//","abc.txt",rf);
		FileRead fr2 = new FileRead("c://FileIO//","abc1.txt",rf);
	
		try {
			fr1.t.join();
			fr2.t.join();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	}
}

  

Note : I am reading 2 files from c:/FileIO

Now when i run the MainClass i get an output that is a mingled output from two files...

However on creating a new file ReadingFile, and defining readFile method in it... I get the proper output.. Suddenly
synchronized keyword starts working.. The MainClass remains as it is

public class FileRead implements Runnable{

	String url1 = null;
	String fileName1=null;
	ReadingFile reading = null;
	Thread t =null;
	FileRead(String url, String fileName, ReadingFile readingFile){
		this.url1 = url;
		this.fileName1 = fileName;
		this.reading=readingFile;
		
		t = new Thread(this);
		t.start();
	}
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
			
		
		reading.readFile(url1,fileName1);
		
	}
		 
}
    

   public class ReadingFile {

	
	synchronized void readFile(String url1, String fileName1){

		try {
			
			FileInputStream fis = new FileInputStream(url1+fileName1);
			BufferedInputStream bis = new BufferedInputStream(fis);
			
			Thread.sleep(5000);
			while(bis.available()>0){
				System.out.print((char)bis.read());	
			}
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

	}
} 

Can you please explain the phenomenom behind this..

[Edward Harned]

Can you please explain the phenomenon behind this..

Can you please read this: http://sscce.org/

[Mike Simmons]

In your main() method, you create one ReadingFile object, and two different FileRead objects. If readFile() is a synchronized method defined in ReadingFile, then every time that method is called, it's synchronized on the same single instance of ReadingFile, and synchronization prevents two method calls from occurring concurrently on the same instance. That gives you the output you expect, apparently.

However if readFile() is a synchronized method defined in FileRead, then when that method is called using two different FileRead objects, synchronization has no effect. This is normal for synchronized instance methods - they only affect concurrent calls using the same instance. They have no effect on calls using different instances.

Note that if readFile() were a static synchronized method in ReadFile, then synchronization would prevent multiple concurrent invocations of the method, period. Static synchronized methods exclude execution of other static synchronized methods in the same class, regardless of what instances may have been involved. That's how they were designed. But if the method is not static, then synchronization is tied to the specific instances involved.

Note - I'm not suggesting you should rewrite readFile() to be static, as that's probably too much work for this example. I just offer this information for comparison. Synchronized instance methods and synchronized static methods behave differently, and that's a difference you need to pay attention to.

[Abhineet Kapil]

Mike , thanku so much..... doubts cleared.. code running properly........