Why synchronization in java?

Consider the following programs

package org.vijayan.sample;

public class Main {

public static void main(String[] args) {

SampleFile file=new SampleFile("test.txt");

FileWriterThread mt1=new FileWriterThread(10000,"MT1", file);

FileWriterThread mt2=new FileWriterThread(20,"MT2", file);

mt1.start();

mt2.start();

}

}

package org.vijayan.sample;

import java.io.File;

import java.io.FileWriter;

import java.io.IOException;

public class SampleFile {

private File file;

public SampleFile(String fileName){

file=new File(fileName);

file.delete();

}

public void writeMessage(int count, String message){

try {

FileWriter writer=new FileWriter(file,true);

for(int i=0;i

writer.write(message+"-"+i);

writer.write("\n");

}

writer.close();

} catch (IOException e) {

e.printStackTrace();

}

}

}

package org.vijayan.sample;

public class FileWriterThread extends Thread {

private SampleFile file;

private int count;

public FileWriterThread(int count, String name, SampleFile math){

super(name);

this.file=math;

this.count=count;

}

@Override

public void run() {

file.writeMessage(count, getName());

}

}

Output of the above program can result

MT1-1026

MT1-1027

MT1-1028

MT1-1029

MT1-1030

MT1-1031

MT1-1032

MT1-1MT2-0

MT2-1

MT2-2

MT2-3

MT2-4

MT2-5

MT2-6

MT2-7

MT2-8

MT2-9

MT2-10

MT2-11

MT2-12

MT2-13

MT2-14

MT2-15

MT2-16

MT2-17

MT2-18

MT2-19

033

MT1-1034

MT1-1035

MT1-1036

MT1-1037

Please note the bold lines carefully. In this case both thread is using same SampleFile Object and both thread is writing to a same file concurrently. Since there is no synchronized keyword for writeMessae() it is possible. if we make this method synchronized like below

public synchronized void writeMessage(int count, String message){

try {

FileWriter writer=new FileWriter(file,true);

for(int i=0;i

writer.write(message+"-"+i);

writer.write("\n");

}

writer.close();

} catch (IOException e) {

e.printStackTrace();

}

}

then the output will change to like the following

MT1-9994

MT1-9995

MT1-9996

MT1-9997

MT1-9998

MT1-9999

MT2-0

MT2-1

MT2-2

MT2-3

MT2-4

MT2-5

MT2-6

MT2-7

MT2-8

MT2-9

MT2-10

MT2-11

MT2-12

MT2-13

Output 2 indicates that though both thread is started running the second thread is waiting until first thread completes its operation. though it is providing desired output we may not want to make the second thread wait until full completion of thread 1. This can be achieved by changing the program to following ways.

package org.vijayan.sample;

import java.io.File;

import java.io.FileWriter;

import java.io.IOException;

public class SampleFile {

private FileWriter writer;

public SampleFile(String fileName){

File file=new File(fileName);

file.delete();

try {

writer=new FileWriter(file,true);

} catch (IOException e) {

e.printStackTrace();

}

}

@Override

protected void finalize() throws Throwable {

super.finalize();

writer.close();

}

public void writeMessage(int count, String message){

try {

for(int i=0;i

synchronized (writer) {

writer.write(message+"-"+i);

writer.write("\n");

}

}

} catch (IOException e) {

e.printStackTrace();

}

}

}

Output:

MT1-1218

MT1-1219

MT2-0

MT2-1

MT2-2

MT2-3

MT2-4

MT2-5

MT2-6

MT2-7

MT2-8

Please note adding a synchronized block requires some extra code and it always needs to be done carefully.