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Not able to get the answer with the help of synchronized method in Java

I have to read the content from the file F1 and F2 and move it to new file (f3) using Multi-threading in Java. The problem I am facing is that when I am using synchronized in method I am not able to get the answer, but when I am not synchronizing the method I was able to get the answer.
Below is the code with synchronized method which is only printing the content of one file:
package com.company;
import java.io.*;
import java.io.FileReader;
public class FileMerge{
public static void main(String[] args) throws IOException, InterruptedException {
WriteToFile pc = new WriteToFile();
//Create a Thread1 to read the content of file 1
Thread t1 = new Thread(new Runnable() {
#Override
public void run() {
try {
pc.file1();
} catch (InterruptedException | IOException e) {
System.out.println(e);
}
}
});
// Create Thread 2 to read the content of file 2
Thread t2 = new Thread(new Runnable() {
#Override
public void run() {
try {
pc.file2();
} catch (InterruptedException | IOException e) {
System.out.println(e);
}
}
});
//Start the Thread
t1.start();
t2.start();
t1.join();
t2.join();
}
public static class WriteToFile{
PrintWriter pw = new PrintWriter("C:/Users/paras.jain/Desktop/des.txt");
public WriteToFile() throws FileNotFoundException {
}
public synchronized void file1() throws InterruptedException, IOException {
// BufferedReader object for file1.txt
BufferedReader br = new BufferedReader(new FileReader("C:\\Users\\paras.jain\\Desktop\\f.txt"));
String line = br.readLine();
//System.out.println("----------" + line);
// loop to copy each line of
// file1.txt to file3.txt
while (line != null) {
pw.println(line);
line = br.readLine();
}
br.close();
pw.close();
}
public synchronized void file2() throws InterruptedException, IOException {
BufferedReader br1 = new BufferedReader(new FileReader("C:/Users/paras.jain/Desktop/f2.txt"));
String line2 = br1.readLine();
//System.out.println("----------" + line2);
// loop to copy each line of
// file2.txt to file3.txt
while (line2 != null) {
pw.println(line2);
line2 = br1.readLine();
//System.out.println("----------" + line2);
}
br1.close();
pw.close();
}
}
}
Output of the above code is :
File 1 line 1
File 1 line 2
File 1 line 3
When I run the code WITHOUT synchronize method I got the following output:
File 1 line 1
File 1 line 2
File 2 line 1
File 2 line 2
File 1 line 3
File 2 line 3
Why is that difference in the output? And how I can I get the second output with the help of Synchronized method?

When the file1() and file2() methods are synchronized, one of them has to complete before the other can run.
Since you start the thread that runs file1() first, it likely wins the race, so file1() must complete before file2() can run.
Unfortunately, file1() closes the PrintWriter, so when file2() runs, all the pw.println(...) calls fails silently.
Why silently? Because that's the way they defined it. See javadoc of PrintWriter:
Methods in this class never throw I/O exceptions, although some of its constructors may. The client may inquire as to whether any errors have occurred by invoking checkError().

How to make a Jtext Component for input and output of a single process ran through ProcessBuilder like netbeans console [duplicate]

I am trying to create a sort of console/terminal that allows the user to input a string, which then gets made into a process and the results are printed out. Just like a normal console. But I am having trouble managing the input/output streams. I have looked into this thread, but that solution sadly doesn't apply to my problem.
Along with the standard commands like "ipconfig" and "cmd.exe", I need to be able to run a script and use the same inputstream to pass some arguments, if the script is asking for input.
For example, after running a script "python pyScript.py", I should be able pass further input to the script if it is asking for it(example: raw_input), while also printing the output from the script. The basic behavior you would expect from a terminal.
What I've got so far:
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextPane;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;
public class Console extends JFrame{
JTextPane inPane, outPane;
InputStream inStream, inErrStream;
OutputStream outStream;
public Console(){
super("Console");
setPreferredSize(new Dimension(500, 600));
setLocationByPlatform(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// GUI
outPane = new JTextPane();
outPane.setEditable(false);
outPane.setBackground(new Color(20, 20, 20));
outPane.setForeground(Color.white);
inPane = new JTextPane();
inPane.setBackground(new Color(40, 40, 40));
inPane.setForeground(Color.white);
inPane.setCaretColor(Color.white);
JPanel panel = new JPanel(new BorderLayout());
panel.add(outPane, BorderLayout.CENTER);
panel.add(inPane, BorderLayout.SOUTH);
JScrollPane scrollPanel = new JScrollPane(panel);
getContentPane().add(scrollPanel);
// LISTENER
inPane.addKeyListener(new KeyListener(){
#Override
public void keyPressed(KeyEvent e){
if(e.getKeyCode() == KeyEvent.VK_ENTER){
e.consume();
read(inPane.getText());
}
}
#Override
public void keyTyped(KeyEvent e) {}
#Override
public void keyReleased(KeyEvent e) {}
});
pack();
setVisible(true);
}
private void read(String command){
println(command);
// Write to Process
if (outStream != null) {
System.out.println("Outstream again");
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(outStream));
try {
writer.write(command);
//writer.flush();
//writer.close();
} catch (IOException e1) {
e1.printStackTrace();
}
}
// Execute Command
try {
exec(command);
} catch (IOException e) {}
inPane.setText("");
}
private void exec(String command) throws IOException{
Process pro = Runtime.getRuntime().exec(command, null);
inStream = pro.getInputStream();
inErrStream = pro.getErrorStream();
outStream = pro.getOutputStream();
Thread t1 = new Thread(new Runnable() {
public void run() {
try {
String line = null;
while(true){
BufferedReader in = new BufferedReader(new InputStreamReader(inStream));
while ((line = in.readLine()) != null) {
println(line);
}
BufferedReader inErr = new BufferedReader(new InputStreamReader(inErrStream));
while ((line = inErr.readLine()) != null) {
println(line);
}
Thread.sleep(1000);
}
} catch (Exception e) {
e.printStackTrace();
}
}
});
t1.start();
}
public void println(String line) {
Document doc = outPane.getDocument();
try {
doc.insertString(doc.getLength(), line + "\n", null);
} catch (BadLocationException e) {}
}
public static void main(String[] args){
new Console();
}
}
I don't use the mentioned ProcessBuilder, since I do like to differentiate between error and normal stream.
UPDATE 29.08.2016
With the help of #ArcticLord we have achieved what was asked in the original question.
Now it is just a matter of ironing out any strange behavior like the non terminating process. The Console has a "stop" button that simply calls pro.destroy(). But for some reason this does not work for infinitely running processes, that are spamming outputs.
Console: http://pastebin.com/vyxfPEXC
InputStreamLineBuffer: http://pastebin.com/TzFamwZ1
Example code that does not stop:
public class Infinity{
public static void main(String[] args){
while(true){
System.out.println(".");
}
}
}
Example code that does stop:
import java.util.concurrent.TimeUnit;
public class InfinitySlow{
public static void main(String[] args){
while(true){
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(".");
}
}
}

You are on the right way with your code. There are only some minor things you missed.
Lets start with your read method:
private void read(String command){
[...]
// Write to Process
if (outStream != null) {
[...]
try {
writer.write(command + "\n"); // add newline so your input will get proceed
writer.flush(); // flush your input to your process
} catch (IOException e1) {
e1.printStackTrace();
}
}
// ELSE!! - if no outputstream is available
// Execute Command
else {
try {
exec(command);
} catch (IOException e) {
// Handle the exception here. Mostly this means
// that the command could not get executed
// because command was not found.
println("Command not found: " + command);
}
}
inPane.setText("");
}
Now lets fix your exec method. You should use separate threads for reading normal process output and error output. Additionally I introduce a third thread that waits for the process to end and closes the outputStream so next user input is not meant for process but is a new command.
private void exec(String command) throws IOException{
Process pro = Runtime.getRuntime().exec(command, null);
inStream = pro.getInputStream();
inErrStream = pro.getErrorStream();
outStream = pro.getOutputStream();
// Thread that reads process output
Thread outStreamReader = new Thread(new Runnable() {
public void run() {
try {
String line = null;
BufferedReader in = new BufferedReader(new InputStreamReader(inStream));
while ((line = in.readLine()) != null) {
println(line);
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("Exit reading process output");
}
});
outStreamReader.start();
// Thread that reads process error output
Thread errStreamReader = new Thread(new Runnable() {
public void run() {
try {
String line = null;
BufferedReader inErr = new BufferedReader(new InputStreamReader(inErrStream));
while ((line = inErr.readLine()) != null) {
println(line);
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("Exit reading error stream");
}
});
errStreamReader.start();
// Thread that waits for process to end
Thread exitWaiter = new Thread(new Runnable() {
public void run() {
try {
int retValue = pro.waitFor();
println("Command exit with return value " + retValue);
// close outStream
outStream.close();
outStream = null;
} catch (InterruptedException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
});
exitWaiter.start();
}
Now this should work.
If you enter ipconfig it prints the command output, closes the output stream and is ready for a new command.
If you enter cmd it prints the output and let you enter more cmd commands like dir or cd and so on until you enter exit. Then it closes the output stream and is ready for a new command.
You may run into problems with executing python scripts because there are problems with reading Process InputStreams with Java if they are not flushed into system pipeline.
See this example python script
print "Input something!"
str = raw_input()
print "Received input is : ", str
You could run this with your Java programm and also enter the input but you will not see the script output until the script is finished.
The only fix I could find is to manually flush the output in the script.
import sys
print "Input something!"
sys.stdout.flush()
str = raw_input()
print "Received input is : ", str
sys.stdout.flush()
Running this script will bahave as you expect.
You can read more about this problem at
Java: is there a way to run a system command and print the output during execution?
Why does reading from Process' InputStream block altough data is available
Java: can't get stdout data from Process unless its manually flushed
EDIT: I have just found another very easy solution for the stdout.flush() problem with Python Scripts. Start them with python -u script.py and you don't need to flush manually. This should solve your problem.
EDIT2: We discussed in the comments that with this solution output and error Stream will be mixed up since they run in different threads. The problem here is that we cannot distinguish if output writing is finish when error stream thread comes up. Otherwise classic thread scheduling with locks could handle this situation. But we have a continuous stream until process is finished no matter if data flows or not. So we need a mechanism here that logs how much time has elapsed since last line was read from each stream.
For this I will introduce a class that gets an InputStream and starts a Thread for reading the incoming data. This Thread stores each line in a Queue and stops when end of stream arrives. Additionally it holds the time when last line was read and added to Queue.
public class InputStreamLineBuffer{
private InputStream inputStream;
private ConcurrentLinkedQueue<String> lines;
private long lastTimeModified;
private Thread inputCatcher;
private boolean isAlive;
public InputStreamLineBuffer(InputStream is){
inputStream = is;
lines = new ConcurrentLinkedQueue<String>();
lastTimeModified = System.currentTimeMillis();
isAlive = false;
inputCatcher = new Thread(new Runnable(){
#Override
public void run() {
StringBuilder sb = new StringBuilder(100);
int b;
try{
while ((b = inputStream.read()) != -1){
// read one char
if((char)b == '\n'){
// new Line -> add to queue
lines.offer(sb.toString());
sb.setLength(0); // reset StringBuilder
lastTimeModified = System.currentTimeMillis();
}
else sb.append((char)b); // append char to stringbuilder
}
} catch (IOException e){
e.printStackTrace();
} finally {
isAlive = false;
}
}});
}
// is the input reader thread alive
public boolean isAlive(){
return isAlive;
}
// start the input reader thread
public void start(){
isAlive = true;
inputCatcher.start();
}
// has Queue some lines
public boolean hasNext(){
return lines.size() > 0;
}
// get next line from Queue
public String getNext(){
return lines.poll();
}
// how much time has elapsed since last line was read
public long timeElapsed(){
return (System.currentTimeMillis() - lastTimeModified);
}
}
With this class we could combine the output and error reading thread into one. That lives while the input reading buffer threads live and have not comsumed data. In each run it checks if some time has passed since last output was read and if so it prints all unprinted lines at a stroke. The same with the error output. Then it sleeps for some millis for not wasting cpu time.
private void exec(String command) throws IOException{
Process pro = Runtime.getRuntime().exec(command, null);
inStream = pro.getInputStream();
inErrStream = pro.getErrorStream();
outStream = pro.getOutputStream();
InputStreamLineBuffer outBuff = new InputStreamLineBuffer(inStream);
InputStreamLineBuffer errBuff = new InputStreamLineBuffer(inErrStream);
Thread streamReader = new Thread(new Runnable() {
public void run() {
// start the input reader buffer threads
outBuff.start();
errBuff.start();
// while an input reader buffer thread is alive
// or there are unconsumed data left
while(outBuff.isAlive() || outBuff.hasNext() ||
errBuff.isAlive() || errBuff.hasNext()){
// get the normal output if at least 50 millis have passed
if(outBuff.timeElapsed() > 50)
while(outBuff.hasNext())
println(outBuff.getNext());
// get the error output if at least 50 millis have passed
if(errBuff.timeElapsed() > 50)
while(errBuff.hasNext())
println(errBuff.getNext());
// sleep a bit bofore next run
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("Finish reading error and output stream");
}
});
streamReader.start();
// remove outStreamReader and errStreamReader Thread
[...]
}
Maybe this is not a perfect solution but it should handle the situation here.
EDIT (31.8.2016)
We discussed in comments that there is still a problem with the code while implementing a stop button that kills the started
process using Process#destroy(). A process that produces very much output e.g. in an infinite loop will
be destroyed immediately by calling destroy(). But since it has already produced a lot of output that has to be consumed
by our streamReader we can't get back to normal programm behaviour.
So we need some small changes here:
We will introduce a destroy() method to the InputStreamLineBuffer that stops the output reading and clears the queue.
The changes will look like this:
public class InputStreamLineBuffer{
private boolean emergencyBrake = false;
[...]
public InputStreamLineBuffer(InputStream is){
[...]
while ((b = inputStream.read()) != -1 && !emergencyBrake){
[...]
}
}
[...]
// exits immediately and clears line buffer
public void destroy(){
emergencyBrake = true;
lines.clear();
}
}
And some little changes in the main programm
public class ExeConsole extends JFrame{
[...]
// The line buffers must be declared outside the method
InputStreamLineBuffer outBuff, errBuff;
public ExeConsole{
[...]
btnStop.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if(pro != null){
pro.destroy();
outBuff.destroy();
errBuff.destroy();
}
}});
}
[...]
private void exec(String command) throws IOException{
[...]
//InputStreamLineBuffer outBuff = new InputStreamLineBuffer(inStream);
//InputStreamLineBuffer errBuff = new InputStreamLineBuffer(inErrStream);
outBuff = new InputStreamLineBuffer(inStream);
errBuff = new InputStreamLineBuffer(inErrStream);
[...]
}
}
Now it should be able to destroy even some output spamming processes.
Note: I found out that Process#destroy() is not able to destroy child processes. So if you start cmd on windows
and start a java programm from there you will end up destroying the cmd process while the java programm is still running.
You will see it in the task manager. This problem could not be solved with java itself. it will need
some os depending external tools to get the pids of these processes and kill them manually.

Although #ArticLord solution is nice and neat, recently I faced the same kind of problem and came up with a solution that's conceptually equivalent, but slightly different in its implementation.
The concept is the same, namely "bulk reads": when a reader thread acquires its turn, it consumes all the stream it handles, and pass the hand only when it is done.
This guarantees the out/err print order.
But instead of using a timer-based turn assignment, I use a lock-based non-blocking read simulation:
// main method for testability: replace with private void exec(String command)
public static void main(String[] args) throws Exception
{
// create a lock that will be shared between reader threads
// the lock is fair to minimize starvation possibilities
ReentrantLock lock = new ReentrantLock(true);
// exec the command: I use nslookup for testing on windows
// because it is interactive and prints to stderr too
Process p = Runtime.getRuntime().exec("nslookup");
// create a thread to handle output from process (uses a test consumer)
Thread outThread = createThread(p.getInputStream(), lock, System.out::print);
outThread.setName("outThread");
outThread.start();
// create a thread to handle error from process (test consumer, again)
Thread errThread = createThread(p.getErrorStream(), lock, System.err::print);
errThread.setName("errThread");
errThread.start();
// create a thread to handle input to process (read from stdin for testing purpose)
PrintWriter writer = new PrintWriter(p.getOutputStream());
Thread inThread = createThread(System.in, null, str ->
{
writer.print(str);
writer.flush();
});
inThread.setName("inThread");
inThread.start();
// create a thread to handle termination gracefully. Not really needed in this simple
// scenario, but on a real application we don't want to block the UI until process dies
Thread endThread = new Thread(() ->
{
try
{
// wait until process is done
p.waitFor();
logger.debug("process exit");
// signal threads to exit
outThread.interrupt();
errThread.interrupt();
inThread.interrupt();
// close process streams
p.getOutputStream().close();
p.getInputStream().close();
p.getErrorStream().close();
// wait for threads to exit
outThread.join();
errThread.join();
inThread.join();
logger.debug("exit");
}
catch(Exception e)
{
throw new RuntimeException(e.getMessage(), e);
}
});
endThread.setName("endThread");
endThread.start();
// wait for full termination (process and related threads by cascade joins)
endThread.join();
logger.debug("END");
}
// convenience method to create a specific reader thread with exclusion by lock behavior
private static Thread createThread(InputStream input, ReentrantLock lock, Consumer<String> consumer)
{
return new Thread(() ->
{
// wrap input to be buffered (enables ready()) and to read chars
// using explicit encoding may be relevant in some case
BufferedReader reader = new BufferedReader(new InputStreamReader(input));
// create a char buffer for reading
char[] buffer = new char[8192];
try
{
// repeat until EOF or interruption
while(true)
{
try
{
// wait for your turn to bulk read
if(lock != null && !lock.isHeldByCurrentThread())
{
lock.lockInterruptibly();
}
// when there's nothing to read, pass the hand (bulk read ended)
if(!reader.ready())
{
if(lock != null)
{
lock.unlock();
}
// this enables a soft busy-waiting loop, that simultates non-blocking reads
Thread.sleep(100);
continue;
}
// perform the read, as we are sure it will not block (input is "ready")
int len = reader.read(buffer);
if(len == -1)
{
return;
}
// transform to string an let consumer consume it
String str = new String(buffer, 0, len);
consumer.accept(str);
}
catch(InterruptedException e)
{
// catch interruptions either when sleeping and waiting for lock
// and restore interrupted flag (not necessary in this case, however it's a best practice)
Thread.currentThread().interrupt();
return;
}
catch(IOException e)
{
throw new RuntimeException(e.getMessage(), e);
}
}
}
finally
{
// protect the lock against unhandled exceptions
if(lock != null && lock.isHeldByCurrentThread())
{
lock.unlock();
}
logger.debug("exit");
}
});
}
Note that both solutions, #ArticLord's and mine, are not totally starvation-safe, and chances (really few) are inversely proportional to consumers speed.
Happy 2016! ;)

Programatically interact with an I/O program in Java

I am writing a program that utilizes a third party mathematics software, "Maxima". This program is a command line interface, and so it can communicate through my Java program with simple I/O routing. I have already figured out how to run the program from within Java, and I have read a lot about how I can reconfigure System.out and how InputStreams/OutputStreams work, but I can't figure out how to do the following (what I think should be a pretty simple task):
Output to Maxima a command from Java, (like the string "5 + 5;")
Retrieve Maxima's output, and deal with it from Java code (like maybe printing the given string + "blah").
Output another command to Maxima from Java...
etc.
-
Below is code which will run Maxima and allow me to interact with it on the Eclipse Console
public static void main(final String[] args) {
// An idea I had for manipulaing how the printstream works.
// Set the system.out to be a custom Prinstream.
// final PrintStream interceptor = new Interceptor(origOut);
// System.setOut(interceptor);
// Run the program:
final String programLocation = "\"C:\\Program Files (x86)\\Maxima-sbcl-5.37.2\\bin\\maxima.bat\"";
final ProcessBuilder pb = new ProcessBuilder();
pb.redirectInput(Redirect.INHERIT); // Inherit I/O
pb.redirectOutput(Redirect.INHERIT);
pb.command(programLocation);
try {
// Start the program and allow it to run in Eclipse's/the program's
// console.
pb.start().waitFor();
} catch (final InterruptedException e) {
e.printStackTrace();
} catch (final IOException e) {
e.printStackTrace();
}
}
This allows for the following style of interaction:

Thanks to the words of wisdom from #RealSkeptic, I think I worked out a solution here.
The key was building a BufferedWriter, and a BufferedReader to interact with the I/O of Maxima. That is:
BufferedWriter w = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
BufferedReader r = new BufferedReader(new InputStreamReader(process.getInputStream()));
Those two lines of code create buffered readers and writers which can input data to Maxima, and read what Maxima output. Here is a (fairly lengthy) use case of this method, which I use to do basically what I asked in the question:
public class TestClass {
public static void main(final String[] args) {
#SuppressWarnings("unused")
final TestClass ts = new TestClass();
}
private BufferedWriter w;
private BufferedReader r;
public TestClass() {
// Start the process using process builder
final String programLocation = "\"C:\\Program Files (x86)\\Maxima-sbcl-5.37.2\\bin\\maxima.bat\"";
final ProcessBuilder pb = new ProcessBuilder();
pb.command(programLocation);
Process process;
try {
process = pb.start();
} catch (final IOException e) {
e.printStackTrace();
process = null;
// killProgram();
}
// Build your own wrappers for communicating with the program.
w = new BufferedWriter(
new OutputStreamWriter(process.getOutputStream()));
r = new BufferedReader(new InputStreamReader(process.getInputStream()));
// Print the five starting messages.
printFromBuffer();
printFromBuffer();
printFromBuffer();
printFromBuffer();
printFromBuffer();
// Run the following three commands in Maxima
runCommand("5+5;");
runCommand("2*65;");
runCommand("quit();");
}
/**
* Runs the given string and prints out the returned answer.
*/
private void runCommand(final String s) {
try {
w.write(s);
w.flush();
printFromBuffer();
printFromBuffer();
} catch (final IOException e) {
e.printStackTrace();
}
}
private void printFromBuffer() {
try {
final String s = r.readLine();
System.out.println(s + " -blah");
} catch (final IOException e) {
System.err.println(e.getMessage());
e.printStackTrace();
}
}
}

Reading and writing in files with Thread and semaphore

I'm new in semaphore and i have any questions. I have a thread which start and read lines from text file A and write them in to other text file B. I wrote this code, but I'm not sure whether thread block critical section and sync properly. Because and others threads can operation with these files.
public static void main(String[] args) {
Thread thread = new Thread(new ThreadManager());
thread.start();
}
Thread class:
public class ThreadManager extends Thread {
private Semaphore semaphore;
public ThreadManager() {
this.semaphore = new Semaphore(1);
}
public void run() {
try {
this.semaphore.acquire();
BufferedReader br = null;
String line;
String fileNme = "threadLog.txt";
ArrayList<String> fileLines = new ArrayList<String>();
int numLine = 0;
File outFile = new File("$$$$$$$$.tmp");
// input
FileInputStream fis = null;
PrintWriter out = null;
try {
fis = new FileInputStream(fileNme);
// output
FileOutputStream fos = new FileOutputStream(outFile);
out = new PrintWriter(fos);
} catch (FileNotFoundException e2) {
// TODO Auto-generated catch block
e2.printStackTrace();
}
BufferedReader in = new BufferedReader(new InputStreamReader(fis));
try {
while ((line = in.readLine()) != null) {
fileLines.add(line);
}
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
if (!fileLines.isEmpty()) {
int middleLine = (int) Math.round(fileLines.size() / 2);
fileLines.add(middleLine, Thread.currentThread().getName());
for (int i = 0; i < fileLines.size(); i++) {
out.println(fileLines.get(i));
}
out.flush();
out.close();
try {
in.close();
new File(fileNme).delete();
outFile.renameTo(new File(fileNme));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
this.semaphore.release();
} catch (InterruptedException e3) {
// TODO Auto-generated catch block
e3.printStackTrace();
}
}

You cannot guarantee synchronized access to a file, given only its filename, using semaphores or synchronized blocks (or whatever). Any other thread (or process) can still open, read, or modify that file, e.g. by creating its own FileOutputStream and passing the same file name.
While you could certainly structure your code to encourage synchronized access to a file, at least within your process, you cannot guarantee it. So you'll have to make some assumptions about the possibility of other processes accessing your file, and define (and document) some rules as to how other threads can access the file and adhere to them.
It looks like you're just making a temporary file so you could also consider File.createTempFile() to reduce the possibility of the same file name being used; adding uniqueness to the file name could help.
While I could definitely go into more detail about specific options, though, your precise use isn't clear from your question, and without more information, the best I can tell you is that synchronization primitives can't be used to 100% guarantee that nothing else is accessing that file at the same time - you have to consider your situation and find a balance between providing this protection through code vs. good documentation and sticking to rules that you define.
By the way, a semaphore with 1 permit is the same as a mutex, in which case you might find that synchronized blocks provide a more appealing syntax.
Also don't forget about ReadWriteLock, another useful tool depending on your access patterns and performance requirements.

Why threads not switching?

I want to get a file contains a letter from other files in random order. I must to do it with Threads. And I don't know why I have in output file content from 1 file, after it content from 2 file and after that content from 3 file. I have Main:
public static void main(final String[] args) throws Exception {
for(int i=1;i<args.length; i++) {
new Thread1( args[i], args[0]).start();
}
}
And class Thread1:
public class Thread1 extends Thread {
String path;
FileWriter fw;
private String desc;
public Thread1( String path, String desc) {
super();
this.desc=desc;
this.path=path;
}
#Override
public void run() {
FileReader f = null;
try {
f = new FileReader(path);
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
int c;
try {
fw = new FileWriter(desc, true);
while((c = f.read()) != -1) {
fw.write(c);
}
fw.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
Please, explain me, why it don't work in way, what I think it should be work.

Please, explain me, why it don't work in way, what I think it should be work.
Your problem is that all of your threads are appending to the same file but using different FileWriter instances. This sounds like it would work but they are all overwriting each other. When you open a file for appending it opens it and positions the write marker at the end of the file. When two threads do this, they both will be at the same marker. If thread #1 writes a character then thread #2 will write a character and overwrite the first.
You could use a single FileWriter and share it with each of your threads. Then you synchronize on it for mutex purposes and do the write.
public Thread1( String path, String desc, FileWriter fw) {
this.fw = fw;
...
}
...
// when you write to it, you will need to synchronize on the writer
sychronized (fw) {
fw.write(c);
}
// don't close it in the threads but close it later after you have joined with
// the threads
Another option is to share a PrintStream which is already synchronized internally.