Say I had 2 Java programs running simultaneously, one writes to a file using PrintWriter and then closes it and repeats this process, and the other program constantly reads the file as it is being written, without closing the BufferedReader until it ran to the bottom of the file. Would the BufferedReader being open stop the file from being written to?/ would this work?/ is there a better way of doing this?
Program 1 (contents of method called around once every 10 minutes):
public void write(String text){
PrintWriter wr = new PrintWriter(new FileWriter(file, true));
wr.println(text);
wr.close();
}
Program 2
BufferedReader br = new BufferedReader(new FileReader(followerFile));
String line;
while((line = br.readLine()) != null){
//do something
}
br.close();
Related
I'm relative newbie to java and am trying to automate command line using java. I tried to search for the solution here, but couldn't find it.
I created a simple test shell script like below for testing my program:
#!/bin/bash
echo "What is your name?";
read name;
echo "Hello, $name"
echo "What is your contact number?";
read num;
echo "Saved contact number $num for $name"
The Java code is below:
import java.io.*;
import java.util.*;
public class CmdLineMain {
public static void main(String args[]) throws InterruptedException, IOException {
List<String> command = new ArrayList<String>();
command.add("./test.sh");
ProcessBuilder builder = new ProcessBuilder(command);
final Process process = builder.start();
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line = "";
BufferedWriter bw = null;
while (process.isAlive()) {
line = br.readLine();
// since stream may be closed earlier, re-open it
bw = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
System.out.println(line);
if (line != null) {
switch (line) {
case "What is your name?":
bw.write("John Doe");
bw.close();
break;
case "What is your contact number?":
bw.write("123456789");
bw.close();
break;
}
}
}
System.out.println("Program terminated!");
}
}
Problem: The second input to the process fails with error:
What is your name?
Hello, John Doe
What is your contact number?
Exception in thread "main" java.io.IOException: Stream Closed
at java.io.FileOutputStream.writeBytes(Native Method)
at java.io.FileOutputStream.write(FileOutputStream.java:326)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
at java.io.FilterOutputStream.close(FilterOutputStream.java:158)
at sun.nio.cs.StreamEncoder.implClose(StreamEncoder.java:320)
at sun.nio.cs.StreamEncoder.close(StreamEncoder.java:149)
at java.io.OutputStreamWriter.close(OutputStreamWriter.java:233)
at java.io.BufferedWriter.close(BufferedWriter.java:266)
at nkh.app.CmdLineMain.main(CmdLineMain.java:34)
Closing the BufferedWriter will close its underlying streams, this includes the process OutputStream which you get through process.getOutputStream(). Thus, once it is closed in one loop, in the next loops you have your BufferedWriter wrapping a closed stream. Instead wrap the output stream only once and reuse that.
Like this:
BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
// while the stream is open and there is something to read
// probably a better condition than `process.isAlive()`
while ((line = br.readLine()) != null) {
switch (line) {
case "What is your name?":
bw.write("John Doe");
bw.newLine();
bw.flush();
break;
case "What is your contact number?":
bw.write("123456789");
bw.newLine();
bw.flush();
break;
}
}
The reason your original code died on close() rather than on the write() that came before it when your new BufferedWriter was wrapping the closed underlying stream was because BufferedOutputStream.write() may not actually write to the underlying stream yet, since it's buffered. Calling flush() should tell the stream to actually write, and as you see in the stack trace close() is calling flush() which is ultimately writing the buffered bytes to the underlying FileOutputStream, which is then realizing that the FileOutputStream is already closed.
I am trying to run a .exe file from a java application and would like to be able to use it (for now) like a terminal where i can write to the .exe then read back from it before writing it again.
My issue is that the code only works when the writer is closed before the reader attempts to read from the inputstream.
String line = "", prev = "";
Scanner scan = new Scanner(System.in);
ProcessBuilder b = new ProcessBuilder("myexe");
b.redirectErrorStream(true);
Process p = b.start();
OutputStream stdin = p.getOutputStream();
InputStream stdout = p.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(stdout));
System.out.println ("->");
while (scan.hasNext()) {
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(stdin));
String input = scan.nextLine();
if (input.trim().equals("exit")) {
writer.write("C");
} else {
writer.write(input);
}
writer.flush();
//writer.close();
while ((line = reader.readLine ()) != null) {
System.out.println ("[Stdout] " + line);
if (line.equals(prev)){
break;
}
prev = line;
}
reader.close();
}
So my question is, am i doing something wrong with the ProcessBuilder? I have read about not reading the output correctly can cause the system to hang. But this doesnt explain why it hangs when the writer is still open?
The issue was actually with my C compiled .exe. When it was running, the application was printing an output, therefore working in cmd terminal however i had not flushed the buffer after each command sent. Once i had done this, the java application would recognise each command as they were sent.
I want to write a java program to retrieve the status of all the services running on different servers (approx 20). For this i am using SC command, i am able to do so using the java program. But now i am stuck in a situation where i want to run the SC command as a different user by using RUNAS, the problem that i am facing is that i am not able to input the password once the command has been executed for the first time. Following is the code that i am using :-
String[] command = new String[3];
command[0] = "cmd";
command[1] = "/c";
command[2] = "runas /noprofile /user:domain\\admin \"sc \\\\serverName queryex type= service state= all\"";
Process p = Runtime.getRuntime().exec(command);
PrintWriter writer = new PrintWriter(new OutputStreamWriter(new BufferedOutputStream(p.getOutputStream())), true);
BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line = reader.readLine();
while (line != null) {
new PrintWriter(p.getOutputStream(),true).println("AdminPassword");
System.out.println(line);
line = reader.readLine();
}
BufferedReader stdInput = new BufferedReader(newInputStreamReader(p.getInputStream()));
BufferedReader stdError = new BufferedReader(new InputStreamReader(p.getErrorStream()));
String Input;
while ((Input = stdInput.readLine()) != null) {
System.out.println(Input);
}
String Error;
while ((Error = stdError.readLine()) != null) {
System.out.println(Error);
}
But i am not been able to print the states of all the services. I am not not sure after providing the password whether i need to capture some other stream or else??
Any help on this?
Thanks
In your while (line != null) loop you open a new PrintWriter for each line you read. You print the admin password to these writers, but never close or flush them.
Try the PrintWriter writer you created above, and flush() it after writing the password, otherwise it will still be in the buffer.
You also create several BufferedReader on the inputStream of the Process, which might interfere with each other.
So: create only one reader resp. writer for the inputStream, errorStream and outputStream of the Process.
I have a Shell Scripts that read the Input
#!/bin/bash
echo "Type the year that you want to check (4 digits), followed by [ENTER]:"
read year
echo $year
I'm executing this shell scripts using JAVA APi
ProcessBuilder pb = new ProcessBuilder("/bin/bash", "/junk/leaptest.sh");
final Process process = pb.start();
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
System.out.println("Program terminated!");
In the Java Console I can see the Output
Type the year that you want to check (4 digits), followed by [ENTER]:
Now the Actual Problem in How to pass the values to the Shell Scripts in my scripts how the varialble "year" can be read
I have edited the code as per the suggestion but doesn't work where we correct it
ProcessBuilder pb = new ProcessBuilder("/bin/bash", "-c", "/junk/leaptest.sh");
final Process process = pb.start();
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;
/*
* OutputStream os = process.getOutputStream(); PrintWriter pw = new
* PrintWriter(os);
*/
BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
while ((line = br.readLine()) != null) {
System.out.println(line);
// pw.println("8999");
bw.write("2012");
}
System.out.println("Program terminated!");
You can use the OutputStream of the Process class:
OutputStream os = process.getOutputStream();
PrintWriter pw = new PrintWriter(os);
pw.println("1997");
What you write to this output stream will become the input stream of the shell script. So read year will read 1987 to the year variable.
EDIT:
I also tried it out and I've managed to find the problem. The 1997 string hasn't reached the script, beacuse PrintWriter buffers the data that was written to it. You either have to flush the PrintWriter stream after the println() with pw.flush() or you have to set the auto-flush property to true upon creation:
PrintWriter pw = new PrintWriter(os, true);
Here is the complete code that was working fine for me:
leaptest.sh:
#!/bin/bash
echo "Type the year that you want to check (4 digits), followed by [ENTER]:"
read year
echo $year
Test.java:
import java.io.*;
class Test {
public static void main(String[] args) {
try {
ProcessBuilder pb = new ProcessBuilder("/bin/bash", "leaptest.sh");
final Process process = pb.start();
BufferedReader br = new BufferedReader(new InputStreamReader(process.getInputStream()));
PrintWriter pw = new PrintWriter(process.getOutputStream());
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
pw.println("1997");
pw.flush();
}
System.out.println("Program terminated!");
} catch(Exception e) {
e.printStackTrace();
}
}
}
Output:
$ java Test
Type the year that you want to check (4 digits), followed by [ENTER]:
1997
Program terminated!
To pass values from java program that executes script to the script use command line arguments. If you want to send information back from script to java program print the value in script, read the script's STDOUT in java program and parse it.
You really almost there. Now you are reading the script output (into while loop) but you are just printing it. Parse the output and do what you need with it.
Think you should parse input stream is to extract your values. Parse it by lines.
You want to set up an OutputStream using getOutputStream aswell, to be able to write data from your Java program into the process.
public abstract OutputStream getOutputStream()
Gets the output stream of the subprocess. Output to the stream is piped into the standard input stream of the process represented by
this Process object.
I think this should work. You need to handle your subprocess' output stream. Read the docs.
BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
bw.write("2012");
I have no idea how to do the following: I want to process a really huge textfile (almost 5 gigabytes). Since I cannot copy the file into temporarily memory, I thought of reading the first 500 lines (or as many as fit into the memory, I am not sure about that yet), do something with them, then go on to the next 500 until I am done with the whole file.
Could you post an example of the "loop" or command that you need for that? Because all the ways I tried resulted in starting from the beginning again but I want to go on after finishing the previous 500 lines.
Help appreciated.
BufferedReader br = new BufferedReader(new FileReader(file));
String line = null;
ArrayList<String> allLines = new ArrayList<String>();
while((line = br.readLine()) != null) {
allLines.add(line);
if (allLines.size() > 500) {
processLines(allLines);
allLines.clear();
}
}
processLines(allLines);
Ok so you indicated in a comment above that you only want to keep certain lines, writing them to a new file based on certain logic. You can read in one line at a time, decide whether to keep it and if so write it to the new file. This approach will use very little memory since you are only holding one line at a time in memory. Here is one way to do that:
BufferedReader br = new BufferedReader(new FileReader(file));
String lineRead = null;
FileWriter fw = new FileWriter(new File("newfile.txt"), false);
while((lineRead = br.readLine()) != null)
{
if (true) // put your test conditions here
{
fw.write(lineRead);
fw.flush();
}
}
fw.close();
br.close();