Java: How to I change the configuration file value in Java easily?

Question

I have a config file, named config.txt, look like this.

IP=192.168.1.145
PORT=10022
URL=http://www.stackoverflow.com

I wanna change some value of the config file in Java, say the port to 10045. How can I achieve easily?

IP=192.168.1.145
PORT=10045
URL=http://www.stackoverflow.com

In my trial, i need to write lots of code to read every line, to find the PORT, delete the original 10022, and then rewrite 10045. my code is dummy and hard to read. Is there any convenient way in java?

Thanks a lot !

Since you will have to read the file, why not read file line by line and keep on writing that line to the new file. and when port comes, you know to write the changed value. — Zohaib
– Zohaib, Commented Dec 21, 2011 at 10:32

Buhake Sindi · Accepted Answer · 2012-09-17 12:57:39Z

5

If you want something short you can use this.

public static void changeProperty(String filename, String key, String value) throws IOException {
   Properties prop =new Properties();
   prop.load(new FileInputStream(filename));
   prop.setProperty(key, value);
   prop.store(new FileOutputStream(filename),null);
}

Unfortunately it doesn't preserve the order or fields or any comments.

If you want to preserve order, reading a line at a time isn't so bad.

This untested code would keep comments, blank lines and order. It won't handle multi-line values.

public static void changeProperty(String filename, String key, String value) throws IOException {
    final File tmpFile = new File(filename + ".tmp");
    final File file = new File(filename);
    PrintWriter pw = new PrintWriter(tmpFile);
    BufferedReader br = new BufferedReader(new FileReader(file));
    boolean found = false;
    final String toAdd = key + '=' + value;
    for (String line; (line = br.readLine()) != null; ) {
        if (line.startsWith(key + '=')) {
            line = toAdd;
            found = true;
        }
        pw.println(line);
    }
    if (!found)
        pw.println(toAdd);
    br.close();
    pw.close();
    tmpFile.renameTo(file);
}

edited Sep 17, 2012 at 12:57

Buhake Sindi

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answered Dec 21, 2011 at 10:40

Peter Lawrey

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1 Comment

Kit Ho Over a year ago

what is the solution of preserver order? need to write the program myself?

charlemagne · Accepted Answer · 2011-12-21 10:34:43Z

My suggestion would be to read the entire config file into memory (maybe into a list of (attribute:value) pair objects), do whatever processing you need to do (and consequently make any changes), then overwrite the original file with all the changes you have made.

For example, you could read the config file you have provided by line, use String.split("=") to separate the attribute:value pairs - making sure to name each pair read accordingly. Then make whatever changes you need, iterate over the pairs you have read in (and possibly modified), writing them back out to the file.

Of course, this approach would work best if you had a relatively small number of lines in your config file, that you can definitely know the format for.

Jose Luis Tato · Accepted Answer · 2022-05-10 00:33:24Z

1

this code work for me.

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Properties;

public void setProperties( String key, String value) throws IOException {
    
    Properties prop = new Properties();
    FileInputStream ip;
    try {
        ip = new FileInputStream("config.txt");
        prop.load(ip);
        
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } 
    
    prop.setProperty(key, value);
    
    PrintWriter pw = new PrintWriter("config.txt");
    
    prop.store(pw, null);

}

edited May 10, 2022 at 0:33

answered May 10, 2022 at 0:26

Jose Luis Tato

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Comments

Jaco Van Niekerk · Accepted Answer · 2011-12-21 10:38:11Z

0

Use the Properties class to load/save configuration. Then simply set the value and save it again.

Properties p = new Properties();
p.load(...);
p.put("key", "value");
p.save(...)

It's easy and straightforward.

As a side, if your application is a single application that does not need to scale to run on multiple computers, do not bother to use a database to save config. It is utter overkill. However, if you application needs real time config changes and needs to scale, Redis works pretty well to distribute config and handle the synchronization for you. I have used it for this purpose with great success.

answered Dec 21, 2011 at 10:38

Jaco Van Niekerk

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4 Comments

Kit Ho Over a year ago

where can i load my target file?

Kent Over a year ago

property file is a good idea. however, OP didn't say it is a property file as config, but a text file. it could be ip=xx port=xx url=xx. now we want to change the port number of ip=x.y.z.n. so the key in the file could be not unique.

Jaco Van Niekerk Over a year ago

I'd drop the file in the same package (directory) as the class and then simply go: p.load(MyConfigReaderClass.class.getResourceAsStream("myconfig.txt"));

user85421 Over a year ago

but beware, this does not preserve the original order of lines (no idea if this is important)

Vadzim · Accepted Answer · 2011-12-21 10:42:04Z

0

Consider using java.util.Properties and it's load() and store() methods. But remember that this would not preserve comments and extra line breaks in the file. Also certain chars need to be escaped.

answered Dec 21, 2011 at 10:42

Vadzim

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Comments

Community · Accepted Answer · 2017-05-23 12:09:42Z

0

If you are open to use third party libraries, explore http://commons.apache.org/configuration/. It supports configurations in multiple format. Comments will be preserved as well. (Except for a minor bug -- apache-commons-config PropertiesConfiguration: comments after last property is lost)

edited May 23, 2017 at 12:09

CommunityBot

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answered Dec 21, 2011 at 10:43

Jayan

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