Question

Basically, I have to overwrite a certain property in a .properties file through a Java app, but when I use Properties.setProperty() and Properties.Store() it overwrites the whole file rather than just that one property.

I’ve tried constructing the FileOutputStream with append = true, but with that it adds another property and doesn’t delete/overwrite the existing property.

How can I code it so that setting one property overwrites that specific property, without overwriting the whole file?

Edit: I tried reading the file and adding to it. Here’s my updated code:

FileOutputStream out = new FileOutputStream("file.properties");
FileInputStream in = new FileInputStream("file.properties");
Properties props = new Properties();

props.load(in);
in.close();

props.setProperty("somekey", "somevalue");
props.store(out, null);
out.close();

Answer 1

The Properties API doesn’t provide any methods for adding/replacing/removing a property in the properties file. The model that the API supports is to load all of the properties from a file, make changes to the in-memory Properties object, and then store all of the properties to a file (the same one or a different one).

But the Properties API is not unusual in the respect. In reality, in-place updating of a text file is difficult to implement without rewriting the entire file. This difficulty is a direct consequence of the way that files / file systems are implemented by a modern operating system.

If you really need to do incremental updates, then you need to use some kind of database to hold the properties, not a ".properties" file.

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Other Answers have suggested the following approach in various guises:

1. Load properties from file into Properties object.
2. Update Properties object.
3. Save Properties object on top of existing file.

This works for some use-cases. However the load / save is liable to reorder the properties, remove embedded comments and white space. These things may matter¹.

The other point is that this involves rewriting the entire properties file, which the OP is explicitly trying² to avoid.

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^{1 – If the API is used as the designers intended, property order, embedded comments, and so on wouldn’t matter. But lets assume that the OP is doing this for "pragmatic reasons".
2 – Not that it is practical to avoid this; see earlier.}