.properties

.properties is a file extension for files mainly used in Java-related technologies to store the configurable parameters of an application. They can also be used for storing strings for Internationalization and localization; these are known as Property Resource Bundles.

Each parameter is stored as a pair of strings, one storing the name of the parameter (called the key), and the other storing the value.

Unlike many popular file formats, there is no RFC for .properties files and specification documents are not always clear, most likely due to the simplicity of the format.

Format
Each line in a .properties file normally stores a single property. Several formats are possible for each line, including,  ,  , and. Single-quotes or double-quotes are considered part of the string. Trailing space is significant and presumed to be trimmed as required by the consumer.

Comment lines in .properties files are denoted by the number sign (#) or the exclamation mark (!) as the first non blank character, in which all remaining text on that line is ignored. The backwards slash is used to escape a character. An example of a properties file is provided below.

In the example above:
 * the number sign (#) and the exclamation mark (!) at the beginning of a line (line 1, 2, etc), marks text as comments and comments they are ignored,
 * an empty line (line 5) is also ignored,
 * "website" (line 8) would be a key, and its corresponding value would be "https://en.wikipedia.org/",
 * all of the whitespace at the beginning of line 44 is excluded. This means that the key "welcome" (line 43) has the value "Welcome to Wikipedia!" and not "Welcome  to Wikipedia!".

Before Java 9, the encoding of a .properties file is ISO-8859-1, also known as Latin-1. All non-ASCII characters must be entered by using Unicode escape characters, e.g. \uHHHH where HHHH is a hexadecimal index of the character in the Unicode character set. This allows for using .properties files as resource bundles for localization. A non-Latin-1 text file can be converted to a correct .properties file by using the native2ascii tool that is shipped with the JDK or by using a tool, such as po2prop, that manages the transformation from a bilingual localization format into .properties escaping.

An alternative to using unicode escape characters for non-Latin-1 character in ISO 8859-1 character encoded Java *.properties files is to use the JDK's XML Properties file format which by default is UTF-8 encoded, introduced starting with Java 1.5.

Another alternative is to create custom control that provides custom encoding.

In Java 9 and newer, the default encoding specifically for property resource bundles is UTF-8, and if an invalid UTF-8 byte sequence is encountered it falls back to ISO-8859-1.

Editing
Editing .properties files is done using any text editor such as those typically installed on various Operating Systems including Notepad on Windows or Emacs, Vim, etc. on Linux systems.

Third-party tools are also available with additional functionality specific to editing .properties files such as:
 * Data Validation
 * Creating new keys
 * Synchronization of key names
 * Automatic bi-directional conversion of non-ASCII symbols to support non-Latin languages
 * Integration with Eclipse

Non-Java uses and exceptions
Apache Flex uses .properties files as well, but here they are UTF-8 encoded.

In Apache mod_jk's uriworkermap.properties format, an exclamation mark ("!") denotes a Negation operator when used as the first non blank character in a line.

Perl CPAN contains Config::Properties to interface to a .properties file.

SAP uses .properties files for localization within their framework SAPUI5 and its open-source variant OpenUI5

There are many Node.js (JavaScript/TypeScript) options available on Npm's package manager.

PHP also has many package options available.